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 ((13)delta) during photosynthesis is relatively large among C(4) species and it is variable. Here the (13)delta of C. squarrosa and its potential drivers at a nested set of scales were examined. Within cohorts of tillers, (13)delta of leaves increased from 5.1 parts per thousand to 8.1 parts per thousand from old to young leaves. At the local scale, (13)delta of mature leaves varied from 5.8 parts per thousand to 8.4 parts per thousand, increasing with decreasing grazing intensity. At the catchment scale, (13)delta of mature leaves varied from 6.2 parts per thousand to 8.5 parts per thousand and increased with topsoil silt content. At the regional scale, (13)delta of mature leaves varied from 5.5 parts per thousand to 8.9 parts per thousand, increasing with growing-season precipitation. At all scales, (13)delta decreased with increasing leaf nitrogen content (N(leaf)). N(leaf) 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 N(leaf) as a major driver of (13)delta in C. squarrosa and, possibly, other C(4) species