Specific leaf area and nitrogen distribution in New Zealand forests: Species independently respond to intercepted light

The foliage of five forested sites across a climatic gradient on the South Island of New Zealand were studied to assess whether the distribution of specific leaf area (SLA) and leaf nitrogen (N) were due to the light environment or intrinsic species characteristics. Within both mixed and single species stands, SLA and N were correlated with relative light interception (τ) but varied greatly. For N, this variation was reduced by expressing N values as relative site N (N/maximum site N) that increased correspondence between N and ln(τ) across sites (r2 = 0.79). However, comparison of mixed species composition to a monoculture stands showed difference in the N-τ especially in stands with high species richness. The nitrogen extinction coefficient (kN) was estimated for canopies based on profiles of measured projected leaf area indices (PLAI) and leaf N, with an average value of 0.17 ± 0.06 indicating a conservative relationship between N and τ in these forests. Site maximum leaf N is correlated positively with annual incident irradiance further corroborating the N and light relationship as well as providing a potential mechanism for estimating canopy N over larger areas.