Species-specific effects of phosphorus addition on tropical tree seedling response to elevated CO₂

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.