In-situ warming and its differential effect on the photosynthetic performance of two dominant species
Climate change is altering the structure and function of alpine ecosystems. The air temperature of the Tibetan Plateau is forecast to increase by 0.6–0.9 °C from 2015 to 2050 , which is within the simulated warming in our experimental setup (Table 1). Leaf traits are closely related to the resource acquisition and utilization efficiency of plants and reflect plant survival strategies to environmental changes. Our results suggested that the leaf traits of E. nutans and P. anserina have different responses to warming. Under increased temperature, the SLA of E. nutans increased, and the LMA of P. anserina increased. Our findings support the hypothesis that the phenotypic plasticity of certain traits in plants can predict the performance of communities under climate change [29, 30]. SLA and LMA are related to plant growth, resource capture, and utilization. SLA impacts light interception, photosynthesis, and plant growth, and is thus indicative of competitive ability and environmental tolerance [31, 32]. The SLA of E. nutans was higher than that of P. anserina, indicating that it possesses a higher net photosynthetic rate and higher leaf light capture area. The higher LMA value of P. anserina is a characteristic that allows it to flourish in alpine environments.
Under certain circumstances, warming can meet the growth requirements of plants. However, it can also change the microclimatic environment of the plant community and directly or indirectly affect plant photosynthetic physiological processes in a variety of ways. Although increased temperature in cold ecosystems, such as our study area, may promote plant growth (Fig. 6), it may also increase interspecific competition. Elymus nutans is a grass (Gramineae) and is the dominant species in alpine meadows, while P. anserina is a forb (Rosaceae) and is widespread and common. The responses of different plants to climate warming differ, and these responses determine the adaptive capacity of species to future climate warming as well as their competitive ability . In our study, Pn in E. nutans was higher than P. anserina in the OAs (Fig. 2). Interestingly, the Pn of E. nutans decreased with increased temperature, while that of P. anserina increased. This is because the air temperature was high in the growing season (July), and thus the leaf temperature of E. nutans exceeded its optimal temperature, leading to a decrease in Pn. These results corresponded with the changes in photosynthetic parameters (Fig. 3).
Photosynthetic parameters are very important for estimating the alpine C budget. Previous studies showed that warming would increase plant C uptake by providing optimal temperature conditions [34, 35]. In our study, P. anserina had a higher photosynthetic rate than E. nutans under warming (Figs. 2 and 3). Shi et al.  suggested that forbs (Vicia unijuga and Allium atrosanguineum) would adapt better to future climate warming than grasses (E. nutans and Koeleria macrantha) in alpine meadows, which is consistent with our findings. In cold climates or in areas with no water restrictions, species will change their optimal photosynthetic temperature to increase photosynthesis under warming [37, 38]. A temperature increase from 15 °C to 20 °C resulted in increased Pnmax, LSP, Rd, and AQY in E. nutans, while a decrease was observed at 25 °C. This suggests that 20 °C is the approximate optimum growth temperature for E. nutans. Conversely, Pnmax, LCP, LSP, Rd, and Vcmax all increased with increased temperature in P. anserina, which implies that P. anserina can survive at a higher temperature. Similar responses have also been reported by Shi et al. , who found that elevated temperature increased the photoinhibition of E. nutans but reduced the photoinhibition of P. anserina. Elmendorf et al.  discovered that the response to long-term warming was opposite by grasses, sedges, and rushes. In the present study, E. nutans demonstrated the highest Pnmax at 20 °C, which thereafter decreased at 25 °C, but was still higher than in P. anserina. A higher Pnmax is associated with higher photosynthetic gain, suggesting that E. nutans had a greater photosynthetic gain. E. nutans demonstrated have greater photosynthetic gain, while P. anserina can survive at a higher temperature, suggesting that the community structure of alpine meadow may change from grass to forb with climate warming. CO2 utilization during photosynthesis is indicative of photosynthetic efficiency, and a higher Rd is indicative of greater consumption of photosynthetic products. Vcmax and Jmax increased with increased temperature in both species, which might be related to the changes in nitrogen distribution and photosynthetic enzyme activity in the leaves .
In alpine regions, lower temperatures and a short growing season are the main limiting factors for plant growth and ecosystem productivity. Studies have shown that in temperature-limited ecosystems, the extension of the growing season under long-term warming will significantly improve net primary productivity by increasing photosynthetic capacity [41, 42]. Peng et al.  concluded that alpine ecosystems with low temperature and relatively high soil moisture tend to absorb more C in a warmer climate. In our study, the above-ground biomass of the alpine vegetation increased under three consecutive years of warming, which is consistent with the increase in community photosynthesis under warming conditions (Fig. 4). Climate warming increased the aboveground biomass of the arctic willow Salix arctica, which had a positive feedback on its photosynthetic activity . Under warming, the tested alpine meadow plants increased their above-ground biomass due to the increased community photosynthetic rate, which is consistent with Chapin et al. . Liang et al.  used a meta-analysis to estimate the effects of warming on leaf photosynthesis of terrestrial plants, and found that the effect of warming on grass was greater than that of forbs, indicating that forbs may accumulate lower biomass and have less competitive than grasses under climate warming.
Community photosynthetic carbon assimilation and its relationship with leaf-level carbon assimilation
The leaf is the smallest unit of a plant community, and a plant community is the basic component of an ecosystem. CAP represents the photosynthesis of both the top and bottom leaf layers. In the present study, we offer a simplified approach for estimating community photosynthesis. We used six easily-measurable parameters to calculate the community photosynthesis and compared these with the observed results (Eq. 3, Fig. 5). Ellsworth and Reich  suggested that LMA is an effective means of combining the effects of canopy structure and light environment on leaf photosynthetic performance. Li et al.  found that under experimental warming, leaf area could determine the response of ecosystem productivity in an Alpine Steppe. In our study, we used fresh leaf mass per unit leaf area (A) and fresh weight of all plant leaves (m) in the scale conversion. Using our equation, the result agrees with the observations under natural conditions (Fig. 5).
Leaf position, leaf age, and different leaf orientations or growth angles (horizontal and vertical) also influence the photosynthetic rate of the leaves . Alpine meadow plants have obvious stratified structures. Grasses (such as E. nutans) largely grow in full-sun conditions, while sedges and forbs (such as P. anserina) mostly grow in shaded environments. In the canopy, the light absorbed by the upper leaf layer is usually more than its saturation level, and the excess light energy is dissipated primarily by heat dissipation, while the lower layer leaves are usually limited by available light . Considering the degree of shading between the plants and the angle of the leaves, the parameter r (the percentage of received effective light by leaves in the community) was used in the estimation of community photosynthetic capacity. As for the different maturity of leaf maturities in the community, we used the parameter k to represent the percentage of healthy leaves, and these parameters were successfully incorporated in the model. Using these parameters, the leaf-level carbon assimilation can be accurately estimated to the community level. Comparing with the canopy model, all of the parameters were more simplified and easier to obtain.