Can livestock coexist with Polylepis australis forests in mountains of central Argentina? Setting thresholds for a land sharing landscape

Abstract

Developing a land sharing strategy requires a thorough understanding of what is the maximum productivity that can be achieved without altering key ecosystem processes. In the central Argentina highlands, Polylepis australis dominates the few remaining patches of forests and its evergreen trees are highly selected by domestic livestock. Grazing exclusion and low herbivory pressure allow P. australis forest expansion and soil conservation, while high herbivory pressure promotes forest retraction and environment degradation. However, the extent to which stocking densities should be reduced to allow forest expansion and stand maturation is still unknown. We aimed at answering this question by studying 15 stands located in paddocks with different management, from grazing enclosures to heavily stocked. In 2003 we selected 12 trees less than 2 m tall at each stand. Almost annually along 10 years, for each tree we registered survival, and if alive, we measured the height and two perpendicular diameters of the canopy. We built indicators of vertical and horizontal biomass accumulation rate along the study period and calculated an average effective stocking density (i.e., animal units per ha of non-rocky surface, AU ha−1) for each stand. We found a piece-wise relationship with two pieces between the accumulation rate of each biomass component and average stocking density. Biomass accumulation was maximum under grazing exclusion (i.e. zero stocking density). Between zero and 0.21 AU ha−1 we detected a strongly negative relationship among stocking density and biomass accumulation. Effective stocking densities lower than 0.12 and 0.06 AU ha−1 promoted a widespread vertical and horizontal biomass accumulation, respectively. Stands with stocking densities higher than 0.21 AU ha−1, had null horizontal and vertical biomass accumulation and higher tree mortality. We suggest that effective stocking densities lower than 0.12 AU ha−1 allow a land sharing strategy in central Argentina highlands, ensuring the conservation of a traditional economic activity and the restoration of Polylepis australis forests. Additionally, in areas where Polylepis australis is absent we suggest active restoration without herbivory pressure. Finally, we suggest that these management recommendations may be studied and promoted throughout the extension of Polylepis spp. forests in South America.