Plant diversity and pastoral value in alpine pastures are maximized at different nutrient indicator values

In alpine environments, very low and very high amounts of soil nutrients are generally associated to the lowest plant diversity and forage quality levels. Both soil nutrient content and forage quality and productivity of a site can be inferred from plant species lists, by attributing each species a nutrient indicator value (N value) and a quality value, and computing respectively average N Value and Pastoral Value (PV) at site scale. We used a wide dataset of vegetation surveys carried out in the pastures of Western Italian Alps to 1) evaluate if N values, PV, and plant diversity (species richness and Shannon diversity index) change along an elevation gradient, from montane/sub-alpine pastures (i.e. the ones located below treeline) to alpine pastures (above treeline), 2) analyze the relationships between N value and plant diversity indexes and between N value and PV, and 3) evaluate whether the N values associated to the highest plant diversity and PV differ. Plant diversity, PV, and N values were higher in the pastures located at lower elevation. Plant diversity and PV showed a unimodal relation with N values, both in the montane/sub-alpine and alpine belts. Plant diversity indexes peaked at intermediate N indicator values, confirming the Intermediate Disturbance Hypothesis, while PV peaked at higher N values, where higher nutrient availability in the soil increased plant species productivity, growth rate, leaf turnover and nutrient concentration, digestibility, and palatability. The overall shape of the curves as well as the N values at which plant diversity and PV values peaked did not considerably change from montane/sub-alpine to alpine pastures. These results suggest that an extensive pastoral management is recommended when plant diversity conservation is the main goal. Conversely, a more intensive management can produce an overall enhancement of forage quality/productivity of alpine pastures, but only if restricted under certain critical N values.