Abiotic and biotic recovery pathways of arid rangelands: lessons from the High Atlas Mountains, Morocco. (Special Issue: Long-term degradation of fragile landscape systems.)

摘要

Dryland rangelands are fragile ecosystems which are vulnerable to overgrazing and other forms of unsustainable land use. Their subsequent degradation is a phenomenon which results in a persistent decrease in productivity. To come to a functional understanding of degradation and restoration pathways, interactions between abiotic and biotic processes need to be disentangled. We studied soil and vegetation response to grazing removal along a steep climatic aridity gradient on the southern slopes of the High Atlas Mountains, Morocco. Data were collected on four altitudinal levels (semidesert, sagebrush steppe, woodsteppe and Oromediterranean shrubland), and for three grazing treatments (grazed, 1-year and 7-year exclosures). Grazing removal led to site-specific, functionally different pathways of rangeland recovery. At least at one of the four sites, we found either an accumulation of sand, the Aeolian component, or total nitrogen in the topsoil. Total standing crop, shrub ANPP and shrub performance (ANPP_rel, i.e. ANPP indexed on initial biomass) also increased with grazing removal. An increased shrub density led to an increased water storage capacity and/or nutrient content of the soil. However, this improvement in plant resources was not connected to the observed increase in shrub performance. Thus we only found evidence for biotic recovery having a positive feedback on abiotic recovery. We conclude that correlations between biotic and abiotic recovery processes have to be interpreted carefully, as they may not necessarily be functionally connected. Moreover, the performance of perennial plants (ANPP_rel) is, if interpreted together with abiotic parameters, a useful indicator for distinguishing functionally different pathways of pasture degradation and restoration in drylands.