Coordinated changes in pliotosyntliesis, water relations and leaf nutritional traits of canopy trees along a precipitation gradient in lowland tropical forest

We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian forests with contrasting mean annual precipitation (1,800, 2,300, 3,100 and 3,500 mm). There was near complete turn-over of dominant canopy tree species among sites, resulting in greater dominance of evergreen species with long-lived leaves as precipitation increased. Mean structural and physiological traits changed along this gradient as predicted by cost-benefit theories of leaf life span. Nitrogen content per unit mass (Nmass) and lightand COa-saturated photosynthetic rates per unit mass (Pmass) of upper canopy leaves decreased with annual precipitation, and these changes were partially explained by increasing leaf thickness and decreasing specific leaf area (SLA). Comparison of 1,800 mm and 3,100 mm sites, where canopy access was available through the use of construction cranes, revealed an association among extended leaf longevity, greater structural defense, higher midday leaf water potential, and lower Pmass, N•iass, and SLA at wetter sites. Shorter leaf life spans and more enriched foliar 5 ^N values in drier sites suggest greater résorption and re-metabolism of leaf N in drier forest. Greater dominance of short-lived leaves with relatively high Pmass in drier sites reflects a strategy to maximize photosynthesis when water is available and to minimize water loss and respiration costs during rainless periods. L. S. Santiago (H) • K. Kitajima • S. S. Mulkey Department of Botany, University of Florida, P.O. Box 118526 Gainesville, FL 32611, USA e-mail: [email protected] Tel.: +1-510-6421054 Fax: +1-510-6426264 S. J. Wright Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Ancón, Republic of Panama Present address: L. S. Santiago Integrative Biology, University of California, 3060 Valley Life Sciences Building, Berkeley, CA 94720, USA Overall, our study links coordinated change in leaf functional traits that affect productivity and nutrient cycling to seasonality in lowland tropical forests.