Regeneration of riparian forests of the Brazilian Pantanal under flood and fire influence
Introduction
Disturbances such as flood and fire can act as biological filters during the process of regeneration (Gentry, 1991a). The ecological roles of flood and fire are important in tropical savanna wetlands, where the riparian woody vegetation is shaped by regularly occurring floods and occasional wildfires (Pettit and Naiman, 2007). Plant community composition and structure is a function of position within the drainage network and additional disturbances such as fire (Swanson et al., 1988, Agee, 1993, Naiman et al., 1998). However, the interaction of both flood and fire does not have to be entirely destructive. For example, North American coastal redwood forests are disturbed by both flooding and fire, but these factors generate growth conditions that support the largest trees in the world (Naiman et al., 1998). Flooding is deleterious for non-adapted woody species (Hook, 1984, Kozlowski, 1984, Kozlowski and Pallardy, 2002), but trees growing in wetlands exposed to periodic and predictable flooding develop adaptations, such as the formation of lenticels, aerenchyma, and adventitious roots (Parolin et al., 2004), suberization of the roots to protect against radial oxygen loss (De Simone et al., 2002, De Simone et al., 2003), and photosynthetic activity under water (Schlüter et al., 1993). These adaptive responses expressed by plants submitted to hypoxia or anoxia (Drew et al., 1981, Lobo and Joly, 1995, Scatena and Menezes, 1996) enable woody species to cope with flood, and to undergo vigorous growth and physiological activities despite flooding (Parolin, 2009, Parolin and Wittmann, 2010).
Depending on their respective sets of adaptations, species are restricted to specific areas with a determined pattern of flooding, sedimentation, and soil texture, leading to clear vegetation zonation along the flooding gradient (Parolin et al., 2004, Wittmann et al., 2008, Wittmann et al., 2010). The same is true to a certain extent with fire, where plants are adapted to survive frequent fire regimes, e.g., by possessing thick bark, fire-resistant seeds, or root crowns that sprout (Dwire and Kauffman, 2003, Heinl et al., 2007a, Ricklefs, 2003). This resistance also depends on the fire behavior (i.e., return interval, intensity, and severity), the types of fuels from arboreal and herbaceous components, leaf moisture (Agee et al., 2002), canopy closure, height of canopy base (Lyons-Tinsley and Peterson, 2012), and accumulated fuel load due to flood frequency (Cronk and Fennessy, 2001, Heinl et al., 2007b, Mitsch et al., 2010).
In floodable savannas, the ecotone belt with riparian vegetation contains trees that are tolerant of flood and fire (Pettit and Naiman, 2007). Species growth and dominance are determined by the local situation, the available propagles, i.e., the soil seed bank (Bradshaw, 1984, Harper, 1967, Harper, 1982, Harper, 1988, Whittaker, 1953). Some studies have found that the structure of these communities is closely related to the frequency and intensity of flood and fire (Bendix and Cowell, 2010, Pettit and Naiman, 2007), and to species responses conditioned by phenology, ability to resprout, seed germination, seedling development, and the competitive effects of other species (Pettit and Naiman, 2007). The interaction of environmental factors determines a gradient of species composition and distribution (Bradshaw, 1984, Cronk and Fennessy, 2001, Harper, 1967, Harper, 1982, Whittaker, 1953).
The role of fire in the flooded forests of the South American Pantanal has not been studied to date. The Pantanal is a pristine environment with a very regular flood pulse (Junk et al., 1989) and less frequent natural wildfires (Macedo et al., 2009). Human presence is low and landscape changes due to human influence are still essentially negligible in many parts of the Pantanal. The Paraguay River has fringing riparian vegetation that is subjected to periodic floods. Plant species are distributed according to gradients of topography, flood, and its duration (Damasceno-Junior et al., 2005). Heavy floods regulate species distribution; tree mortality is greater in years when localized floods occur on higher, less frequently flooded levees (Damasceno-Junior et al., 2004). The role of fire in these flooded forests has not been studied to date. Our objective is to verify if fire and inundation can interact to determine variations in richness, density, and composition of woody species regeneration in riparian forests of the Paraguay River. We expect that fire acts together with flood in a non-destructive but selective way, resulting in modification of species composition, species density, and richness of the riparian forests.
Section snippets
Study area
We collected data in the riparian forests of the Paraguay River, approximately 14 km upriver from the town of Corumbá (18°57′54.70″S and 57°39′23.53″W; 18°52′36.99″S and 57°40′3.75″W) (Fig. 1). The climate in the region is tropical megathermic with dry winters and rainy summers (November–March), and a mean annual rainfall of 1070 mm. The mean annual temperature is 21.0–30.6 °C, and mean relative annual humidity is 76.8% (Soriano, 1997). The predominant soils in the region are gleysoils with
Results
We recorded 50 species in the sampled areas that were distributed in 30 families and 44 genera; 25 were liana species, 17 were trees, 7 were shrubs, and one palm (Table 1). The most abundant species in unburned areas were Tabernaemontana siphilitica, Paullinia spicata, Myrcia splendens, I. vera, Dolichandra ungis-cati, Albizia inundata, Anemopaegma flavum, and Odontocarya tamoides (Fig. 4). The main species in burned areas were M. splendens, P. spicata, T. siphilitica, Genipa americana,
Discussion
Flood and fire can be considered as biological filters in the Brazilian Pantanal, which act together primarily by regulating the number of individuals and composition of species in the riparian areas (Table 2). The fire acts as a biological filter during the dry season killing plants with different intensity which depends on the kind of fuel (Agee et al., 2002) and position of woody debris (Pettit and Naiman, 2007). The inundation is associated with microtopography, which is responsible for
Acknowledgements
We thank the Brazilian agency CAPES for a scholarship to M.T. de Oliveira and a research grant to A. Pott; INAU/CNPq (Instituto Nacional de Ciência e Tecnologia de Áreas Úmidas), for support of the project and a research grant from Science Without Borders program to G.A. Damasceno-Junior; Marco A.P.L. Batalha, Fernando A. Ferreira, and three anonymous reviewers for valuable critiques of the work; Jens Oldeland for help with the generalized linear models and Alexandre de Matos Martins
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