It has been shown that diverse forests are more productive than less diverse forests, but the overall cause remains unknown. In forest ecosystems with heterogeneous nutrient supplies (e.g. different soils) how do tree species-soil associations affect ecosystem level nutrient use efficiency and productivity? To address this question, we measured aboveground net primary productivity (ANPP) and phosphorus use efficiency (PUE) within forest stands on infertile schist soil and fertile basalt soil on the Atherton Tablelands, Australia. Concurrently, we measured the same variables for 52 tree species occurring within the experimental site. Soil P pools and radiation use efficiency (RUE = ANPP / % intercepted radiation) were also measured.
Results/Conclusions
PUE was markedly variable (CV = 44%) among species. Species associated with infertile soil (infertile soil specialists) had 39% greater PUE than species common on both soil types (soil generalists). 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 (intrinsic) and phenotypic (plastic) traits accounted for 49% and 29% of the total PUE variance. These intrinsic differences between species and plastic differences within species significantly contributed to higher tree-level and stand-level PUE on schist soils than on basalt soils. Moreover, P-efficient trees (PUE > 8 kg biomass / g P uptake) on schist soils contributed markedly more to total stand-level species richness (schist = 73%, basalt = 20%), basal area (schist = 86%, basalt = 18%), and biomass production (schist = 82%, basalt = 10%) than did P-efficient species on basalt soils. Although PUE of stands on schist (7.1 kg g-1) was markedly higher than stands on basalt (3.4 kg g-1), RUE did not decline significantly on schist soils (i.e. RUE was not traded for higher PUE). This lack of a strict tradeoff between RUE and PUE indicates that other, as yet unknown, opportunity costs to high PUE probably exist in these forests. Despite the importance of PUE in maintaining high production on P-limited soils, large amounts of PUE redundancy exists – many species can attain similar PUE values by plastic response or genetic similarity. This PUE redundancy among species is likely to reduce the impacts of species loss on ecosystem-level functioning, with significant losses of ecosystem functioning occurring as species richness declines below ca. ten species.
