Responses of alpine plant communities and soils along natural and induced snow melt gradients, southern South Island, New Zealand

Abstract

Theory predicts that disturbance affects both the biotic and abiotic components of ecosystems, and the highest community diversity occurs at intermediate levels of, and time since, disturbance ('the intermediate disturbance hypothesis (IDH)'). I tested these predictions using snow accumulation as a proxy for disturbance, determining the relationships between plant communities, snow cover and soil nutrients in natural and artificial snowbanks. I asked whether the IDH is applicable temporally and spatially, and whether soil characteristics reflected the gradient.

Alpine plant communities and soil nutrient composition were studied along the snow melt gradients induced by a 52-yr old snow fence and two natural snowbanks in the high-alpine zone of the Old Man Range, Southern New Zealand. Percent covers of vascular plant species and ground cover variables were recorded around the snow fence in 2003 and 2011, and across natural snowbanks in 2011. Plant species were the grouped into growth form categories (graminoids, forbs and woody species), and diversity indices (Species richness, Shannon-Weaver, Simpson and Pielou´s Evenness) were calculated. Soil nutrient levels and pH from the rooting zones were collected (in 2011) and analyzed. Plant communities responded both spatially and temporally along the snow melt gradients caused by the snow fence and snowbanks in relation to species richness, growth form and other diversity indices. Plant species composition and distribution patterns varied between induced and natural snowbanks, and some species increased (p < 0.05), others decreased (p < 0.05) and others were indifferent (p > 0.05) to the effects of increasing snow. At the snow fence, the cover of graminoids and forbs increased, and woody species decreased with increasing snow depth (p < 0.05). I found no such variation across the natural snowbanks (p > 0.05). Along with increasing snow cover, the covers of bare soil and pavement decreased at the snow fence, rocks and pavement increased at snowbanks, and dead plant matter decreased at all sites (all p < 0.05). Diversity was overall greater around the snow fence than at the natural snowbanks. Species richness reached a peak at the intermediate levels of snow, whereas the other diversity indices increased with increasing snow at both snowbanks and snow fence. Soil nutrient levels were greater under all depths of snow at snowbanks, and in some cases were double or triple the levels found at the snow fence. Most soil parameters decreased with increasing snow at the natural snowbanks (p < 0.05), as opposed to the snow fence, where the variation was much less pronounced. Soil pH did not vary along either gradient (p > 0.05). The IDH was spatially supported by the ´species richness´ index, as opposed by other diversity indices along the snow melt gradients studied. Also, the temporal patterns followed by plant communities as a response to snow accumulation gradients in the alpine zone also supported the IDH concept. The gradients of snow accumulation appeared to be important drivers of the biotic and abiotic components across the alpine landscape. In all, snow appeared to be the main driver of plant communities, and soil nutrient composition a result of the combined effects of topography and snow.