Effects of drought and heat on the productivity and photosynthetic characteristics of alpine meadow plants on the Qinghai-Tibetan Plateau

Alpine meadow plants, adapted to humid and cold environments, are highly sensitive to environmental factors such as drought and heat. However, the physiological responses of individual alpine meadow species to drought and heat stress remain unclear. In this study, four representative species of typical functional groups in an alpine meadow of the Qinghai-Tibet Plateau were selected as experimental materials. Heat (H1, H2), drought (D1, D2), and combined stress (D1H1, D2H2) treatments were implemented to reveal the biomass and physiological characteristics’ response to a constant drought and heat environment. Our results showed that the leaf water content (LWC) of Kobresia humilis and Poa annua increased significantly under heat stress and the compound stress (P<0.05). The effect of a single factor on LWC was greater than that of multiple factors. The aboveground biomass (AGB) of Oxytropis ochrocephala and Saussurea pulchra decreased significantly under compound stress (P<0.05). The response patterns of the net photosynthetic rate (Pn) and transpiration rate (Tr) of K. humilis and P. annua under various stress treatments were similar; as were those of O. ochrocephala and S. pulchra. The stomatal conductance (Gs) variation in K. humilis, P. annua, O. ochrocephala, and S. pulchra were the same under three kinds of stress treatments. The photosynthetic characteristics were more sensitive to the effects of composite than those of single factors. The drought × heat × species treatment had a significant influence on various indexes except on height (Ht) and the belowground biomass (BGB) (P<0.01). Within a certain range, daytime temperature (DT) promoted the Ht and increased the LWC of the plants, while it inhibited their AGB and intercellular CO2 concentration (Ci). The Pn, Tr, and Gs were more sensitive to soil moisture than to DT. The results help improve understanding of the physiological response regularity of representative alpine meadow plant species to continuous drought and high temperature conditions at the species level, and provided experimental data and theoretical basis to identify the decisive factors of stress response.