Phosphorus economics of tropical rainforest species and stands across soil contrasts in Queensland, Australia: Understanding the effects of soil specialization and trait plasticity

1. Edaphic specialization among species may lead to greater productivity and resource use efficiency across heterogenous landscapes than could be achieved in the absence of specialization. Although this idea has been tested conceptually and in garden experiments, it has rarely been examined in undisturbed forests. 2. To address this gap in our knowledge, we measured aboveground net primary productivity (aboveground biomass increment plus litterfall; ANPP) and phosphorus use efficiency (ANPP/P uptake) for stands on infertile schist soil and fertile basalt soil on the Atherton Tablelands, Australia. We also measured aboveground biomass production and estimated P use efficiency (PUE) for 52 tree species within these stands. Soil P fractions and radiation use efficiency (ANPP/percent intercepted radiation) were also measured. 3. Phosphorus use efficiency was markedly variable (CV = 44%) among species across soil types. Phosphorus use efficiency of obligate specialists on infertile soil was twice as high as species common on both soil types. Plastic responses within species were also significant, with trees on infertile soil having 45% greater PUE than trees on fertile soil. At the ecosystem level, genotypic and phenotypic traits accounted for 49% and 29% of the total PUE variance. Phosphorus-efficient trees (PUE >8 kg biomass g^-1 P uptake) on schist soils contributed more to stand-level species richness (schist = 73%, basalt = 20%), basal area (schist = 86%, basalt = 18%) and production (schist = 82%, basalt = 10%) than did P-efficient species on basalt soils. 4. Forest on schist soils achieved higher PUE than forest on basalt soils by partitioning more P to leaves rather than wood and by retaining P for longer periods of time before losing it via tissue senescence. These PUE traits enabled forest on schist to achieve similar ANPP and radiation use efficiency (i.e. PUE was not traded for radiation use efficiency). It is possible that opportunity costs of high PUE may exist among other life-history traits. 5. These results suggest that plasticity in traits that confer P conservation is significant, but limited, and that maximum P conservation at the landscape level must be achieved via genetic differences between species. Although this highlights the importance of genetic conservation in forests, it also demonstrates that high P conservation is possible with relatively few, but markedly plastic species.