Warming-induced C4 expansion is better explained by water and nitrogen than by photosynthesis

Abstract

Differences in leaf structure determine the intense competition for resources between C3 and C4 species under climate warming. Competitive advantage is not only reflected in photosynthetic rate, but also in the water and nitrogen content of the leaves. However, the competitive mechanisms between C3 and C4 species in temperate desert steppe under long-term warming remain unclear. Therefore, after six to eight years of warming (ambient temperature, the control treatment, warming 2 ℃ and 4 ℃, T1 and T2 treatments), we estimated the importance values of C3 and C4 species in the plant community, and monitored the photosynthetic rate, nitrogen and water content of plants and soil. Results showed that long-term warming increased the importance values of C4 species in temperate desert steppe. Compared to the control treatments, the importance value of C4 species increased by 29.30% and 26.97% in T1 and T2 treatments, respectively. Variation partitioning analysis and structural equation model showed that leaf photosynthetic rate had a non-significant effect on the distribution of C3 and C4 species. The proportion of C3 and C4 species in plant communities was mainly influenced by soil water and nitrogen content. In summary, this study suggests that the competitive advantage of C3 species will be replaced by C4 species under long-term warming. The nitrogen and water content in soils explain the distributional of C3 and C4 species in temperate desert steppe.