Re-examining phosphorus limitations on photosynthesis and productivity in forests around the world

Tropical forests have played a key role in absorbing anthropogenic emissions of carbon dioxide, yet too little is known about (present or potential) phosphorus (P) limitation on carbon (C) uptake. We have re-examined a recently published data set that was designed to quantify the dependence of photosynthetic capacities (V_cmax25, J_max25) on leaf N and P concentrations – from an alternative perspective, considering leaf nutrients as consequences, rather than causes, of leaf function. We found that leaf N per unit area can be expressed a linear combination of components related to leaf mass per area and V_cmax25, whereas leaf P per unit area can be expressed a linear function of J_max25. Globally, both V_cmax25 and J_max25 increase with light intensity, but decrease with temperature – both responses supported by experiments. V_cmax25 decreases with soil C:N ratio, while J_max25 increases with soil pH. Together, these various environmental factors explain almost 50% of global variation in photosynthetic capacities. These findings provide a promising route towards an optimality-based approach to modelling leaf traits and their relationships to properties of climate and soils.