Light fluctuations and crown traits were studied for saplings of four tree species in a Costa Rican rain forest. Light fluctuations (1988–1994) were assessed by annual light estimations above saplings, using a visual crown position index. Crown traits in 1994 and growth between 1994 and 1995 were measured. Crown position values varied between 1.5 and 2.5. Of the 70 saplings 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 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 Simarouba), or they were not significantly correlated with light levels (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 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 three of the four 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 three species, light levels had no discernible effect on growth and survival (due in part to low variation in crown position in two of these species), whereas leaf area had a positive effect on both.