Abstract

Of all the environmental factors affecting plants, light is perhaps the most spatially and temporally heterogeneous. This heterogeneity takes on special importance in tropical forests in particular because here light is considered to be the single-most limiting resource for plant growth and reproduction. Accordingly, the life cycle and physiological responses of many tree and understory species have been shown to be closely keyed to changes in light availability (Bazzaz and Pickett 1980, Denslow 1980, 1987, Strauss-Debenedetti and Bazzaz 1991, Canham et al. 1994, Sipe and Bazzaz 1994, Gratzer et al. 2004). Although less studied, herbaceous and shrub communities also exhibit heterogeneity in light environments (Tang et al. 1989, Washitani and Tang 1991, Ryel et al. 1994, 1996, Derner and Wu 2001). Much of the emphasis in ecology has been on the community and ecosystem consequences of environmental heterogeneity. For light in particular, the role of gaps in forest regeneration and diversity has received much attention. Several current models of forest dynamics have as their foundation differential species responses to heterogeneous light (Friend et al. 1993, Acevedo et al. 1996, Pacala et al. 1996, Shugart 1996, Bolker et al. 2003). Considerable attention has also been given to the responses of individuals in terms of their ability to capture and use light as a spatially and temporally heterogeneous resource (Caldwell et al. 1986, Chazdon 1992, Newell et al. 1993, Pearcy et al. 1994). At the level of the individual, the concepts of heterogeneity and plasticity are intertwined. Phenotypic plasticity has a central explanatory role in how individuals across populations differ in form and function in response to spatial heterogeneity. At the same time, physiological constraints on organism responses to temporal heterogeneity determine to a large extent the nature of the adaptive responses to it.