Ecological responses of two pioneer species to a hydrologicalconnectivity gradient in riparian forests of the lowerParana River

Sylvina Lorena Casco • Juan Jose Neiff •

Alicia Poi de Neiff

Received: 25 February 2009 / Accepted: 8 February 2010! Springer Science+Business Media B.V. 2010

Abstract The vegetation of the Parana River

floodplain has physiognomic, structural and ecolog-ical characteristics that are distinct from those of the

surrounding landscapes across the river (Forest,

Chaco Savannahs, Pampean steppe). Few speciesare able to live in the alternately flooded and dry soil.

The distribution of each species is strongly condi-tioned by the water regime in each area of the

floodplain, and its location in the topographical

gradient of the islands allows us to determine thepossibility of colonising the bars and islands of the

river course to determine their tolerance to hydro-

logical variations and to foresee the changes that cantake place as a result of disturbances in the hydro-

logical regime. Here, we present the occurrence of

willow (Salix humboldtiana Willd.) and South Amer-ican alder (Tessaria integrifolia Ruiz et Pav.) in bars

and islands of different topographies in a section of

the Parana River downstream from the confluence ofthe Paraguay and Parana rivers. The results indicate

that both species have similar niches in relation to

hydrological fluctuations. However, willow was sig-nificantly more frequent at of 48.49 m a.s.l., while

palo bobo reached its highest frequency at 49.29 m

a.s.l. The difference between the modes of thedistribution curves of each species was 0.80 m.

Canopy trees such as willow and palo bobo areadapted to flooded soil conditions for 77.8 and 40%

of their respective lives and survive with a long-

lasting inundation phase (306 and 366 days, respec-tively). However, many trees in the Parana River

floodplain died when the flood period extended for

more than 1 year.

Keywords Riverine forests ! River pulse !Parana floodplain ! Ecohydrology !Vegetation gradient ! Fluvial landscape

Introduction

A classic approach in ecology based on the contin-

uum concept indicates that species composition

changes gradually along environmental gradients(Gleason 1926; Austin 1999; Austin and Smith

1989), with each species having an individualistic

distribution. Based on this assumption, Lindenmayerand Fischer (2006) suggest that all species in a given

landscape respond differently to landscape changes

S. L. Casco (&) ! J. J. Neiff ! A. P. de NeiffCentro de Ecologıa Aplicada del Litoral, ConsejoNacional de Investigaciones Cientıficas y Tecnicas,Ruta 5 km 2,5. C.C 291 (3400), Corrientes, Argentinae-mail: sylvina.casco@gmail.com

S. L. Casco ! A. P. de NeiffDepartamento de Biologıa. Facultad de Ciencias Exactasy Naturales y Agrimensura, Universidad Nacional delNordeste, Avda. Libertad 5460 (3400), Corrientes,Argentina

123

Plant Ecol

DOI 10.1007/s11258-010-9734-9

and that each species may have its own unique spatialdistribution.

Hydrology is the primary factor governing physical

and biotic processes in riverine wetlands (Tabacchiet al. 1998). One of the prevalent gradients in wetlands

is the continuum of depth and frequency of flooding

(Brinson 1993). The composition and distribution ofriparian forests are determined by the timing, depth

and duration of flooding in both temperate (Brinson

and Verhoeven 1999; Mitsch and Gosselink 1993) andtropical floodplain forests (Worbes 1997; Neiff 2005).

The degree of tolerance to flooding varies among

forests (Kozlowski 2002); thus, minor variations in thefrequency and duration of flooding as well as in the

texture of soils determine changes in their distribution

along environmental gradients.The pioneer gallery forests of Tessaria integrifolia

Ruiz et Pav. (South American alder or palo bobo) and

Salix humboldtiana Willd. (willow or sauce) have awide distribution in the floodplains of South Amer-

ican tropical and subtropical rivers (Hueck 1972;

Worbes 1997; Casco 2003; Kandus and Malvarez2004), occupying more than 30,000 km2 of the island

and lateral banks of the tributary rivers coming from

Los Andes mountains (Reboratti and Neiff 1987).Several authors have indicated that the two species

rarely coexist in the same stand in the lower Parana

River (Neiff 1986; Reboratti and Neiff 1987; Mal-varez 1997). According to Lewis and Franceschi

(1979), palo bobo and willow forests correspond to

different successional stages of the hydrosere, wherewillow forests are replaced by palo bobo in

subsequent stages of the ecesis.

The objectives of this study were (1) To analysethe distribution patterns of S. humboldtiana and

T. integrifolia in a natural alluvial floodplain system

along a hydrogeomorphic gradient; (2) To determinethe frequency of pulses and the number of flooding

days for the topographic positions that set the

boundaries of their distribution. This study wasdesigned to answer the following questions: First, in

which topographic position of the hydrogeomorphicgradient do mature populations (canopy trees [10-cm diameter at breast height) occupy the floodplain?

Second, is the adjustment between tree distribution andconnectivity gradient similar for both species?

Riparian wetlands have been recognised for their

importance in the maintenance of biodiversity, streamintegrity, wildlife habitat, and movement corridors

(Mitsch and Gosselink 1993; Gregory et al. 1991).The results of our study will be useful to determine

(a) the possibility that these trees may occupy new

islands; (b) the possible negative impacts that couldbe generated as a consequence of modifications in the

hydrological regime by civil works (impoundments,

drainage, others); (c) the possibility of finding a broadgeneral distribution model for both species in the

lower Parana using data from many sites.

Methods

Site description

The lower stretch of the Parana River has uniqueconditions because the west bank is affected by the

high level of suspended solids from the Andes

mountains in the Bermejo River, a tributary of theParaguay River, while the east bank is influenced by

the transparent water of the Upper Parana River

(Fig. 1). The water of both rivers remains unmixedfor 300 km downstream from the Parana–Paraguay

confluence (Orfeo 1995). In the study area, the main

channel has a braided design, with more islands nearthe west bank than the east. The alluvial floodplain of

the Parana River stretches over 8 km on the west bank

and is free of any man-made alterations created when ameandering channel is cut off from the main channel.

Soil texture

Sediments from the Paraguay and Bermejo rivers

predominate in the west-bank islands, and muddy claysoils from the High Parana River predominate in the

east-bank islands (Orfeo and Stevaux 2002). The soils

of these islands are incipient, formed by small amountsof pedologic material transported and accumulated in

successive floods by the horizontal water flux. There-

fore, these soils, in contrast with those from upland, donot have a vertical pattern of differentiation. Each

layer is constituted by materials of different origin,

Fig. 1 Study area. a Parana, Paraguay and Bermejo Rivers inthe context of the large rivers. b Location of the east and westbanks of the Parana River. c Satellite image from Google Earth(2008 Terra Metrics; Image 2008 Digital Globe; 2008 EuropaTechnologies). Letters indicate the sites of sampling. dSatellite image from Google Earth with the study sitesindicated by numbers

c

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Image Site Geographic PositionA 1 27°18´32´´S; 58°38´43´ WB 2 27°21´14´´S; 58°41´46´ W

C

3456789

1011121314151617

27°28´14´´S; 58°52´39´ W27°28´56´´S; 58°53´56´ W27°29´00´´S; 58°53´28´ W27°28´56´´S; 58°53´16´ W27°28´53´´S; 58°52´59´ W27°28´58´´S; 58°52´58´ W27°29´05´´S; 58°52´59´ W27°29´07´´S; 58°53´21´ W27°29´13´´S; 58°53´81´ W27°29´26´´ S ; 58°52´59´ W27°29´26´´S; 58°53´44´ W27°29´36´´S; 58°33´03´ W27°29´43´´S; 58°53´08´ W27°29´26´´S; 58°53´44´ W27°29´47´´S; 58°53´47´ W

D

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27°30´03´´S; 58°53´54´ W27°30´01´´S; 58°53´48´ W27°30´01´´S; 58°53´13´ W27°29´39´´S; 58°53´12´ W27°30´37´´S; 58°53´45´ W27°31´13´´S; 58°53´21´ W27°31´16´´S; 58°53´23´ W27°31´02´´S; 58°53´35´ W27°30´42´´S; 58°51´28´ W27°30´59´´S; 58°51´13´ W27°31´33´´S; 58°52´28´ W27°31´40´´S; 58°52´15´ W27°31´29´´S; 58°51´33´ W27°31´29´´S; 58°50´56´ W

E

3233343536394043444546

27°31´46´´S; 58°50´41´ W27°31´50´´S; 58°50´42´ W27°31´49´´S; 58°51´08´ W27°32´06´´S; 58°51´11´ W27°32´09´´S; 58°50´30´ W27°32´33´´S; 58°51´10´ W27°32´33´´S; 58°50´23´ W27°32´45´´S; 58°51´01´ W27°32´47´´S; 58°50´35´ W27°32´51´´S; 58°50´53´ W27°33´06´´S; 58°50´49´ W

F

373841424749

27°32´24´´S; 58°52´28´ W27°32´30´´S; 58°52´33´ W27°32´40´´S; 58°53´09´ W27°32´49´´S; 58°53´09´ W27°33´11´´S; 58°53´28´ W27°33´28´´S; 58°52´01´ W

G5051

27°34´13´´S; 58°50´29´ W27°35´00´´S; 58°49´43´ W

H5253

27°36´48´´S; 58°49´20´ W27°37´28´´S; 58°48´57´ W

I54555657

27°56´44´´S; 58°49´19´ W27°56´45´´S; 58°49´21´ W27°57´09´´S; 58°49´29´ W27°57´10´´S; 58°49´28´ W

J 58 28°00´20´´S; 58°51´29´ W

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E F

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High Paraná River

Paraguay River

East bankWestbank

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granulometry and packing according to the intensityand duration of floods (Neiff 2005).

The alluvial soils of the studied area are Fluvents,

within the order Entisols, with profile A–C and atypical laminated structure (Soil Survey Staff 2006).

There are differences in the proportions of textural

composition between the two river banks. On thewest bank, the soils are dominated by silts and clays

with a scarce proportion of fine sand (Table 1).

However, the proportions of particles of differentsizes vary between the different profiles and horizons.

The proportion of particles smaller than 2 lm

diameter (clay) varies between 6 and 45%, whileparticles between 2 and 20 lm are dominant in most

of the horizons. On the east bank, the soils are

dominated by fine sand between 56 and 125 lm insize and silt, and the fraction smaller than 2 lm (clay)

varies between 6.8 and 16.72%. On both banks, the

proportion of organic matter decreased regularly withdepth, and the pH was more acidic on the east bank

than on the west bank (Table 1).

Hydrology

In the last century, the regime of the Parana River wasvery irregular (Neiff 1990), with four extraordinary

floods (above 8 m in the Corrientes gauge) that

occurred in June 1905, July 1983, June 1992 and May1998. The first, with an absolute maximum value of

9.03 m, was a centenary flood that caused mortality of

Table 1 pH values, proportion of organic matter (OM) and particle size in the soil profiles in the forests of both river banks

Particle size (%)

Depth (cm) pH OM (%) Fine sand Silt Clay

500 250 125 56 36 20 2 lm

West bank

Profile 1

A1 0–16 7.80 2.50 0.50 1.00 1.00 6.00 46.00 45.00

C1 16–46 8.50 1.00 1.00 2.00 8.00 19.00 48.00 21.00

C2 46–105 8.60 0.40 0.50 3.00 10.00 23.00 50.00 12.00

Profile 2

A1 0–25 8.40 2.00 0.50 1.00 6.00 29.00 59.00 13.00

C1 25–76 8.50 0.60 0.50 5.00 31.00 6.00 47.00 10.00

C2 76–115 8.30 0.60 1.00 3.00 5.00 16.00 40.00 35.00

Profile 3

A1 0–21 8.00 2.20 0.10 1.00 45.00 29.00 17.00 8.00

C1 21–37 8.60 0.50 – 6.00 31.00 9.00 47.00 6.00

C2 37–60 8.70 0.50 0.50 1.00 1.00 2.00 50.00 45.00

C3 60–110 8.30 1.00 0.50 1.00 1.00 4.00 70.00 20.00

East bank

Profile 1

A1 0–22 6.94 1.50 1.26 15.90 45.10 20.50 0.60 9.60 7.03

C1 22–45 6.50 0.21 0.98 10.50 63.50 10.00 0.80 7.50 6.80

C2 45–85 6.94 0.44 0.54 17.08 41.51 20.10 0.60 12.50 7.70

C3 85–150 7.20 0.09 0.50 9.50 44.60 11.40 2.10 19.10 11.97

Profile 2

A1 0–24 6.70 1.90 0.59 20.49 38.17 19.02 8.49 5.07 8.17

C1 24–60 6.30 0.50 0.50 18.77 38.06 21.50 8.74 4.63 7.80

C2 60–75 7.20 0.20 0.26 3.31 19.58 27.33 19.70 13.02 16.72

C3 75–95 6.90 0.40 0.17 10.52 30.18 24.09 14.50 8.48 12.04

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40% of the riparian forests and more than 60% of the

pioneer forests because of their lower position in thetopographic gradient of the studied area (Neiff et al.

1985). Thus, we assume that the analysed canopy trees

(younger than 20 years) were affected by changes inthe water level of the Parana River between January

1984 and December 1999. In this period, the waterlevel of the Parana River varied between 1.74 and

8.39 m, with a mean overflow value of 4.28 m

(Fig. 2). During the extraordinary flood of 1992, theriver reached 8.64 m in the Corrientes gauge, whereas

during the last ENSO event of 1998, it reached 8.39 m.

Study species

From its confluence with the Paraguay River to theDelta, the lower Parana River floodplain extends for

1,100 km. Adjacent monotypic stands of willow and

palo bobo are located in the lowest sites, which areflooded for most of the year (islands in the main

channel and sand bars). The mature trees of Tessariaintegrifolia and Salix humboldtiana measure up to 12–13 m in height with spherical crowns, with diameters at

breast height of 15–50 cm, roots that extend up to

1.5 m deep and only one layer with trees of the sameage (Neiff 2005). Total annual litter fall reaches

8.15 t ha-1 yr-1 (Neiff and Poi de Neiff 1990), and

the rate of leaf decomposition is fast (Poi de Neiff et al.2006).

We used ecological response (Keddy 2000) as the

distribution pattern of a species in the field withneighbours present.

Sampling

A total of 432 points in 58 sites located along a

60-km segment of the Lower Parana River betweenCorrientes and Empedrado were included in this

study (Fig. 1). The sites were chosen on the basis of

the presence of dense stands of willow and palo boboon both banks of the river and on islands of the main

channel. Between September 1998 and November

1999, we registered the occurrence of canopy trees,defined as stems [ 10 cm DBH (diameter at breast

height): 1.5 m above ground, in willow and palo bobo

stands at each point using the water table as thereference level (Neiff 1986). Owing to disparities in

the slopes of the studied floodplain, the topographicposition of each point, rather than distance from the

river, was used to indicate the connectivity of each

point. The hydrologic connectivity was defined bydetermining the date of initial connection to the river

in relation to the water level of the Parana River in

the gauges located near the study sites (Neiff and Poide Neiff 2003). Afterward, the topographic position

value was corrected based on the knowledge that the

zero level at the Corrientes gauge is 42.39 m abovesea level (m a.s.l.).

Data analysis

For the distribution curves, the occurrence of canopy

trees of both species in each topographic position wasgrouped at 20-cm intervals. In order to model the

occurrences of both species along the topographic

0.00

1.00

2.00

3.00

4.00

5.00

6.00

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Wat

er le

vel (

m)

Time

Fig. 2 Water levelfluctuations of the ParanaRiver at Puerto Corrientesbetween 1984 and 1999

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gradient, Poisson regression models were fitted usingInfoStat (2008) statistical software. The adjusted

models were all dependent on the topographic

position of the counts. This variable was transformedto the distance from the topographic position of each

count to the mean topographic position of all counts

(around 48 m a.s.l.). These distances were included inthe model as a third-order polynomial. Three models

were fitted. One evaluated the effect on the topo-

graphic distributions of count according to species.This was done through the inclusion in the regression

model of a species effect and species* distanceinteractions. The other two models included bothriver banks and their interactions with distance. The

outputs of these models were summarised graphi-

cally, plotting observed count and model-expectedcounts against topographic position. Observed as well

expected counts were identified according to species

and river banks, respectively.The daily hydrometric information provided by a

gauge at the National Division of Navigable Ways

and Ports in Corrientes was used to analyse waterlevel fluctuations. The frequency of pulses, number of

flooded days and number of emergent soil days (Neiff

1996) for three topographic positions along theboundaries of the distribution curves (mode and

extremes of the distribution) were calculated using

Pulse software (Neiff and Neiff 2004).

Results

Distribution patterns of both species

In the study area, canopy trees of S. humboldtianawere found between 45.69 and 51.09 m a.s.l., whereas

those of T. integrifolia were distributed between 46.89and 50.89 m a.s.l. The adjusted model shows a

significant difference between species for the

expected number of trees based on topographicposition (species* topographic position interaction,

P \ 0.0001). Figure 3 shows that the observed countsfor willow were significantly more frequent at of

48.49 m a.s.l., while palo bobo reached its highest

frequency at 49.29 m a.s.l. The difference between themodes of the distribution curves of each species was

0.80 m (Fig. 3). The trend lines (expected counts)

indicated that both species curves were slightlyskewed towards the highest topographic positions.

Our results suggest that willow canopy treesgrowing in the topographic position of 48.49 m a.s.l

are flooded when the water level of the Parana River

reaches 6.1 m on the Corrientes gauge. At thisoverflow level, PULSO software estimated that

S. humboldtiana canopy trees were affected by 23

pulses between 1984 and 1999, the number offlooding days reached 434 and the days with emerged

soils were 5,410 (over a total of 5,844 days in a 16-

year period).The mode of palo bobo canopy trees (Table 2)

received nine pulses and were subject to 170 days

with flooded soil. In the lower position of thetopographic gradient, canopy trees remained with

flooded soil for 4,549 (willow) and 2,339 (palo bobo)

days distributed in 58 and 76 pulses, respectively(Table 2). On both banks, the frequency of the pulses

was significantly different among the three topo-

graphic positions that set the mode and the limits ofthe distribution (Table 2). However, the number of

flooding days and the number of dry soil days at the

lower position of the gradient were significantlydifferent from the other two topographic positions

(Table 2).

The duration of flooded soil was longer in thewillow forests than in those of palo bobo in the

lowest positions of the gradient and in the modal

positions (Table 2). In the high positions, the palobobo trees were subjected to 5 days of flooded soil

during the 16 years, while the willows in this position

were not subjected to flooded soil (Table 2).

Comparison between river banks

When both banks of the river were analysed alone,

there were differences in the distribution curves of

willow (banks*topographic position interaction,P \ 0.0001, Fig. 4) and palo bobo (banks*topo-

graphic position interaction, P \ 0.0001, Fig. 5).

On the west bank, willow was found between 45.8and 50.8 m a.s.l. and palo bobo between 47.8 and

49.3 m a.s.l. The topographic position at which theexpected counts reached their maximum was 47.3

(willow) and 48.8 m a.s.l. (palo bobo). On the east

bank, the distribution of expected counts of willow(Fig. 4) was skewed to the lowest topographic

position and palo bobo was not found. In the islands

near to the centre of the river course, the frequency of

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the willow forests was bigger in the highest positions

(24.61% at 50.8 m.a.s.l).

Discussion

Because willow and palo bobo occupy bare soils and

form monospecific stands, it seems evident that theirdistribution is strongly conditioned on physical and

chemical factors. In this floodplain landscape, topo-

graphic position is a crude indicator of the positionalong the complex gradient, but it also includes

information about flood/drought periods and the

trees’ resilience to extreme hydrological phases.The distribution curves of S. humboldtiana and

T. integrifolia along a hydrogeomorphic gradient in

the lower Parana River were platykurtic, showing theeurytypic condition of species, i.e. adapted to the

hydrological variability of the river. Both species can

occur in the proximal floodplain if conditions suitablefor the germination and survival of plants are present.

These particular conditions occur during a very

limited window of time for a certain point of thetopographic gradient due to the frequent water level

42.39 43.77 45.15 46.53 47.92 49.30 50.68 52.06

Topographic position (m a.s.l.)

0.0

17.8

35.7

53.5

71.4

89.2

107.1N

umbe

r of

tree

s

Salix humboldtianaTessaria integrifolia

Trend line

Fig. 3 Distribution of Salix humboldtiana and Tessaria integrifolia in the topographic gradient of the study area

Table 2 Ecohydrologic attributes of the Parana River in forests of willow and palo bobo from January 1984 to December 1999(5,844 days) within the topographic positions that set the boundaries of the distribution curves of willow and palo bobo

Salix humboldtiana Tessaria integrifolia

Topographic position (m a.s.l) 45.69 48.49 51.09 46.89 49.29 50.89

Overflow level (m) 3.3 6.1 8.7 4.5 6.9 8.5

Frequency of pulses 58a 23b 0c 76b 9a 1a

Range of potamophase (amplitude, in days) 208–366 0–74 0 53–306 0–66 0–5

Number of flood days 4549b 434a 0a 2339b 170a 5a

Number of emerged soil days 1295a 5410b 5844b 3505a 5674b 5839b

Different letters indicate significant differences at P \ 0.0001

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fluctuations. Therefore, the beginning of the ecesis

phase is strongly conditioned by the frequency of theriver pulses, i.e., the hydrological connectivity of

each point of the floodplain with the river flow.Our results suggest that willows and palo bobo

have different distribution curves. The centres or

modes of their distributions along a connectivitygradient are not physiological optima defined by the

maximum growth rate, but are centres of maximum

population success, expressed by their maximumfrequencies, in competition with other species’ pop-

ulations (Whittaker 1978; Stiling 1999). These

distribution curves are only a model of adjustmentbetween the vegetation distribution and the connec-

tivity gradient.

The slightly asymmetric trend curves for bothspecies could indicate different tolerances to flood

and drought conditions. Response curves predomi-

nantly skewed towards more mesic conditions werefound, for example, in the distribution of eucalypt

species (Austin and Smith 1989).

Mature S. humboldtiana are able to survive understressful, flooded conditions and are well established

on both banks of the Parana River. In contrast,

canopy trees of T. integrifolia have a narrower

distribution range and are more successful in the

clay-loam soils of the west bank. As various authorshave noted, palo bobo develops extensive riverine

forests in the basins of the Bermejo and PilcomayoRivers (Reboratti and Neiff 1987; Neiff 2005) in

comparably textured soils, but is not observed in the

upper stretch of the Parana River gallery forest (DeSouza et al. 2004; Campos 2004).

Besides the differences indicated in the size of the

particles, the soils of the islands on the east bank havehigh iron and aluminium content derived from clays

transported by the High Parana River (Reboratti and

Neiff 1987). Mesocosm experiments have demon-strated that some aquatic plants are limited by the low

concentration of phosphorus in the Upper Parana

(Kobayashi et al. 2008), which could be the cause ofthe absence of T. integrifolia on the islands of the east

bank that receive water from the Upper Parana.

Our results indicate that S. humboldtiana growsalong both river banks, but with different distribu-

tions in the topographical gradient depending on the

type of silt in which the trees grow. Direct evidencefrom in vitro experiments is necessary to elucidate

the role of soil chemical composition in species

distribution. The results of such analyses may explain

42.39 43.77 45.15 46.53 47.92 49.30 50.68 52.06

Topographic position (m a.s.l.)

0.0

10.8

21.6

32.4

43.2

54.0

64.8

75.6N

umbe

r of

tree

s

Island course

East bank

West bankTrend line

Fig. 4 Distribution of Salix humboldtiana at the study sites

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why the frequency of this species is different for the

same level of hydrological connectivity when S. hum-boldtiana grows in the silts of the east bank or thewest bank of the river.

It seems that colonisation of Salicaceae in fluvial

environments is strongly conditioned by the presenceof saturated soil and good insolation (McLeod and

McPherson 1973; Niiyama 1990). Liotta (2001) states

that willow forests of the Parana River delta areconstituted by contemporary trees that have occupied

the bare soil of sedimentary bars during the short

period in which the soils remain saturated, and thusthey are soft. This author also found a relationship

between the flood period of the soil and the diameter

of trees, with the thickest trees (the oldest ones) beingpresent on the highest islands.

The critical stage for both species is germination,

since they cannot germinate in flooded soil or withlittle humidity (Neiff 2005). The presence of a certain

landscape or population of riverine vegetation is

mainly dependent on the conditions of the physical

medium being conducive to germination, especially

through water-saturated soil and full heliophany.After ecesis, the abilities of plants to resist droughts

and floods are high, even in extreme hydrological

conditions, since organisms have developed struc-tures and processes that allow them to persist in this

landscape (McLeod and McPherson 1973; Neiff

2005).Although the environmental heterogeneity of riv-

erine forests may represent a wide range of sites for

plant growth (Tabacchi et al. 1998), a pulse regimewith long-lasting inundation phases may restrict the

establishment of trees. At the study sites, canopy trees

of willow and palo bobo are adapted to flooded soilconditions during 77.8 and 40% of their lives,

respectively, and survive long-lasting inundation

phases (306 and 366 days, respectively). However,many trees in the Parana River floodplain died when

the length of the flood period, and thus the duration of

42.39 43.77 45.15 46.53 47.92 49.30 50.68 52.06

Topographic position (m a.s.l.)

0.0

14.8

29.7

44.5

59.4

74.3

89.1

104.0N

umbe

r of

tree

s

Island course

West bankTrend line

Fig. 5 Distribution of Tessaria integrifolia at the study sites

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123

continuously flooded soil, extended for more than1 year (Neiff 1999). The influence of the length of the

inundation phase on species composition has been

mentioned for the Amazon floodplain, where thelower tree line experiences an average annual inun-

dation of 230 days (Worbes 1997).

There was a clear relationship between the occur-rence of canopy trees, the frequency of pulses and the

number of flood days that can be estimated through

the pulse FITRAS function (Neiff and Neiff 2004).The civil works that modify the pulse regime (dams,

drainage, etc.) can alter the intensity and duration of

the flood and drought phases, thus affecting thecentral part of the distribution or any of the quartiles

at the ends of the curves.

Acknowledgments We are very grateful to Julio Di Rienzoand the InfoStat team for their advice concerning the statisticalanalysis, and the anonymous reviewer and the Associate Editorwho provided useful comments on the manuscript. This studywas supported by grants PICT 12755 ANPCYT and PIP 6316CONICET.

References

Austin MP (1999) A silent clash of paradigms: some incon-sistencies in community ecology. Oikos 86:170–178

Austin MP, Smith TM (1989) A new model for the continuumconcept. Vegetatio 83:35–47

Brinson MM (1993) Changes in the functioning of wetlandsalong environmental gradients. Wetlands 13:65–74

Brinson MM, Verhoeven JTA (1999) Riparian forests. In: HuntML Jr (ed) Maintaining biodiversity in forest ecosystems.Cambridge University Press, New York, pp 265–299

Campos JB (2004) Spatial characterization of the vegetation.In: Thomaz SM, Agostinho AA, Hahn NS (eds) The upperParana and its floodplain. Physical aspects, ecology andconservation. Backhuys, The Netherlands, pp 369–380

Casco SL (2003) Poblaciones vegetales centrales y su vari-abilidad espacio-temporal en una seccion del Bajo Paranainfluenciada por el regimen de pulsos. Tesis Doctoral.Universidad Nacional del Nordeste, Corrientes, Argentina

De Souza MC, Romagnolo MB, Kita KK (2004) Riparianvegetation: ecotones and plant communities. In: ThomazSM, Agostinho AA, Hahn NS (eds) The upper Parana andits floodplain. Physical aspects, ecology and conservation.Backhuys, The Netherlands, pp 353–367

Gleason HA (1926) The individualistic concept of the plantassociation. Bull Torrey Bot 53:7–26

Gregory SV, Swanson FJ, McKee WA, Cummins KW (1991)An ecosystem perspective of riparian zones. Bioscience41:540–550

Hueck K (1972) As florestas da America do Sul: ecologia,composicao e importancia economica. Editora da Uni-versidade de Brasılia e Editora Polıgono, Sao Paulo

InfoStat (2008) InfoStat version 2008. Grupo InfoStat, FCA.Universidad Nacional de Cordoba, Argentina

Kandus P, Malvarez I (2004) Vegetation patterns and changeanalysis in the lower delta islands of the Parana River(Argentina). Wetlands 24:620–632

Keddy PA (2000) Wetland ecology: principles and conserva-tion. Cambridge University Press, London

Kobayashi JT, Thomaz SM, Pelicice FM (2008) Phosphorus aslimiting factor for Eichhornia crassipes growth in theupper Parana River floodplain. Wetlands 28:905–913

Kozlowski TT (2002) Physiological-ecological impacts offlooding on riparian forest ecosystems. Wetlands 22:550–561

Lewis JP, Franceschi EA (1979) Notas sobre la dinamica de lavegetacion del Valle del rıo Parana. Ecosur 6:145–163

Lindenmayer DB, Fischer J (2006) Habitat fragmentation andlandscape change. Island Press, Washington

Liotta J (2001) Rasgos biologicos de Salix humboldtianaWilld. y regimen de pulsos de inundacion. Interciencia26:397–403

Malvarez AI (1997) Las comunidades vegetales del Delta delrıo Parana. Su relacion con factores ambientales y patro-nes del paisaje. Tesis doctoral. Universidad de BuenosAires, Buenos Aires, Argentina

McLeod KW, McPherson JK (1973) Factors limiting the dis-tribution of Salix nigra. Bull Torrey Bot Club 100:102–110

Mitsch WJ, Gosselink JG (1993) Wetlands, 2nd edn. VanNostrand Reinhold, New York

Neiff JJ (1986) Las grandes unidades de vegetacion y ambienteinsular del rıo Parana en el tramo Candelaria-Ita Ibate.Rev Cienc Nat Lit 17:7–30

Neiff JJ (1990) Ideas para la interpretacion ecologica delParana. Interciencia 15:424–441

Neiff JJ (1996) Large rivers of South America: toward the newapproach. Verh Int Ver Limnol 26:167–181

Neiff JJ (1999) El regimen de pulsos en rıos y grandes hume-dales de Sudamerica. In: Malvarez AI (ed) Topicos sobrehumedales subtropicales y templados de Sudamerica.Universidad de Buenos Aires, Buenos Aires, pp 97–146

Neiff JJ (2005) Bosques fluviales de la cuenca del Parana. In:Arturi MF, Frangi JL, Goya JF (eds) Ecologıa y Manejode los Bosques de Argentina. EDULP, La Plata, pp 1–26

Neiff JJ, Neiff M (2004) Pulso. Software disenado para estu-diar fenomenos recurrentes en el tiempo. www.neiff.com.ar. Accessed 12 Nov 2008

Neiff JJ, Poi de Neiff A (1990) Litterfall, leaf decompositionand litter colonization of Tessaria integrifolia (Composi-tae) in the Parana River floodplain. Hydrobiologia203:45–52

Neiff JJ, Poi de Neiff A (2003) Connectivity processes as abasis for management of aquatic plants. In: Thomaz SM,Bini LM (eds) Ecologia e Manejo de Macrofitas Aquati-cas. Editora da Universidade Estadual de Maringa, Mar-inga, pp 127–144

Neiff JJ, Reboratti HJ, Gorleri CM, Basualdo M (1985) Im-pacto de las crecientes extraordinarias sobre los bosquesfluviales del Bajo Paraguay. Bol Com Espec Rıo Bermejo4:13–30 (Chaco)

Niiyama K (1990) The role of seed dispersal and seedling traitsin colonization and coexistence of Salix species in aseasonally flooded habitat. Ecol Res 5:317–331

Plant Ecol

123

Orfeo O (1995) Sedimentologıa del rıo Parana en el area deconfluencia con el rıo Paraguay. Tesis Doctoral Univers-idad Nacional de La Plata, Buenos Aires, Argentina, 286 p

Orfeo O, Stevaux J (2002) Hydraulic and morphologic char-acteristics of middle and upper reaches of the ParanaRiver (Argentina and Brazil). Geomorphology 44:309–322

Poi de Neiff A, Neiff JJ, Casco SL (2006) Leaf litter decom-position in three wetland types of the Parana Riverfloodplain. Wetlands 26:558–566

Reboratti HJ, Neiff JJ (1987) Distribucion de los alisales deTessaria integrifolia (Compositae) en los grandes rıos dela Cuenca del Plata. Bol Soc Bot 25:25–42

Soil Survey Staff (2006) Keys to soil taxonomy, 10th edn.USDA-Natural Resources Conservation Service,Washington

Stiling P (1999) Ecology: theories and applications, 3rd edn.Prentice Hall, Upper Saddle River, NJ

Tabacchi E, Correll DL, Hauer R, Pinay G, Planty TabacchiAM, Wissmar RC (1998) Development, maintenance androle of riparian vegetation in the river landscape. FreshwBiol 40:497–516

Whittaker RH (1978) Ordination of plant communities. Dr. WJunk bv Publishers, The Hague, Boston

Worbes M (1997) The forest ecosystem of the floodplain. In:Junk WJ (ed) The central Amazon floodplain. Springer-Verlag, Heidelberg, pp 223–265

Plant Ecol

123