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

Light fluctuations and crown traits were studied for saplings of four tree species in a Costa Rican rain forest. Light fluctuations (1988-1994) were a ssessed by annual light estimations above saplings, using a visual crown po sition index. Crown traits in 1994 and growth between 1994 and 1995 were me asured. Crown position values varied between 1.5 and 2.5. Of the 70 sapling s only four were found at higher light levels (crown position of 3 or 3.5), but for 1-2 y only. Over the 6 y of investigation, 55-75 % of the saplings experienced no or only one light fluctuation, and 25-45 % two or three flu ctuations. Crown traits in 1994 were either most strongly correlated with l ight levels in 1993 (Lecythis) and with light levels in 1991 and 1992 (Dipt eryx and Simarouba), or they were not significantly correlated with light l evels (Minquartia). It is hypothesised that: (1) the saplings require 1-3 y to establish a crown trait in response to light levels in the forest; and (2) and species that can economically produce a leaf can adjust crown trait s more quickly in an environment dominated by low light levels than species that are less economic. Crown trait responses may track environmental chan ges in three of the four species, in particular in Lecythis. In this latter species, leaf area had no significant effect on growth and survival, where as light level had a positive effect. Conversely, in the other three specie s, light levels had no discernible effect on growth and survival (due in pa rt to low variation in crown position in two of these species), whereas lea f area had a positive effect on both.