Tree survival and growth in central Argentina's highlands: impact of wildfires and land management

Abstract

High-mountain forests of subtropical and tropical South America are dominated by Polylepis trees. Their fragmentation is believed to result from human caused wildfires, livestock grazing and deforestation. We assessed survival and growth of adult Polylepis australis trees in central Argentina, examining the effects of wildfires, land management, and tree cover. We monitored 880 adult trees over 15 years (2003-2018) in 139 permanent plots distributed in conservation and ranching areas. We found an average tree survival of 99.2% yr-1, a net height growth of 1.4 cm yr-1, and 17.3% crown dieback. Fire affected 16% of plots in ranching areas and 25% in conservation areas. Fire damage reduced survival by 3% yr-1, reduced growth by 4 cm yr-1 and increased crown dieback 15% as compared to unburned plots. Land management had no direct impact on tree survival, growth, or dieback but had a moderate indirect effect through fire: conservation areas, which experienced more fires, showed slightly lower survival and growth, and slightly higher dieback compared to ranching areas. Tree cover was not related to survival but was positively associated with growth and dieback. Trees located in high-elevation areas had a 2% higher survival rate and 2 cm lower growth than those located at low elevations, with trees at mid-elevations showing the lowest dieback. Larger trees exhibited lower survival and growth, and increased dieback. No logging evidences were observed. We conclude that wildfires were the primary human impact on P. australis trees. Therefore, fire reduction policies could be a key management strategy.