A field experiment on climatic and herbivore impacts on post-fire tree regeneration in north-western Patagonia

1. Wildfires are predicted to increase in many ecosystems in relation to globally increasing temperatures but future patterns of post-fire vegetation change are largely unknown, particularly when there are synergistic effects from introduced biota. In the late 1990s northern Patagonia, Argentina, experienced extreme droughts which led to severe wildfires affecting a range of Andean ecosystems. 2. We experimentally examined how variations in moisture, temperature and herbivory by livestock affect post-1999 fire patterns of the three main tree species. Over two years we monitored, in three forest types, the survival and growth of tree seedlings in a factorial warming (+2 degrees C)/livestock exclosure/watering experiment. 3. Seedling survival in the warmed treatments and in the controls was nil for the evergreen Nothofagus dombeyi and the conifer Austrocedrus chilensis at the two low-elevation experimental sites. Survival of the subalpine Nothofagus pumilio in the warmed treatments at high elevation tended to be lower than in the control; for all treatments of warming alone there were no significant differences compared with the controls. 4. In all three forest types, increased water availability was essential for higher rates of tree seedling survival. Doubling water availability during the growing season resulted in up to fourfold increases in seedling survival and up to threefold increases in seedling biomass. 5. In the subalpine forest, livestock reduced seedling survival by c. 30% in non-watered treatments compared with watered treatments, probably due mainly to soil desiccation and to consumption of or damage to facilitating plants. In contrast, at lower elevation, where livestock pressure was lower, seedling survival of N. dombeyi and A. chilensis tended to be higher in unfenced sites, possibly due to reduced competition from highly palatable shrub species. 6. General circulation models predict a warming-drying trend in northern Patagonia during the twenty-first century. The resulting increase in wildfire is likely to be followed by inadequate tree regeneration and conversion from forest to shrubland cover types. This and similar studies suggest that under relatively slight changes in regional climate, increased fire occurrence interacting synergistically with moisture limitations will result in long-lasting displacements of forest by more xeric vegetation shrublands.