Search

Aim: Ecosystem carbon use efficiency (CUEe) is a core parameter of ecosystem process models, but its relationships with climate are still uncertain, especially for ecosystems with harsh environments. Large inconsistencies in climate impacts on the CUEe have been reported among various spatial scales. The goal of this study was to examine whether warming promotes or restricts the CUEe and whether the CUEe responds to a warming gradient in a linear or nonlinear manner. Location: Tibetan Plateau. Time period: 2000–2018. Major taxa studied: Alpine grassland ecosystem. Methods: We integrated multiple‐source data of carbon fluxes and CUEe, including warming experiments at a site scale, eddy covariance observations at a landscape scale and synthesized warming experiments and ecosystem process models at a regional scale. Next, we deployed a statistical model to examine the warming impacts on the CUEe across scales; the effects of biotic and abiotic factors on the CUEe and its components were summarized based on the results of standardized major axis tests and routines, structural equation modelling and nonlinear models. Results: This study reported a suppressive warming impact on the CUEe, which followed a nonlinear curve with severe inhibition in the high‐level warming treatment. With a warming threshold of 1.5–2.0°C, CUEe response patterns transitioned from no change to a significant decrease. The restriction effects can be ascribed to the joint adverse and asymmetric effects of warming on CUEe components under multiple‐level warming. Warming‐modified relationships among CUEe components and the nonlinear effects of biotic and abiotic factors led to the nonlinear responses of CUEe to warming. Main conclusions: This study revealed suppressive and nonlinear effects of warming on the CUEe, including especially dramatic CUEe decreases with high‐level warming. These findings are critical for optimizing model parameters and improving predictions of the carbon sequestration capacity of alpine grasslands. [ABSTRACT FROM AUTHOR]

Effects of climate warming and changing precipitation on ecosystem carbon fluxes have been intensively studied. However, how they co‐regulate carbon fluxes is still elusive for some understudied ecosystems. To fill the gap, we examined net ecosystem productivity (NEP), gross ecosystem productivity (GEP,) and ecosystem respiration (ER) responses to multilevel of temperature increments (control, warming 1, warming 2, warming 3, warming 4) in three contrasting hydrological growing seasons in a typical semiarid alpine meadow. We found that carbon fluxes responded to precipitation variations more strongly in low‐level warming treatments than in high‐level ones. The distinct responses were attributable to different soil water conditions and community composition under low‐level and high‐level warming during the three growing seasons. In addition, carbon fluxes were much more sensitive to decreased than to increased precipitation in low‐level warming treatments, but not in high‐level ones. At a regional scale, this negative asymmetry was further corroborated. This study reveals that future precipitation changes, particularly decreased precipitation would induce significant change in carbon fluxes, and the effect magnitude is regulated by climate warming size. [ABSTRACT FROM AUTHOR]

• Alpine grasslands on the tibetan plateau possess low resistance to drought;. • The alpine ecosystems exhibit strong post-drought regrowth;. • Post-drought regrowth compensates for the prior loss to a certain extent;. • Observations across varied spatial scales turned out consistent conclusions. One-third of the global fossil fuel CO 2 emissions is offset by carbon uptake of terrestrial ecosystems, while its strength is highly sensitive to drought events. It is predicted that frequencies of drought events would increase under a changing climate, which entails improving our understanding about their effects. Here, we combined direct observations at plot (experiment sites) and landscape (eddy-covariance, EC) scales with remote sensing observations at a regional scale, and evaluated the linkages between ecological resistance (summer drought loss, SDL) and resilience (post-drought regrowth, PDR). The study was conducted for an alpine grassland ecosystem on the Tibetan Plateau, which is highly vulnerable to climate changes. The results showed that alpine grasslands possess low resistance to drought. A summer drought of 2015 reduced net primary productivity (NPP) or net ecosystem productivity (NEP) by 25.4 g C m −2, 48.6 g C m −2 and 14.2 Tg C at the plot, landscape and regional scale relative to the baseline, respectively. In another dry summer of 2017, NEP was 11.0 g C m −2 and 7.5 g C m −2 lower than the baseline at the landscape and plot scale, respectively. To be noted, NEP or NPP completely recovered and exceeded the baseline due to rewetting induced PDR, compensating for the prior SDL to a certain extent. In 2015, the SDL of NEP or NPP was compensated by 39.0%, 17.3% and 10.6% due to the PDR effects at the plot, landscape and regional scale, respectively. The PDR of NEP in 2017 offset 23.6% and 70.7% of the prior SDL at the landscape and plot scale, respectively. Overall, these results demonstrated that weakened ecosystem function due to drought (e.g., low resistance) does not preclude rapid ecosystem recovery and regrowth (e.g., high resilience), which compensates for the prior loss to a certain degree. [ABSTRACT FROM AUTHOR]

Climate warming is predicted to considerably affect variations in soil organic carbon (SOC), especially in alpine ecosystems. Microbial necromass carbon (MNC) is an important contributor to stable soil organic carbon pools. However, accumulation and persistence of soil MNC across a gradient of warming are still poorly understood. An 8‐year field experiment with four levels of warming was conducted in a Tibetan meadow. We found that low‐level (+0–1.5°C) warming mostly enhanced bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and total MNC compared with control treatment across soil layers, while no significant effect was caused between high‐level (+1.5–2.5°C) treatments and control treatments. The contributions of both MNC and BNC to soil organic carbon were not significantly affected by warming treatments across depths. Structural equation modeling analysis demonstrated that the effect of plant root traits on MNC persistence strengthened with warming intensity, while the influence of microbial community characteristics waned along strengthened warming. Overall, our study provides novel evidence that the major determinants of MNC production and stabilization may vary with warming magnitude in alpine meadows. This finding is critical for updating our knowledge on soil carbon storage in response to climate warming. [ABSTRACT FROM AUTHOR]