PHOTOSYNTHETIC RATES IN RELATION TO LEAF PHOSPHORUS-CONTENT IN PIONEER VERSUS CLIMAX TROPICAL RAIN-FOREST TREES

In Guyana dense rainforest occurs on intensely weathered acid soils, l ow in soil phosphorus. To investigate whether low P availability limit s photosynthesis of trees growing on these soils more than N does, lea f P and N content, and their relationship with the photosynthetic capa city (A(sat) mu mol CO2 m(-2) s(-1)) were studied for nine pioneer and climax tree species in a range of light climates. Light environment w as described using hemispherical photographs. For both pioneer and cli max species, leaf P content (r(2) = 0.71 and 0.23, respectively) is a more important determinant of A(sat) than leaf N content (r(2) = 0.54 and 0.12, respectively). Pioneer species have a higher leaf P and N co ntent than climax species. At similar P or N content, pioneers have a higher A(sat) than climax species. The saplings studied had a relative ly high A(sat), considering their low P concentration (15-30 mu mol P g(-1)). All species studied had a constant leaf P and N concentration and photosynthetic capacity across light climates, because specific le af mass (g m(-2)) increased similarly with light availability. This ac climation to a change in light environment makes a possible limitation of A(sat) by P or N independent of light environment.