Ecological gradients within the riparian forests of the ...

Plant Ecology 152: 101–118, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.

101

Ecological gradients within the riparian forests of the lower Caura River, Venezuela Judith Rosales1 , Geoffrey Petts2 & Claudia Knab-Vispo3 1 Universidad

Nacional Experimental de Guayana, Centro de Investigaciones Ecológicas de Guayana, Avda. Las Americas, Puerto Ordaz, Edo. Bol´ıvar, Venezuela; 2 University of Birmingham, School of Geography and Environmental Sciences, Edgbaston B15 2TT, Birmingham, U.K.; 3 Instituto de Investigaciones Ecol´ ogicas Orinoco-Esequibo, Fundación La Salle, Tumeremo, Edo. Bol´ıvar, Venezuela Accepted in revised form 5 July 2000

Key words: Guayana Region, Hydrogeomorphology, Plant ecology, Riparian forests, Tropical rivers, Varzea-Igapó

Abstract The distribution of woody species within the 260 km-long riparian corridor of the Lower Caura River, a nutrientpoor tributary of the Orinoco River, draining the Guiana Shield in Venezuela is related to environmental variables. TWINSPAN clustering and a Canonical Correspondence ordination were used on abundance data for the 110 most common tree species in 51 sample plots. Four groups are identified: (i) upstream of La Mura Rapids, narrow floodplains in constrained valleys with steep slopes show marked differences between sites dominated by channel lateral accretion (levees, Group B) and overbank deposition (swamps, Group A), (ii) downstream of the rapids, levees (Group C) are differentiated from a more heterogeneous floodplain (Group D) influenced by a backwater effect caused by the ponding of the Caura River by the Orinoco. A Multiple discriminant analysis of these groups resulted in a function including depth of inundation, ratio of alkaline/alkaline earth major cations, and soil phosphorous content, which accounted for 83% of the variance between the four groups. Inundation level and phosphorous content were also the most significant variables in the ordination, within which the first two axes explained 48% of the species- environment relationships. Tree density, species richness and diversity (H’) are shown to change significantly along the lower Caura with highest values associated with levees in sectors upstream of the La Mura Rapids; effects of terrestrialization and intermediate disturbance are proposed to explain these patterns. Floristic elements typical of both Amazonian Igapó and Várzea forests are shown to occur along the whole riparian corridor of the lower Caura, but the majority occur downstream of La Mura Rapids. The intermediate nutritional status of the Caura River and a hydroecological confluence effect associated with higher flooding depths and stronger biogeochemical gradients along the lower reach are suggested to explain the co-occurrence of Igapó and Várzea species.

Introduction The maintenance of two-way lateral exchanges between a river channel and its floodplain is seen as one of the requirements for self-sustaining river systems (Petts & Amoros 1996). Riparian vegetation plays an important role in sustaining fluvial ecosystems through complex interactions with hydrology, geomorphology and limnology (Frissel et al. 1986;

Gregory et al. 1991; Wassen & Grootjans 1996; Petts & Amoros 1996; Naiman & Decamps 1997). Case studies from Europe and North America (Hupp 1986; Nilsson et al. 1989; Tabacchi et al., 1996) show relationships between species distribution patterns and diversity with hydrological and geomorphological characteristics of specific reaches along longitudinal gradients. Analysis of the existing literature on South American riparian forest ecosystems

102 indicates that channel dynamics (Salo et al. 1986), the flood pulse (Junk et al. 1989) and nutrient status are the most important factors influencing local patterns of species composition, diversity, structure and function (Rosales et al. 1999). However, there is a lack of systematic studies devised to quantitatively analyse the interactions between patterns of variation in these variables along environmental gradients in different sectors of these rivers. Many studies conducted on riparian forest vegetation in the Amazon basin, and a few in the Orinoco, have focused on the factors regulating vegetation community and species patterns along lateral gradients. Also they emphasise differences in flooded forests between rivers with different contrasting nutrient status, Igapó and Várzea (Prance 1979; Sioli 1984; Kubitzki 1989; Colonello 1990a, b; 1991; Rosales 1990; Campbell et al. 1992; Worbes 1997; Junk 1997a, b; Rosales et al. 1999). The Igapó and Várzea forest types occur in the floodplains of rivers with nutrient-poor waters and nutrient-rich waters, respectively. Salo et al. (1986), Kalliolla et al. (1991) and Puhakka & Kalliolla (1995) addressed longitudinal patterns within riparian forests, but they did not undertake any systematic analysis of vegetation and environmental data along the Amazonian rivers studied . Duivervoorden & Lips (1993) presented riparian forest data from the Middle Caqueta in the Amazon basin that gives some insights to longitudinal patterns of variation, but their study addresses the landscape scale. In the Orinoco basin, a longitudinal study of the confluence of the Mapire River (nutrient-poor-waters) with the Orinoco River (nutrient-rich waters) showed the existence of Várzea to Igapó forests along a biogeochemical gradient (Rosales 1990), which Rosales et al. (1999) highlight as driven by a confluence effect. This paper presents a systematic study of longitudinal patterns of variation in riparian forests along the 260 km-long corridor of the Lower Caura River, above its confluence with the Orinoco River. It seeks to characterise the patterns of variation in terms of (1) species distribution, (2) forest profile structure and diversity, and (3) relationships with landforms, flooding depth and soil variables. Previous botanical studies along the lower Caura (Williams 1942; Rosales 2000) identified a change in floristic composition around the lowermost rapids in the Caura, La Mura Rapids, about 100 km upstream of the Caura River mouth. This study investigates this lower reach of the river in more detail, examines the dominance of Várzea and Igapó species, and explores the influence of any confluence effect –

ponding of the Caura by the Orinoco – on the observed species, landforms and soil patterns.

Study area and methods The study area is located in the Caura River basin downstream Salto Pará waterfalls, between 6◦ 150 and 7◦ 470 N latitude and 64◦ 150 and 65◦ 350 W longitude (Figure 1). Most (90%) of the river basin is forested; 92% of the area has humid to very humid conditions, with rainfall averaging between 2000 and 3000 mm, reaching over 4000 mm in some southern areas. A transition to a drier climate occurs about La Mura Rapids, and rainfalls below 1500 mm occur throughout the northern 8% of the catchment. (Martínez 1996; Marin & Chaviel 1996). River discharge is about 3500 m3 s−1 , and shows a highly seasonal pattern in flood levels (Figure 2). However, the seasonality close to the river mouth is modified by the influence of the Orinoco River (Vargas & Rangel 1996). The Caura River basin south of La Mura Rapids, has a heterogeneous geology including sedimentary sandstones, acid volcanic and meta-sedimentary rocks, granites and gneisses. North of the rapids is an extensive alluvial plain of Pleistocene and Holocene age (Rincon & Estanga 1996; Heredia 1996). Confined bed-rock controlled channels dominate the stream network upstream from the La Mura Rapids. The high stream power of the upper river relates to the high discharge and a high topographic gradient. Tepui massifs in the south reach maximum altitudes of 2300 m asl (Jaua-Sarisarinama), and along the western border of the basin there are mountain massifs of 1800 (Sierra Uasadi) and 2400 m asl (Sierra de Maigualida). The central valley, however, is dominated by a relief of rolling hills ranging from 200–400 m asl in the south to 40–100 m in the north. The Caura River water has a brown colour and the concentrations of nutrients and suspended materials are low. However, it is not typical of the blackwater tributaries of the Amazon and Orinoco basins, because in comparison with some of them, like the Negro and Caroni rivers, the concentration of dissolved carbon is very low, and the pH and concentration of principal cations is high (Vegas-Vilarrubia et al. 1987; Lewis & Saunders 1990; Garcia 1996). Nevertheless, the Caura contrasts with the Orinoco, which is a white-water river rich in nutrients and transporting high loads of sediments. In a comparison of lakes of floodplains in the Orinoco and the Caura rivers, Hamilton & Lewis

103

Figure 1. The lower Caura study area showing location of survey transects and sectors.

(1990) reported important differences in calcium (1– 3 mg l−1 vs. 0.4–0.7 mg l−1 , respectively), and conductivity (15–32 µS cm−1 vs. 9 to 12 µS cm−1 , respectively). Rosales (2000) differentiated a series of sectors in the riparian corridor of the Caura River downstream of large tributary junctions and major rapids or waterfalls. The lower Caura, which is the focus of this paper, includes the first six sectors upstream from the Orinoco confluence (Figure 1) each comprising a suite of geomorphic units (Table 1). Sectors 1 to 3 are associated with unconstrained channels downstream of La Mura Rapids, dominated by vertical accretion and overbank deposits with back-water habitats. These sectors are characterised by seven geomorphological units: confluence swamps, backswamps, abandoned channels, levees, point-bar systems, alluvial islands, and lateral sand bars. Upstream of La Mura Rapids,

Sectors 4 to 6 are associated with constrained channels including: (a) erosional and in-channel units (rocky margins and rocky islands, bank benches, and lateral sand bars), and (b) vertical accretion and overbank deposits plus in-channel units (levees, backswamps, confluence swamps, slack-water areas and point-bars). Sampling Based on side-looking airborne radar (SLAR) images, scale 1:250 000, sixteen sites were selected along the main channel, representing the major bends of the river. At each site, linear transects perpendicular to the channel on both sides of the river were surveyed and the geomorphic units identified. Flood depth for the year prior to field work was estimated by the trash line height in the riparian vegetation in each transect and measured in the high water season of October 1995 with the aid of a dugout canoe.

104 Table 1. Definition of geomorphic units where riparian forest communities were sampled in the lower Caura. From Rosales (2000). Unit In-channel origin A2. Point-bars C. Rocky margins, rocky islands Vertical accretion – overbank deposits D.Natural levees E. Backswamps E1. Confluence swamps

Non-active floodplains T. Terrace

Characteristics

Bars deposited in the convex margin of meander bends. Lateral growth of a point-bar gives rise to systems of ridges and swales. Formed by erosion on margins of bed-rock dominated channels. Frequent in multi-thread sectors.

Flood units formed by the vertical accretion of sandy and silty sediment along the channel margin. Units formed by vertical accretion of fine (usually organic) sediment behind levees. Similar to backswamps but occurring at confluences between the banks of the tributary and the main channel.

All parts of the riparian corridor flooded by high magnitude, low frequency flood events.

Figure 2. Water level fluctuations in the lower Caura River (San Luis gauging station) compared with the water levels in the Orinoco River (Musinacio gauging station).

Along each transect, plots of 0.1 ha (20 × 50 m) incorporating 10 subplots of 10 m × 10 m, were chosen to represent the main geomorphic units. A total of 54 plots were surveyed. The number of plots per transect varied between 2 and 14 in relation to the width of the floodplain, which increased from upstream to downstream. The presence of human use, such as mango plantations, required the addition of two extra sites. With the assistance of local plant specialists, all trees above 10 cm DBH in the 54 survey plots were identified using local names and collection numbers, and their diameter at breast height (DBH) was measured. A reference of 318 riparian plant specimens collected in the lower Caura during low and high water periods of 1994 and 1995 permitted the botanical identification of the majority of species encountered in the survey plots. Duplicates of specimens collected have been deposited in the Herbarium of Guayana (GUYN), Herbarium of Venezuela (VEN), Ovalles Herbarium (MYF) and Herbarium of the Missouri Botanical Garden (MO). In order to analyse the variation in soil physicochemistry, composite soil samples were taken from the four corners of each plot after removal of the root mat (topmost 20 cm of the mineral soil). The soil samples were air-dried and sent to the Corporacion Venezolana de Guayana Soils Lab (CVG-Hato Gil) for the fol-

105 lowing analyses: texture (Bouyucos), organic matter (Walkey-Black), exchangeable aluminium and acidity (Yuan), exchangeable cations (NH4OAC extraction, atomic absorption), and Cationic Exchangeable Capacity CEC (1 N NH4OAC extraction pH 7), pH 1:1 in water, and available phosphorous (Bray I). For methods see Jackson (1976) and Soil Survey Laboratory Staff (1992). Two additional variables were calculated from the data, the ratio alkaline/alkaline earth major cations (Ca+Mg/Na+k) and the sum of major cations (Ca+Mg+Na+K). Data analysis All species with individuals above 10 cm DBH were used for general analysis. The relative importance of each species was evaluated by calculating: (i) the Importance Value (I.V.) of Curtis per plot (sum of relative number of individuals, relative frequency between subplots and relative basal area) and (ii) the frequency of occurrence of each species in the 54 plots. For the whole forest community, tree density (number of individuals per hectare) and basal area (m2 ha−1 ) per plot were used as a measurement of forest structure. Diversity was assessed per plot using species richness (total number of species) and two diversity indexes – Shannon–Wiener H 0 and Evenness J . All the mentioned methods are detailed in Greig-Smith (1983). Floristic patterns were assessed in two ways: (1) a species-area curve documenting the change in species number along the riparian corridor by nesting the number of new species present for each plot from upstream, commencing at Sector 6, to downstream, and (2) a two- way indicator analysis TWINSPAN (Gauch 1979) based on the abundance of the 110 locally and regionally most important tree species per plot. In this context, species that reached one of the five highest Importance Values in at least one survey plot were defined as locally important, while species that ocurred in at least 20% of the survey plots were considered as regionally important. For the TWINSPAN, and the further statistical analyses, only 51 plots were considered given that one plot from transect 10 belonged to an under-represented forest in a rocky margin geomorphic unit and two other plots from transect 11 were not considered enough to represent the variability of Sector 3. The multivariate vegetation-environment relationships were examined using Canonical Correspondence Analysis CANOCO (Ter Braak 1988) based on the

same species abundance data used in the TWINSPAN. Independent environmental variables per plot included values of inundation depth and soil characteristics: percentages of organic matter, silt and clay, concentrations of exchangeable aluminium, acidity, calcium, potassium, magnesium, sodium, available phosphorus, cationic exchange capacity, and pH. A logarithmic transformation of the species abundance and down weighting of the rare species was done to enhance the visibility of species-environment relationships. The CANOCO analysis differentiated four groups of plots that integrated both the major gradients in floristic variation of the riparian forests and the geomorphic units with a reasonable number of sample plots to perform statistical comparisons. Using SPSS software, parametric statistics were used after studying the normality of the variables. The importance of the environmental variables in explaining the four groups defined using CANOCO was analysed with Stepwise Multiple Discriminant Analysis. Differences between the means of the four groups in terms of structural (basal area and density) and richness-diversity variables (number of species, H 0 of Shannon–Wiener and J Evenness) were analysed with ANOVA. Post-hoc comparisons using Tukey (equal variances) and Dunnett’s T3 (unequal variances) were used to calculate the significance of the differences at the 95% confidence level. A species-based analysis using Stepwise Multiple Regression was conducted to determine the environmental variables that correlated significantly with the densities of each of the 110 most important species.

Results Floristic patterns The species-area curve (Figure 3) shows the presence of a floristic discontinuity in the riparian forests around La Mura Rapids. A relative saturation of the species/area curve with 191 species is found after the first 2.7 ha of nested plots in sectors 6 to 5 upstream of La Mura Rapids. New floristic elements are progressively added from sector 4 until the curve flattens in sector 2, and the species number reaches an approximate maximum of 258 species at 5.4 ha. Only four additional species are found if data from 15 plots of 0.1 ha are added from flooded forest communities in the borders of backswamps, abandoned channels and oxbow lakes in sector 2 (Knab–Vispo unpublished

106 data). The rate of species gain is accentuated between sectors 4 and 2, where new floristic elements appear. However, these are not only characteristic of forests downstream of La Mura Rapids, but also of the relatively rare forest sampled at the rocky riparian margin in sector 4. TWINSPAN differentiated species between sample plots located upstream and downstream of La Mura Rapids (Figure 4). Eschweilera tenuifolia and Euterpe precatoria are the indicator species for the upstream riparian communities and Piranhea trifoliata is the indicator species for downstream communities. These two main floristic assemblages are subdivided into eight classes, each associated with specific geomorphic units. Multivariate species-environment relationships The Canonical Correspondence Analysis placed the 51 plots into a multidimensional space according to their similarity in floristic composition. The first four axes of this ordination account for 67% of the variance in the species-environment relationships. Figure 5 shows the spatial arrangement of the survey plots on the plane defined by the first two axes which together account for 48% of the variance. Inundation depth, soil phosphorous, organic matter, and silt are most significantly correlated with the first two axes, while soil clay and calcium are significant for the 3rd and 4th axes. Along the first two axes of the ordination, higher inundation depths and soil phosphorous content seem to explain the species composition of the floristic assemblage downstream of La Mura Rapids. From this figure, the four groups of plots that represent the major species-environment differences in the forest communities along the riparian corridor of the lower Caura are shown and include: (i) Upstream of La Mura Rapids, Group A represents different types of swamps (TWINSPAN 1, 2 and 3) and Group B represents levees and terraces (TWINSPAN 4); (ii) Downstream of La Mura Rapids, Group C separates levees (TWINSPAN 5 and 6) from a more diverse Group D including point bar interiors, backswamps and confluence swamp (TWINSPAN 7 and 8). The Stepwise Multiple Discriminant Analysis, using these four groups A–D gives two canonical functions accounting for 100% of the variance. The first function accounts for 82.8% of the variance and the significant variables were (in order of decreasing importance), inundation depth, the ratio alkaline-alkaline earth cations and available phosphorous.

Structure and diversity Figure 6 shows the average values of density and basal area for the four groups. Group B has significantly higher density values than the other three, which have similar values. Inundation depth was the most significant variable in the multiple regression. Tree density has a significant negative correlation with inundation depth. In terms of basal area, there are no significant differences between the groups. Except A, all the groups have maximum values above 35 m2 ha−1 and are highly heterogeneous. Basal area has a significant positive correlation with cation exchange capacity and a negative correlation with organic matter. Regarding richness and diversity (Figure 7), the number of species is significantly higher in Group B. The diversity index (H 0 ) gave significant differences between all groups. Concerning evenness J , Group D shows the lowest values but it is not significantly different from C, whereas B shows the highest values nevertheless it is not significantly different from A. Inundation depth and available phosphorous concentration are most significant in explaining the species number and the diversity index (H 0 ). However, evenness was explained by a significant combination of inundation depth, organic matter, and available phosphorous content. Assessment of the Várzea – Igapó gradient A list of 90 species with at least 10 individuals in the 51 sample plots, and their average densities in the above defined four groups of plots (abundance/number of plots in the group), was established in Table 2. This list highlights the species found in the literature as typical Várzea or Igapó species, and also shows the sign of significant relationships (α > 0.5) resulting from the stepwise regression analysis of abundance data per plot with the environmental variables. Two main floristic assemblages occur: (i) species occurring exclusively (58%) or mostly (12%) upstream of La Mura Rapids, and (ii) species occurring exclusively (17%) or mostly (7%) downstream of La Mura Rapids. Only one species occurred in similar densities above and below La Mura. Species that were documented exclusively, or were most common, in plots on levees and terraces above La Mura (Group B) clearly tend to be correlated negatively with inundation (21 of 37 = 57% of the species) and/or silt (6 of 37 = 16%). Some of them are also positively correlated with exchangeable acidity (6 of 37 = 16%). Only five of these species (9%) showed no significant correlation with

107

Figure 3. Species - area curve for the Lower Caura joining new tree species occurring in 0.1 ha plots at transects from upstream to downstream direction (Sector 6 to Sector 1).

any of the measured environmental variables. On the contrary, 57% (16 of 28) of the species that occur exclusively or most commonly in the confluence swamps or backswamps above La Mura (Group A), did not show any significant correlation with the measured environmental variables. The only remarkable pattern in the remaining species seems to be a trend towards positive correlation with organic matter in four species (14%). All species exclusive to or most common on levees below La Mura (Group C) show significant environmental correlations. 64% (7 of 11) are positively correlated with the ratio of alkaline to alkaline earth cations and 56% (6 of 11) are positively correlated with available phosphorus and/or potassium levels.

Of the fourteen species that occur in highest densities in plots below La Mura (Group D), 43% are positively correlated with inundation depth, and 36% with available phosphorus, 21% are negatively correlated with the sum of cations and only two species in that group did not show any significant environmental correlation.

Discussion The results support the existence of two major phytogeographical subregions along the riparian corridor of the lower Caura River, the lowermost rapids of La Mura being an effective boundary between them. That boundary is reflected in both the riparian forest com-

108

Figure 4. Classification of plots by the TWINSPAN analysis. Indicator species for the two types of sectors and eight geomorphic units are represented.

Figure 5. Ordination of species and environmental data for 51 plots in sectors 1 – 6 of the lower Caura, determined by Canonical Correspondence Analysis. A line differentiate plots upstream and downstream of La Mura Rapids. Numbers refer to end classes generated by TWINSPAN and symbols show different geomorphic units to which the forest communities are grouped into swamp (A and D) and levees and terraces (B and C) separated by dashed lines. The dominant environmental variables, as well as the variance explained by the different axes are shown on the right.

Brownea longipedicellata Huber Trichilia quadrijuga Kunth. Micropholis melinoniana Pierre Virola sebifera Aubl. Sterculia pruriens (Aubl.) Schum. Ecclinusa guianensis Eyma Alexa confusa Pittier Cordia bicolor A.DC. Maquira coriacea (Karst) C.C.Berg Protium aracouchini (Aubl.) Marchand Catostemma commune Sandw. Jacaranda copaia (Mart. ex DC.) ssp. spectabilis Gentry Rinorea flavescens (Aubl.) Kuntze Eugenia feijoi O.Berg Phenakospermum guyannense (L.C.Rich.) Endl. ex Miq. Dipteryx odorata (Aubl.) Willd. Peltogyne paniculata Benth. Andira surinamensis (Bondt.) Splitg. ex Pulle Protium heptaphyllum (Aubl.) Marchand Cupania cinerea Poepp. & Endl. Guarea trunciflora C.DC. in A.DC. Attalea maripa (Aubl.) Mart. Gustavia coriacea Mori Burseraceae Chrysobalanaceae Sloanea latifolia (Rich.) Schumann Euterpe precatoria Mart. Dialium guianense (Aubl.) Sandw. Amphirrhox latifolia Mart. Licania sp. Simaba cedron Planch. Licania sp. 1 Micrandra minor Benth. Clathrotropis brachypetala (Tul.) Kleinh. Brownea coccinea Jacq. Tabebuia capitata (Bur. & K.Schum.) Sandw.

Species

1.7 2.3 2.3 1.3 0.7 1.3 0.8 0.2 0.5 0.3

0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.6 0.6 0,5 0.5 3.9 3.3 2.5 2.4 2.4 1.9 1.6 1.5 1.5 0.6

1.0 0.8 0.8

4.4 2.7 2.4 2.0 1.9 1.9 1.8 1.8 1.5 1.4 1.0 1.0

Above La Mura A B 1 2 3 4

Below La Mura C D 5 6 7 8

− − −

− −

− − −

−

− − − −

−

−

−

− −

− −

IN

−

−

− −

−

−

SILT

−

+

−

CLAY

+

SAND

+

+

+

+

+

+

+ +

ns

ns

ns

ns ns

−

+

− +

+

Significant variables in the regression (0.05) EAC PH ALKR CA CEC SCAT OM

AL

P

K

NA

Table 2. Densities of species within the Groups of riparian forest communities associated with swamps-like (A and D) and levees and terraces (B and C) geomorphic units identified in Figure 5 and significant environmental variables in the multiple regression. Species are ordered following TWINSPAN classes 1 to 8. In bold the indicator species, ∗ Várzea species, ∗∗ Igapó species. ns = no significant correlation. IN = inundation depth.

109

Licania sp. 2 Caraipa densifolia Mart. Mimosaceae Gustavia poeppigiana Berg Abarema adenophora (Willd.) Britton & Rose Inga splendens Willd. Calycolpus goetheanus (DC.) O.Berg Protium unifoliolatum Engl. Swartzia picta Benth. ∗∗ Macrolobium angustifolium (Benth.) Cow. ∗ Virola surinamensis (Rol.) Warb. Eugenia florida DC. Elvasia elvasioides (Planch.) Gilg. Eschweilera subglandulosa (Steudel ex Berg) Miers Croton cuneatus Kl. ∗∗ Amanoa guianensis Aubl. Mouriri sagotiana Triana Mouriri acutiflora Naudin ∗∗ Ocotea cymbarum H.B.K. ∗∗ Macrolobium multijugum (DC.) Benth. Zygia latifolia (L.) Fawc. & Rendl. Cathedra acuminata (Benth.) Miers ∗∗ Panopsis rubescens (Pohl) Pittier Mabea taquari Aubl. Hydrochorea corymbosa (Rich.) Barn. & Grim. Swartzia leptopetala Benth. Homalium guianense (Aubl.) Oken Trichilia sp. ∗∗ Pouteria reticulata (Engler) Eyma Vochysia venezuelana Stafleu ∗ Vitex capitata Vahl Luehea seemanii Tr. et Planch. ∗ Spondias mombin L. ∗∗ Mabea nitida Spruce ex Benth. ∗ Gustavia augusta L. Bixa urucurana Willd. ∗∗ Trichilia mazanensis Macbride ∗∗ Byrsonima japurensis Adr. Juss. Nectandra aurea Rohwer

Species

Table 2. Continued

2.0

1.0

2.0

2.5 2.0 1.0 8.0 7.0 6.5 5.0 4.0 4 2.5 1.0 17.5

9.0 9.0 7.5

1.0

1.0

3.0

1.0

18.0 31.0 2.0 5.0

5.0

1

0.3

0.3

1.7 2.3

1.2 0.2

4.3 1.0 1.0 0.2

0.3

0.1

0.3

2.1 0.3 0.2 0.3 0.7

0.1 0.7 0.7 0.2

1.2 0.2 0.6 0.8 0.4 0.4 1.0 0.7 0.1 0.6 5.6

1.1 2.6

0.5 2.0 0.7 0.5 1.3

2.8 6.2

6.2 4.2 3.2 1.8 1.8 1.0 0.8 0.8 18.0 11.0

Above La Mura A B 2 3 4

2.5 1.5 0.5

1.5

9.5 6.5 1.5 1.5

1.0

0.5 1.0 1.5 5.5 5.0 1.8 3.8 2.5 1.9

0.1

0.6 0.4 1.6

0.6 0.3 0.3 0.1

0.5 0.6

0.4

1.8 0.2

1.6

0.2

0.2

1.8

1.2 0.4 0.4

0.2

0.1 2.4

0.8

0.1


−

−

−

IN

+

−

+

−

SILT

−

CLAY

−

SAND

+

+ + + + + + −

ns

ns

ns ns ns ns ns ns

ns ns ns ns

ns

ns ns ns +

Significant variables in the regression (0.05) EAC PH ALKR CA CEC SCAT

−

+ +

+

+

OM

+

+

AL

+ +

+ +

−

P

+

+ +

+

+

+

K

NA

110

+ +

+

−

−

− −

−

ns ns +

+

0.1 0.4 0.6

0.2 2.0

1.4 2.2

1.5 3.3 2.5 1.8

+ − + +

+

0.8 5.4 5.4 3.1 2.3 2.3

0.4

+

2.0

0.4 14.4 7.6 6.6 2.0 1.6 1.8

1.0

0.1 0.3 0.1

1.0

1.1 0.5 1.8 0.1 0.3 0.6 0.3 0.2

Brosimum guianense (Aubl.) Huber ∗ Piranhea trifoliata Baill. ∗ Etaballia dubia (Benth.) Rudd. Pouteria orinocoensis (Aubr.) Penn. Symmeria paniculata Bentham ∗ Homalium racemosum Jacq. ∗ Sclerolobium guianense Benth. ∗∗ Acosmium nitens (Vog.) Yakovlev ∗ Alchornea schomburgkii Kl. ∗ Ruprechtia tenuiflora Benth. in Hooker ∗∗ Duroia micrantha (Ladbr.) Zarucchi & Kirkbride Ouratea steyermarkii Sastre ∗∗ Campsiandra taphornii Sterg. ∗∗ Couepia paraensis (Mart. & Zucc.) Benth. Connarus venezuelanus Baill.

Species

Table 2. Continued

Above La Mura A B 1 2 3 4


IN

+

SILT

CLAY

SAND

Significant variables in the regression (0.05) EAC PH ALKR CA CEC SCAT

OM

AL

P

+

+

K

+

+

NA

111

Figure 6. Comparison of forest structure (D= density and BA = basal area) between the four groups of communities associated with A to D groups defined in Figure 5. Density mean values in group B and basal area in Group D are significantly different (α = 0.05) to the mean values of the other groups. Equations above the graphs indicate the results of multiple regression analysis using inundation and soil environmental variables (IN = inundation depth, CEC = capacity of exchangeable cations, OM = organic matter).

munity characteristics (species composition, structure and diversity) and the environmental variables assessed along the corridor. The boundary is associated with the convergence of the following factors: (1) A bioclimatic limit between the humid environments to the south and semiarid environments to the north.

112 (2) A geomorphological change between constrained channel sectors developed in the hard crystalline rocks of the Guiana Shield to the south, and the unconstrained channel sectors developed on the fluviolacustrine sediments to the north. (3) A hydrological confluence effect driven by a backwater process when the Orinoco River impounds the Caura River. Floristic patterns

Figure 7. Comparison of species richness (S), diversity (H’) and evenness (J) variables between the four groups of communities associated with A to D groups defined in Figure 5. Mean values of S for Groups A and B are significantly different (α = 0.05) from the mean values of the other groups; Groups C and D are significantly different from Groups A and B. Values of H’ are significantly different between all the groups. Mean values of J for Groups A and B are significantly different from D; also significant differences are found between the means of Groups B and C. Equations above the graphs indicate the results of multiple regression analysis using inundation and soil environmental variables (IN = inundation depth, P = available phosphorous, OM = organic matter).

The bioclimatic gradient partially explains the changes in species composition of the riparian forests of levees and terraces subject to low flood depths that, especially upstream of La Mura, show a strong resemblance to the floristic composition of the adjacent non-flooded forests. New species incorporated downstream of La Mura Rapids have been reported for the Llanos-Caribean Phytogeographical Province (Aymard et al. 1997). This division also agrees with the limits given by Huber (1994) between the Central Guayana Province and the Llanos. In addition, a comparison of the flora with local studies in the lower Caura ‘tierra firme’ forests (Marin & Chaviel 1996; Salas et al. 1997; Aymard et al. 1997; Knab-Vispo 1998) indicates an increased frequency of species typical of semi-deciduous forests dominating the dissected landscape of the surrounding ‘tierra firme’ forests downstream of the area around the confluence of the Nichare River. Some species, such as Eschweilera subglandulosa and Euterpe precatoria, which are very frequent on the levees and terraces upstream of La Mura Rapids, were also reported by Aymard et al. (1997) in non-flooded forests downstream of these rapids. In the case of other geomorphic units like swamps, with deeper flooding depths, a bioclimatic influence can not be demonstrated. Different species found in this group upstream and downstream of La Mura Rapids, such as the typical Igapó species Byrsonima japurensis, Panopsis rubescens, Couepia paraensis ssp. glaucescens, were reported by Rosales et al. (1999) to be part of the common floristic taxa of riparian flooded forests shared by the Orinoco and the Amazon River basins. They are found in floodplains under a variety of bioclimates ranging from very humid to semiarid conditions. Diversity and structural variation Many studies have described flooding tolerance as a requisite for species to be successful in wetland

113 habitats (Gill 1970; Mitsh & Gosselink 1993; Fernandez et al. 1999). Therefore, diversity and structural variables have been found by many authors to be highly dependent upon inundation characteristics in different riparian vegetation environments in the world (Brinson 1990; Medley 1992; Tabacchi et al. 1996; Naiman & Decamps 1997) and also in the Orinoco and Amazon rivers (Rosales, 1990; Worbes 1997; Ferreira 1997; Ferreira & Prance 1998). As will be explained in the following sections, results of this study support these findings and reflect not only a flood intensity gradient but also a potential flooding disturbance at both the sector and regional scale, and the local geomorphological units scale. Flooding intensity The data indicates: (1) a lower species richness in the riparian forests of unconstrained sectors downstream of La Mura, which experience higher flooding depths, and (2) higher species richness in the constrained sectors upstream of La Mura under lower flooding depths. Although flood duration was not measured, it is expected that because of the backwater effect of the Orinoco, it will be higher downstream of La Mura, where a more specialised floristic assemblage dominates the forests of the riparian corridor. The 56 species recorded within the sectors downstream of La Mura Rapids is comparatively very low in relation to the 190 species found upstream, but of the same order of magnitude (50–70 species) as the total number of species reported in different studies of the Amazon and Orinoco rivers with long flood duration (Keel & Prance 1979; Rosales, 1990; do Amaral et al. 1997; Worbes 1997). Values of richness and diversity at the plot level in general decreased with the increase in flood depth, a result comparable with other studies along flood intensity gradients. For example, in an Igapó forest of the Central Amazon, Ferreira (1997) showed comparable significant differences to those found in this study in the means of species diversity (Shannon–Wienner index) at three plots having values of 3.1, 2.3 and 1.6 for 2.1, 4.8 and 8.6 m of mean inundation levels respectively. The results of the Caura study were also similar to those found for similar habitats in central and western Amazonia (Duivenvoorden & Lips 1993; Ferreira & Prance 1998). For the lower Caura as a whole, regional species richness, or the sum of the species downstream and upstream of La Mura Rapids, resulted in 258 riparian forest species in fifty-four 0.1 ha plots. This number is close to the 274 species found in sixty-seven 0.1 ha

plots near the Nichare mouth, which to a large degree represented the local variability of ‘tierra firme’ forests (Knab-Vispo 1998). It is within the reported numbers of 250–300 species for the Várzea in the region of the Lower Solimoes (Klinge et al. 1995) and the 157–252 for the floodplain in the region of the lower Caqueta River (Duivenvoorden & Lips 1993). The similarity of these values probably indicates that levels of regional diversity are similar for different riparian forests in the tropical humid environments of the Amazonian and Guayana phytogeographical regions. Flood disturbance Flood disturbance, by silting and mechanical damage, has been shown to be a major factor promoting successional changes (Kalliolla et al. 1991; Bornette & Amoros 1996). At the population level, flood frequency and timing can also be related to successional changes in a forest community. All of these factors affect the species recruitment and rate of extinction, and dominance patterns of local populations, therefore influencing plant diversity and structural patterns along successional gradients as has been thoroughly reviewed by Bazzaz (1996). This author indicates that those patters can be explained by the intermediate disturbance hypothesis (Connell 1978) which proposes that under gradients of disturbance, theoretical evidence indicates that in early-successional habitats with frequent disturbance only few tolerant-species are present whereas with very infrequent disturbance, the forces of competitive exclusion eliminate many of the species, and with intermediate frequency of disturbance, the number of species present in the community is at its maximum. Observing the results of this study in terms of the high variability in the structural variables, density and basal area, the data reflects comparable trends to those presented by Worbes et al. (1992) along successional sequences in a Várzea forest in the Central Amazon. The high variability found in each of the four groups of this study could also be indicative of different ages in different forest communities. Worbes et al. showed how basal areas between younger and older stands are higher at medium age sites (60–100 years), whereas in stands between 10 to 50 years old, basal area increased exponentially from around 10 to 40 m2 ha−1 , and in older stands they reached values close to 40 m2 ha−1 . In terms of density, these authors found similar values to our data with maximum values of density in younger stands. In this study, although levees showed higher densities than backswamps, most of the point-

114 bar edge communities are nearly impenetrable scrubs. The data reflects similar trends as those shown by Worbes et al. for the youngest, tall shrubland, woody communities associated with point-bar edges. The range of plots on levees may represent successional stages in the vertical accretion of this geomorphic unit, with young forms characterised by high hydraulic disturbance (shear stress and sediment accretion). The communities on levees share a floristic composition with both the adjacent, ‘tierra firme’ forests and the frequently flooded forests. This could reflect a terrestralization process, with highest diversity in older communities (Worbes 1997). However, diversity values for levees in this study exceed those for ‘tierra firme’ forests in the same region (Knab-Vispo 1999). Therefore, flood disturbance might be considered intermediate in levees in terms of velocity, sedimentation rates, depth and duration than in other units of the floodplain. A biogeochemical Varzea to Igapo gradient? Kubitzki (1989) concluded that water chemistry is an important discriminator of plant communities within flooded forests along the Amazon. He further argued that the Várzea has a floristic composition similar to that of the non-inundated Amazonian forests. In this study, results indicate that contrary to what is expected for a black-water river, typical Várzea and Igapó species both occur along the lower Caura corridor. If one supports Kubitzki’s arguments, then the riparian forests of the Caura might be older in origin, and related to an ancient floristic domain, than the Varzea environments in the Amazon. According to the oligotrophic black-water characteristics of the Caura River waters, it could be expected to have an Igapó floristic group dominating the flooded forests. Both Igapó and Várzea groups are common below La Mura Rapids, reflecting the higher flood depths, but Virola surinamensis, cited as common in Várzea forests of the Amazon (Prance 1979; Kubitzki 1989), has its highest density in the swamps above La Mura and the same is true for Panopsis rubescens, cited as common in Igapó by the same authors! In general, there is no strict separation of Igapó and Várzea species in the lower Caura. For example, on the levees below La Mura, two Várzea species Spondias mombin and Gustavia augusta, as well as a Igapó species Byrsonima japurensis reach their highest densities. The same is true for the confluence swamps below La Mura, where the three Várzea species Sclerolobium

guianense, Alchornea schomburgkii and Ruprechtia tenuifolia and two Igapó species Acosmium nitens and Campsiandra taphornii reached their highest densities. Furthermore, while three Várzea species occur in highest densities on point bar interiors and backswamps below La Mura, Igapó species are not completely absent from this geomorphic unit. A study of flooded forests at the confluence of the Negro River with the Solimoes in the Amazon basin, documented the co-occurrence of floristic elements from both Várzea and Igapó (Do Amaral et al. 1997), which led the authors to call this forest a ‘seasonal Várzea & Igapó’ or ‘mixed-water’ inundation forest, one of the types previously acknowledged in the riparian forests classification of Prance (1979). In addition, at the confluence of the Mapire River, a small tributary on the left margin of the Orinoco River, Rosales (1990) reported a longitudinal gradient from Várzea–Igapó floristic components. In general, comparisons with literature on Várzea and Igapó forests indicate the presence of a mix of highly flood-tolerant species of both Várzea and Igapó floristic groups in the lower Caura riparian forests downstream of La Mura Rapids. In this study, the occurrence of some Várzea species such as Piranhea trifoliata and Symmeria paniculata exclusively downstream of La Mura Rapids reflects the higher flood pulse. The Várzea species Gustavia augusta occur on the levees characterised by lower inundation depths but also with higher alkaline/alkaline earth ratios and phosphorous levels. In the Mapire study (Rosales 1990), the Várzea species Piranhea trifoliata occurred only at the mouth, where flooding depth and available phosphorous were highest and Gustavia augusta was found also upstream of the confluence, with lower flooding depths but where the accretion of sediments was related to relatively high nutrient concentrations. Igapó species such as Acosmium nitens and Panopsis rubescens were found in that study, associated with lower flooding depths and more organic and extremely acidic soils. The mix of floristic components found in the present study seems to relate to high variability in flood depths, as well as in some soil variables. Várzea environments have been related in the literature to richer soils (Furch 1997). In Table 3 we contrast some comparable soil variables from Igapó and Várzea forest types of the Amazon and Orinoco with those of the four main groups of riparian forests of the Caura. Caura soils seem to be in general richer in magnesium than those commonly reported for Igapó, but

115 Table 3. Means for some important environmental variables in the riparian forests of the lower Caura and comparison with other flooded Forests of the Orinoco basin and Central Amazon. Caura groups

N. Plots Inundation (m) Phosphorous (ppm) Organic matter (%) Silt (%) Clay (%) Sand (%) K (cmol kg−1 ) Na (cmol kg1 ) Mg (cmol kg−1 ) Ca (cmol kg−1 ) P

major cations Ratio alkaline/ alkaline earth

Mapire ∗

Central Amazon∗∗ Varzea Igapo ilha taruma marcha mirin ntaria

A

B

C

D

Igapo (BI-PCA)

9

19

10

13

8

8

10

3.5 (0.77) 8.2 (5.51) 3.62 (1.72) 24.22 (9.56) 38.13 (19.05) 37.64 (26.55) 0.26 (0.16) 0.004 (0.0005) 0.53 (0.09) 0.0198 (0.01) 0.82 (0.19) 2.63 (1.20)

0.64 (0.83) 12.09 (8.41) 2.89 (1.00) 20.84 (9.55) 30.27 (15.39) 48.88 (23.34) 0.20 (0.18) 0.003 (0.0004) 0.55 (0.06) 0.0198 (0.01) 0.78 (0.10) 3.37 (1.59)

2.79 (1.37) 30.50 (24.57) 2.91 (0.95) 28.40 (6.95) 40.60 (13.84) 30.00 (20.12) 0.18 (0.14) 0.005 (0.010) 0.56 (0.05) 0.0198 (0.008) 0.77 (0.15) 4.70 (2.57)

6.11 (1.91) 24.51 (25.20) 4.18 (1.40) 24.31 (7.25) 52.18 (15.44) 20.43 (21.35) 0.19 (0.13) 0.03 (0.07) 0.52 (0.11) 0.0201 (0.08) 0.77 (0.18) 3.30 (1.79)

5.33 (2.07) ∗∗∗ 2.04 (0.76) 6.14 (0.88) 17.31 (6.26) 70.44 (12.55) 10.06 (12.93) 0.23 (0.18) 0.1 (0.01) 0.1 (0.04) 0.02 (0.01) 0.45

0.34 (0.13) 0.18 (0.07) 2.84 (0.64) 12.5 (3.6) 15.86

0.18 (0.07) 0.08 (0.05) 0.14 (0.04) 0.08 (0.07) 0.48

0.36

29.50

0.85

∗ Rosales (1990). ∗∗ Furch (1997). ∗∗∗ Method Murphy–Riley.

not rich enough to be considered Várzea. The higher magnesium concentrations also influences the alkaline/alkaline earth ratio, which is significant in the discriminant analysis of the forest groups and is correlated with the densities of Várzea species such as Gustavia augusta and Spondias mombin. A relatively rich biogeochemical status of the Caura River in relation to other black-water rivers is also reported by Johnson et al. (1991) and confirmed in a study of soil samples from in-channel units along longitudinal gradients of riparian confluences in several tributaries of the Orinoco draining the Andes, Llanos, and Guayana geological regions (Rosales et al. 1999b).

A backwater process associated with a confluence effect of the Orinoco flows upon the Caura can explain the accumulation of phosphorous, clay and organic matter (from both Caura and Orinoco sources) to which species densities are responding. Phosphorous, which is a very important regulator for plant species (Medina & Cuevas 1994), proved to be a major variable both in the ordination and the species distributions. But not all phosphorous in the soil is always available for the plants. Following Mitsh & Gosselink (1993), differences in phosphorous availability in the Caura may be related to the organic matter and clay variability. It can be a consequence of precipitation

116 of insoluble phosphates under aerobic conditions, or adsorption of phosphate onto clay particles or aluminium hydroxides and oxides. Soils in levees and point bars with coarser textures and lower organic matter percentages than swamps, have probably relatively more available phosphorous than less permeable and organic soils. Irrespective of the geomorphic unit, swamp conditions promote a generalised deposition of colloidal material throughout the confluence zone when low velocities are maintained during the months of maximum flows. The presence of both Várzea and Igapó species along the corridor of the lower Caura, which could not be explained by simple correlations with flood depth or water quality, is most likely complicated by the history of the basin. However we can conclude that some active hydro-ecological processes are important regulators of the observed variability in species composition, structure and diversity in the riparian forests of the lower Caura, and need to be the subject of further detailed studies. The dominant processes appear to be: (i) hydraulic disturbance driven by velocity and sediment deposition gradients, in the confined bedrock channels upstream of La Mura Rapids, and (ii) a hydraulic backwater process downstream of La Mura Rapids and upstream of the Orinoco causing increased flood depths and duration that drive biogeochemical gradients within the riparian soils.

Acknowledgements The first author wants to acknowledge CONICIT and The British Council for sponsoring her studies at Birmingham University. Universidad Nacional Experimental De Guayana (UNEG) and Dirección de Vegetación, Ministerio del Ambiente y de los Recursos Naturales Renovables (MARNR) supported the fieldwork for this project. Logistical assistance came from Servicio Forestal de Venezuela SEFORVEN and MARNR-Hidrología Guayana, CVG-Electrificación del Caroní (CVG-EDELCA), Asociación para la Conservación del Ambiente y la Naturaleza (ACOANA) and the Cauraventura touristic camp. The assistance of many botanists from the Herbarios Regional de Guayana, Nacional de Venezuela, Ovalles, and Missouri Botanical Garden in the taxonomical identification is appreciated. Also acknowledged is the assistance with the fieldwork of the specialists in the local flora; German Rodriguez (Nichare), Felix Flores (Las Majadas), Jose Luis Valles (Mapire) and Rafael Diaz

(El Palmar). Franklin Uscategui and Hernan Castellanos also helped in some of the expeditions. Many people from the local Ye’kwana and criollo populations helped very much to facilitate the work in the field. Comments from Nigel Maxted, Klaus Kubitzki, Stan Gregory and David Tikner are also appreciated. Kevin Burkhill helped with the drawing of some of the figures.

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