Phosphorus efficiency of Bornean rain forest productivity: Evidence against the unimodal efficiency hypothesis

Plant communities on nutrient-poor soils are thought to use nutrients more efficiently to produce biomass than plant communities on nutrient-rich soils. Yet, increased efficiency with declining soil nutrients has not been demonstrated empirically in lowland tropical rain forests, where plant growth is thought to be strongly limited by soil nutrients, especially phosphorus (P). We tested for higher P uptake and use efficiency across a 16-fold soil P gradient in lowland Borneo by measuring the P content of aboveground net primary productivity (fine litter production plus new tree growth; ANPP) for 24 months. Extractable soil P was positively related to litter production, tree growth, and ANPP. Efficiency of P response (ANPP/available soil P), uptake (P uptake/available soil P), and use (ANPP/P uptake) increased monotonically with declining soil P and was significantly higher on P-rich soil than P-poor soil. Increased P uptake and use efficiency with declining soil P enabled higher than expected plant productivity on low P soils and thus strongly influenced spatial patterns of aboveground productivity throughout this lowland landscape. A complementary P use efficiency index, the integrated canopy P(Pc) use efficiency of production (ANPP/Pc× residence time of Pc), was similar across the P gradient, but underlying dynamics varied significantly with soil P: on rich soils, ANPP/Pcwas high and Pcresidence time was low, while the converse held on poor soils. These contrasting strategies enabled rapid tree growth on nutrient-rich soils, where P limitation is relatively weak, and higher P conservation on nutrient-poor soils, where P limitation is relatively strong. The occurrence of contrasting P use strategies on high and low P soils has important implications for understanding spatial patterns of aboveground productivity, P cycling, and canopy tree species composition across the P gradient.