Light fluctuations, crown traits, and response delays for tree saplings in a Costa Rican lowland rain forest.

摘要

Light fluctuations and crown traits were studied for saplings of 4 tree species (Lecythis ampla, Dipteryx panamensis, Simarouba amara [Quassia simarouba] Minquartia guianensis) in a Costa Rican rain forest. Light fluctuations were assessed by annuallight estimations above saplings made in 1988-1994, using a visual crown position index for which increasing index corresponds with increasing light availability from no direct light (1) to crown fully exposed to vertical and lateral light (5). Crown traits in 1994 and growth between 1994 and 1995 were measured. Crown position values mostly varied between 1.5 and 2.5; of the 70 saplings only 4 were found at higher light levels (crown position of 3 or 3.5), but for 1-2 yr only. Over the 6 yr of investigation, 55-75% of the saplings experienced no or only one light fluctuation, and 25-45% 2 or 3 fluctuations. Crown traits in 1994 were either most strongly correlated with light levels in 1993 (Lecythis) and with light levels in 1991 and 1992 (Dipteryx and Quassia), or they were not significantly correlated with light levels (Minquartia). It is hypothesised that (1) the saplings require 1-3 yr to establish a crown trait in response to light levels in the forest, and (2) species that can economically produce a leaf can adjust crown traits more quickly in an environment dominated by low light levels than species that are less economic. Crown trait responses may track environmental changes in 3 of the 4 species, in particular in Lecythis. In this latter species, leaf area had no significant effect on growth and survival, whereas light level had a positive effect. Conversely, in the other 3 species, light levels had no discernible effect on growth and survival (due in part to low variation in crown position in 2 of these species), whereas leaf area had a positive effect on both.