Size-dependent changes in light requirements of tropical trees: weak light-growth relationships in seven Caribbean rainforest species preclude testing a general hypothesis

In tropical forests light is presumed to be the most important limiting resource for trees below the forest canopy, and interspecific variation in shade tolerance an important driver of tree community composition. It has been hypothesized that tree light requirements may increase with tree size, but to date no study has explicitly quantified ontogenetic changes in light requirements for tropical trees. Here we make use of a field measure of whole-plant light compensation point (WPLCP) to quantify size-related shifts in light requirements of seven Dominican rainforest tree species, at two distinct size categories: saplings and pole-size juveniles. Although our dataset was large (representing 429 trees remeasured over a 13-month period), relative tree growth rate was only weakly related to estimated light levels ( r 2 = 0–0.36) across all species/size groups, and WPLCP could thus only be estimated for saplings of three species, and juveniles of one species. Both sapling and juvenile WPLCPs were significant for only one species, where we observed a size-dependent decrease in light requirements, a result contradicting our hypothesis. However, generalizations possible from our dataset are limited, as WPLCP could not be accurately determined within species/size groups, and did not match a priori qualitative shade-tolerance rankings. Overall, our data neither confirm nor deny a hypothesized ontogenetic increase in light requirements of tropical trees. We suggest that the highly dynamic canopies of Dominican rainforests due to strong, persistent winds on the island make estimates of understory light levels using hemispherical photography unreliable: a limitation that is likely present, though less pronounced, in many forest ecosystems.

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