Plant affinity to extreme soils and foliar sulphur mediate species-specific responses to sheep grazing in gypsum systems

Plants growing on extreme soils have mainly been described in relation to their adaptations to edaphic conditions, although herbivores may also be an important factor in these ecosystems. Gypsum soils occur in drylands often where livestock practices occur. However, it is unknown whether plant traits related to gypsum soil constraints are associated with resistance to herbivory. In order to assess whether gypsum specialist species might be favoured at higher grazing levels and to detect the traits involved, we evaluated the responses of gypsum specialists vs. generalists to three intensities of livestock pressure. We analysed the relative cover shifts of species along a livestock gradient, and variation in canopy height, canopy area, leaf carbon (C), nitrogen (N), and sulphur (S), specific leaf area (SLA) and leaf dry matter content (LDMC). We found that gypsum-specialists responded by increasing or maintaining their cover at medium and high grazing pressure, whereas most generalists responded by decreasing it. Gypsum-specialists showed higher leaf S than generalists, regardless of grazing intensity. All species showed similar patterns for traits linked to loss of above-ground biomass when grazing increased. Plant affinity to gypsum soils mediates vulnerability to grazing with foliar S possibly being a defence trait.