Ecosystem structure and soil-surface conditions drive the variability in the foliar δ¹³C and δ¹⁵N of Stipa tenacissima in semiarid Mediterranean steppes

We evaluated the effects of ecosystem composition and structure (species richness and diversity, cover and spatial attributes of vegetation), abiotic factors (climate and topographical features) and the condition of the bare-ground areas (evaluated using soil-surface indicators) on the performance of Stipa tenacissima [evaluated using foliar δ¹³C, δ¹⁵N, nitrogen concentration and the carbon-to-nitrogen (C:N) ratio] in 15 steppes of SE Spain. Foliar δ¹³C values of S. tenacissima showed a low degree of variation in the studied steppes, with average values ranging from -24.1 to -22.9‰. Higher variation was found in the δ¹⁵N values, which ranged from -5.5 to -2.4‰. The nitrogen concentration and the C:N ratio varied between 5.0 and 8.0 mg g ^-1, and between 55.4 and 85.3, respectively. The δ ¹³C values became less negative with increasing spatial aggregation of perennial vegetation, while the C:N values increased with increasing perennial vegetation cover. The δ¹⁵N values became more negative with increasing infiltration in the bare-ground areas, but the nitrogen concentration was not related to any of the environmental variables measured. Our results suggest that the relative importance of ecosystem structure and soil-surface conditions in the bare ground areas was higher than that of abiotic factors as determinants of the performance of S. tenacissima. The results also show that even subtle changes in these ecosystem features may lead to modifications in plant performance in semiarid S. tenacissima steppes, and thus to modifications in the associated ecosystem functions in the mid- to long-term.