Trees increase their P:N ratio with size

Aim: Phosphorus (P) tends to become limiting in ageing terrestrial ecosystems, and its résorption efficiency is higher than for other elements such as nitrogen (N). We thus hypothesized that trees should store more P than those other elements such as N when tree size increases and that this process should be enhanced in slow-growing late-successional trees. Location: Catalan forests. Methods: We used data from the Catalan Forest Inventory that contains field data on the P and N contents of total aboveground, foliar and woody biomasses of the diverse mediterranean, temperate and alpine forests of Catalonia (1018 sites). We used correlation and general linear models to analyse the allometric relationships between the nutrient contents of different aboveground biomass fractions (foliar, branches and stems) and total aboveground biomass. Results: Aboveground forest P content increases proportionally more than aboveground forest N content with increasing aboveground biomass. Two mechanisms underlie this. First, woody biomass increases proportionally more than foliar biomass, with woody biomass having a higher P:N ratio than foliar biomass. Second, the P:N ratio of wood increases with tree size. These results are consistent with the generally higher foliar résorption of P than of N. Slow-growing species accumulate more total P aboveground with size than fast-growing species, mainly as a result of their large capacity to store P in wood. Main conclusions: Trees may have developed long-term adaptive mechanisms to store P in biomass, mainly in wood, thereby slowing the loss of P from ecosystems, reducing its availability for competitors and implying an increase in the P:N ratio in forest biomass with ageing. This trend to accumulate more P than N with size is more accentuated in slow-growing, large, long-lived species of late successional stages. In this way they partly counterbalance the gradual decrease of P in the soil.