Physiological linkage in co-variation of foliar nitrogen and phosphorus in tropical tree species along a gradient of soil phosphorus availability

In order to understand the stoichiometric balance between foliar nitrogen (N) and phosphorus (P) on P-poor soils, we investigated how foliar N and P attributes (i.e. N and P concentrations in green and senesced leaves, N and P resorption efficiencies) of 30 tropical tree species co-vary along a gradient of soil P availability across three forests on Mount Kinabalu, Borneo. We found strong and positive correlations between foliar N and P in the concentrations and resorption efficiencies within each forest and across the three forests. Slopes of standardized major axis between foliar N and P concentrations for both green and senesced leaves were not different among the three forests, although the values of the scaling exponent in the relationships of foliar N to P across the three forests were significantly lower than 1. We suggest that down-regulation of N concentrations in green leaves on P-poor soils is one of several possible mechanisms explaining why N concentrations decrease with decreasing P concentrations in both green and senesced leaves toward a lower P availability in soils. On the other hand, the physiological and ecological reasons why N and P resorption efficiencies are positively correlated with each other across tree species remain unclear.