Substrate conditions, foliar nutrients and the distributions of two canopy tree species in a Costa Rican secondary rain forest.

摘要

A 28-year-old secondary lowland rain forest in Florencia de San Carlos, Costa Rica, was dominated by two tree species of contrasting ecologies: Vochysia ferruginea, a species typical of well-drained but infertile soils with high Al saturation, and Cordia alliodora, a species requiring soils of at least moderate fertility. The two species exhibited markedly different spatial distributions in the forest and a study was made to determine whether these different distributions were related to variationin substrate conditions and the nutrient requirements of the two species. Two soil types were present: Ultisols (Typic Haplohumults) and Inceptisols (Typic Dystropepts) and topography was of low hills. The study formed part of an analysis of site qualityfor timber production in the two species and therefore used standard forestry methods for such analyses. Thirty-six plots of 20 X 20 m were established using predetermined criteria of site uniformity, presence of at least four dominant trees of at leastone of the study species, and absence of disturbance. The abundance (number of individuals >10 cm d.b.h.) of each species was determined in each plot. Soil samples were taken in 27 of the 36 plots, 9 plots being considered to replicate conditions already sampled, and 14 soil chemical and physical variables were measured using standard methods. Foliar nutrient analyses were carried out for trees of both species with crowns fully exposed to sunlight, distributed across the gradient of soil conditions. Variations in species abundances were not related to soil type, but plots in which V. ferruginea was more abundant tended to be on steeper slopes with soils of higher exchangeable acidity and lower concentrations of Mn; C. alliodora was more abundant on gentler topography where soils had much lower exchangeable acidity but higher Mn. Differences between the two species in foliar nutrient concentrations were marked and supported previous interpretations of their nutrient requirements; foliar nutrients of V. ferruginea were typical of tree species of moist tropical forest on infertile soils, and those of C. alliodora typical of a species requiring more fertile soils. Factors such as the distributions of seed trees at site abandonment may affect the spatialdistributions of tree species in secondary forests such as that studied. The relationships of the distributions of the two species to substrate variation, however, and their evidently different nutrient requirements, support the hypothesis that variation in the composition and structure of the forest studied is at least partially related to exchangeable acidity and its dominant cation, Al.