Photosynthetic rates in relation to leaf phosphorus content in pioneer versus climax tropical rainforest trees

In Guyana dense rainforest occurs on intensely weathered acid soils, low in soil phosphorus. To investigate whether low P availability limits photosynthesis of trees growing on these soils more than N does, leaf P and N content, and their relationship with the photosynthetic capacity (A_sat, μmol CO₂ m^-2 s^-1) were studied for nine pioneer and climax tree species in a range of light climates. Light environment was described using hemispherical photographs. For both pioneer and climax species, leaf P content (r²=0.71 and 0.23, respectively) is a more important determinant of A_sat than leaf N content (r²=0.54 and 0.12, respectively). Pioneer species have a higher leaf P and N content than climax species. At similar P or N content, pioneers have a higher A_sat than climax species. The saplings studied had a relatively high A_sat, considering their low P concentration (15-30 μmol P g^-1). All species studied had a constant leaf P and N concentration and photosynthetic capacity across light climates, because specific leaf mass (g m^-2) increased similarly with light availability. This acclimation to a change in light environment makes a possible limitation of A_sat by P or N independent of light environment.