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.

中文翻译：

---

土壤侵蚀促进了巴塔哥尼亚草本草原的灌木侵占

先前在巴塔哥尼亚东北部的研究表明，主要是由于绵羊过度放牧，草本草原正在被灌木侵占，以Chuquiraga avellanedae为主的灌木草原所取代。草本草原呈现厚沙壤土至壤质砂 A 层，位于粘土层上，而灌木草原在土壤表面或附近呈现粘土层。剩余 A 地层的厚度减少和沙漠路面的形成是水土流失的证据。由于先前的研究表明灌木侵占与沙质 A 地平线的侵蚀同时发生，我们假设灌木侵占有利于土壤表面附近粘土地平线的存在。我们预计C. avellanedae当粘土层更接近土壤表面且 A 层减少时，出苗率和定植率会更高。我们进行了田间和温室实验，以评估沿 A 地平线厚度梯度的幼苗出苗和建立。我们发现地表相对薄的沙层的存在对灌木的出现和建立都有负面影响。夏季干旱不是幼苗的关键时期。相反，沙质土壤在冬末的幼苗死亡率最高，这表明这些土壤比粘土更容易受到冻胀的影响。总之，由于沙土和黏土之间冻胀敏感性的差异，巴塔哥尼亚草原东北部的土壤侵蚀将间接促进灌木侵占。