Climate warming is significantly altering plant communities globally, particularly in high-altitude and cold regions. These changes in species composition and structure are expected to have profound effects on ecosystem functions, including carbon cycling. However, the mechanistic links between warming-induced shifts in plant community structure and ecosystem carbon sequestration remain poorly understood. This study addresses this knowledge gap by focusing on plant height, a key functional trait directly linked to light capture, competitive ability, and carbon acquisition strategies. Previous research has demonstrated shifts in plant community composition towards taller species in response to warming in various cold regions, but the consequences of these shifts for ecosystem carbon cycling remain unclear. This study utilizes both a manipulative warming experiment and a large-scale field survey across the Qinghai-Tibetan Plateau (QTP) to investigate the impact of warming on plant community height, species composition, and ultimately, net ecosystem productivity (NEP) and soil carbon. The QTP provides an ideal setting for this research, as its unique environment and sensitivity to climate change allow for the observation of direct effects of warming on plant community structure and function. The research questions posed are: (1) How does warming change plant community composition and height? (2) Do these changes accelerate or counteract the direct warming impact on NEP? and (3) How will changes in plant community height under warming impact soil carbon content?

The existing literature highlights the significant impacts of climate warming on plant species composition and functional traits, especially in cold and high-elevation regions. Studies have shown that warming can lead to shifts in community composition, with varying responses among plant functional groups. The trait-based approach, focusing on functional traits rather than taxonomic identities, has proven valuable in understanding plant community responses to climate change and their impact on ecosystem functions. Plant height, in particular, is a crucial trait influencing light capture and competition, and its role in ecosystem C cycling has been highlighted in several studies. Synthesizing existing warming experiments, researchers have found that increased plant community height is often linked to shifts in species composition, with taller species gaining a competitive advantage. However, a critical gap exists in our understanding of how warming-induced changes in plant community height and composition affect ecosystem carbon sequestration, particularly the direction, magnitude, and sensitivity of ecosystem carbon fluxes to climate warming. This study aims to address this gap by providing empirical evidence from a warming experiment and a large-scale field survey.

This study employed a combined approach using both a manipulative warming experiment and a large-scale field survey along a 1500 km transect across the Qinghai-Tibetan Plateau (QTP). **Warming Experiment:** A randomized complete block design was used with three warming treatments (control, low-level warming (+1.5 °C), and high-level warming (+2.5 °C)) and five replications. Infrared radiators were used to maintain the warming treatments continuously. Ecosystem CO2 fluxes (NEP, GEP, ER) were measured using an infrared gas analyzer and a transparent canopy chamber twice monthly during the growing season (May-September) from 2014-2017. Soil temperature was measured concurrently. Aboveground net primary production (ANPP) was measured annually by clipping all living plants in quadrats. Plant height for each species within each quadrat was measured. The community weighted height (CWH) was calculated using the percent ANPP of each functional group (grass, sedges, legumes, forbs) and their corresponding mean heights. **Regional Field Investigation:** A field survey was conducted at 45 sites along a 1500 km transect across the QTP during the peak growing season in 2019. Plant community composition, ANPP, and plant height were measured using the same methods as the warming experiment. Soil samples were collected to measure total C, total nitrogen, and available phosphorus. Mean annual temperature and precipitation data were obtained from the Loess Plateau Scientific Data Center. **Statistical Analysis:**Two-way ANOVA and repeated-measures ANOVAs were used to analyze the effects of warming on plant height and CWH. A surface plot was used to visualize the interactions between soil temperature and CWH on ecosystem carbon fluxes. Rolling regression analysis examined changes in the temperature sensitivities of ecosystem fluxes with CWH. A linear mixed-effect model and partial regressions were used to analyze the effects of environmental variables and CWH on NEP, controlling for confounding factors such as precipitation and soil nutrients.

The warming experiment demonstrated that warming significantly increased plant community weighted height (CWH). This increase was driven by both an increase in the mean height of different functional types and a shift in community composition toward taller species (grasses and sedges). The proportion of grass and sedge increased significantly with increasing soil temperature, while the proportion of forbs and legumes decreased. A positive relationship was found between NEP and CWH, with both changes in height and composition positively correlating with NEP. Taller species exhibited higher chlorophyll content and larger stomata, indicating enhanced photosynthetic capacity. Analysis showed that the temperature sensitivity of NEP, GEP, and ER increased with CWH, implying that taller plant communities exhibited greater sensitivity to temperature changes, fixing more CO2 with a given temperature rise. The transect study supported these findings, showing that CWH was positively associated with NEP and soil total C content, even after controlling for precipitation and soil nutrient status. Across investigation sites, CWH was also positively correlated with community-level chlorophyll content, stomatal size, leaf C content, and leaf area index (LAI), all of which enhanced NEP. The increase in soil total carbon content was not significant during the experimental period, but the study notes a trend consistent with long-term warming experiment and regional investigations indicating increased soil total carbon.

The findings from both the warming experiment and the large-scale field survey strongly support the hypothesis that warming promotes taller plant communities in alpine ecosystems, leading to enhanced ecosystem carbon sequestration. The increased carbon uptake is likely attributed to the higher carbon uptake capacity of taller plant communities and the positive correlation between plant height and traits related to light capture and carbon uptake (LAI, leaf mass per unit area). The observed shifts in community composition, with taller species becoming dominant, further contribute to this effect. The study's findings demonstrate that plant community height is a crucial trait influencing the response of alpine ecosystems to climate warming and its impact on carbon cycling. The study highlights the importance of considering both direct warming effects and indirect impacts through changes in plant community structure and traits when modeling ecosystem carbon fluxes. The increased temperature sensitivity of NEP in taller communities suggests that future warming may lead to substantially increased carbon sequestration in alpine regions if these communities continue to increase in height.

This study provides compelling evidence that warming-induced increases in plant community height enhance ecosystem carbon sequestration in alpine ecosystems. The findings demonstrate the critical role of plant height as a key functional trait mediating the ecosystem response to climate change. The increased temperature sensitivity of carbon uptake in taller communities highlights the importance of incorporating vegetation dynamics and trait-based mechanisms into climate change models to accurately predict future ecosystem carbon fluxes. Further research should investigate the long-term dynamics of soil carbon sequestration in response to these shifts in plant communities and explore other key traits that may influence ecosystem function under climate change.

The warming experiment was conducted over a relatively short time frame (4 years), which may limit the ability to fully assess the long-term impacts of warming on soil carbon sequestration. While the transect study provided a larger spatial scale, potential confounding effects of water and nutrient availability could still influence the observed relationship between CWH and NEP. Although partial regressions addressed these issues, residual influences might still exist. The study focused primarily on the QTP, and further research is needed to determine the generalizability of these findings to other alpine ecosystems with potentially different environmental conditions and plant species.