Diameter, height, crown, and age relationships in eight neotropical tree species

Abstract

Diameter, height, crown shape, and crown area were measured on 23-42 trees ranging in size from saplings to large adults for each of eight common dicotyledonous tree species in a neotropical forest on Barro Colorado Island, Panama. Six species were canopy trees, one species was an emergent tree, and the remaining species was an understory tree. Crown areas and shapes were quantified by eight radii measured every 45 degree from the trunk to the vertically projected edge of the crown. Crown areas were calculated from the areas of circles with the average radius; crown shapes were measured by the coefficients of variation of the eight crown radii. Age-diameter relationships were estimated from diameter growth increments over an 8-yr period. Observed height-diameter relationships were compared to expectations based on the theories of elastic similarity, constant stress, and geometric similarity. Slopes of log-transformed height-diameter relationships differed from the theoretically expected value of 2/3 for elastic similarity in three of eight species; two canopy species had higher slopes, but not as high as the expected value of 1.0 for geometric similarity; one canopy species was shorter, but the slope was greater than the slope of 0.5 predicted by the constant stress theory. The theory of elastic similarity also predicts that canopy mass to trunk mass ratio should remain constant during tree growth. Observed ratios based on proxy variables for the masses were constant in six of eight species the two deviant species had heavier crowns in large trees than expected from their diameters. Crown shapes were much more variable in some species than others. The understory species had much lower r-2 values for height-diameter and crown-diameter relationships, suggesting that these relationships may be more variable in trees that live in the less windy understory than in trees that reach the canopy. The allometric relationships for the species in this study were not unique for each species, suggesting that it may be possible to model the allometric relationships of many species with fewer equations than species. If the primary adaptive forces acting on tree species in a diverse forest are their physical environment and the sum of competitive interactions with an ever-changing mix of neighbors, then different species may have similar resource allocation patterns. Thus, using the age-size relationships, we found three groups of species among the seven canopy species. Two groups had three species each and one species (Ocotea whitei) was in a group by itself. The understory species, Faramea occidentalis, was also in a group by itself.