Tropical forest productivity is often thought to be limited by soil phosphorus (P) availability. Phosphorus availability might therefore constrain potential increases in growth as the atmospheric CO2 concentration increases, yet there is little experimental evidence with which to evaluate this hypothesis. We hypothesized that while all species would respond more strongly to elevated CO2 when supplied with extra P, individual species' responses would also depend on their habitat associations with either high- or low-P soils. We further hypothesized that this effect would be exacerbated by a reduction in transpiration rate under elevated CO2, as transpiration may aid in P acquisition. We used a pot experiment to test the effects of P addition on the physiological and growth response to elevated CO2 of eight tropical tree species with contrasting distributions across a soil P gradient in Panamanian lowland forests. Seedlings were grown in an ambient (400 ppm) or elevated (800 ppm) CO2-controlled glasshouse in either a high or low P treatment to quantify the effects of P limitation on relative growth rate, transpiration, maximum photosynthetic rate, and foliar nutrients. We found evidence of limitation by P and CO2 on growth, photosynthesis, foliar nutrients, and transpiration. However, species' P affinities did not predict relative growth rate or transpiration responses to elevated CO2 in either the low P or high P treatments. Transpiration rates decreased under elevated CO2, but foliar P was greater for some species under elevated CO2, suggesting a greater capacity for upregulation of P acquisition in species associated with low P soils. Our results show that tropical forest responses to elevated CO2 will be species-specific and not necessarily explained by P affinities based on distribution, which poses challenges for predictions of community-wide responses. This article is protected by copyright. All rights reserved.
