Variation in carbon isotope discrimination in Cleistogenes squarrosa (Trin.) Keng: Patterns and drivers at tiller, local, catchment, and regional scales

Understanding the patterns and drivers of carbon isotope discrimination in C ₄ species is critical for predicting the effects of global change on C ₃/C ₄ ratio of plant community and consequently on ecosystem functioning and services. Cleistogenes squarrosa (Trin.) Keng is a dominant C ₄ perennial bunchgrass of arid and semi-arid ecosystems across the Mongolian plateau of the Eurasian steppe. Its carbon isotope discrimination ( ¹³Δ) during photosynthesis is relatively large among C ₄ species and it is variable. Here the ¹³Δ of C. squarrosa and its potential drivers at a nested set of scales were examined. Within cohorts of tillers, ¹³Δ of leaves increased from 5.1‰ to 8.1‰ from old to young leaves. At the local scale, ¹³Δ of mature leaves varied from 5.8‰ to 8.4‰, increasing with decreasing grazing intensity. At the catchment scale, ¹³Δ of mature leaves varied from 6.2‰ to 8.5‰ and increased with topsoil silt content. At the regional scale, ¹³Δ of mature leaves varied from 5.5‰ to 8.9‰, increasing with growing-season precipitation. At all scales, ¹³Δ decreased with increasing leaf nitrogen content (Nleaf). Nleaf was positively correlated with grazing intensity and leaf position along tillers, but negatively correlated with precipitation. The presence of the correlations across a range of different environmental contexts strongly implicates Nleaf as a major driver of ¹³Δ in C. squarrosa and, possibly, other C ₄ species.