Soil erosion facilitates shrub encroachment in Patagonian herbaceous steppes

Abstract

Previous studies in northeastern Patagonia suggested that, mainly as a consequence of overgrazing by sheep, the herbaceous steppes are being replaced by shrub steppes dominated by Chuquiraga avellanedae as a consequence of shrub encroachment. The herbaceous steppe presents a thick sandy loam to loamy sand A-horizon lying on a clay-horizon while the shrub steppe presents the clay-horizon at or near the soil surface. The thickness reduction of the remaining A-horizon and the formation of desert pavements are evidence of soil erosion. As previous studies suggested that shrub encroachment co-occurs with the erosion of the sandy A-horizon, we hypothesized that shrub encroachment is favored by the presence of an clay-horizon near the soil surface. We expected that C. avellanedae emergence and establishment would be higher as the clay horizon is closer to the soil surface and the A-horizon is reduced. We performed field and greenhouse experiments to evaluate seedling emergence and establishment along a gradient of A-horizon thickness. We found that the presence of a relatively thin sandy layer on the surface had a negative influence on both shrub emergence and establishment. The summer drought was not a critical period for the seedlings. Conversely, the highest seedling mortality took place during late winter in sandy soils, suggesting that these soils are more susceptible to frost heaving than clay soils. In conclusion, soil erosion in the northeastern Patagonian steppes would indirectly facilitate shrub encroachment due to differences in frost heaving susceptibility between sandy and clay soils.