Interactions between past grazing pressure and topography predict patchy shrub encroachment in Páramo grasslands

Background/Question/Methods:

Known for exceptionally high levels of floral diversity and endemicism, páramo ecosystems were exposed to varying degrees of grazing and burning during the later half of the 20^th century.  Release from these activities can lead to shrub encroachment, however, this phenomena can be patchy in post-agricultural páramo. Other studies suggest that topographic and soil variables (e.g. soil moisture, pH, bulk density, organic matter) are important factors in shrub encroachment. The goal of this study was to determine whether interactions among these factors and past grazing/burning pressure predict which patches are prone to shrub encroachment.  We sampled forty plots in recently abandoned páramo in Zuleta, Ecuador and collected the following data from each plot: 1) percent shrub cover, 2) edaphic conditions (pH, bulk density, soil moisture, and organic matter from the upper and lower A horizons), and 3) topography (elevation, slope, aspect, and topographic position).  Interviews with former ranchers were used to develop proxies for grazing and burning pressure.  These proxies (distance from the cattle release point, distance from streams, distance from paths) were then quantified for each plot using GIS.  These data were used to construct a recursive partitioning model predicting percent shrub cover.

Results/Conclusions:

At elevations >3740 masl, shrub cover was generally high, especially when pH <4.5.  Below 3740 masl, shrub cover was higher if 1) the plot was > 1.2km from the cattle introduction point, or 2) <300 m from a stream.   The importance of pH as a predictor at higher elevations probably reflects the influence of shrub litter on pH rather than the effects of pH on shrub establishment.  The importance of distance from cattle introduction points reflects a legacy of past grazing and burning, whereas the greater shrub cover near streams may reflect cattle-induced propagule dispersal of shrubs from streamside montane forests. In spite of a low-moderate prediction error rate, our model suggests that legacies of past grazing and burning interact with elevation to predict shrub cover, and that grazing legacies can either hinder or help shrub encroachment depending on whether cattle were serving as dispersal vectors.