Aims

Grassland-to-shrubland transition is a common form of land degradation in drylands worldwide. It is often attributed to changes in disturbance regimes, particularly overgrazing. A myriad of direct and indirect effects (e.g., accelerated soil erosion) of grazing may favor shrubs over grasses, but their relative importance is unclear. We tested the hypothesis that topsoil “winnowing” by wind erosion would differentially affect grass and shrub seedling establishment to promote shrub recruitment over that of grass.

Methods

We monitored germination and seedling growth of contrasting perennial grass (Bouteloua eriopoda, Sporobolus airoides, and Aristida purpurea) and shrub (Prosopis glandulosa, Atriplex canescens, and Larrea tridentata) functional groups on field-collected non-winnowed and winnowed soils under well-watered greenhouse conditions.

Results

Non-winnowed soils were finer-textured and had higher nutrient contents than winnowed soils, but based on desorption curves, winnowed soils had more plant-available moisture. Contrary to expectations, seed germination and seedling growth on winnowed and non-winnowed soils were comparable within a given species. The N₂-fixing deciduous shrub P. glandulosa was first to emerge and complete germination, and had the greatest biomass accumulation of all species.

Conclusions

Germination and early seedling growth of grasses and shrubs on winnowed soils were not adversely nor differentially affected comparing with that observed on non-winnowed soils under well-watered greenhouse conditions. Early germination and rapid growth may give P. glandulosa a competitive advantage over grasses and other shrub species at the establishment stage in grazed grasslands. Field establishment experiments are needed to confirm our findings in these controlled environment trials.

中文翻译：

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在温室条件下在完好无损和被风蚀的土壤上发芽和旱地草和灌木的早期建立

目的

草地到灌木丛的过渡是全世界旱地土地退化的一种常见形式。它通常归因于干扰制度的变化，特别是过度放牧。放牧的无数直接和间接影响（例如，加速的土壤侵蚀）可能比灌木更有利于灌木，但它们的相对重要性尚不清楚。我们检验了以下假设：风蚀使表层土壤“风吹”会不同地影响草和灌木苗的建立，从而促进灌木的吸收超过草。

方法

我们监测发芽和幼苗对比多年生草（生长格兰马草属eriopoda，鼠尾airoides，和芒草菊）和灌木（腺牧豆树，四翅滨藜，和拉雷亚三齿）上现场收集的非淘和淘土官能团下充分浇水温室条件。

结果

非风吹过的土壤比风吹过的土壤质地更细，养分含量更高，但是根据解吸曲线，风吹过的土壤具有更多的植物水分。与预期相反，在给定的物种内，风吹过和非风吹过的土壤上的种子发芽和幼苗生长是可比的。N个₂ -定影落叶灌木P.腺是第一次出现，完整萌发，并有所有物种的最大的生物量积累。

结论

与未浇水的温室条件良好的土壤相比，风吹过的土壤中草和灌木的发芽和幼苗的早期生长没有受到不利或差异的影响。在放牧草地的建立阶段，早期发芽和快速生长可能使灰头果狼比草和其他灌木树种具有竞争优势。需要现场建立实验来确认我们在这些受控环境试验中的发现。