Estación Biológica Cocha Cashu
Edaphic specialization of three Geonoma palm species in a Peruvian tropical lowland forest.
Supported by The Marina Riley Scholarship Program
The high species diversity of plants in tropical rainforests has been well documented by systematists. However, the mechanisms by which such diversity is maintained are a source of on going debate. The "niche diversification" hypothesis proposes that much of the tropical plant diversity depends on habitat and microhabitat specialization (Connell 1978, Denslow 1987, Welden et al. 1991, Clark et al. 1993). Habitat and microhabitat specialization with regard to topography and soil factors have been shown to be important for several plant groups, including tropical trees, melastomataceous shrubs, herbs, pteridophytes and palms (Lieberman et al. 1985, Kahn and Castro 1985, Poulsen and Baslev 1991, Basnet 1992, Tuomisto and Ruokolainen 1994, Clark et al. 1998, Svenning 1999). Palms are usually among the ten most species-rich tree families in Amazonian moist or wet forest. The importance of forest palms to Neotropical people has often been remarked. Their economic importance relies on their use as construction material, as well as their commercial, edible, technological and medicinal value. This abundant species group has also significant impacts on forest structure, confirming the need for effective conservation of this supremely useful family.
The genus Geonoma is one of the most complex and biggest genera of neotropical palms. It represents an important group of understory plants with a very interesting leaf variation. This genus is species-rich, and therefore it is likely that it is represented in most forest habitats making it suitable to show different palm edaphic specializations. I propose to study the patterns of distribution and abundance of three Geonoma species in relationship to topography, soil drainage and texture in a Peruvian lowland rain forest. In this way, it can be possible to elucidate any edaphic specialization of these palms at the habitat and microhabitat levels. Consequently, the results may provide some evidence in support of the niche diversification hypothesis.
Study species and site Fieldwork is going to be carried out at "Cocha Cashu" Biological Station in Manu National Park. This park is located in the Amazon Basin of Southeastern Peru (Cuzco and Madre de Dios Departments, 11° 52'S, 71° 21'W). Chavez (1996) published a list of the palm species in Cocha Cashu and Pakitza stations. Based on her work the three Geonoma species identified will be sampled: G. acaulis, G. brongniartii and G. deversa. Experimental design The mentioned species will be inventoried in the four main forest types described by Pitman et al. (1999): terra firme forest, mature floodplain forest, swamp forest and primary successional floodplain forest. Three 300 x 5 meter long transects will be established in three different hills or valleys to avoid pseudoreplication on each forest type. Transects will be subdivided into subplots of 20m x 5m size. In the center of each subplot, the texture on the topmost 10cm of the mineral soil will be estimated. The texture classes recorded will be from one to seven according to the following definitions: clay, clayey silt, sandy clay, clayey sand, fine sand, medium sand and course sand. Methods to find the proportion of sand, silt and clay will follow Page et al. 1982. Topographic position within each subplot will be recorded as: slope crest, steep slope, moderate slope, gentle slope, base of slope and flat (Clark et al. 1995). The degree of waterlogging of the soil will be estimated based on the abundance of depressions that gathered water after rains. The cover of such temporary pools within each subplot will be estimated on a relative scale: 0, 1-10%, 11-20%, 21-50% and 51-70% and 71-100% (Tuomisto and Ruokolainen 1994). Each subplot will be surveyed for the three palm species. Only individuals with stems taller than 0.10m of leaves longer than 0.50m will be considered. The abundance of each species on each subplot will be recorded as: absent, present but not abundant and abundant. The latter category will be defined as a subplot having at least 5 individuals or if the species is clonal at least 2 separate clones. Data Analysis The Mantel test will be used to assess correlations among 5 distance matrices constructed for all the subplots: i) soil-type matrix, ii) topography matrix, iii) drainage matrix, iv) palm composition distance matrix and v) habitat-type matrix. Logistic and ordinal logit regression will be used to analyze the distribution and abundance of each species in terms of the habitat and microhabitat variables (Svenning 1999).
SIGNIFICANCE OF STUDY
This study will contribute to the body of knowledge currently available regarding the distribution of plants with respect to edaphic factors. If the palm varieties show certain edaphic specialization at the habitat and microhabitat levels this study will support the "niche diversification" hypothesis of high biodiversity in the tropical lowland rainforests. Inconsistency of results may indicate that other factors may predominate over those analyzed, such as, the ability to tolerate shade or sunlight, density-dependent susceptibility to herbivores or pathogens, pollinators or seed dispersers, physical disturbance, etc. The understanding of species distribution is a first step towards a further development in population, community and ecosystem ecology. This study will also provide baseline ecological data for a site in one of the richest parts of the Amazon Basin. The Manu Biosphere is considered as one "hotspot" of biodiversity and thus merits special attention form scientists and conservationists. The proposed study is part of a PhD thesis that will include more species of the genus and analysis in other Peruvian forests. Furthermore, the thesis project includes the improvement of the molecular phylogeny of the genus (Asmussen 1999) in order to find an evolutionary relationship of these edaphic traits. The results of the whole study will provide insight on how the evolutionary relationships of these palm species reflect the derivation of habitat and microhabitat specialization.
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