Carbon isotope composition of tree leaves from Guanacaste, Costa Rica: comparison across tropical forests and tree life history

Despite the progress made in understanding the ecophysiology of tropical plants during the past two decades (Lüttge 1997, Mulkey et al. 1996), questions regarding relationships between the environment and physiological diversity remain. It is now recognized that tropical climate can be quite variable (see Coen 1983) which could lead to significant functional diversity (increased variation in life history traits) among species due to the tight association between gas exchange physiology and the environment (see Enquist & Leffler 2001, Guehl et al. 1998, Huc et al. 1994, Martinelli et al. 1998, Sobrado 1993). It remains unclear, however, how the subtleties of variation in tropical climate and tree life history traits are related to the functional diversity of tropical communities (Borchert 1994, 1998). The stable carbon isotope composition (δC) of plant tissue can be used to quantify certain aspects of functional diversity among species. δC has been suggested as a surrogate for water use efficiency (WUE, the ratio of carbon gained to water lost during gas exchange) (Farquhar et al. 1988). Generally plants with high δC are thought to have high WUE while those with low δC have low WUE. Several researchers have examined δC values in tropical forests (see Martinelli et al. 1998) however, additional studies are necessary to fully understand the extent of δC variation and its correlates.