New Zealand’s Nothofagus treelines: An analysis of treeline patterns and associations with the abiotic environment

Background/Question/Methods

Alpine treelines are conspicuous features of mountain systems worldwide and have interested ecologists for decades.  Globally, the uppermost treeline boundary represents a ubiquitous bioclimatic limit, where tree growth is limited by low temperature. However, at regional-to-local scales, treeline boundaries vary considerably in terms of a number of characteristics, including elevation, boundary shape and spatial configuration, and the degree of contiguity in the landscape. This variability is largely indicative of how a suite of abiotic factors modulate the overall effect of temperature at treeline.  In this study, I use available and derived spatial datasets, GIS techniques, and statistical modelling to characterize and examine patterns of variation in the abrupt Nothofagus treelines of New Zealand.   In the analysis, the main question was: to what extent are treeline patterns related to variability in climate, substrate, topography, and disturbance across the country?

Results/Conclusions

Nothofagus treelines display complex patterns of variation across New Zealand in terms of seven distinct pattern metrics: elevation, horizontal and vertical sinuosity, adjacent vegetation type, degree of contiguity, orientation relative to main slope aspect, and degree of compactness in the local landscape.  Each of these pattern metrics was strongly associated with different components of abiotic variability, although disturbance and climatic factors were shown to be most influential.  Multivariate clustering of the treeline pattern metrics revealed seven clear Nothofagus treeline clusters that were spatially structured when mapped; abiotic factors explain about 50% of the variation in multivariate treeline pattern.  Results suggest that New Zealand’s mountains are characterised by distinct, spatially-defined, abiotic environments which, in turn, have a substantial influence on Nothofagus treeline variability.