Shrub-grass interactions in semi-arid rangeland, and their combined effects on soil-mound growth and maintenance

Background/Question/Methods

Interactions between vegetation and soil are key elements to ecosystem function in drylands, with roles in both spatial pattern formation and plant interactions. We focused on relations between annual plants and shrub-dominated mounds of a shrubland in the semi-arid northern Negev, Israel. Ecosystem engineering through landscape modulation has been studied extensively in this area, focusing mainly on animals and woody plants, while regarding annual plant productivity as a response variable. However, the effects of low-laying herbaceous plants on sediment transport may be significant in pattern formation and ecosystem function. We examined these effects using field experiments and analyses of climatic conditions in situ.  

Recent changes in rainfall amount have given rise to a significant loss of shrub cover over most of the landscape, with high mortality rates of N. mucronata dwarf-shrubs. Due to the interactions between shrubs and annuals, and their combined geomorphic effect, such phenomena can have significant consequences for community and ecosystem processes. We evaluated the control h of mound development and shrub state over annual plant community attributes. A multi-scale observation approach was used in an attempt to relate mechanisms acting at patch-scale and smaller (< 1m²) to patterns observable at the meso-scale of slopes (~ 100m²).  

Results/Conclusions

Positive correlation between mound size and annual plant density and cover alludes to a positive feedback interaction between mound development and annual vegetation growth. A micro-scale field experiment of sediment dispersal following natural rain events revealed understory herbaceous vegetation reduces sediment loss from shrub-dominated mounds. By integrating the results obtained from this experiment with measurements of shrub-mound morphology, we propose that annual plants are involved in a positive feedback between sediment capture and plant density growth. We suggest that modulation by annual plants affects mound morphology, an effect that has profound impacts on the rate and pattern of run-off in the landscape, and therefore also on landscape evolution and susceptibility to erosion.

 A combination of among-patches community similarity analysis and within-patch incidence-abundance analysis has highlighted the effect of the recent loss of shrub-cover on spatial community dynamics. At the patch-scale, shrub canopy loss acts as a disturbance, leading to community reorganization. At the watershed-scale, the combination of environmental stress and patch-scale disturbances has led to an abundance reduction or loss of several annual plant species. Results indicate a possible ecosystem state-shift, which might lead to a reduction in resources, productivity, and biodiversity, due to positive feedback processes.