Wetlands Ecology and Management 12: 575–585, 2004. # 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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Aquatic plant community composition and distribution along an inundation gradient at two ecologically-distinct sites in the Pantanal region of Brazil Ronald H. Fortney1,*, Michael Benedict2, Johan F. Gottgens2, Timothy L. Walters2, Brenda S. Leady2 and James Rentch3 1
Department of Civil and Environmental Engineering, West Virginia University, West Virginia 26506, USA; 2Department of Earth, Ecological and Environmental Sciences, University of Toledo, Toledo, Ohio 43606, USA; 3Division of Forestry, West Virginia University, Morgantown, WV 26506, USA; *Author for correspondence (e-mail:
[email protected]; phone: 1-304-293-3031 ext. 2659; fax: 1-304-293-7109) Accepted in revised form 16 November 2003
Key words: Aquatic macrophytes, Brazil, Diversity, Floating vegetation, Floodplain, Pantanal, Richness
Abstract In spite of its size and biological significance, we know little about the ecology of the Pantanal, a 140,000 km2 floodplain in west-central Brazil. Increasing human pressures make this lack of understanding particularly critical. Using transects and 1 m2 circular plots, we documented floristic composition and interactingenvironmental conditions associated with littoral herbaceous vegetation along inundation gradients at two ecologically-distinct sites in the Pantanal. We recorded water depth and percent cover for each species in Baı´a Piuval, a bay in the Bento Gomes River (Mato Grosso), and in a bay in the Acurizal Reserve (Mato Grosso do Sul). Baı´a Piuval and Acurizal plots contained a total of 22 and 18 macrophyte species, respectively. At both sites Eichhornia azurea and Salvinia auriculata occurred most frequently as dominant or co-dominant species. Chi2 analysis, used to quantify zonations along depth gradients, generated four different groups of species ( p < 0.05) for Baı´a Piuval. For Acurizal, two significantly different groups ( p < 0.05) occurred with an intermediate assemblage of species that could be assigned to either group. Canonical correspondence analysis, used to analyze species distributions, showed a pattern consistent with the Chi2 results for Baı´a Piuval but not for Acurizal. Higher species richness and diversity occurred where dry season and low water levels coincided and richness was generally highest in proximal plots where water depths were lowest. Our results are consistent with the few other plant ecological studies reported for the Pantanal. This study can be considered additive to needed baseline data on biota and ecology of this region of South America.
Introduction Floodplain plant communities in the neotropics are biologically diverse and productive (Ada´moli 1986). They are inextricably linked to nearby river systems, where rivers serve as a means for transporting organic matter and nutrients into and out of floodplains (Junk et al. 1989; da Silva and Pinto-Silva 1989; Sparks 1995). Among the most
important plant communities occurring on floodplains in the neotropics are the floating aquatic mats and associated emergent herbaceous-dominated communities. Floating and rooted plant communities composed of aquatic macrophytes typically develop around the margins of bays and lakes and along edges of slow-moving rivers. Aquatic-macrophyte mats are especially common in the Pantanal, one of the largest freshwater
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Figure 1. Map of the Pantanal wetland region of Brazil showing the location of the two study sites, Baı´a Piuval and Acurizal.
wetlands in the world. The Pantanal is an extensive savanna-like wetland, with an expanse of 140,000 km2. It is located mostly in western Brazil, extending into contiguous portions of Bolivia and Paraguay (Figure 1). Water flowing through this broad, flat basin is collected by the south-flowing Paraguay River along the Pantanal’s western edge (Klammer 1982). Low mountains and elevated plateaus and plains surround the basin. Three important aspects of the Pantanal flood regime influence the floodplain vegetation: flood depth, duration, and periodicity (da Silva 1990; Heckman 1994; Junk and da Silva 1995; Naranjo 1995). The Pantanal has pronounced annual wet and dry seasons, with the wet season typically
beginning in November and ending in March. Flooding results from riverine overflow, nonchannelized floodplain inflow, and direct precipitation during the wet season (Hamilton et al. 1995). Once floodwaters accumulate, the drainage system is inadequate to quickly transport surface waters out of the Pantanal. As a result, flooding is sustained for extended periods following the wet season (December–April), with water that accumulated during the wet season requiring several months to drain out of the basin. Water depths increase up to 5 m during the annual flood event throughout some portions of the Pantanal. This seasonal periodicity in precipitation and the distinctive flood-pulse cycles are the key
577 environmental factors that control the type and distributional pattern of vegetation in the Pantanal. According to Prance and Schaller (1982), savanna and mixtures of grassland with patches of semideciduous forest are the most common vegetation types on the floodplain, a vegetation pattern that reflects the wide shifts in seasonal precipitation rates. Aquatic and semi-aquatic vascular macrophyte communities can develop anywhere surface water persists for several weeks. This includes bays, sheltered river littoral zones, and lakes. In these aquatic environments, floating and rooting macrophytes, including Eichhornea crassipes, Eichhornea azurea, and Salvinia auriculata, predominate in the deeper waters, while in more shallow areas that are typically inundated for relatively short periods of time Cyperus giganteus, Pontederia spp., Paspalum spp., Polygonum spp., and Ludwigia spp. are the commonly-occurring taxa (Pott et al. 1992; Hamilton et al. 1995; Penha et al. 1999; Pott and Pott 2000). In permanent water bodies, mats of floating vegetation may develop. These mats of macrophytes are mixtures of rooted, free-floating, and epiphytic hydrophytes. They move vertically with the rise and fall of floodwaters. Portions of the mats frequently break off and float to other parts of a bay or downstream. On the upland or seasonally-inundated side of these the littoral aquatic environments, a transition occurs from communities of rooted-emergent hydrophytes dominated by a mixture of forbs and graminoids to semi-aquatic shrub or graminoiddominated communities where periods of soil inundation and saturation only occur during a part of the year. In the Pantanal, ecological studies of the floating-macrophytic and rooted-emergent plant communities have been limited. Pott et al. (1989, 1992); da Silva and Esteves (1993) and Sanches et al. (1999) studied the floristic composition and the structure of aquatic macrophytic communities in ponds and bays. Further, Prado et al. (1994) focused on the floristic changes within macrophytic communities as hydrological conditions change seasonally. They concluded that very distinct macrophytic species aggregations could occur. However, they found less clearly defined spatial distribution of species along inundation gradients. In a productivity study of a common macrophyte in a northern section of the Pantanal, Pontederia
lanceolata (now Pontederia parviflora), Penha et al. (1999) concluded that productivity and the standing stock of the species are related to flood patterns of the area and the life cycle of the plant. Because of the size and diversity of the physical environment of the Pantanal, many studies of aquatic-macrophytic communities are needed to understand the diverse phytosociological patterns, community dynamics, and importance of physical environmental factors. These studies should include floristic composition, structure, interacting environmental conditions, and human impacts. Rather than giving a comprehensive floristic record, our study focuses on characterizing representative communities of macrophytes and interacting-environmental conditions in two ecologically-distinct areas in the Pantanal. We have three objectives: (1) compare the composition of littoral herbaceous vegetation in two ecologically-distinct sites within the Pantanal; (2) determine if a correlation exists between water depth and plant community composition along a topographical-inundation gradient; and (3) collect baseline data at permanently-established study sites that will be used to evaluate long-term phytosociological changes in aquatic communities. For objective 2, we predict that the inundation gradient is an important factor influencing the spatial distribution of aquatic-macrophyte species.
Study area Two sites in different regions of the Pantanal were selected for study. The first site, Baı´a Piuval, is a bay in the Bento Gomes River on the Fazenda Ipiranga (16 220 S and 56 340 W) in the northern section of the Pantanal near the municipality of Pocone in the state of Mato Grosso (Figure 1). Because the site is near the northern-most extent of the Pantanal, flooding typically occurs during the austral summer rainy period, with water levels receding to (more or less) permanently inundated bays and lakes by mid-July. The amplitude of water level fluctuations between the lowest (usually in August) and the highest level (usually in March) is 130 cm (Figure 2). Because we sampled the inundated floodplain in August, the plant community composition we report is that of a floodplain under permanent or near-permanent inundation.
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Figure 2. Stage (dotted line) and precipitation (solid line) in the northern Pantanal (left) and near Acurizal (right). Stage data for the northern Pantanal derived from Hamilton (1999) and for Acurizal from Schaller (1983). Precipitation data for the northern Pantanal are from records kept at the Fazenda Ipiranga near Pocone (unpubl.). Precipitation at Acurizal was reported by Schaller (1983).
The topography of this region of the Pantanal is level to nearly level, with typically gradual slopes from the seasonally inundated floodplain environments into contiguous lakes and bays. This results in a broad littoral zone where the shoreline can move laterally tens of meters annually. The vegetation adjacent to Baı´a Piuval is characterized by discontinuous patches of scrub-shrub cover, with a transition to communities of rooted herbaceous vegetation and floating mats of aquaticmacrophytes. The site, including the shallow edges of the bay, is grazed by cattle. The second site is in the western edge of the Pantanal in a bay connected to the Paraguay River near the Acurizal Reserve (17 500 S and 57 320 W) in the state of Mato Grosso do Sul (Figure 1). Because this site is near the main stem of the Paraguay River, the peak water levels generally extend well into the winter following the summer rainy season (Figure 2). Further, high water occurs over a longer period because the bay is connected to the Paraguay River. Peak water levels at the Baı´a Piuval site, on the other hand, decrease rapidly following the austral summer wet season, as indicated by the precipitation and stage data (Figure 2). The topography of the Acurizal region is markedly different from that of the Fazenda Ipiranga. The Amolar Mountains flank the western side of the bay, resulting in a more abrupt transition from aquatic to upland habitats than occurs at Baı´a Piuval. Consequently, throughout the littoral zone of the Acurizal site, the water depths under the floating mats are deeper than those at Baı´a Piuval. Depths at Acurizal range from 110 to 247 cm, while at Baı´a Piuval depths range from 0 to 97 cm. The minimum water depth of 110 cm
at Acurizal creates a sampling region under permanent or near-permannent inundation, analogous to the conditions at our Baia Piuval site. A narrow scrub-shrub community fringes the Acurizal bay on most sides. No sustained grazing has occurred in this area for about 13 years.
Methods At both study sites, the vegetation was sampled in randomly located 1 m2 circular plots along transects that were oriented perpendicular to the shoreline. The center points of the plots were no closer than 3 m and no farther apart than 5 m. The proximal end of each transect on the shoreline was located where the substrate became saturated; the distal end was established at the open water edge of a mat. Transects were located in an attempt to sample representative plant assemblages. For each 1 m2 plot, we recorded the water depth in centimeters and the estimated percent surface area covered by each vascular plant species. Cover classes were based on the Daubenmire (1968) system, using the following percent cover class ranges: 1 ¼
