ASSESSING THE GROWTH OF TROPICAL RAIN FOREST TREES: ISSUES FOR FOREST MODELING AND MANAGEMENT

Abstract

Growth performance was assessed for a diverse suite of canopy and emergent tree species in a lowland neotropical rain forest (the La Selva Biological Station, northeastern Costa Rica). Species were evaluated based on annual diameter measurements of large samples of individuals in all post-seedling size classes, over a 12-yr period. The study species were seven non-pioneers (Minquartia guianensis, Lecythis ampla, Hymenolobium mesoamericanum, Simarouba amara, Dipteryx panamensis, Pithecellobium elegans, and Hyeronima alchorneoides) and two pioneers (Cecropia obtusifolia and C. insignis). For each species, inherent growth capacity through ontogeny was estimated as the mean of the five largest annual increments (from different individuals) in each juvenile and adult size class. At all tree sizes, species differed highly significantly in this measure. In the small sapling sizes (≤4-cm diameter), the two pioneers showed markedly higher maximum growth than all the non-pioneers, in line with current understanding. This growth advantage, however, was quickly lost with increasing tree size. For eight of the nine species, the ontogenetic patterns of maximal growth closely corresponded to species groupings previously based on juvenile microsites (forest stage, light levels). Average growth rates, however, which are the combined result of environmental and biotic factors, did not show this correspondence. The groups based on microsite occupancy and growth potential provide a starting point for treating the great numbers of species of tropical rain forest trees as many fewer functional types, a sine qua non for modeling these complex forests. The exceptional growth capacity of one of our focal non-pioneer species (Simarouba) suggests that it represents a hitherto unrecognized functional type among tropical rain forest trees. The data from this long-term study indicate that several assumptions of current forest-dynamics models are not applicable to trees of this biome. For most species (1) diameter growth during regeneration was not correlated with tree size; (2) adult sizes were not characterized by a constant maximal basal-area increment; and (3) growth capacity did not decline toward zero as trees approached maximum size. Evaluation of overall increment distributions revealed a broad adult size range of exclusively high growth in the emergent Hymenolobium; for two additional species, sectors of the adult size range showed an absence of very low or negative growth. At most sizes, however, all nine species displayed the full range of growth increments, from slightly negative to observed maxima. Describing the performance of tropical rain forest canopy trees will require similar analysis of growth data obtained from many individuals and intervals, and stratified across juvenile and adult life stages. More realistic forest-dynamics models will depend on such evaluations of many tree species, in diverse tropical rain forests. Such models are greatly needed, both for designing management for this biome and for predicting its responses to changing climate and atmospheric conditions.