Universidade Federal de Mato Grosso-Instituto de Biociências. CEP Cuiabá, Mato Grosso, Brazil

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1 The Impact of Flooding on Distribution of Woody Plant Communities in the Pantanal of Poconé, Mato Grosso, Brazil. Nunes da Cunha, C. 1 and Junk, W. J. 2 1 Universidade Federal de Mato Grosso-Instituto de Biociências. CEP Cuiabá, Mato Grosso, Brazil 2 Max-Planck-Institut für Limnologie, AG Tropenökologie,24306 Plön, Postfach Germany Abstract The distribution of tree species along the flood gradient was studied on a transect through a levee at the Cassange River, tributary of the Cuiabá River near Poconé. 31 species were detected that formed 8 communities separated by simple Euclidean Distance Coefficient and Unweighted Pair-Group Method using Arithmetic Averages (UPGMA). An analysis of the flood tolerance of 85 most common tree species occurring in the Pantanal of Poconé, including data from other studies indicate that 30% are restricted to dry habitats, 65% have a wide range of distribution, and only 5% are restricted to habitats subjected to long-term flooding. It is assumed that pluriannual pronounced dry and wet periods favor species that tolerate a wide variety of hydrological conditions. Highly flood adapted species, which are very frequent in the Amazon River floodplain are rare in the Pantanal of Mato Grosso because they do not tolerate pronounced dry periods. 1 Introduction The Pantanal of Mato Grosso is situated in the central part of South America between 16 and 22 South and 55 and 58 West. The climate is tropical with a pronounced dry season during May to September and a rainy season during October to April. In the northern part of the Pantanal, where our study sites were established, rainy and dry seasons coincide with high and low water periods. In addition to the annual water level fluctuations, there are pluriannual fluctuations that create prolonged periods of heavy droughts and floods. Woody plants have a long live span and are therefore influenced not only by the annual flood pulse but also by pluriannual hydrological and climatic events. A large diversity in landscape units associated with edaphic, topographic, and hydrological peculiarities results in specific habitats, which allow the occurrence of a large variety of plants and animals with different levels of tolerance to periodic flooding and drought. The lowest parts of the Pantanal locally called campos are covered during low water period by semi-aquatic grasses, sedges, and herbaceous plants and short living terrestrial plants with flood tolerant seeds. Higher lying areas, covered by water during 4-6 months are colonized by flood resistant woody species. Elevations, e.g., paleo-levees locally called capão and cordilleira, termite mounds locally called murunduns, and levees reach during normal floods about 1m above the flood level. They form dry islands in the floodplain and are covered by drought resistant tree species. They serve as refuges for terrestrial plants and animals. Only during extremely high floods, these islands become shallowly flooded for short periods. The flood pulse is the driving force in floodplain systems (JUNK et al. 1989). Basing on this concept we postulate that the woody vegetation in the Pantanal is distributed along the flood gradient according to its flood and drought tolerance, and that the level of tolerance in combination with specific morphological end physiological peculiarities (e.g., height, shade tolerance) leads to specific forest communities. In the following study we analyze tree species composition along a transect cutting through a levee to evaluate species distribution along the flood gradient. Data from other inventories are used to present a general model for tree distribution in the northern part of the Pantanal of Mato Grosso and to explain occurrence and survival strategies of trees considering the paleo-climatic history of the area. 2 Methods Levees can be considered as very characteristic forested landscape units and will be used here as example for the presentation of the floristic composition of a forest along the flood gradient that is typical also for other forested areas, studied by us, e.g., paleo-levees, and termite mounds. - Topographic characterization Zero base line was determined as the limit between the periodically flooded grass land and the occurrence of woody vegetation. The highest point of the levee reached about 2.80 m above the base line. - Hydrologic characterization A hydrograph registered the depth of flooding and the days of inundation. 557

2 Maximum flood depth during the study period from September 1992 to July 1994 was 90 cm, longest flood period reached 170 days. According to observations of local inhabitants, the top of the levee becomes flooded only in years of exceptionally high floods. - Data collection and analyses For the description of the vegetation a transect of 440 m length was designed crossing a levee along the Cassange River, affluent of the Cuiabá River. The transect was subdivided into 44 pairs of plots of a size of 10 x 10 m each, corresponding to a total area of 8,800 m 2. All woody species with a diameter of more than 0.1 cm in breast height were registered, and diameter and height were measured. Phenological observations were made throughout the study period. The 44 plots were clustered to determine distinguishable groups of species by classification analysis using simple Euclidean Distance Coefficient and Unweighted Pair Group Method using Arithmetic Averages (UPGMA). A total of 31 species were included in the classification analysis. Fig. 1: a) dendrogram of cluster analysis showing the different forest types. b) Distribution of the forest types along the hydrotopographyc gradient of the Cassange River levee. 558

3 Fig. 2: Tree species distribution along the flood gradient according to habitat preferences in the Pantanal of Poconé. S = dry habitats, flooded only during exceptionally high floods for short periods. T = transition zone, flooded up to two months yr -1. A = low lying habitats, flooded during more than two months yr Results and discussion The dendrogram (Fig. 1a) differentiates between eight forest groups or communities. Two major types can be distinguished that separate forests growing in frequently flooded areas and those growing in mostly dry areas. Species of the periodically flooded areas are mostly evergreen, those of the dry areas are often deciduous. The plots of the dry areas show a high level of similarity, however, several communities can be distinguished, being the one on the highest part of the levee (C2) more deciduous than the others (C3, C4, and C5). C4 has a high density of the evergreen palm Sheelea phalerata. The communities C3 and C5 could not be separated in the previous visual field classification but are separated by the classification analysis. One community (C6) occupies the area with short and shallow floods, being considered as a transition zone. The community C1 represents a low lying forest bordering the Cassange River, and the communities C8 and C7 are similarly low lying forests on the opposite site of the levee, bordering the flooded savanna (campo) (Fig. 1b). Three groups of species can be distinguished: 1) species, which only occur in frequently flooded habitats, 2) species, which have a wide distribution along the topographic gradient, and 3) species, which occur exclusively in habitats, which are flooded only during extreme floods, for short periods. Amplifying this analysis to the 85 most common woody species in the Pantanal of Poconé (NUNES DA CUNHA & JUNK 1999, 2001), we found, that 30% are restricted to high lying areas, that are rarely flooded, 65% have a wide range of distribution, and only 5% are occurring only in areas subjected to long lasting inundation (Fig. 2). 4 Conclusions - Woody species of the Pantanal are distributed along the topographic gradient according to their tolerance to periodical flooding and drought. Different communities can be distinguished. - The elevated number of species on the rarely flooded habitats can be explained by the proximity of the nonflooded cerrado, bordering the Pantanal. It provides drought tolerant semideciduous species, which are tolerant to short and shallow flooding. - The number of flood adapted species is low. Hydrophilous species from the species rich floodplain forests of the Amazon basin could easily tolerate flood 559

4 conditions in low lying habitats, however, many of them could probably not survive the pluriannual dry periods, which affect these habitats every couple of decades. - The large climatic variability of the Pantanal, which results in pluriannual prolonged dry and wet periods, favors the occurrence of species with a broad tolerance to periodical flooding and drought. 5 References JUNK, WJ, BAYLEY, PB, SPARKS, RE, 1989: The Flood Pulse Concept in River-Floodplain Systems. In: Dodge, DP, (ed): Proceedings of the International Large River Symposium (LARS), Can Spec Publ Fish Aquat Sci 106, NUNES DA CUNHA, C, JUNK, WJ, 1999: Composição florística de capões e cordilheiras: localização das espécies lenhosas quanto ao gradiente de inundação no Pantanal de Poconé, MT Brasil. Anais do II Simpósio sobre Recursos Naturais e Sócio-economicos do Pantanal. Manejo e Conservação. EMBRAPA, Corumbá, Brasil, pp NUNES DA CUNHA, C, JUNK, WJ, 2001: Distribution of woody plant communities along the flood gradient in the Pantanal of Poconé, Mato Grosso, Brazil. International Journal of Ecology and Environmental Sciences 27,

5 Editors Reinhard Lieberei 1, Helmut K. Bianchi 2, Vera Boehm 1, Christoph Reisdorff 1 1 Universität Hamburg, Institut für Angewandte Botanik, Ohnhorststr. 18, Hamburg, Germany 2 GKSS-Forschungszentrum Geesthacht GmbH, Max-Planck-Straße 1, Geesthacht Germany Layout Printing Helmut K. Bianchi, GKSS, Karsten Bittner, Documedia, Geesthacht, Germany GKSS-Forschungszentrum Geesthacht GmbH, Geesthacht, Germany ISBN X Lieberei, R., Bianchi, H-K., Boehm, V., Reisdorff, C., (eds.) 2002: Neotropical Ecosystems, Proceedings of the German-Brazilian Workshop, Hamburg GKSS-Geesthacht. The publishers give permission to copy single contributions from the Proceedings for personal use exclusively. Copies may be passed on only with the correct specification of the source. The research cooperation has been carried out under the auspices of the German - Brazilian Govemmental Agreement on the Cooperation in Scientific Research and Technological Development. The issuance of the Proceedings and the production of the CD-ROM was sponsored (Code ) by the Federal Ministry of Education and Research The responsibility for the contents of the contributions is solely the authors'.