Widespread changes in forest structure and distribution have been documented in Patagonia over the past century. Reconstructions of forested area in Patagonia over the last 75 to 100 years show an expansion of forests at high and low elevation. At low elevation, expansion has coincided with the implementation of fire suppression, potentially shifting the vegetation from open woodlands to dense forest, especially along the forest/steppe margin. In contrast, at high elevation, a combination of climate and fire suppression is thought to be responsible for forest expansion.We employed LPJ-GUESS, a Dynamic Global Vegetation Model (DGVM) to investigate the role of climate, atmospheric carbon dioxide (CO2), and fire on simulated forest cover during the 20th century. We established a series of experimental simulations to evaluate the individual and interactive effects of climate, CO2, and fire, running combinations of fixed and dynamic CO2 with historical climate data (1901-2016). We simulated current potential vegetation composition and distribution for southern Chile and southwestern Argentina (40oS - 45oS).
Results/Conclusions
Simulating observed climate and observed CO2 from 1930 to 2010 showed an increase in forest cover under changing climate and CO2 because of higher carbon assimilation and net primary production. The model results were compared with a remote sensing derived biomass map and ‘greening’ indices from the normalized difference vegetation index and revealed agreement in greening at the high and low elevations. The simulations also captured the distribution of temperate rainforest west of the Andes and the high-elevation forest but underestimated forest cover along the eastern forest-steppe transition. At low CO2, biomass was low, thereby reducing available fuel for burning and suggesting that high CO2 is necessary to accurately simulate present-day productivity. The use of pre-industrial climate (PI) and PI CO2 caused a decrease in fire frequency and reduction of simulated biomass that does not match present-day distributions. Climate is the primary driver and CO2 fertilization is the secondary driver of forest expansion. CO2 also mitigates climate-induced drought stress due to increases in water-use efficiency.