Root Productivity in an Amazonian Rain Forest

Root Productivity in an Amazonian Rain ForestAuthor(s): Carl F. Jordan and Gladys EscalanteSource: Ecology, Vol. 61, No. 1 (Feb., 1980), pp. 14-18Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1937148 .

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Ecology, 61(1), 1980, pp. 14-18 ? 1980 by the Ecological Society of America

ROOT PRODUCTIVITY IN AN AMAZONIAN RAIN FOREST'

CARL F. JORDAN Institute of Ecology, University of Georgia, Athens, Georgia 30602 USA

AND

GLADYS ESCALANTE Centro de Ecologia, Instituto Venezolano de Investigaciones Cientificas,

Apartado 1827, Caracas 101, Venezuela

Abstract. Average rate of root biomass accumulation in the surface root mat of an Amazonian rain forest near San Carlos de Rio Negro, Venezuela was 117 g m-2 yr-1, and total root biomass increment was 201 g m-2 yr-1. Root growth was higher in the surface root mat when fresh litter was present. Root growth rates relative to shoot growth rates at San Carlos were similar to ratios for a temperate forest near Oak Ridge, Tennessee, USA. However, turnover rates of roots were higher at the San Carlos site. High turnover rates result in a relatively large proportion of the roots in smaller size classes, which have a large surface area in relation to their volume and thus are efficient nutrient traps. High efficiency of nutrient trapping is important in the nutrient-poor San Carlos forest.

Key words: Amazonian ecosystem; biomass production; nutrients; root productivity; root:shoot ratios; root turnover; San Carlos project; tropical rain forest.

INTRODUCTION

Root productivity is one of the most difficult ecosystem parameters to measure. Most of the first estimates for forests came from whole tree harvests in stands of known age. Root production was considered to be the root biomass divided by the age of the stand (Bray 1963).

Estimates of root production also have been made under the assumption that the root:shoot production ratio is equal to the root:shoot biomass ratio (Harris et al. 1977, Hermann 1977). These biomass ratios vary from about 1.0 for seedlings to 0.2 for mature trees, but are influenced by factors such as soil type and amount of shading (Bray 1963, Lyr and Hoffman 1967).

Recent studies of root production have shown that a large amount of small roots (<5-mm diameter) are sloughed off every year (Coleman 1976). This root production would not appear in any study based only upon biomass changes of entire trees or root:shoot biomass ratios. When production of small roots is taken into account, root production in forest, prairie and desert ecosystems is actually greater than aboveground production (Coleman 1976). Because of the difficulty of separating rootlets from mineral soil, few studies of this type have been undertaken.

As part of a study of ecosystem structure and func- tion in the Amazon Territory of Venezuela (Medina et al. 1977), we have been measuring root productivity of a "tierra firme" (never flooded) forest growing on

oxisols near our field station at San Carlos de Rio Ne- gro, 1054'N., 67'03'W. The underlying kaolinitic clay in this soil is mixed with concretions up to 2 cm in diameter, at depths <0.5 m. The uppermost mineral soil horizon is sand and varies between 10 and 30 cm in depth. On top of the sand is a layer of roots and humus ranging from 5 to 40 cm in thickness (Fig. 1). Roots in this mat comprise 36.5% of the total root biomass, but 58% of roots <6 mm in diameter (Stark and Spratt 1977). We have observed that more new roots seem to be present during the wetter months of March through December in spite of the fact that, on the average, no month receives less than 100 mm of rain. In other regions of the Amazon Basin, oxisols may not have a layer of sand on top, and the surface root mat may be reduced in thickness or even lacking.

After a dead leaf has fallen to the forest floor in this forest, roots from the surface root mat begin covering the leaf (Fig. 2a). Root growth onto leaves seems to occur faster when there are other leaves or humus forming a cover, perhaps because a cover provides a moist microhabitat. After several months the leaf be- gins to disappear, (Fig. 2b, 2c) and after perhaps a year, all that is left is a shell of roots where the leaf previously existed (Fig. 2d). Roots from the root mat also invade fallen logs (Fig. 3), but the time required for roots to replace the wood completely may be several years or more.

Since the root mat is readily observable and acces- sible, we designed several experiments to measure root growth in this root mat. In addition, observations resulting from other experiments permitted us to obtain other estimates of the root growth.

I Manuscript received 15 March 1979; revised and accept- ed 8 May 1979.



February 1980 RAIN FOREST ROOT PRODUCTIVITY 15

FIG. 1. Root mat on the tierra firme soil near San Carlos. Scale is in centimetres. Small concretions are visible in the mineral soil and on the bottom of the pit.

METHODS

Experiment 1: marked leaf and twig

In this method, root growth in response to freshly fallen litter was measured, and this rate was multiplied by the litter fall rate to obtain root production. We collected freshly fallen leaves from two of the most common tree species in the forest and twigs <5 cm in diameter, determined their wet mass, attached each one with a copper wire to a numbered aluminum tag, and placed 120 leaves and 120 twigs on the forest floor on 8 June 1976. Leaves and twigs were subsampled for moisture content. On 24 March 1977, 289 d later, the leaves and twigs were harvested by carefully cut- ting the attached roots at the exact point where they joined the remaining leaves and twigs. The samples were then brought back to the laboratory, roots were separated from leaves and twigs, oven dried, and the mass determined. We then calculated root growth per gram of leaf and twig. We know that the rate of leaf fall in this forest varies between 20 and 60 g. m-2 mo-1 and averages 614 go m-2 yr-1, and wood fall averages 340 g- m-2 yr-1 (P. G. Murphy, personal communication). We multiplied leaf fall rate by the growth rate onto leaves, and wood fall rate by the growth rate onto wood to determine rate of root growth per square metre of forest floor.

This method assumes that root growth in the mat occurs only in response to fresh litter input. If the roots on the bottom of the mat are growing, this growth would not be included. Further, the method does not account for fine roots which may grow and then are sloughed before the final harvest. The method also does not account for root growth in the mineral soil. However, the root growth in the mineral soil must range from a rate equal to that on top of the root mat to zero. Since we know the proportion of root biomass in the mat and in the mineral soil from Stark and Spratt (1977), we can put limits to the range of total root productivity by assuming for the lower limit that the

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i: 2. _,,enc la l: d a

FoiGio. 2. Sequenceofvleafdlitatero decomposition a.hootn

root shell where material is missing. d. Center of figure shows remnant leaf veins with white fungal "ghost."

root growth per gram of root biomass in the mineral soil is zero, and for the upper limit that the root growth per gram of root biomass in the mineral soil is equal to that in the root mat.

Another problem with the method was the disap- pearance of some leaves. We were able to recover



16 CARL F. JORDAN AND GLADYS ESCALANTE Ecology, Vol. 61, No. I

only 74 out of the original 120 leaves placed on the forest floor. Leaves may have been consumed by soil arthropods, or ants may have dissected and carried them away.

Experiment 2: litter bags

As part of a litter decomposition experiment, we put out 32 litter bags made of fiberglass screen and stuffed with fresh litter, and collected them at periodic inter- vals for 15 mo. After the bags and their contents were dried, the remaining litter and the new roots inside the bag were separated, dried, and the mass determined.

Experiment 3: screens method without litter In this method we placed 48 fiberglass screens each

40 by 40 cm on top of the forest floor. After 289 d we carefully excised all the roots that had grown upward through the screen, dried them and determined their mass. This method has the same disadvantages as the marked leaf and twig method, in that it measures only growth on top of the root mat. An advantage of the method is that rates of litter fall do not have to be known. Since all the roots on top of the 1600-cm2 screens are harvested after a known interval, harvested root weights are directly convertible into root growth rates on a mass per unit area per unit time basis.

Experiment 4: screens method with litter

This experiment was carried out in the same manner as screens method without litter, except that litter was placed on top of the screens to a depth equal to the depth of litter in the surrounding area.

Experiment 5: lateral growth into cleared surface area

In July 1975, a series of pits 1.5 m on each side was dug as part of another experiment. The mineral soil taken out of the pits was piled to one side about 1 m from the pits. Of the total number of pits originally dug, six were not needed, and were filled in with mineral soil. This resulted in a square patch of about 2.25 m2 without a root mat, where each hole had been. In July 1978, all the roots that had grown into these 2.25- mI areas were harvested, separated from the litter to which they were attached, dried, and the mass determined. Total root mass divided by area of each plot gave root growth per unit area.

A major objection to this method could be that root growth into the initially bare area would be unusually rapid, due to lack of root competition for the fresh litter falling into the square. On the other hand, due to the large size of the squares relative to root size, the method might underestimate growth; there is no source for roots in the center of the square and there- fore the area in the center cannot exhibit any root growth during the first part of the experiment. All root

FIG. 3. Cross section of fallen log, showing invasion of roots. Knife in lower left corner gives scale.

growth is along the edges. Also, as in previous methods, this method does not account for root growth in the mineral soil.

Experiment 6: lateral growth into pits In December 1975, a series of 17 pits was dug, each

40 cm on a side and 40 cm deep. In July 1978, all the roots that had grown into these pits were harvested, separated from the litter to which they were attached, dried, and weighed.

This method has the same problem as lateral growth into cleared surface area, in that there was no initial root competition in the interior of the pit. However, in contrast to the larger pits, the centers of the pits were completely filled with roots at the time of harvest, thereby diminishing the error due to lack of surface area from which new roots could grow. In contrast to all the previous methods, the measured root growth represented all root growth, not just that in the root mat, because these pits extended below the zone of almost all the root biomass (Stark and Spratt 1977).

RESULTS AND DiSCUSSIoN

Calculated rates of root biomass accumulation var- ied relatively little (Table 1). A series of analyses of variance was performed on the data used to calculate Table 1. Using a 90%/ level of confidence, experiments 3 and 5 had rates significantly lower than the average for the rest of the group, and experiment 2 had a rate significantly higher. There were no statistically signif- icant differences between root growth rates as measured by experiments 1, 4 and 6.

Except in the case of experiment 6 where root growth in the mineral soil was measured, the total root increment in Table 1 was calculated assuming that belowground roots grow at the same rate as aboveground roots. There is general agreement between measured total root growth in experiment 6 and calculated total growth in experiments 1 and 4. Such agreement, however, does not prove that above- and belowground



February 1980 RAIN FOREST ROOT PRODUCTIVITY 17

TABLE 1. Description and results of root biomass accumulation studies at San Carlos de Rio Negro, Venezuela. Average is based on experiments 1, 4, and 6 only, since the others yielded results significantly different (see text).

Biomass accumulation Length (g m-2 yr-1 + SE) of ex-

Experi- periment Aboveground Total ment Description (mo) roots roots

I Root growth into leaf and wood litter 9 123 ? 13* 212 ? 22t 2 Root growth into litter bags 15 143 ? 16 247 ? 28t 3 Root growth through screens (no litter placed on screens) 9 35 ? 3 60 ? 5t 4 Root growth through screens (litter placed on top of screens) 10 103 ? 29 178 ? 50t 5 Root growth into cleared surface area 36 88 + 19 152 ? 32t 6 Root growth into pits 30 124:: 213 ? 33

Average of experiments 1, 4, and 6 117 201

* Growth into wood litter accounts for 12% of total root growth. t Aboveground increment divided by 0.58 to calculate total increment (because 0.58 of the biomass of roots <6 mm in

diameter occurs in the surface root mat; see text). t Total root increment multiplied by 0.58 to calculate increment in aboveground root mat.

root growth rates are equal. The agreement could be coincidental or an artifact of experiment 6.

The highest rate of root growth was into the litter bags (experiment 2). St. John and Machado (1978), in an experiment in which Amazonian rain forest plants were grown in vessels with sterilized leaf litter and leaf litter inoculated with bacteria and fungi, found that the plants in the inoculated vessels produced twice as many root tips. They concluded that the higher root tip production in the inoculated vessels was due to a hormone produced by the biotic component. However, since experiment 6 showed that rates of root growth below ground where there is little organic matter are comparable to aboveground rates, humidity as well as organic matter must be a control- ling factor.

The lowest rate of root growth was through the screens not covered with a litter layer (experiment 3). The slow growth probably is caused by the lack of litter and the resultant lower humidity and also perhaps lack of a biotic component.

In experiment 5, where roots had to fill in the cleared areas of >2 ml the lack of a source of roots in the center of the pits probably is the reason for the low production.

None of the experiments reported here were designed to detect fine roots which grow and then are sloughed off. T. St. John (personal communication) working in a similar forest type near Manaus, Brazil carried out an experiment similar to experiment 1 de- scribed here. His study differed in that instead of wait- ing 9 mo to determine root coverage of leaf litter set out as "bait," he made a careful inspection of the leaves every month. He reports that in some cases roots covered the leaves and both the leaves and roots disappeared by the end of the experiment. He attri- butes this observation to root sloughing, similar to that reported by Harris et al. (1977) in a temperate forest.

Recent reviews of root productivity such as that of

Hermann (1977) imply that the problem of root sloughing causes inaccuracies in root: shoot production ratios based on biomass at harvest time. Part of the problem is with the definition of the word production. Total root biomass production includes sloughed roots, while root biomass increment or root biomass accumulation would simply be net annual gain in standing crop. Total shoot production would include sloughed leaves, bark and branches, while annual increment would be net change in wood only. To be consistent, root:shoot ratios should either compare total production or net annual increment.

If we compare the total root biomass accumulation rate from Table 1 with our preliminary total aboveground wood accumulation of about 600 g m-2 yr-1, determined by a dendrometer study of 120 trees which was begun in 1975 and is still continuing, we get a root:shoot increment ratio of 0.33. If we calculate total production by assuming that total root production including sloughing is 2.8 times aboveground wood accumulation, as occurred in a temperate zone forest (Harris et al. 1977), then total root production should

TABLE 2. Root and shoot biomass and biomass accumulation ratios for a temperate forest at Oak Ridge, Tennessee, and a rain forest at San Carlos, Venezuela. Oak Ridge data are from Sollins et al. (1973), and Harris et al. (1977). San Carlos data are from Stark and Spratt (1977), Jordan and Uhl (1978) and this report.

Oak San Ridge Carlos

Root biomass (g/m2) 4 200 5 600 Shoot biomass (g/m2) 13 400 33 500 Root:shoot biomass ratio 0.31 0.17 Root biomass increment (g m-2 yr-) 100 200 Shoot biomass increment (g m-2 yr-') 300 600 Root:shoot increment ratio 0.33 0.33 Root turnover (yr) 42 28 Shoot turnover (yr) 44 55



18 CARL F. JORDAN AND GLADYS ESCALANTE Ecology, Vol. 61, No. 1

be 1680 g m-2 yr-1. If we calculate shoot production to be the sum of annual wood increment (600 g m-2 yr-1), wood sloughing (340 g m-2 yr-1 determined by a 2 x 800 m transect sampled quarterly for 3 yr) and leaf sloughing (600 g m-2 yr-1 determined by 42 litter traps sampled monthly for 4 yr), then the root:shoot production ratio approaches unity. In the Oak Ridge temperate zone forest, total annual shoot production including all litter fall approaches 800 g m-2 yr-1 (Sollins et al. 1973) while total root production including sloughing is about 900 g m-2 yr-1 (Harris et al. 1977), giving a root:shoot production ratio close to one for the temperate zone also.

A comparison of root: shoot biomass ratios and biomass accumulation ratios between the temperate forest at Oak Ridge, Tennessee and the tropical forest of San Carlos (Table 2) is instructive. Biomasses of roots at the two sites are roughly similar, but shoot biomass at San Carlos is much higher, resulting in a higher root:shoot biomass ratio at Oak Ridge. The production relationships are different. Although root growth at San Carlos is double that at Oak Ridge, shoot increment is also double, resulting in equal root:shoot increment ratios for the two sites.

If we divide biomass by production we get turnover time for roots and shoots for each site. Turnover time assumes a steady state, that is, each compartment is losing biomass through death and decay of individuals at the same rate that other individuals are accumulat- ing biomass. This assumption may not be correct for the Oak Ridge forest which was about 48 yr old at the time of the study. Nevertheless, comparisons are instructive (Table 2). Root turnover rate at the Oak Ridge site shoot turnover rate, while turnover of roots in the San Carlos forest is more rapid both relative to local shoot turnover and relative to root turnover in Oak Ridge.

The relatively higher root turnover in San Carlos may be due more to the fact that the San Carlos forest is nutrient poor than to the fact that it is tropical. We have already shown how efficient the root mat at San Carlos is at trapping nutrients from soil solution (Stark and Jordan 1978). Because newer, smaller roots have a larger surface area per unit volume than larger older roots, new roots probably play a relatively important role in nutrient trapping. Rapid turnover of roots would result in a greater proportion of small-sized roots.

ACKNOWLEDGMENTS

This study was part of the San Carlos Venezuela rain forest project. Funds for this project come in part from the United States National Science Foundation, the Organization of American States, UNESCO, and the Venezuelan Govern- ment. The project is a UNESCO Man and the Biosphere (MAB-1) project. The work is coordinated through Centro de Ecologia, Instituto Venezolano de Investigaciones Cientifi- cas, Caracas, Venezuela. We thank Drs. David Coleman and Ted St. John for reviewing the manuscript. The pictures in Fig. 2 were taken by Dr. Rafael Herrera.

LITERATURE CITED

Bray, J. R. 1963. Root production and the estimation of net productivity. Canadian Journal of Botany 41:65-72.

Coleman, D. C. 1976. A review of root production processes and their influence on soil biota in terrestrial ecosystems. Pages 417-434 in J. M. Anderson and A. Macfadyen, editors. The role of terrestrial and aquatic organisms in decomposition processes. Blackwell, Oxford, England.

Harris, W. F., R. S. Kinerson, and N. T. Edwards. 1977. Comparison of belowground biomass of natural deciduous forests and loblolly pine plantations. Pages 29-37 in J. K. Marshall, editor. The belowground ecosystem: a synthesis of plant-associated processes. Science Series 26, Range Science Department, Colorado State University, Fort Col- lins, Colorado, USA.

Hermann, R. 1977. Growth and production of tree roots: a review. Pages 7-28 in J. K. Marshall, editor. The belowground ecosystem: a synthesis of plant-associated processes. Science Series 26, Range Science Department, Col- orado State University, Fort Collins, Colorado, USA.

Jordan, C. F., and C. Uhl. 1978. Biomass of a "tierra firme" forest of the Amazon Basin. Oecologia Plantarum 13:255- 268.

Lyr, H., and G. Hoffman. 1967. Growth rates and growth periodicity of tree roots. Pages 181-236 in J. Romberger and P. Mikola, editors. International Revue Forest Re- sources 2. Academic Press, New York, New York, USA.

Medina, E., R. Herrera, C. Jordan, and H. Klinge. 1977. The Amazon project of the Venezuelan Institute for Sci- entific Research. Nature and Resources 13(3):4-7.

Sollins, P., D. E. Reichle, and J. S. Olson. 1973. Organic matter budget and model for a southern Appalachian Lir- iodendron forest. Eastern Deciduous Forest Biome-Inter- national Biome Program-73-2. Oak Ridge National Labo- ratory, Oak Ridge, Tennessee, USA.

Stark, N., and C. F. Jordan. 1978. Nutrient retention by the root mat of an Amazonian rain forest. Ecology 59:434-437.

Stark, N., and M. Spratt. 1977. Root biomass and nutrient storage in rain forest oxisols near San Carlos de Rio Negro. Tropical Ecology 18:1-9.

St. John, T., and A. D. Machado. 1978. Evidencia da acao microorganismos na ramificacao de raizes. Acta Amazon- ica 8:9-11.



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Root Productivity in an Amazonian Rain Forest

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