Leaf Litter Decomposition and Monodominance in the Peltogyne Forest of Maracá Island, Brazil1

Abstract

The forest type dominated by Peltogyne gracdipes (Caesalpiniaceae) on the riverine Marad Island is the least speciesrich of any recorded for Brazilian Amazonia. Because the forest has high soil and foliar Mg concentrations, and Mg is known to be toxic to plant growth at high concentrations, this study tested the hypothesis that dominance by Peltogyne is related to Mg leaf litter amounts and decomposition. We predicted that decomposition of PeItogyne leaves would differ from that of other species, and that their decomposition would result in a pulse of Mg release, Three plots (50 X 50 m) were established in each of three forest types: Peltogyne-rich forest (PRF; dominated by Pgracilipes), Peltogynepoor forest (PPF), and forest without Peltogyne (FWP). Three leaf litter decomposition experiments tested if decomposition of mixed leaf litter in coarse-mesh (CM) litterbags differed among forests (experiment 1); whether or not decomposition and nutrient release of Ecclinusa pianensis, Lucbeopsis duckeana, and Peltogyne in CM litterbags differed among forests and species (experiment 2); and using fine-mesh (FM) litterbags, investigated the differences in the influence of faunal activity on Ecclinusa and Peltogyne decomposition (experiment 3). Decomposition was independent of the presence and dominance of Peltogyne, since decomposition rates in both PRF and FWF' were in general lower than in PPF. These differences appeared to be related to faunal activity. The decomposition of Peltogyne leaves was lower than that of the other species tested and was more affected by microbial and physical action. It is possible that the monodominance of PeItogyne is related to its deciduousness and faster decomposition in the dry season, which coincides with a large leaf fall. Magnesium was lost quickly from the PeItogyne leaves and the resultant pulses of Mg into the soil during the heavy rains at the beginning of the wet season may be deleterious for other species that are not adapted to high solution Mg concentrations. Results obtained were consistent with the hypothesis that PeItogyne dominance is related to the pattern of its leaf decomposition and the seasonal pulses of toxic Mg. RESUMOA floresta dominada por Peltogyne gracili$es (Caesalpiniaceae) na Ilha fluvial de Marad t a mais pobre em esptcies reportada para a Amazania brasileira. Devido a esta floresta possuir altas concentrapks de Mg no solo e nas folhas, e pel0 fato de Mg ser conhecidamente t6xico para o crescimento das plantas quando em alms concentra+es, este estudo testou a hip6tese de que a dominhcia de Peltogyne esd relacionada corn as quantidades de Mg e corn a decomposieo das folhas da serrapilheira. Esperou-se encontrar um padrzo de decomposigo diferente para folhas de Peltogyne, e que as folhas de Peltogyne apresentassem urn pulso de liberago de Mg. Tr& parcelas (50 X 50 m) foram estabelecidas em cada um dos tres tipos florestais: floresta r i a em PeItogyne (PRF; dominada por Pgrmilipes), floresta pobre em Peltogyne (PPF) e floresta sem Peltogyne (FWF'). Tres exper...

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