Fire-Mediated Forest Encroachment in Response to Climatic and Land-Use Change in Subtropical Andean Treelines

Abstract

We used dendroecological techniques to analyze the effects of rainfall and grazing on fire regime and its implications for tree regeneration in subtropical mountains of northwestern Argentina during the 20th century, a period characterized by increasing rainfall and decreasing land-use intensity. We dated fire scars and establishment of Alnus acuminata (the dominant tree species) in six watersheds along a 600 km latitudinal range. We correlated fire frequency with rainfall records and performed Superposed Epoch Analyses to assess the relationship between rainfall and fire events during the century, and in two sub-periods: 1930-1965 (low rainfall, high grazing) and 1966-2001 (high rainfall, low grazing). We performed permutation analyses to assess the association between fire events and tree establishment, and to describe the spatial distribution of fires and forests in relation to hillslope aspect. Rainfall was associated with regional fires at interannual and decadal scales: fire probability increased after growing seasons with above-average rainfall and through the century, in concurrence with rainfall increase. The climatic control of fire was stronger under lower land-use intensity. Tree establishment was temporally associated with fire events, which occurred mainly in north facing slopes, where grassland cover is more extensive and forest colonization more likely. These results suggest that fire is limited by the availability of fine fuels, which is enhanced by high rainfall and reduced grazing; and tree establishment is limited by the competition with grasses. Consequently, increasing rainfall and decreasing grazing favored higher fire frequency, thus promoting forest encroachment during 20th century.

Scientific Foundations for an IUCN Red List of Ecosystems

“…These include systems with strong top-down trophic regulation [58], [124], [127], [128], systems with many mutualistic or facilitation interactions [126], [129], systems that are strongly dependent on mobile links [130] and systems where disturbance regimes impose top-down regulation and positive feedbacks operate between the biota and the disturbance [131], [132].…”

Do Wildfires Promote Woody Species Invasion in a Fire-Adapted Ecosystem? Post-fire Resprouting of Native and Non-native Woody Plants in Central Argentina

“…In this work, we assumed a long fire history in our study region according some antecedents within other mountain ecosystems of Argentine North region with a dry season (Grau and Veblen 2000;Aráoz and Grau 2010) and because in our study region all woody plants resprout after fires albeit resprouting is not necessarily an adaptation to fires (Gurvich et al 2005;Torres et al 2014). Considering the diverse geographical origins of non-native species, our hypothesis is that native woody species would respond to fires relatively better as compared to non-native woody species which should present a greater diversity of responses to fire.…”

Land-Use Redistribution Compensated for Ecosystem Service Losses Derived from Agriculture Expansion, with Mixed Effects on Biodiversity in a NW Argentina Watershed

“…Contemporary socioeconomic changes lead to rural-urban outmigration from marginal agriculture lands and to the development of off-farm economies, reducing land pressure in these areas. This, in turn, facilitates revegetation in certain scenarios [55,56], including this study area [7,23].…”

“…In addition, regional climate change (rainfall increase, [22]). might have influenced these forest cover trends, by facilitating both forest and agriculture expansion, respectively, in the highlands and lowlands: water availability for irrigation is a limiting factor for agriculture in the area, while woody encroachment over grasslands in montane areas is also favored by increasing rainfall, particularly in the case of water demanding species, such as Alnus acuminata (the most important tree species expanding in the highlands, [23]). As a result of agriculture modernization, subsistence livestock experienced reductions in the last two decades [24,25], while market-oriented cattle production increased by almost 50% [26].…”