Multiple‐scale negative impacts of warming on ecosystem carbon use efficiency across the Tibetan Plateau grasslands

Abstract

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.

Precipitation manipulation and terrestrial carbon cycling: The roles of treatment magnitude, experimental duration and local climate

“…We selected terrestrial C cycling variables from published studies and grouped these variables into six categories (Supporting Information Table S1): (1) ecosystem C fluxes [gross ecosystem productivity (GEP), ecosystem respiration (ER) and net ecosystem productivity (NEP)], which are typically used to evaluate whether an ecosystem is a C sink or source (Quan et al., 2019); (2) ecosystem C use efficiency (CUE), defined as the ratio of NEP to GEP, which is usually applied to assess the capacity of the ecosystem for C sequestration and is a crucial parameter in ecosystem process models (Chen, Zhang, et al., 2021); (3) net primary productivity (NPP), representing the net C sequestered by plants (i.e., assimilated C minus C released by autotropic respiration) (Roxburgh et al., 2005), which includes ANPP and BNPP; (4) biomass, including aboveground biomass (AGB) and root biomass (BGB); (5) shoot‐to‐root ratio (S/R), calculated as AGB/BGB or ANPP/BNPP, which is widely used to assess changes in the allocation of C in plant biomass or C allocation in response to climate change (Song et al., 2019); (6) soil respiration (Rs) and soil microbial biomass C (MBC), which are two crucial variables for assessing soil C losses and substrate availability for microbial respiration, respectively.…”

Interannual characteristics and driving mechanism of CO<sub>2</sub> fluxes in alpine shrubland ecosystem during growing season at the southern foot of Qilian Mountains

“…此外, 温度升高会导致土壤水分蒸发的增加, 加剧植被 受干旱胁迫的程度, 导致植被生产力降低 [11] . 这些相互 对立的生物代谢过程之间的平衡决定了高寒生态系统 对气候环境的反馈作用, 气候变化对青藏高原不同植 被类型生态系统源/汇动态的影响目前还不十分明 确 [12] . 研究表明, 高寒生态系统的CO 2 通量主要受生长季 碳平衡动态的控制 [7,9] .…”

“…. 在植被生长旺盛期, 植被的自养呼吸强烈, 可能 导致在这一时期植被自养呼吸对生态系统呼吸的贡献 率更大, 所以使得在生长季旺盛期高寒灌丛的月Re主 要受GDD的影响 [6,7] , 而在植被非生长季旺盛期, 植被 的自养呼吸较弱, 高寒灌丛生态系统呼吸可能更依赖 于土壤呼吸, 导致在非生长季旺盛期的月Re主要受T s 影响 [12,14,19,21] .…”

“…线性回归分析表明, 2003~2016年高 寒灌丛在生长季的月NEE与月GPP和月Re都呈极显著 负相关(P<0.001)(图3(b), (c)). 并且, 与月Re(r 2 =0.47)相 比较, 月GPP(r 2 =0.88)对月NEE的控制作用更强, 这与 前人的研究结果相似 [12,15,18] , 在生长初期、末期植被光合生产能 力较弱, 而青藏高原高寒草地土壤中含有大量未分解 的有机质, 具有较强的土壤呼吸, 导致这个时期Re对 NEE的控制作用更强; 而在植被生长-旺盛期, 雨热同 期的条件使得植被具有较高水平的光合生产能力 [21,25] , 致使GPP对NEE的控制作用更强. 季的光合生产能力和碳汇强度.…”

Interannual characteristics and driving mechanism of CO2 fluxes during the growing season in an alpine wetland ecosystem at the southern foot of the Qilian Mountains

“…Global climate change is predicted to have a substantial influence on the stability of the grassland ecosystem, an ecosystem whose vegetation is dominated by grasses ( Piao et al, 2012 ; Chai et al., 2019 ; Chen et al., 2021a ). The carbon process of the alpine grassland ecosystem is extremely sensitive to and complex in the face of continuous global warming ( Shen et al., 2015 ; Wang et al., 2017 ).…”