Plant height as an indicator for alpine carbon sequestration and ecosystem response to warming

The study addresses how climate warming-driven changes in plant community structure and traits, particularly plant height, influence ecosystem carbon cycling and sequestration in cold, high-elevation regions. Prior work shows climate warming shifts species composition and alters community-level traits, but the consequences for carbon fluxes remain unclear due to difficulties linking community changes to ecosystem functions via mechanisms. Plant functional traits provide a mechanistic framework to relate vegetation changes to ecosystem processes. Height is a key trait in cold, temperature-limited ecosystems, where warming can increase growth and light competition, favoring taller species and increasing community height. Syntheses across tundra and cold biomes indicate community height increases with warming, mainly via turnover to taller species, and height correlates with carbon acquisition traits. However, empirical evidence connecting warming-induced changes in community composition and height to the direction, magnitude, and temperature sensitivity of ecosystem carbon fluxes is lacking. This study combines a manipulative warming experiment and a 1,500 km transect survey on the Qinghai-Tibet Plateau to link warming, community structure and traits, and NEP, addressing: (1) how warming changes community composition and height; (2) whether these changes amplify or offset direct warming effects on NEP; and (3) how warming-driven changes in community height affect soil carbon.

Previous syntheses and long-term monitoring in cold and high-latitude systems show that community-weighted plant height tends to increase with warming, largely due to shifts toward taller species and canopy closure that intensify light competition (for example, global assessments of tundra warming and multi-decade trait datasets). Height plays a dominant role in light competition and is strongly correlated with traits governing carbon cycling (e.g., chlorophyll content, LAI, leaf mass per area), suggesting that height-associated ecosystem functions, especially carbon cycling, are likely to change rapidly under warming. Despite this, few studies have explicitly linked plant community traits to the climate sensitivity of ecosystem carbon fluxes, leaving uncertainties in predicting ecosystem stability and carbon budgets under climate change.

Warming experiment: Conducted in an alpine meadow on the Eastern Qinghai-Tibetan Plateau (32°48′N, 102°33′E; ~3,500 m a.s.l.) with mean annual precipitation 753 mm (May–September ~80%) and mean annual temperature 1.1 °C. Dominant species included Deschampsia caespitosa, Kobresia setchwanensis, Carex schneideri and Anemone rivularis. Randomized complete block design: 3 treatments (control, low-level warming +1.5 °C, high-level warming +2.5 °C) × 5 blocks; plot size 3 × 2 m; warming via continuously operating infrared radiators (Kalglo MSR-2420) suspended 1.5 m aboveground (1,000 W for low; 2,000 W for high); control plots had dummy heaters. Measurements: During growing seasons (May–Sep) 2014–2017, ecosystem CO2 fluxes were measured twice monthly using a transparent canopy chamber (0.5 × 0.5 × 0.5 m) connected to an LI-6400XT infrared gas analyzer. NEP was derived from the time series of CO2 concentration; ER measured by covering chamber with an opaque cloth; GEP calculated as NEP + ER. Soil temperature (10 cm depth) measured at flux times; precipitation from nearby station. Community composition and traits: ANPP measured annually (mid-August) by clipping within 0.1 × 1 m quadrats; biomass sorted by species and oven-dried. Species heights measured within permanent 0.5 × 0.5 m quadrats subdivided into 25 subquadrats; one healthy individual per species per subquadrat measured; averages computed. Functional groups: grasses, sedges, legumes, forbs. Community composition index (CCI) = (P_grass + P_sedge)/(P_legume + P_forb). Community-weighted height (CWH) computed as sum over functional groups of percent ANPP × mean height. Plant functional traits (community-weighted chlorophyll content, stomatal size, leaf C content) assessed (details in Supplementary Methods). Regional transect: Field survey of 45 sites along a ~1,500 km QTP transect (July–August 2019). Site climate spans mean annual temperature −3.5 to 1.8 °C and precipitation 71.9 to 461.5 mm; average elevation 4,576 m. At each site, ten 0.5 × 0.5 m quadrats were used to assess composition, ANPP, and species heights using the same protocols; soil (0–10 cm) sampled from three quadrats for total C and N (LECO macro-CN) and available P (molybdenum–antimony spectrophotometry). Climate data (1987–2017) obtained via Kriging interpolation from the Loess Plateau Scientific Data Center. Remote-sensing NEP (negative NEE) used without extra validation. Statistical analyses: Two-way ANOVA tested effects of warming treatment, functional type, and interaction on mean plant height; repeated-measures ANOVA tested effects on CWH. Linear regressions related ST to functional-group proportions and to CCI, and related CWH to CCI and ST. Surface plots (R visreg) visualized partial relationships of ST, CWH, and C fluxes. Rolling regressions (R zoo) estimated temperature sensitivities (slopes of ST–flux relationships) along the CWH gradient. Linear mixed-effects models (R nlme) evaluated effects of environmental variables on CWH and NEP (experiment: ST, MAP as fixed; block random. Transect: MAT, MAP, soil total N, available P fixed; site random). Partial residuals/regressions tested specific partial relationships (e.g., NEP–CWH controlling for covariates). Analysis of covariance compared slopes. Additional models (structural equation, ridge regression) assessed drivers while accounting for collinearity (Supplementary).

- Warming increased growing-season soil temperature by 1.41 °C (low) and 2.44 °C (high) on average and significantly increased community-weighted height (CWH). Both increases in mean height within functional groups and shifts in composition toward taller groups contributed to higher CWH. - Functional-group responses: With increasing soil temperature, proportions of grasses and sedges increased while forbs and legumes decreased; the community composition index (CCI) rose with soil temperature. Mean heights (± s.d.) by group: grasses 36.82 ± 8.52 cm, sedges 29.76 ± 7.82 cm, legumes 20.43 ± 6.10 cm, forbs 19.28 ± 3.72 cm. CWH was positively related to CCI (R² = 0.37, P = 2.56 × 10⁻⁷) and to soil temperature (R² = 0.294, P = 7.58 × 10⁻⁶). - Ecosystem carbon uptake: NEP increased with CWH in the warming experiment; both changes in height and composition were positively correlated with NEP. Taller species exhibited higher chlorophyll content and larger stomata; at the community level, higher CWH was associated with higher chlorophyll content, larger stomata, and higher leaf C content, traits that positively related to NEP. - Temperature sensitivity: Partial response surfaces and rolling regressions showed that temperature sensitivities of NEP, GEP, and ER increased with CWH. Reported relationships of sensitivity vs. CWH: NEP R² = 0.201, P = 0.0114; GEP R² = 0.936, P = 7.06 × 10⁻⁹; ER R² = 0.974, P = 1.28 × 10⁻²⁴. Slopes for GEP and ER were not significantly different, indicating a stable trade-off while sensitivities increased. - Regional transect (45 sites): After controlling for precipitation, soil N, available P, and site effects, CWH increased with mean annual temperature (R² = 0.183, P = 0.0034). NEP increased with CWH after controlling for MAT, MAP, soil N, available P, and site (R² = 0.136, P = 0.0127). CWH was positively related to community-weighted chlorophyll content (R² = 0.212, P = 0.0014), stomatal size (R² = 0.24, P = 6.36 × 10⁻⁴), and LAI (R² = 0.147, P = 0.0094); leaf C content showed a positive but marginal relationship (R² = 0.069, P = 0.0823). Soil total C increased with CWH and NEP across sites (Supplementary). - Soil carbon: In the experiment, soil total C showed a modest increasing trend under high-level warming (+0.25%) over the study period (P > 0.05), consistent with modeling and transect evidence indicating higher soil C with taller communities and higher NEP. - ANPP: No significant warming effect on ANPP in the experiment (P = 0.061) or in the transect (P = 0.423); nevertheless, NEP and soil C remained positively related to CWH after controlling for ANPP, and mass-specific C uptake increased with CWH.

The findings demonstrate that warming-induced shifts toward taller plant communities enhance ecosystem carbon uptake in cold, high-elevation meadows. Taller communities exhibit trait syndromes (higher chlorophyll content, larger stomata, higher LAI) that increase photosynthetic capacity, explaining stronger NEP with increasing community height. Community restructuring, quantified by a higher CCI (more grasses and sedges, fewer forbs and legumes), raises CWH and contributes comparably to NEP as the direct thermal effect, thereby amplifying warming-driven CO2 uptake. Height also modulates the temperature sensitivity of carbon fluxes: ecosystems with taller communities display greater sensitivities of GEP and ER to temperature without altering their balance, leading to higher NEP sensitivity and greater CO2 sequestration for a given warming increment. Regionally, across the QTP, taller communities at warmer sites, independent of water and nutrient status, are associated with higher NEP and soil C, supporting the generality of the trait-based linkage between community structure and ecosystem carbon sequestration. These results underscore the importance of incorporating vegetation trait dynamics—particularly community height—into models to more accurately predict carbon flux responses and feedbacks to climate warming in temperature-limited biomes.

By integrating a manipulative warming experiment with a 1,500 km regional survey on the Qinghai-Tibet Plateau, the study shows that climate warming promotes plant community height and that taller communities enhance net ecosystem productivity and likely soil carbon sequestration in this cold, high-elevation region. Changes in community height also increase the temperature sensitivity of ecosystem CO2 uptake, implying that taller-canopy ecosystems will sequester more carbon for the same temperature increase. Plant community height emerges as a key, mechanistic trait linking climate warming, community composition, plant functional traits, and ecosystem carbon balance. Future research and modeling should explicitly account for vegetation dynamics and associated trait changes to improve projections of ecosystem carbon cycling and climate feedbacks.

- The warming experiment did not explicitly quantify intraspecific variation in traits (e.g., chlorophyll content, stomatal size, leaf C) under warming, focusing instead on community-weighted values. - NEP in the transect was derived from remote sensing (negative NEE) without additional site-specific validation, potentially introducing uncertainty. - Potential confounding effects of water and nutrients in the transect could bias NEP–height relationships; partial regressions and mixed models were used to control for these, but residual confounding cannot be fully excluded. - Soil carbon responses integrate multiple opposing processes (inputs vs. microbial decomposition); during the experimental period, soil C increases were modest and not statistically significant, and mechanisms remain unresolved. - ANPP did not significantly change with warming, suggesting compensatory dynamics among functional groups; longer-term observations may be needed to resolve biomass vs. efficiency contributions. - The affiliation for one author’s superscript (9) was not provided in the text segment, limiting complete reporting of affiliations.