A Functional Analysis of New Zealand Alpine Vegetation: Variation in Canopy Roughness and Functional Diversity in Response to an Experimental Wind Barrier

Abstract

There is an increasing interest in describing species assemblages in terms of function or texture, as an alternative to floristics. However, the use of objective criteria for evaluating plant function remains uncommon. A procedure for determining plant fractal dimensions was applied to the measurement of canopy roughness of alpine plants, as a major functional character in high-alpine cushionfield vegetation in southern New Zealand. The study was carried out within a 50 x 30 m area located around an experimental snow fence. The snow fence was erected on an originally uniform site 32 years prior to this study. Since its construction it has caused visible changes in snow lie and in the composition of vegetation on its leeward side, apparently because of its ameliorating effect on microenvironmental parameters. Floristic patterns across the study area, revealed by a DCA ordination of species frequency data, showed a strong association with snow depth, corresponding to the sheltering effect of the snow fence. Vegetation canopy roughness increased along a gradient of increasing snow depth, consistent with the expectation that plants growing in the most exposed alpine environments would possess smooth canopies that promote laminar air flow over their surface and thus enhance plant/air temperature differentials. Functional diversity in terms of canopy roughness similarly increased with increasing snow depth, consistent with expectations of community structure theory. Species richness and Shannon-Wiener species diversity showed similar relations. It is concluded that plant canopy roughness, measured by means of fractal dimensions of canopy profiles, has value for predicting the physiognomy of alpine vegetation from the microenvironment. It is also practical to measure. It should be considered for future studies examining vegetation texture in the alpine zone.