Untangling the influence of biotic and abiotic factors on the elevational variation of plant intrinsic water-use efficiency and nitrogen availability in an alpine-gorge region on the eastern Qinghai-Tibetan Plateau

Despite the sensitivity to climate change in the alpine-gorge region on the eastern Qinghai-Tibetan Plateau, comprehensive information on the elevational variation patterns of plant intrinsic water use efficiency (iWUE) and plant nitrogen (N) availability in this region is still lacking. To explore the elevational patterns and their drivers of plant iWUE (calculated from leaf δ¹³C) and plant N availability (assessed by leaf δ¹⁵N) of different plant growth forms, plant iWUE and leaf δ¹⁵N of 654 observed samples of 158 species from seven typical vegetation types distributed along the elevation were investigated, and the effects of taxonomic identity, leaf functional traits, soil and climatic factors on them were explored by linear mixed-effects model. It was found that both plant iWUE and leaf δ¹⁵N differed significantly among different plant growth forms, with plant iWUE showed shrubs > herbs > trees and leaf δ¹⁵N showed herbs > shrubs > trees. The plant iWUE and leaf δ¹⁵N showed a U-shaped pattern along elevation, and the elevation patterns of different plant growth forms were consistent. The plant iWUE of different growth forms first decreased and then increased with increasing humidity index, and the humidity index breakpoints of different plant growth forms were different. The leaf δ¹⁵N of herbs and shrubs showed a significantly positive correlation with humidity index, whereas the leaf δ¹⁵N of trees had an insignificant correlation with humidity index. The elevation variations of plant iWUE and leaf δ¹⁵N of different plant growth forms were jointly influenced by biotic (i.e., plant taxonomy and leaf functional traits) and abiotic (i.e., soil and climate) factors, but were mainly determined by biotic factors. Our results showed that the significant coupling between plant iWUE and leaf δ¹⁵N did not vary by plant growth forms, thereby suggesting a close link between carbon and N cycles. This study deepens our knowledge regarding elevation variations and their drivers of plant iWUE and leaf δ¹⁵N, and confirms the importance of biotic factors on plant iWUE and leaf δ¹⁵N variations along elevation gradients in the alpine-gorge region.