Forest Productivity and Efficiency of Resource Use Across a Chronosequence of Tropical Montane Soils

We tested the hypothesis that plants adjust to nutrient availability by altering carbon allocation patterns and nutrient-use efficiency (NUE = net primary production [NPP] per unit nutrient uptake), but are constrained by a trade-off between NUE and light-use efficiency () = NPP per unit intercepted light). NPP, NUE and ) were measured in montane Metrosideros polymorpha forest across a 4.1 x 10⁶ yr space for time substitution chronosequence in which available soil N and P pools change with site age. Although the range of N and P availability across sites was broad, there was little difference in NPP between sites, and in contrast to theories of carbon allocation relative to limiting resources, we found no consistent relationships in production allocation to leaves, fine roots or wood. However, canopy nutrient pools and fluxes were correlated with the mass of fine roots per unit soil volume and there was a weak but positive correlation of NPP with LAI. Patterns of ) and NUE across the soil developmental sequence were opposite to each other. ) increased as nutrient availability and nutrient turnover increased, while NUE decreased in response to the same influences but reached its highest values where either N or P availability and turnover of both N and P were low. A negative correlation between ) and NUE supports the hypothesis that a trade-off exists between ) and leaf characteristics affecting NUE.