GROWTH RATE OF TECTONA GRANDIS AND CEDRELA ODORATA IN
MONOCULTURE AND MIXED SPECIES SYSTEMS IN BELIZE, CENTRAL AMERICA
By
JUANITA GARCIA-SAQUI
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2007
© 2007 Juanita Garcia-Saqui
To my mother Mrs. Juanita Garcia, You Are My Star.
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ACKNOWLEDGMENTS
I would like to thank my parents, Mr. Jorge Garcia and Mrs. Juanita Garcia, for
their encouragement throughout the duration of my study. I would like to especially
thank my mother for teaching me that patience and courage are big attributes toward
achieving one’s goals in life; she believed in me from the beginning.
I would like to thank my adviser, Dr. Shibu Jose, and my committee members, Dr.
Michael Bannister and Dr. Richard Stepp, who guided me throughout the duration of my
study. I could not have done this without the help of the staff in the School of Natural
Resources and the Environment, and, to them, I am grateful. I would also like to extend
sincerest gratitude to my husband, Mr. Pio Saqui, for his patience, words of
encouragement, and for keeping me on tract. I would also like to acknowledge the
assistance of Dipl. Ing. Sylvia Baumgart, Project Manager/Coordinator OAS Agro-
Forestry Research Project, for providing technical assistance while I was in the field. You
all made my life and my studies easier at the University of Florida.
v
TABLE OF CONTENTS
page
ACKNOWLEDGMENTS ................................................................................................. iv
LIST OF TABLES............................................................................................................ vii
LIST OF FIGURES ......................................................................................................... viii
ABSTRACT....................................................................................................................... ix
CHAPTER
1 INTRODUCTION ........................................................................................................1
History of Agroforestry ................................................................................................2 Role of Agroforestry..............................................................................................6 Monoculture Systems vs. Mixed Species Systems................................................7
Ecological basis for mixed species systems ...................................................9 Mixed canopy...............................................................................................10 Deeper rooting system..................................................................................10 Improving soil quality ..................................................................................11 Available light ..............................................................................................13 Mixed species system as a pest management strategy .................................14
Current Project............................................................................................................16
2 GROWTH RATE OF TECTONA GRANDIS AND CEDRELA ODORATA IN MONOCULTURE VERSUS MIXED SPECIES PLANTATIONS IN BELIZE ......18
Problem Statment........................................................................................................19 Mixed Species Systems in Belize........................................................................20 Study System.......................................................................................................22 Characteristics of Cedrela odorata and Tectona grandis ...................................23
Materials and Methods ...............................................................................................23 Study Area ...........................................................................................................23 Experimental Design and Measurements ............................................................24 Statistical Analysis ..............................................................................................26
Results And Discussion ..............................................................................................26 Survival Rate of T. grandis and C. odorata ........................................................26 Growth.................................................................................................................27 Foliar Nutrients....................................................................................................29 Soil Fertility.........................................................................................................30
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Conclusion ..................................................................................................................32
3 CONCLUSIONS AND RECOMMENDATIONS.....................................................41
LIST OF REFERENCES...................................................................................................47
BIOGRAPHICAL SKETCH .............................................................................................56
vii
LIST OF TABLES
Table page 2-1 Stem volume index of T. grandis and C. odorata in mixed species system
compared to those in monoculture systems, four years after planting in the Cayo District, Belize..........................................................................................................38
2-2 Foliar analysis results per treatment. Chemical characteristics of the leaves from C. odorata and T. grandis at age four in mixed and monoculture systems in the study site in the Cayo District, Belize. .....................................................................39
2-3 Chemical characteristics of the top soil to 15 cm depth for monoculture and mixed species systems, Cayo District, Belize. .........................................................40
viii
LIST OF FIGURES
Figure page 2-1 Map of Belize showing project/research site in the Cayo District. .........................34
2-2 Graphical description of the monthly weather patterns (temperature (Co) and rainfall (mm)) of the study region in Belize during the study period May - August 2005 and 2006..............................................................................................35
2-3 The growth increment per month (measured during a 3 months active growing season in 2005) of C. odorata and T. grandis in mixed species and monoculture plots at the study site in the Cayo District, Belize. ..................................................36
2-4 Final height (m), GLD (cm) and SVI (cm3) of C. odorata and T. grandis at four years after planting in mixed species systems versus monoculture systems at the study site in Cayo District, Belize. ...........................................................................37
ix
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science
GROWTH RATE OF TECTONA GRANDIS AND CEDRELA ODORATA IN
MONOCULTURE AND MIXED SPECIES SYSTEMS IN BELIZE, CENTRAL AMERICA
By
Juanita Garcia-Saqui
August 2007
Chair: Shibu Jose Major: Interdisciplinary Ecology
This study seeks to understand the importance of mixed species agroforestry
systems in Belize. It investigated the growth patterns of two hardwood tree species:
Cedrela odorata L. (Cedar) and Tectona grandis L. (Teak) grown in mixed-species and
monoculture plots to determine which type of system provides the best growth pattern.
The hypothesis was that hardwood tree species grown in managed mixed-species system
would grow better because of complimentary interactions between species.
The results showed that the hardwood trees grew faster in mixed-species systems
than in the monoculture treatment. However, C. odorata was found to be more prone to
attacks by Hypsypla grandella Zellar (shoot borer) in the mixed species system than in
the monoculture plots, which reduced their height growth when compared to the
monoculture plot. Despite the H. grandella attacks of C. odorata, the mixed species
system had higher land equivalent ratio (LER) compared to the monoculture treatment,
indicating that mixing species was advantageous over growing the species in
x
monoculture. An investigation to compare soils in both systems revealed that the mixed
system improved soil fertility (higher cation exchange capacity) compared to the
monoculture treatment. Future research should examine soil and canopy nutrient
dynamics in detail so that the underlying mechanisms for the observed yield advantage in
mixed species system can be unveiled.
1
CHAPTER 1 INTRODUCTION
Belize, a Central American country, with a population size of ~290,000, is located
to the south of Mexico, east of Guatemala and bordered to the east by the Caribbean Sea.
It lies between 15°45' and 18°30' N latitude, and 87°30' and 89°15' W longitude. The
total land area is 22,960 sq km (8,867 square miles) of which 95% is located on the
mainland and 5% is distributed over more than 1060 islands. Total national territory
(including territorial sea) is 46,620 sq km (approximately 18,000 square miles) (FRA,
2000). This strategic geographical location allows Belize to be considered part of the
Caribbean economy and also integrated within Central America. The location and long
history of peaceful existence attracted an influx of Central American immigrants
(Latinos) throughout the 1980s and at a reduced rate after the mid 1990s until the present
(Zisman, 1996; Barry and Vernon, 1995).
The displaced immigrants have settled mostly in the rural areas and are now in the
forefront of local small scale agricultural production. They mostly produce vegetables
and short-term cash crop. In their quest to produce enough agricultural products, the
Latinos (also known as Mestizos) are using an intensified mode of slash and burn
agriculture, which has harmful effects on the natural environment.
The harmful effects of an intensified mode of slash and burn can be noted, firstly
by the land being cultivated repeatedly without allowing for adequate fallow periods.
Secondly, the agricultural system is being used for monoculture agricultural crops,
instead of mixed species systems (Arya and Pulver, 1993). Logically, the reasons for such
2
hard choices are financial. Monoculture systems are deemed to yield financial benefits
within short time periods. Lands that are productive are repeatedly used, leading to
degraded and almost sterile soil conditions. It is imperative that alternative forms of
agriculture are designed; those that will allow the farmers to cultivate the same amount of
land, producing a variety of products with less impact on the environment and on the soil.
History of Agroforestry
Farmers make nutrients, water and sun light available to the plants and animals
they nurture, and they do this as simply and efficiently as they can. In intensive
agriculture the task is not so much to tap naturally existing resources, but to increase their
supply to support more biotic growth, to maintain the proper conditions over longer
duration and to replenish and regulate the supply of those elements that are exhausted.
Planting several crops together in mixed stands rather than monocropping is an age-old
practice, but practiced rarely in modern intensive agriculture. In addition to multiple
products, ground cover crops used in these mixed systems provide a stratified cover
protecting the field surface from rain and direct sunlight that can contribute to soil
degradation (Netting, 1993).
It is common knowledge that population growth demands an increase of food
production and also an increase in construction material (Ruark, Schoeneberger, Nair,
2003). Boserup’s “The Conditions of Agricultural Growth” (1965) is the most cited
reference on agricultural intensification. She discusses the process of raising production
at the cost of monoculture work at lower efficiency of labor. Her influential work brought
an alternative viewpoint to the relationship between population growth and food
production, one that questioned the traditional, classical-economic approach to
agriculture based on the Malthusian paradigm. Boserup’s model defined population
3
growth as the independent variable that induces intensification and increases food
production. Therefore, it is based on the frequency that the land is cropped, portraying
agriculture as dynamic and related to a broader array of land use activities and landscape
changes which is also applicable to timber production. Boserup’s model further relates
intensity to frequency of cultivation, proposed in five progressive categories of
intensification: 1) forest fallow (20–25 year cycle), 2) bush fallow (6–10 years), 3) short
fallow (1–2 years), 4) annual cropping (yearly), and 5) multi-cropping (sequential).
Therefore, her model takes into consideration the environment, the use and importance of
technology and socio-economic impacts allowing for the relative elasticity, manageability
and variability of human societies. This mode of production provides a more optimistic
view than that of the Malthusians about human adaptive capacity to population growth
and environmental limitations (Brondizio and Siquiera, 1997).
Conklin (1957) is also a proponent of intensification via mixed species systems.
In his studies with the Hanunoo agriculture he challenged the simplistic view of
subsistence agriculture by showing the complexity and diversity of crop association and
the efficiency of labor input and output yields in these systems. On the other hand Netting
(1963, 1965) also showed the efficiency (biological) of intercropping or intensification
techniques with his work among the Kofyar of Nigeria. Nye and Greenland (1960)
showed the relationship between soil and shifting agriculture thereby providing a
scientific basis for understanding the efficiency of shifting agriculture and its impact on
soils. From their investigation they concluded that there was minimal soil loss through
this system of cultivation. Therefore these studies support the fact that intercropping or
4
mixed species systems form of cultivation is a dynamic land use system with a flexible
productive capacity.
A study done by Wilken (1987) and later by Keys (2005) provided a better
understanding of traditional agriculture and resource management practices in Mexico
and Central America. Wilken was particularly intrigued by the degree of specialization
developed by small farmers to cope with environmental and sociological limitations of
areas that are considered inappropriate for agriculture. On the other hand, McGrath
(1987) reviewed the role of biomass in mixed species cultivation and suggested that the
use of length of fallow rather than vegetation-soil complex be used to measure energy
input and intensification into this system, since burning might lead to the loss of biomass
from the system. Guillet’s (1987) studies in Peru added to this information by showing
that both intensification through mixed species and intercropping systems of production
and deintensification can occur simultaneously at the regional or the community level,
noting the importance of considering the coexistence of agriculture and other crops (e.g.
trees) within a broad scope of land use.
The following work led to the emergence of two interdisciplinary approaches.
One was identified as Farming System Research (Turner and Brush, 1987) and the other
as agroecology (Altieri and Hecht, 1990). Farming System Research focuses on
technology while agroecological studies target the understanding of ecological
relationships within agricultural systems. By their very nature agroecosystems are very
manipulable. Both approaches were interested in looking at agricultural changes in the
context of socioeconomical and ecological changes. However, agroecosystems studies
5
are broadened by a scheme that considers production and technology as well as intensity
measures with emphasis on yield and ecological stability.
Research on agricultural intensification may be summarized under two headings:
(1) intensification analysis based on small farm agriculture and resource management,
and (2) land use analysis that places agriculture within a broader spatial and temporal
landscape proposing scales of analysis at the local, regional and global levels (Netting,
1963, 1965 and 1993). Netting’s (1993) evaluation of the importance and efficiency of
small farmer intensive farming is an example of the first trend. He redefines
intensification in the light of sustainability and productivity. His conclusion was that the
disruption of small farmer agriculture in favor of modern energy-intensive technology
has recurrently deintensified agriculture and has promoted more extensive land use
systems. The second trend of agricultural studies mentioned resulted from research that
integrates approaches and methods of ecological and socioeconomic and landscape
ecology. These were deemed necessary due to the need to understand agriculture and
economics from a broader, regional scale (Brondizio et al., 1994; Moran et al., 1994a)
and to understand the impact of land use strategies in regional-scale processes (Kummer
and Turner, 1994; Ojima et al., 1994; Skole et al., 1994).
The process of integration has resulted from various unifying interests such as the
increased demand for food in less developed countries the effects of deforestation on
global biogeochemical and hydrological cycles, and the loss of biological and crop
diversity (Ruark et al., 2003; FAO, 2001; Zimmerer, 1996; Watts, 1987). Therefore by
mapping processes of human disturbance onto a landscape, translating them to the spatial
6
domain, it becomes possible to derive quantitative measures of diversity and intensity
(Behrens et al., 1994).
Role of Agroforestry
Agroforestry has evolved as a conceptual framework in agriculture and forestry
over the last 40 years as an alternative response to rural development projects. However
it has been carried out for centuries worldwide as an agricultural practice. It includes a
countless variety of systems ranging from swidden-fallow to silvopastoral activities. Nair
(1990) reported more than 150 different agroforestry systems in a global inventory
carried out by ICRAF (International Center in Agroforestry) which included traditional
and newly developed systems (Gholz, 1987). The management strategies identified in
these systems that mimics gardening and native vegetation has long provided a
diversified resource pool for Tropical countries. However, it is only until recently that
understanding of native agroforestry systems has come about. Moreover, the main
challenge to these systems has been to find ways to increase surplus production without
exponential increase of labour input, since it is based on progressive management that
incorporates previous unmanaged areas into the resource pool (Rosenberg and Marcotte,
2005; Roosevelt, 1989; Balee and Gelly, 1989).
Mixed species agroforestry illustrates the potential for intensification of this system
when opportunity such as market demand is favorable. Comparative analysis of food
production systems need to integrate a larger array of variables; since intensification
occurs when there is internal population dynamics and opportunities offered by external
sources. Therefore in the use of mixed species systems in which intensification will
occur, this intensification is defined as a dependent variable of sustainability that
accounts for the ability to maintain production over time, without constraining change in
7
the production systems in the future (Brondizio and Siqueira, 1997). Hence the
management strategy should be one that will allow the intensive use of the system
without depletion of it nutrients via the use of leguminous trees and proper draining
systems.
If mixed species type of agroforestry is practiced properly, it can be
environmentally sound, ecologically viable, sociologically acceptable and economically
feasible. Mixed species agroforestry is not a new form of agriculture in Belize C.A. It has
been practiced perhaps for centuries by the indigenous people. For example, the Maya
have used this form of agriculture in cultivating their crops while also producing timber
and non-timber forest products, including medicinal plants (Levasseur and Olivier, 2000).
Agroforestry has also been used in the form of home gardens where several species are
grown together (Levasseur and Olivier, 2000; Steinberg, 1998).
One of the agroforestry systems receiving much attention presently is the mixed
species forest plantations (Jose et al., 2006). Mixed species plantations offer multiple
market and non-market commodities or benefits such as food, fodder, timber, carbon
sequestration, and soil enrichment among others.
Monoculture Systems vs. Mixed Species Systems
Monoculture systems have been traditionally used globally to increase productivity.
However, pollution due to over fertilization has created great interest in finding ways to
decrease the amount of fertilizers being used in agricultural systems. Monoculture
systems also lead to soil degradation because of over use and extensive mechanization of
farming techniques. Pests have also been a major challenge in monoculture systems
which require a vast amount of pesticides that makes the safety of the items produced
questionable and also leads to contamination of water resources (Bruntland, 1987).
8
Furthermore as the cropping system moves from a random mix of plants to a
monoculture, the biodiversity of the system decreases. Eventually, the productivity of the
whole system can decrease (Vandermeer, 1989; Altieri, 1999) because of competition for
resources (Jose et al., 2006).
Mixed species systems of many species on the other hand are better suited for the
environment. The exception to this is the palm trees grown in monoculture systems in the
Amazon. Studies have shown that these palm trees grow better in monoculture systems
than in mixed species systems (Pollak, Mattos, Uhl, 1995). Several studies have
indicated that mixed species systems not only produce as much or even more than
monoculture systems but they also are better able to prevent soil erosion, leaching of soil
nutrients and pollution. Other advantages of mixed species systems are that herbivores
are deterred from finding their hosts, nitrogen is utilized more efficiently and it also
reduces evaporation (Netting, 1963, 1965; Vandermeer, 1989; Smith et al., 1997; Stanley
and Montagnini, 1999; and Cadisch et al., 2002).
Mixed species agroforestry systems may offer three kinds of benefits to farmers.
These are a) increased productivity, b) increased stability and c) increased sustainability.
The first benefit regarding total productivity of yield can be higher (i.e. output of valuable
products) per unit of land through reduced damage by pests and diseases.
There are several mechanisms that need to be studied in order to understand the
advantages of mixed species systems. These mechanisms include advantages of a mixed
canopy, deeper rooting system, improvement of soil quality, pest control, resource
partitioning and sharing. These mechanisms or factors are briefly discussed in the
following sections.
9
Ecological basis for mixed species systems
The ecological foundation for mixed species agroforestry systems lies in the
structural and functional diversity the plantings create at both the site and landscape
levels. Mixed species plantings can help add structural and functional diversity to
landscapes and, if strategically located, they can help restore many ecological functions
(Ruark et al., 2003). Mixed species systems are common worldwide. Most agricultural
production especially in developing countries is done using this type of production
(Arnon, 1972; Alas, 1974; Nair, 1990). Since it is so common it has attracted a lot of
attention, especially because there are so many systems which are referred to as mixed
species or intercropped systems. Vandermeer (1989) lists a total of 55 combinations, but
Nair (1990) reported more than 150 systems. According to Lamberts (1980) there are
several reasons for cultivating in mixed species systems, and these include
• increased productivity or yield advantages; • better use of available resources (land, water, nutrients, labour, time); • reduction in damage caused by pests (diseases, insects, weeds); • food and cash-flow (economics, human nutrition, greater stability etc.).
This aspect of mixed species systems look at the organism and environment
interactions in which the organism and the environment affect one another. Hence a plant
may influence its neighbour by changing its environment resulting in an effect and
response reaction by the individuals involved. The changes need not be negative, but
requires a response from the other plants such that a plant may either deplete a resource
or may enhance it making it available for its neighbour. However, there are changes that
may result in negative effects such as nutrient extraction that leads to depletion of a
resource or production of shade which may not be good for another individual. A positive
10
interaction may occur such as the case when trees prevent soil erosion, and deep roots
prevent soil nutrient leaching from the system (Vandermeer, 1989).
Mixed canopy
According to Gathumbi (2004) a mixed species system can have a denser canopy
than that of a monoculture system when different species occupy different canopy
positions and levels, allowing it to capture light that would otherwise be inefficiently
utilized in monoculture systems (Morales and Perfecto, 2000; Kelty, 2006). Mixed
canopy may also reduce weed competition, by reducing incident light on the forest floor
making the plant unable to photosynthesize and eventually die. It can also reduce water
loss by evaporation directly from the bare soil, with the use of cover crops leaving more
water for productive transpiration. Furthermore, evaporation of transpired water or
precipitation intercepted by the canopy may further contribute to understory temperature
reductions (Huxman and Smith, 2001; Unwin et al., 2006) and the formation of
microclimates suitable for other organisms in this way providing habitat for organisms
(Ruark et al., 2003).
Deeper rooting system
A mixed species system may also have a denser and perhaps deeper rooting system
allowing maximum use of soil, thereby increasing the potential for water and nutrient
uptake (because different species may use different soil depths) (Akinnifesi et al., 1996;
Morales and Perfecto, 2000). Coupled with better soil physical properties and the
reduction of runoff it may conserve water, leading to enhanced soil biological activity
and nutrient cycling. Through this process it increases the availability of nutrients which
can be readily absorbed by the plant roots at different soil levels since plant roots obtain
most of their nutrients from the soil solution (Holcomb, White and Tooze, 1982; Smith et
11
al., 1997 and Cadisch et al., 2002b). Therefore, intercropping or mixed cropping in small
plots may have the potential to increase total yield compared to those of monoculture
plots using the same resource base (Mead and Willey, 1980). This can also result in more
efficient use of farm resources, thereby increasing economic returns (Hiebsch and
McCollum, 1987). If planned with consideration for each species' response to mixed
conditions, mixed designs can be more productive than monoculture systems (Smith,
1986; Binkley et al., 1992; Cannel et al., 1996). Both T. grandis and C. odorata are tall
trees 30 to 40 m in height. The T. grandis roots extend to a dept of ~20 feet into the soil
column while C. odorata roots have been reported to be superficial with a tendency to
become deeply rooted if the soil is loose or coarse.
Improving soil quality
One of the main conceptual foundations of mixed species system is that trees and
other vegetation improve the soil beneath them. Observations of interactions in natural
ecosystems and subsequent scientific studies have identified a number of facts that
support this concept. Mixed species agroforestry systems have the ability to contribute
significantly to maintaining or improving soil and water quality in a region. However the
degree to which these and other ecological functions can be provided will depend on
plant species composition and their physical structure both above- and below-ground
(Wang et al., 1991; Stanley and Montagnini, 1999; Cusack and Montagnini, 2004; Jose et
al., 2004).
Water relations are very important because water is the medium through which
many of the resources are transported. Nitrogen dissolves in water and moves through
mass flow. On the other hand potassium and phosphorous are easily adsorbed on the
surface of soil particles and when this happens these nutrients move slow in the soil
12
(Vandermeer, 1989; Brady and Weil, 2002). This means that these nutrients can be
immobilized and become unavailable for plant uptake. However if the soil is fertile
allowing organisms to live within it, they can break down those minerals and make them
available for plant uptake (Brady and Weil, 2002; Mooney et al., 2002).
According to Ruark et al. (2003) three main tree-mediated processes have been
identified through research which determines the extent and rate of soil improvement in
mixed species systems. These are 1) increased N input through biological nitrogen
fixation by nitrogen-fixing trees, 2) enhanced availability of nutrients resulting from
production and decomposition of substantial quantities of tree biomass, and 3) greater
uptake and utilization of nutrients from deeper layers of soils by deep-rooted trees (Nair
et al., 1999).
In addition, a mixture of species, each with different nutrient requirements and
different nutrient recycling properties, may be overall less demanding on site nutrients
than pure stands because of their niche separation (Binkley et al., 1997; Jose et al., 2006).
This indicates that the trees are using the nutrients in different proportions and during a
different time period in their growth patterns. In a study of mixed versus monoculture
plantations in Costa Rica, Montangnini et al., (1995) and Montagnini and Porras (1998)
found that the growth of dominant species was faster in mixed than in pure plantations,
and that mixed plantations had high volume and biomass production in comparison with
pure stands. In another study, the mixed plantations had intermediate values of soil N, P
and K, but lower soil Ca and Mg relative to pure plantations (Stanley and Montagnini,
1999) which supports the fact that nutrients are used differently throughout a mixed
13
species system hence there is more nutrient availability in a mixed species system than in
a monoculture plot.
Available light
It is common knowledge that the rate of photosynthesis is an increasing function
of the intensity of light, so that the rate of photosynthesis increases rapidly when a low
level of light is elevated and increases slowly when a high level of light is elevated
(Vandermeer, 1989; Chapin et al., 2002). T. grandis produces large broad leaves which
can be dominant and may decrease the amount and quality of light that reaches the plants
at the lower level. Another factor that would affect light penetration is the overlap of
canopies leading to reduced light intensity. However in a system where the trees are
planted at a distance of 5 m to counteract this effect it will not cause a major effect. On
the other hand this canopy effect will also be evident after the trees have grown and their
canopies have overlapped. In young trees (5 years) this effect is not evident because the
canopy formed still has more space to expand without causing negative effects.
Furthermore, the physiological and morphological traits such as differences in root versus
shoot allocation and differences in leaf structure in the trees of the mixed species systems
are also playing an important role in the capture of sunlight and resources at different
levels in the system (Medhurst et al., 2006).
The light intensity filtering through a mixed species system may be sufficient
enough for shade tolerant and C3 plants to grow in the understory. Hence it is easier for a
mixed species system to utilize available solar energy more efficiently than a
monoculture system, especially when careful planning is involved (Vandermeer, 1989;
Chapin et al., 2002; Unwin et al., 2006). However, it must remain clear that light
14
available for photosynthesis diminishes as it moves from the canopy to the lower layers
of the system.
Mixed species system as a pest management strategy
There are many studies on the effect of intercropping on pest attacks. Although
studies abound, they are often contradictory due to the difficulty of pointing out the
ecological factors that can affect insect-plant relations (Kelty, 2006; Rämert et al., 2002).
In one of his studies Andow (1991) analyzed 209 studies involving 287 pest species on
mixed species system versus monocultures. When compared with monocultures, the
population of pest insects was lower in 52% of the studies (149 species) and higher in
15% (44 species). The population of natural enemies of the pests was higher in the
intercrop in 53% of the studies and lower in 9%.
In another study conducted in the Organization of American States (OAS) funded
project site in Belize it was determined that there were more pest attacks in the hot pepper
monoculture plots than in the mixed species plot (Imhof, 2004). The results of such
studies therefore imply a complex situation in which the specific agro-ecological
situation is important. However, in order to develop mixed species cropping as a tool it is
necessary to understand the underlying mechanisms involved.
Plants belonging to the same or a very close taxonomic group have the tendency to
share common pests. In agroforestry systems, aligophagous and polyphagous insect pests
are expected to thrive if both components belong to the same or a closely allied
taxonomic group. An insect feeding on a plant with a certain biochemical make-up will
adapt more easily to closely related plants with similar biochemical constituents than to
species that have entirely different constituents because of taxonomic differences. A
mixed species agroforestry system comprised of plant species belonging to different
15
taxonomic groups is expected to be less affected by insect pests than a system composed
of closely related species as in a monoculture system (Vandermeer, 1989).
Under natural conditions, even insects with a limited host range have been
observed to feed on taxonomically diverse species of plants. In a mixed species system,
therefore, the plants assembly should consist of species that do not double as host for
insect pests of other plants in the system - whether crops or woody perennials. Some
insects utilize different host plants as food in their larval stages from those eaten in the
adult stage. So an even greater range of plants in a mixed species system may be attacked
by different stages of an insect pest.
If all or most plants in a mixed system are palatable to a polyphagous pest, then it is
likely that the insect will stay longer and become more numerous, causing greater
damage. Therefore, monophagous pests can be controlled altogether by not including
their host plants in the system. The host range of oligophagous pests can also be
narrowed by eliminating palatable species form the assemblage and replacing them with
non-host plants. Therefore what’s important to remember is that farmers need to be
knowledgeable about such pest relationships in order for them to be able to make
informed choices when preparing their fields.
There are two existing hypothesis proposed initially by Aiyer (1949) and redefined
later by Root (1973) and proposed as three hypothesis by Vandermeer (1989) in reference
to the presence of pest in an area; (1) the disruptive crop hypothesis in which a second
species disrupts the ability of a pest to efficiently attack its proper host (specialist
herbivore) (2) the trap crop hypothesis in which a second species attracts a pest that
would normally be detrimental to the principle species (generalist herbivore) (Hokkanen,
16
1991) and 3) the enemies hypothesis in which natural enemies are more effective and
numerous in diverse systems reducing the pests through predation or parasitism.
Therefore it can be said that the first hypothesis more closely explains observed pest
attack on the study conducted in the trees funded by the OAS in Belize although the exact
mechanism of concentration on the resource is not defined. However, there are currently
several competing explanations which are probably best summarized by Finch and
Collier (2000). These authors speculate that insect pests settle on plants only when
various host plant factors such as visual stimuli, taste and smell are satisfied. This is more
likely to occur in monocultures than in mixed species systems where the chance of
encountering a ‘wrong’ stimulus is much increased. Therefore, the complexity of the
overall picture makes it difficult to specifically design intercrops for pest control without
practical knowledge of pest and crop biology which sometimes can be obtained through
research and traditional ecological knowledge (Hokkanen, 1991).
There is a list of pests and diseases which a mixed species system is said to control.
These include a reduction in insect attacks, nematodes, diseases, herbivore attacks,
aphids, and weed control (Risch et al., 1983; Leibman, 1986; Andow, 1991) among
others in which a specialist pest is said to be deterred from its host through the disruptive
effect of a mixed species system of plant (Trenbath, 1976).
Current Project
The objective of the study was to examine growth and soil fertility status of
monoculture and mixed species plantations of Cedrela odorata L and Tectona grandis L
in Belize. We hypothesized that mixed species plantations will have higher soil fertility
and better growth compared to monoculture plantations because of the synergistic
17
interactions when species are mixed together. The following chapter describes the study
in detail.
18
CHAPTER 2 GROWTH RATE OF TECTONA GRANDIS AND CEDRELA ODORATA IN
MONOCULTURE VERSUS MIXED SPECIES PLANTATIONS IN BELIZE
Introduction
The forests of Belize cover slightly more than 85% of its surface area which is very
different when compared to neighboring countries in Central America that have not been
as successful as Belize in protecting their forests. According to the FAO the rate of
deforestation in Belize is relatively low (i.e., approximately 0.25% per year compared to
other countries in the region) (FAO, 1997). The vast majority of these forests, situated in
central and southern Belize, are tropical rainforests which constitute an important
reservoir of biodiversity worldwide. Furthermore, they are an important source of wood,
medicinal plants, and all kinds of natural products (Bruntland, 1989; Boot, 1997).
Although Belize has been able to preserve most of its environmental resources to a
much greater extent than other Central American and Caribbean countries, its economy
has been and will remain highly dependent on environmentally-based industries (FAO,
2001). The forests of Belize are by no means undisturbed; they are dynamic and range
from freshly burned milpas to dense tropical forests.
The main cause of forest degradation in Belize is slash-and-burn agriculture (with
no fallow period), as practiced by the Maya and the Mestizo population (Arya and Pulver,
1993). Slash and burn agriculture in Belize leads to permanent agriculture; in which
crops are rotated initially, but become permanent at a later stage. It is in this system that
the mixed species is mostly centered on, because trees are planted for several purposes
including food, fodder, timber and medicinal purposes among other uses.
19
Problem Statment
Belize has a low population density of hardwood trees species such as Swietenia
macrophylla King and Cedrela odorata L, growing naturally in the forests (Levasseur
and Olivier, 2000). In spite of this situation there is an increasing demand for hardwood
lumber locally. This demand is further intensified by the rise of new industries like
tourism, which is currently the main income generating industry in Belize (World Fact
Book, 2007). Among the various forms of tourism, ecotourism seems to demand the most
natural resources including timber. Those operations build structures that are aesthetically
appealing to foreign travelers, often requiring the use of local construction material. This
form of tourism is placing an even greater demand on lumber for the construction of
resorts instead of alternative building materials (Primack et al., 1997).
In 2001 and 2002 Belize experienced a catastrophic infestation of the Southern
Pine Bark Beetle, which totally destroyed the country’s largest managed forest
ecosystem, Mountain Pine Ridge Forest Reserve. This forest was 31-years-old, and
consisted of the Caribbean White Pine (Pinus caribaea) species which was selectively
harvested for timber (GOB, 1996). The plantation is now unproductive and in a
regeneration phase. The demand for hardwood remains which has forced the logging
industry to revert to harvesting of timber in natural forest stands. This option makes
forest reserves and other unprotected forests vulnerable. With forest reserves not well
monitored (because of lack of personnel) by the Forestry Department of the Government
of Belize (GOB), many designated forest reserves are being illegally harvested. C.
odorata and S. macrophylla are the two hardwood species targeted. These species are
harvested uncontrollably in various sizes, in some cases, before the trees are at the
regulated DBH size or mature enough to reproduce. Other hardwood trees species are
20
also been extracted for domestic and export uses. However, C. odorata is the preferred
hardwood species domestically and has become scarce (GOB, 1996; Primack et al.,
1997).
As logging becomes unproductive, there is a shift towards agriculture, which leads
to increased negative impacts such as deforestation and soil degradation. This keeps
exerting pressure on forest managers and scientists to seek alternative ways to meet
demands for timber. The demand for timber is projected to increase consistently. One
promising alternative to increase timber production is agroforestry in the form of mixed
species systems.
Mixed Species Systems in Belize
Traditionally C. odorata (Cedar) and leguminous trees (i.e. Lonchocarpus castilloi
(Black cabbage bark) and Sweetia panamensis (Billy Webb)) grow naturally in the sub-
tropical forests of Belize. A more recent timber species of choice is Tectona grandis
(Teak). These tree species are now being incorporated into agricultural farming systems,
for purposes other than timber production such as protection against soil erosion, nutrient
cycling, and shade. Similarly, cash crops varieties such as hot peppers and papayas are
intercropped in these systems. However, when these trees are grown in managed mixed
species agroforestry systems (Nair, 1984), especially in plots that includes other
leguminous plant species such as Lonchocarpus castilloi (Black cabbage bark) and
Sweetia panamensis (Billy Webb) and ground cover crops such as the perennial Arachis
pintoi (pinto peanut), competition for essential nutrients and sunlight may be less severe
than in monocultures because of the niche separation (Vandermeer, 1989; Jose et al.,
2006).
21
Mixed species systems are commonly used in Belize in subsistence farming.
However, there is a common perception that these systems are more time consuming to
care for and demands higher investment at the initial plantation phase. Furthermore, it is
perceived that mixed species systems are not as productive as monoculture systems, and
that the trees do not trive as well as in monoculture. It would seem to farmers that the
results are not the same as those obtained in a monoculture plantation. Hence, few
hardwood trees have been intercropped in these systems.
The ecological aspect of a mixed species system is seldom taken into consideration
when establishing hardwood tree plantations. However, several studies have shown that
mixed-species plantations have a high potential for accelerating the process of natural
succession and establishing a stand of ecologically and economically desirable trees
(Montagnini, 1999; Dommergues and Rao, 2000). Other studies have also shown that
mixed-tree plantations could be an effective tool for alleviating site degradation, acting as
a catalyst for forest regeneration and rehabilitation (Lugo, 1988; Parrotta, 1995).
The idea behind a mixed species systems is to capitalize on the beneficial
interactions between the species planted while avoiding negative interactions. Results
from studies done in Belize show that more favorable credit rates, labor saving
technology, and intensive shade management have the strongest potential to increase
smallholder income (Levasseur and Olivier, 2000). If mixed species systems were
continuously implemented adding annual shade tree crops such as fruits, nuts, or spices
there is potential to significantly improve smallholder income. Furthermore, since the
systems are usually small plots (~5ha), labor is mostly carried out by family members;
22
thereby reducing the monetary input into establishing and managing these mixed species
systems.
In order for mixed species production systems to become an important land use
practice in Belize, the farmers have to be convinced about the benefits of these systems.
Scientific evidence is now available to show that the spatial and temporal heterogeneity
created by the mixed species plantings can help enhance resource availability and
capture, increase production, reduce risk of intensive agricultural and forestry practices,
and achieve system stability and sustainability (Lefroy et al., 1999; Nair, 2001). Mixed
species plantations also yield more diverse forest products than monoculture stands,
helping to diminish farmers’ risks in unstable markets. Lastly mixed species cropping
could also be an important tool for pest and disease management in agriculture or farming
systems as has been shown by studies done by Morales and Perfecto (2000) in
Guatemala; Montagnini et al. (1995) in Costa Rica; and by Kelty (2006) and Risch et al.
(1983) who reviewed published articles on mixed species systems and pest management.
Unfortunately, there is no documentation on the performance of mixed versus
monoculture systems for popular species in Belize. Therefore it is not likely to be widely
adopted by commercial farmers until the potential benefits have been fully evaluated and
can be shown to outweigh those of the monoculture systems.
Study System
This study examined the growth and soil chemical properties of mixed and
monoculture plantations of C. odorata and T. grandis in Belize. We hypothesized that
• C. odorata and T. grandis trees growing in mixed species treatment will produce more volume than in monoculture treatment;
• survival and growth of selected tree species will be better in mixed-species than in monoculture treatment;
23
• growing C. odorata and T. grandis in mixed species plots will improve soil quality by increasing soil organic matter and nutrient content.
Characteristics of Cedrela odorata and Tectona grandis
The species used in the current study are different in their physiology and perhaps
can compliment each other in the use of resources. C. odorata has monopodial growth,
with orthotropic branches that form an open crown. It has large, pinnately compound
leaves that can be up to a meter long, with 10–20 pairs of leaflets, each about 40 cm2
(Hiremath et al., 2002). On the other hand, T. grandis has an umbrella like crown with
large, thick leaves whose leaves are simple and opposite, hence it allows the infiltration
of sufficient light for photosynthesis by the understory plants. The rooting pattern of the
trees is also of interest since it determines the extent to which these trees are tapping the
soil nutrients. T. grandis root extend to a dept of ~20 feet into the soil column while C.
odorata roots are mostly superficial but becomes deeply rooted if the soil is loose or
coarse.
Materials and Methods
Study Area
The study was carried out in the Cayo District (Figure 2-1) in Belize (16°50' and
17°45' north latitude, and 88°50' and 89°1' west longitude). The soil varies in texture
from sandy loam to sandy clay but invariably contains angular quartz grit. It is acidic and
has very low contents of available plant nutrients (King, Baillie, Abell, Dunsmore, Gray,
Pratt, Versey, Wright, Zisman 1992) except for calcium. The area experiences a mean
annual rainfall of 1400 mm with a mean temperature of 26°C (Figure 2-2) (Hydromet
Services, 2006).
24
Mean rainfall in this area during the study period ranged from 75 mm to 286 mm
per month between May through August with the wettest month being in August and the
driest month in July 2005. Rainfall pattern differed in 2006 during which there was more
rainfall during the months of June and July declining in August (Figure 2-2).
This research was carried out in conjunction with an ongoing agroforestry research
project funded by the Organization of American States. Hardwood trees species grown in
the western parts of Belize in the Cayo District provide an excellent case study for the
potential of hardwood tree species systems to address the needs of limited timber
resources in Mesoamerica and the Caribbean.
Experimental Design and Measurements
The experiment was set up as a completely randomized design with five
replications. Each plot was 471.5 m2 and contained 40 trees. The two treatments were
monoculture (C. odorata or T. grandis alone; i.e. 40 trees of the same species per plot)
and mixed species plantation (C. odorata and T. grandis mixed together in the same plot
for a total of 40 trees per plot; i.e. 20 trees of each species). The mixed species plot also
had ground cover of Arachis pintoi, Swietenia macrophylla and the leguminous trees,
Lonchocarpus castilloi, and Sweetia panamensis. However only C. odorata and T.
grandis were used in this research. The crops that have been traditionally intercropped in
the mixed species plots have been hot peppers (Capsicum chinense Jacq) and papayas
(Carica papaya). The hardwood trees studied were pruned after the first three years.
Each hardwood tree and leguminous trees were systematically planted at a distance of 5
m from each other (the papaya and hot peppers were intercropped in the space between
the trees) The trial did not receive any irrigation, fertilizers and pest control.
25
Height and ground line diameter (GLD) were measured on a weekly basis from
May through August in 2005 and in August 2006. Tree height was determined using a
height pole, while diameter was measured with a digital caliper. Stem volume index
(SVI) was then calculated using the following formula: SVI=GLD2 x height.
Volume yield of monoculture and mixed species plantations was compared using
the Land Equivalent Ratio (LER) using the following formula:
LER = SVI C. odorata (mix) / SVI C. odorata (monoculture) + SVI T. grandis
(mix) / SVI T. grandis (monoculture).
The LER is the ratio of the area needed under monoculture to a unit area of intercropping
at the same management level to give an equal amount of yield. Therefore it compares
the performance in intercrop to the performance in monoculture of a particular species;
i.e., in this case T.grandis and C.odorata in mixed versus monoculture systems.
Soil samples (0-15 cm) were collected from the mixed-species and the monoculture
plots twice (2005 and 2006), but data were pooled since no differences were found
between years. Soil samples from the mixed species plot appear as one soil because the
soil was taken from the plot consisting of both trees. Soil samples were processed by air
drying overnight, removing root parts and large stones, and crushing remaining soil until
finely ground. Samples were analyzed for organic matter, pH, CEC, K, Ca and Mg
content. Ca, K, and Mg were extracted using neutral normal ammonium acetate, and
analyzed by atomic absorption spectrophotometer. Soil pH was measured in a 2:1 soil:
water slurry made with deionized water. Organic matter was analyzed by
dichromate/colorimetric method while Cation Exchange Capacity was calculated from
26
the cation results. All analyses were performed at a commercial lab (A & L Southern
Agricultural Laboratories, Inc., Pompano, Florida).
Leaf samples were also collected during the first year for foliar chemical analyses.
The foliar tissue samples were first washed and oven dried (75°C) for 48 hr. The dried
tissue samples were ground in a Wiley Mill. Foliar samples were digested by a wet
ashing procedure using sulfuric acid and hydrogen peroxide. The digest is then filtered
and fractioned. The different elements were analyzed using atomic absorption and UV-
Visual Spectroscopy (Wolf, 1982). All analyses were performed at a commercial lab (A
& L Southern Agricultural Laboratories, Inc., Pompano, Florida).
Statistical Analysis
Data was analyzed using ANOVA within the framework of a randomized complete
design using SAS. The dependent variables were tested for normality using the Shapiro–
Wilk statistic. Duncan’s test was used for mean separation if ANOVA revealed
significant differences at α=0.05.
Results And Discussion
Survival Rate of T. grandis and C. odorata
Survival of trees varied between treatments. Five years after planting, there was a
survival rate of 77 % in monoculture treatment. On the other hand, the survival rate of the
mixed species treatment was a little higher at 80%, considering that most mortality
occurred on the C. odorata trees due to the Hypsipyla grandella Zeller infestation.
Individually, the survival of C. odorata in the mixed species treatment was 75% while
that of the monoculture treatment was 77.5%. The survival of T. grandis in the mixed
species treatment was 85% while on the monoculture treatment it was 77.5%. The
27
overall results indicate that the survival rate of trees in the mixed species treatment was
better than those in the monoculture treatment.
Growth
T. grandis grown in mixed species system had greater growth increment (Figure
2-3) compared to monoculture during 2005 and 2006 (Figure 2-4). Similar growth
pattern was recorded for GLD and SVI. Height and GLD of T. grandis at the end of the
growing season in 2006 showed significant differences (p<0.0001) between the
monoculture and mixed species systems. T. grandis trees in the mixed species plots were
40% taller than those in the monoculture plots. Again, similar patterns were observed for
both GLD and SVI. For example, SVI of T. grandis in mixed species system was 0.91
m3 per ha. compared to 0.49 m3 per ha in monoculture (Table 2-1).
Height increment of C. odorata was significantly different between the
monoculture and mixed species systems as depicted by the different letter in Figure 2-3
and 2-4. The growth rate of the C. odorata trees was better than that experienced by the
trees in the mixed species system. Height increment in the mixed species system was
impacted mostly by a shoot borer, H. grandella, which attacked the leading shoots
(Cornelius and Watt, 2002), which had to be pruned regularly. GLD, however, was
greater in the mixed species system compared to monoculture. It was noted that the most
growth occurred during the beginning of the growing season (wet/rainy season). The
growth pattern observed was similar to what Worbes (1999) found in Venezuela in a
study done on T. grandis and C. odorata among other tropical trees and by Dunisch et al.
(2002a) and Dunisch et al., (2003) in studies done in Central Amazon on the growth
increment of C. odorata.
28
Stem volume index (SVI) per ha was also computed and compared (Table 2-1).
The comparison of SVI revealed significant difference between T. grandis in mixed
versus monoculture plots (p<0.0001), but there was no difference between C. odorata in
mixed vs. monoculture plots (p<0.001). The growth rate recorded during this study was
similar to those obtained in other studies for T. grandis and C. odorata, despite the H.
grandella attacks in these plots (Montagnini et al., 1999; Parrota, 1999).
The LER for the mixed species system was 1.47, which indicates that there is some
sort of facilitation occurring because of a modification of some environmental factor(s) in
a positive way by one or both of the species being intercropped. The mixed species
system could be beneficial compared to monoculture plantations of the same species. It
appears that intercropping does not affect the growth rate of C. odorata trees although the
height of the trees in the mixed species plot was severely affected by the H. grandella
attack. However, T. grandis growth was significantly improved as a result of mixing
species. So, overall, mixed species system had higher production potential than
monoculture. This is similar to the findings of other researchers in which they found that
mixed plantations had overall greater growth in the mixed species systems compared to
the monoculture ones (Montagnini et al., 1993, 1995; Montagnini and Porras, 1998)
which can be a result of the leguminous trees intercropped within the system.
According to Vandermeer (1989) if an experimental plot yields an LER greater
than 1.0 it is certainly true that facilitation has been demonstrated in the system. Hence
competition may be minimal, because competition cannot be eliminated totally from such
a system since resources are certainly being used, but possibly from different levels of the
soil column and at different times of their growing period.
29
Vandermeer (1989) suggests that the LER measurement takes its name from its
interpretation as relative land requirements for mixed species versus monocultures.
Therefore, what we are looking at is whether the monoculture plots are producing the
same amount of biomass as the mixed species systems or vice versa. If the LER is greater
than 1.0 then the mixed species system is more efficient. If it is less than 1.0 then
monocultures are more efficient systems of production of the hardwood trees.
Foliar Nutrients
According to Goncalves et al. (1997), Montagnini and Sancho (1994), Foèlster and
Khanna (1997), soil nutrients may be generally abundant early in stand growth as a result
of low plant uptake, stimulation of nutrient mineralization, and low immobilization in
plant biomass, but as plantations grow, decreased nutrient availability can result from
immobilization into woody biomass and detritus pools, and decreased mineralization
(Binkley et al., 1997; Foèlster and Khanna 1997; Wadsworth 1997). Therefore,
alternatives to conserve site nutrients may include preferential planting of tree species
that do not place high nutrient demands on the site (Bruijnzeel, 1984; Wang et al., 1991;
Montagnini and Sancho, 1994). Information available is usually on the effect of trees on
monoculture systems while the effect of different trees on mixed species system is
minimal.
Table 2-2 shows the range in concentration in the leaf for the elements N, P, K, Ca
and Mg observed during the research. However, there is insufficient data to evaluate the
treatment differences in leaf composition for these elements in T. grandis and C. odorata.
Although limited samples prevented a statistical comparison, it appears that there is more
calcium, magnesium and nitrogen in the leaves of the C. odorata in the mixed species
plots than those of the monoculture plots (Table 2-2). The higher nitrogen may be due to
30
the presence of nitrogen fixing trees and the nitrogen fixing cover crop in the mixed
species plots. However, there was more phosphorous and potassium in the leaves of the
C. odorata in the monoculuture plot than in those of the trees in the mixed system plot.
The result for leaves of the T. grandis shows that there is again more nitrogen along with
more potassium in the mixed species plot compared to the monoculture plot. The nutrient
level of phosphorous, calcium and magnesium are the same for trees in monoculture
versus those in the mixed species system.
The mixed species plot has a diverse amount of trees and other crops (hot peppers
and papaya) intercropped annually which would mean that more nutrients are being
utilized in the mixed species plot. However, the results of the foliar analysis indicate that
although there is extraction of the resources through harvest of the crops the mixed
species plot still has overall more available nutrient resources in the mixed species plot
compared to the monoculture plot.
Soil Fertility
Contrary to our expectation, there was no significant difference in soil organic
matter, pH and K between soils of the monoculture C. odorata and the mixed species
treatment. However, there was difference noted in Mg which was higher in the C.
odorata monoculture plot compared to mixed species plot (Table 2-3). Marked
difference in soil CEC between the mixed and the monoculture systems was also
observed with mixed species systems exhibiting higher CEC than monoculture system.
The fact that soils under mixed species had higher CEC perhaps points to the higher soil
fertility in the system.
The soil under monoculture T. grandis had lower CEC, and lower organic matter
compared to the mixed species plot. However, the other nutrients (pH, K, Mg and Ca)
31
were not different between the two treatments. The higher growth observed in T. grandis
could be a result of the higher CEC in the mixed species plot (Table 2-3).
Higher soil fertility in mixed species systems compared to monocultural systems
has been reported in other studies done in Puerto Rico by Parrotta (1999), and also in
Costa Rica by Montagnini (2000). They found that soil fertility was higher in mixed-
species plantations of Strephnodendron microstachyum, Vochysia guatemalensis,
Jacaranda copaia and Callophylum brasiliense than in pure stands of C. brasiliense and
V. guatemalensis. Stanley and Montagnini (1999) obtained similar results in a study on
nutrient accumulation in pure and mixed plantations in Costa Rica. This implies that
there is perhaps faster nutrient cycling and turnover in mixed species systems than in
monocultures, making the soil fertility higher in the former. It is also possible that the
mixed species system has a better nutrient use efficiency. As suggested by Jose et al.
(2006) and Binkley et al. (1997), in a mixture of species, each with different nutrient
requirements and different nutrient recycling properties, there may be overall less
demand for on site nutrients because of the niche separation. This means that each plant
utilizes the resources at a different stage in their life cycle or at different rates. On a
study done by Stanley and Montagnini (1999) in plantations of mixed versus
monoculture, the mixed plantations had intermediate values of soil N, P and K, but lower
soil Ca and Mg relative to pure plantations which was similar to those obtained in this
experiment. It is possible that Ca and Mg are removed from the soil and stored in the
plant biomass during the active growing season, and are subsequently removed when the
plants are harvested.
32
Conclusion
The results obtained during this research have led to the conclusion that when C.
odorata and T. grandis are grown in mixed species systems with leguminous trees they
have the capacity to produce a higher yield compared to monoculture treatment.
Although we did not measure litterfall which should have given an indication of exactly
how much litterfall occurred in both systems it was evident that there was efficient
cycling of nutrients in the mixed species system because of the higher soil C.E.C in the
mixed species system compared to the monoculture plot. Similar results have been
obtained by several researchers who have investigated similar treatments in the tropics
(Binkley et al., 2003; Montagnini, 2000; Stanley and Montagnini, 1999; Parrotta, 1999)
in which they found that mixed species systems produce a more complex array of
litterfall releasing nutrients back to the soil at different rates because of the composition
of the leaves. Therefore, mixed-species plantations have the potential for out-producing
monocultures, but actual yields depend on soils, silviculture, and species. It is important
to know more about these interactions to provide a solid foundation for mixed-species
management of plantations (Binkley et al., 2003; Dommergues and Subba Rao, 2000).
It was expected that the presence of L. castilloi and S. panamensis, both N2-fixing
trees and the cover crop A. pintoi, would result in higher productivity of T. grandis and
C. odorata because of their ability to fix nitrogen and make it available for the trees.
However, these expected effects could not be ascertained because of the limited foliar
and soil analysis data. However there was a tendency of higher yields per plant in the T.
grandis and C. odorata associated with L. castilloi and S. panamensis than in the
monoculture system, possibly due to an indirect effect of nitrogen fixation by the
leguminous trees. Based on the growth recorded throughout the study period it can be
33
surmised that the two species are coexisting probably because their niches do not overlap
sufficiently for them to become competitive.
This research did not investigate the sociological component but it is imperative
that we recognize that an agroforestry system cannot operate without farmers. Farmers
are a rural population; they produce for themselves but also produce for the markets.
Their economy depends on family labor but they often employ themselves and employ
others as needed. Therefore, as rural producers they are an important social category of
contemporary societies and need to be recognized especially by the political authorities
who control the economic and development policies in rural area (Nettings, 1993).
34
Figure 2-1. Map of Belize showing project/research site in the Cayo District.
35
0
50
100
150
200
250
300
350
may june july august
month
tem
pera
ture
(C) r
ainf
all (
mm
)Rainfall 05temperature 05 Rainfall 06 temperature 06
Figure 2-2. Graphical description of the monthly weather patterns (temperature (Co) and
rainfall (mm)) of the study region in Belize during the study period May - August 2005 and 2006. Rainfall varied throughout the study period but temperature remained constant.
36
0
0.2
0.4
0.6
0.8
1
1.2
1.4
C.odorata T.grandis
SVI (
cm3 )
0
0.5
1
1.5
2
2.5
3G
LD (c
m)
mono mixed
0
0.5
1
1.5
2
heig
ht (m
)
aa
aa
a
a
a
b
b
b
b
b
Figure 2-3. The growth increment per month (measured during a 3 months active
growing season in 2005) of C. odorata and T. grandis in mixed species and monoculture plots at the study site in the Cayo District, Belize. For each variable, differences among treatments are statistically significant when followed by different letter.
37
0
0.2
0.4
0.6
0.8
1
1.2
1.4
C.odorata T.grandis
SVI (
cm3 )
0
0.5
1
1.5
2
2.5
3G
LD (c
m)
mono mixed
0
0.5
1
1.5
2
heig
ht (m
)
aa
aa
a
a
a
b
b
b
b
b
Figure 2-4. Final height (m), GLD (cm) and SVI (cm3) of C. odorata and T. grandis at four
years after planting in mixed species systems versus monoculture systems at the study site in Cayo District, Belize. The difference among treatments are statistically significant when followed by different letter. Error bars represent S.E of the mean.
38
Table 2-1. Stem volume index of T. grandis and C. odorata in mixed species system compared to those in monoculture systems, four years after planting in the Cayo District, Belize. Differences among treatments are statistically significant when followed by a different letter using Duncan’s test to note statistical difference (p<0.05).
SVI (m3/ha) Treatments
T. grandis C. odorata
Monoculture system 0.4999257 a 0.146872 a
Mixed species system 0.9100141 b 0.1656123 a
39
Table 2-2. Foliar analysis results per treatment. Chemical characteristics of the leaves
from C. odorata and T. grandis at age four in mixed and monoculture systems in the study site in the Cayo District, Belize.
Leaf parameters Monoculture C. odorata T. grandis
Mixed C. odorata T. grandis
Nitrogen (% ) 4.5 3.8 4.7 3.9 Phosphorous (%) 0.31 0.19 0.16 0.14 Potassium (%) 3.58 2.47 2.59 2.50 Magnesium (%) 0.16 0.23 0.17 0.13 Calcium (% ) 1.80 1.40 1.90 1.40
40
Table 2-3. Chemical characteristics of the top soil to 15 cm depth for monoculture and
mixed species systems, Cayo District, Belize. S.E is shown in parenthesis. For each variable, differences among treatments are statistically significant when followed by different letter using Duncan’s test to note statistical difference (n=9, p<0.05).
Soil parameters Monoculture C.odorata T grandis
Mixed C.odorata T.grandis
Soil pH (%) 6.5(.1) a 6.9(.45)a 7(.2) aa Organic matter (%) 7.0(.45) a 5.5(.55) a 6.4(.95) ab C.E.C (meq/100g) 31.9(3.4) a 40.9(5.3) a 46.4(7) bb Exchangeable K (%) 0.55(.05) a 1.1(.1) a .9(.35) aa Exchangeable Mg (%) 11.75(.85) a 10.3(.45) a 9.7(1.9) ba Exchangeable Ca (%) 81.8(2.65) a 86.0(2.85) a 88.6(2.45) ba
41
CHAPTER 3 CONCLUSIONS AND RECOMMENDATIONS
It is well known that a mixed species plantation have many advantages for farmers,
among them being that the farmers can grow short-term cash crops and obtain some
annual income while they wait for the long-term benefit from wood production from the
associated trees. However, in the case where C. odorata is being used we must reiterate
that the C. odorata is highly susceptible to the attack of the shoot borer H. grandella
Zeller, which is considered to be one of the most severe forest pests in Latin America and
the Caribbean (Hilje and Cornelius, 2001). On the research site, the C. odorata grown in
the mixed species system is constantly being attacked by the larvae which results in many
branches which makes the tree unsuitable for commercial timber production.
The use of intercropping can be a viable alternative for many farmers. Therefore, it
is very important and necessary that information about the factors that affect or enhance
C. odorata and T. grandis growth is made available to those interested in this system of
production. The current study attempted to do so by investigating the growth rate of C.
odorata and T. grandis in mixed and monoculture system to determine which system
produced the highest yield.
Mixed species systems are very important systems that if planned properly can
provide many benefits for the farmers. Based on this research it can be concluded that in
terms of growth rate in height and diameter of C. odorata and T. grandis, mixed species
systems are better than monocultures. In mixed species systems, the layers that are
formed by the tree canopies provide a microclimate of their own.
42
In the mixed species system under investigation the C. odorata trees suffered a lot
of damage from the H. grandella larvae attack. This was as a result of intercropping S.
macrophylla and C. odorata in the same plot at the same time since both trees are host for
the H. grandella. Furthermore, this plot is at the edge of a natural forest where other
mature C. odorata and S. macrophylla trees are growing. Therefore, there is a potential
source of food which keeps the larvae present throughout the year in this treatment. On
the other hand the monoculture treatment is located about 200 meters away from the
mixed species treatment and the land around it has been cleared. Therefore, there are no
mature C. odorata trees in their vicinity. Furthermore, this plot was established leeward
from the mixed species plots.
So what are the implications for farmers today? There are several positive findings
that will aid farmers in their decision to continue or to start farming using mixed species
systems. Montagnini (1994), Montagnini et al. (1995), Parrota (1999) and Jose et al.,
(2006) have shown that mixed species systems are not only good for the environment, but
can be economically better than monocultures.
Farmers need to contact the experts at the Agriculture and Forestry department in
their home country and also the leading experts in research such as the Universities or
other local NGOs who can provide support and advice in the planning and preparation of
their fields. According to Vandermeer (1989) the basic structure of a plantation imposes
certain inevitable microhabitat features at level of the ground. It is through an analysis of
these necessary features that we can gain an idea of how the system should be designed to
eliminate competition between species which might require the same resource at the same
time for their growth. On the other hand plants or crops which are host to polyphagous
43
pest should also be avoided to eliminate or decrease pests, or insect attack to the crops or
trees.
When planning their overstory crop the farmer must take into consideration the
shade effect to the lower level plants. Hence, canopy structure should be considered so
that the system does not produce too much shade for the understory crops. Hence if
hardwood trees are being used they should be trees with different canopy structures. For
example C. odorata produces a canopy with orthotropic branches that form an open
crown, while S. macrophylla produces a closed canopy and T. grandis produces a canopy
that forms an umbrella-like crown allowing the infiltration of sunlight to the understory
plants. A plantation using similar trees would allow enough sunlight to penetrate through
the layers for photosynthesis to occur on the plants at the lower level. Once the effect of
the overstory species is known each species in the understory should then be
characterized with respect to the daily light environment needed for their survival.
Every mixed species system should have a ground cover crop which provides the
first resource for the farmer. Many leguminous crops such as beans, peanuts, peas etc.
can be used as ground cover crops. These plants will provide food for the family and will
also aid in the recycling of nitrogen making it available for plant. A ground cover crop
also protects the soil from erosion and maintains a suitable microhabitat for organisms in
the soil and the same time acting as a filter reducing the time it takes for the residues of
dangerous chemicals to enter the ground water reservoirs thereby allowing the
breakdown by bacteria of these harmful chemical residues. Of much importance also is
the fact the a ground cover crop will reduce or act as a weed control, thereby reducing the
44
amount of labor that would be needed in a system with no ground cover crop
(Vandermeer, 1989).
Results from this research indicate that mixed species can provide higher yield.
Many studies done in tropical countries such as Mexico (Primack et al., 1997), Puerto
Rico (Parrota, 1999) and Costa Rica (Montagnini, 1999) and now Belize have indicated
that better growth rate is obtained from trees growing in mixed species systems compared
to monoculture systems. Intercropping of T. grandis and C. odorata with the leguminous
trees and cover crop has shown an increase of volume production in both hardwood trees
in the mixed species systems when compared to those of the monoculture systems
producing an LER of 1.47. This result could lead to important changes in cultivation
practices mainly for growers wishing to produce timber with minimal insecticide and
fertilizer input. When nitrogen fixers used as ground cover are intercropped with other
crops they can help protect the crop from insect pests, enhance the rate of organic matter,
reduce erosion, supply extra nitrogen to the system and decrease the germination and
development of weeds (Coolman and Hoyt, 1993; Ramert, 1995).
There is also a variety of leaf litter or organic matter produced from these systems,
since plants intercropped are different and they may shed leaves that decompose at
different rate thereby making nutrients available during different intervals. This also
means that these soils are hosts to a variety of organisms which are beneficial to the
system as nutrient recyclers. A very important factor is that you minimize the amount of
fertilizers needed to produce your crops. Leguminous trees should also be intercropped
as much as possible to maintain a constant flow of nitrogen through the system.
45
Mixed species systems have been investigated as a strategy for pest control.
Although the results are inconclusive in our trial, based on the literature it can be said
with some certainty that mixed species systems do reduce pests in most instances. More
research is needed to point to the exact process involved in this strategy. Several studies
have shown a decrease in pests when crops are intercropped when compared to
monocultures. The most outstanding element in this is the fact that the farmer minimizes
risks when using mixed species systems.
Mulch can be produced from leguminous trees within a mixed species treatment.
This mulch which is sometimes referred to as green manure or green fertilizer can be
placed strategically at the base of the plants that are being cultivated and allowed to
decompose to be recycled back into the system. The decomposition of the mulch would
release the nutrients within the mulch and make it available for plant uptake. Hence,
natural fertilizer is added to the system without the fear of pollution. Therefore, this not
only is economically better, but ecologically viable and sociologically sound since the
soils and waterways remain clean. Furthermore, the produce is also free of synthetic
fertilizers and chemicals.
More organic matter means healthier soils and healthier soils means better yield.
The recycling of organic matter occurs in a cycle. Leaves are shed from the trees or
crops and are broken down by the organism in the soil. These organisms then recycle the
nutrients within the soil through excretions and also when they die and decompose.
These nutrients are then taken up by the plants which use them and return them to the soil
as litter fall. The organisms then go through the same process making the nutrients
available for plant uptake. Therefore, it must be stressed that a fertile soil is very
46
important for plant production and also for the survival of organisms which are the
recyclers of nutrients. In summary, mixed species systems have many advantages for
farmers and can be practiced as an alternative land use strategy.
47
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Zisman Simon, 1996. The Directory of Belizean Protected Areas and Sites of Nature Conservation Interest.
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BIOGRAPHICAL SKETCH
Juanita Garcia-Saqui was born in Stann Creek, Belize C.A. She is the fourth
daughter and fifth child born to Reverend Juana Garcia-Gutierrez and Jorge Garcia.
Juanita Garcia began her education at the Light of the Valley Baptist Primary School.
Because of financial constraints, Juanita had to stay out of school for two years before
entering high school. Therefore after two years following the Baptist Primary School,
she was enrolled at the Ecumenical High School. In 1992, she graduated from
Ecumenical High School, and because of financial constraints, she was forced to enter the
work force. She worked for six years, after which she returned to the University of
Belize in 1999 where she obtained an associate’s degree in natural resources management
in 2001. After her associates degree, Juanita pursued and obtained a bachelor’s degree in
biology in 2003.
While pursuing her bachelor’s degree, Juanita also worked as an assistant
laboratory technician for the Micro propagation laboratory at the University of Belize,
where she assisted in the tissue culturing of native orchids of Belize. After obtaining
her bachelor’s degree Juanita took full responsibility of the micro propagation laboratory
and worked appointed by the University of Belize as a consultant for the OAS project in
Belize which is affiliated with the University of Belize.
In 2004, Juanita was accepted with funding to the University of Florida in the
School of Natural Resources and Environment, where she majored in interdisciplinary
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ecology with a concentration in forest resources and conservation. Juanita has been
married to Mr. Pio Saqui for eight years.
