Factors influencing altitudinal patterns of C3 plant foliar carbon isotope composition of grasslands on the Qinghai-Tibet Plateau, China

Foliar δ13C of 219 dominant C3 species samples from 82 alpine grassland sites on the Qinghai-Tibet Plateau were related to altitude, precipitation and temperature, atmospheric pressure and life-form and taxonomic groupings. Foliar δ13C values were within those previously found for C3 plants and the mean was higher than those of lower altitude areas. Averaged for all samples foliar δ13C increased with altitude at an average rate of 0.78‰ km−1. Foliar δ13C was higher for herbs than shrubs and was attributed to the absence of shading of herbs by taller woody plants on the Plateau. Significant differences were found between classes and families. Growth season temperature (GST) accounted for variation of foliar δ13C more effectively than mean annual temperature (MAT) but growth season precipitation (GSP) did not account for more of the variation than mean annual precipitation (MAP). Drought Index (DI) considered for all samples indicated that water availability did not critically affect carbon isotope discrimination but differences between the variation of foliar δ13C for the genera Kobresia and Stipa in relation to DI were shown. When considered separately, decreasing temperature and precipitation with altitude in the region were associated with a reduction of carbon isotope discrimination. However, when temperature and precipitation were accounted for together with atmospheric pressure their effects were not significant. This concurs with the recent finding that in the absence of significant moisture limitation, the reduction of carbon isotope discrimination with increasing altitude reflects an atmospheric pressure-related adaptation and not an adaptation related to temperature.