Positive fire feedbacks contribute to shifts from Nothofagus pumilio forests to fire-prone shrublands in Patagonia

Abstract

Question: Under climate change and increased ignitions by humans, burning of forests in which severe fires were naturally infrequent may result in environmental changes that increase the probability that they will burn again. On the eastern slopes of the northern Patagonian Andes, after fire-resistant Nothofagus pumilio forests burn they are typically replaced by fire-prone shrublands dominated by resprouting shrubs. We examine fuel properties and microclimatic conditions at the community level as potential fire feedback mechanisms mediating switches from fire-resistant N. pumilio forests to fire-prone shrublands. Location: Northwestern Chubut province, Patagonia, Argentina. Methods: We characterized the volume and vertical distribution of fine fuels, understorey woody and semi-woody plant composition, stand structure and microclimatic conditions in unburned and burned N. pumilio forest and shrublands 14-29 yr after severe fire. Results: Fuel amount and arrangement in unburned N. pumilio forests are unfavourable for fire activity compared with post-fire N. pumilio forests and shrublands. Unburned N. pumilio forests presented vertical discontinuities in fine fuel distribution and lesser amounts of fine fuels near the ground in comparison to fuels in shrublands. Floristic understorey composition of unburned and burned shrublands was very similar, while composition of unburned and burned N. pumilio forests showed clear differences. Additionally, microclimatic conditions following burning of N. pumilio forests and shrublands were significantly warmer and drier than in the unburned forest, and more frequently exceeded thresholds associated with fire activity in this region. Conclusions: Positive feedbacks from initial burning of otherwise fire-resistant N. pumilio forest will accelerate the rate of fire-induced conversion of forests to non-forest assemblages. Once transformed to the alternative state of shrublands, return to a forest cover is unlikely due to increased probability of burning in shrublands, as well as the unfavourable effects of warmer and drier conditions on tree establishment. After fire-resistant Nothofagus pumilio forests burn in the northern Patagonian Andes, they are typically replaced by fire-prone shrublands dominated by resprouting shrubs. We show that fuel properties and microclimatic conditions at the community level are likely fire feedback mechanisms mediating switches from fire-resistant N. pumilio forests to fire-prone shrublands.