Disturbances by humans are one of the main drivers of change in contemporary desert ecosystems. Restoration treatments such as topsoil application and outplanting can be implemented in response to disturbances in order to maintain soil stability and a diverse plant community. Fertile islands - nutrient enriched areas beneath perennial shrubs - are fundamental features of deserts that can facilitate annual plant growth. A major uncertainty in desert ecology is how much time is required for fertile islands and nurse plant effects (the facilitation of one plant by another) to develop below maturing perennial plants. By studying naturally recruited perennial plants no older than age 10 years using a unique study design including sites where soils were severely disturbed, homogenized, and denuded of perennial plants 10 years earlier and comparing with undisturbed desert, this study assessed soil, annual plants, and soil seed banks beneath Ambrosia dumosa shrubs. Influences on fertile island development of the restoration treatments of applying salvaged topsoil and outplanting were also assessed.
Results/Conclusions
In disturbed areas, soil nitrogen was three times higher beneath shrubs than in interspaces. This effect is consistent with fertile island development seen in undisturbed areas. However, soil from fertile islands in disturbed areas only had two-thirds as much total nitrogen as fertile islands in undisturbed areas. Disturbance had a significant effect on the plant community. Plant cover and species richness of native plants tended to be four times and 1.6 times higher in undisturbed areas respectively, while exotic plants had higher cover in disturbed areas regardless of treatment type. Both native and exotic plants grew in association with the fertile island instead of in interspaces between shrubs. In addition, the soil seed bank was larger and more species-rich in fertile islands compared with interspaces. Areas that had topsoil applied contained larger soil seed banks than disturbed, unrestored sites. This study demonstrates that the fertile island effect can develop within 10 years below shrubs across all treatments including disturbed/unrestored, disturbed/restored, and undisturbed controls. Topsoil reapplication may be beneficial to maintaining abundant seeds in the soil seedbank, but whole plot analyses of vegetation cover and species richness should be conducted to determine differences in vegetation among treatment types.