Variable community responses to herbivory in fire-altered landscapes of northern Patagonia, Argentina

Landscapes in northern Patagonia have undergone dramatic changes in fire regimes over the last century. Superimposed on this changing vegetation mosaic are impacts from introduced herbivores. In this paper we identify synergistic interactions developing into positive fire-herbivory feedbacks that maximise vegetation change. Analyses of vegetation changes over 30 years at low altitude (c. 800m) montane forest indicates that fire-fragmented forest has coalesced despite relatively heavy grazing pressure. During recent fire suppression, vegetation shifted in dominance from short-lived resprouting species (mostly shrubs) to obligate seed-dispersed trees, some of them requiring initial facilitation by shrubs. Transitions from shrubland and grassland to forest were restricted to c. 20–30m from the nearest forested patches. Beyond this distance, shrublands and grassland are stable in time. In contrast, post-fire responses of high altitude (>1 000m) subalpine forests show a much higher sensitivity to herbivory. Experimental evidence suggests strong fire severity X herbivory interactions affect tree seedling mortality. Severe fires and/or repeated fire create conditions where trees are unable to establish due to unfavorable microenvironments and because fire-opened forests are more heavily used by herbivores. These patterns are consistent with century-long trends of increase in montane forest at the expense of shrublands and increase in shrublands over former subalpine forests.