Species‐specific effects of phosphorus addition on tropical tree seedling response to elevated CO 2

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, the responses of individual species 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 (RGR), transpiration, maximum photosynthetic rate and foliar nutrients. We found evidence of limitation by P and CO₂ on growth, photosynthesis, foliar nutrients and transpiration. However, the affinity of a species for P, defined as the species distribution relative to P availability, was not correlated with RGR 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. A free Plain Language Summary can be found within the Supporting Information of this article.