Interactions among fire legacies, grazing and topography predict shrub encroachment in post-agricultural páramo

Abstract

Woody encroachment in grasslands is a global phenomenon driven by complex interactions between climate, grazing and fire management. Alpine shrub encroachment is of particular concern for biological conservation because high-elevation grasslands harbor high levels of biodiversity and species endemism. Paramo grasslands of the high Andes are exceptionally high in floral diversity, but traditional agricultural practices have resulted in widespread livestock grazing and anthropogenic burning. Fire suppression has frequently been identified a driver of woody expansion in other grasslands, and conservation initiatives that aim to decrease burning and grazing in paramos may inadvertently lead to shrub encroachment. We tested whether interactions among fire and grazing legacies, topography and edaphic conditions predicted the patchy distribution of encroaching shrubs in an Ecuadorian paramo 10 years after release from burning and grazing. Interviews with land-users identified proximity to roads, footpaths and riparian areas as proxies for fire frequency and grazing pressure. A recursive partitioning model of shrub cover revealed that woody abundance was generally lower at lower elevations, especially near the access road (where fire frequency and grazing pressure were high). Within the low-elevation areas, shrub cover was highest near streams (where grazing pressure was high). These results suggest that (1) the effects of fire and grazing legacies depend on the spatial patterns of grazing, and (2) legacy effects interact with topography to help explain patchy shrub encroachment.