Grassland-to-shrubland transitions are a widely-observed form of land degradation in arid grasslands. This physiognomic transformation is commonly ascribed to changes in disturbance regimes (e.g., overgrazing) mediated by topoedaphic and climatic factors. Myriad direct and indirect effects of grazing may tip the balance to favor shrubs over grasses, but their relative importance is not clear. Here, we test the hypothesis that post-grazing wind erosion changes surface soils to favor shrub recruitment over that of grasses. Aeolian erosion in drylands has been documented to deplete nutrient-rich fines and coarsen soil texture, thus reducing water and nutrient availability. If this ‘winnowing’ of topsoils differentially effects grass and shrub seedling establishment to promote shrub recruitment and/or reduce grass recruitment, then shifts from grass to shrub dominance would be expected to occur with grazing. We tested this hypothesis in a greenhouse bioassay experiment that 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 intact soil (unwinnowed) and soil where upwind plant cover had been experimentally removed for 27 years and subject to winnowing at a site in the northern Chihuahuan Desert.
Results/Conclusions
Unwinnowed (control) soil was finer textured and had higher nutrient content than winnowed soil. Available water capacity was comparable, but water potential of winnowed soil exceeded that of unwinnowed soil. P. glandulosa (deciduous N2-fixing shrub) seeds were first to emerge and complete germination, followed by the grasses B. eriopoda, A. purpurea, and S. airoides. Germination/emergence was lowest on A. canescens (suffruticose, semi-deciduous) and L. tridentata (fruticose, evergreen). However, soil type (control vs. winnowed) had no discernible influence on these patterns. Contrary to expectations, grasses and shrubs grown on winnowed soil were generally taller, with lower root:shoot ratio and greater numbers of leaves than those on unwinnowed soil. This is consistent with the winnowed soil having higher water potential than the unwinnowed under the same watering regime. Overall, our results indicate that wind-eroded surface soils do not adversely impact germinability and early growth of contrasting plant functional type relative to that on non-winnowed soils. If anything, grass and shrub seedlings performed better on winnowed soils under well-watered greenhouse, small container conditions. Field establishment experiments are needed to confirm these greenhouse findings.