Differential access to organic nitrogen appears to influence competition between tree species with different types of mycorrhiza. The only ectomycorrhizal (EM) canopy trees in New Zealand’s native forests are five species of southern beech (Nothofagaceae), nearly all other species having arbuscular mycorrhizas (AM). New Zealand Nothofagaceae are relatively light-demanding, yet environmental modelling indicates they are able to suppress a variety of AM species on sites where their climatic envelopes overlap. We asked if dominance of Nothofagaceae resulted in lower leaf area index (LAI) and better-lit understories than in AM mixed conifer-broadleaved forests, when climate and other site factors are held constant. We addressed this hypothesis by using the LAI-2200C Plant Canopy Analyzer to measure LAI and understorey light at 44 forest sites distributed throughout the two main islands of New Zealand.
Results/Conclusions
LAI ranged from 3.5 to 6.9, and increased linearly with MAT; percentage light transmission to the understorey spanned an order of magnitude and decreased exponentially with increasing MAT. However, at a given MAT, LAI of stands dominated by Nothofagaceae averaged 0.8 to 0.9 units lower than conifer-broadleaf forests, resulting in nearly two-fold higher understorey light levels in the former. Annual water deficit, vapour pressure deficit and thermal amplitude also influenced LAI and understorey light, but had less explanatory power. Our results suggest that the ability to tolerate deep shade is of little value in cold environments, and that AM and EM nutrient economies have different consequences for canopy structure and light use in New Zealand forests. Competition for light is probably the main driver of the dynamics of lowland conifer-broadleaved forests, whereas competition for nutrients may be more important in New Zealand’s EM southern beech forests.