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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]

Aim Carbon use efficiency [net primary production ( NPP)/gross primary production ( GPP) ratio] is a parameter related to the allocation of photosynthesized products by plants and is commonly used in many biogeochemical cycling models. But how this parameter changes with climates is still unknown. Faced by an aggravated global warming, there is a heightened necessity in unravelling the dependence of the NPP/ GPP ratio on climates. The objective of this study was to examine how ongoing climate change is regulating global patterns of change in the NPP/ GPP ratio. The study finding would elucidate whether the global vegetation ecosystem is becoming more or less efficient in terms of carbon storage under climatic fluctuation. Location The global planetary ecosystem. Methods The annual NPP/ GPP ratio of the global terrestrial ecosystem was calculated over a 10-year period based on Moderate Resolution Imaging Spectroradiometer data and an ecosystem productivity model. The temporal dynamics of the global NPP/ GPP ratio and their dependence on climate were investigated. Results The global NPP/ GPP ratio exhibited a decreasing trend from 2000 to 2009 due to decreasing NPP and stable GPP over this period. The temporal dynamics of the NPP/ GPP ratio were strongly controlled by temperature and precipitation. Increased temperature lowered the NPP/ GPP ratio, and increased precipitation led to a higher NPP/ GPP ratio. Conclusions The NPP/ GPP ratio exhibits a clear temporal pattern associated with climatic fluctuations at a global scale. The associations of the NPP/ GPP ratio with climatic variability challenge the conventional assumption that the NPP/ GPP ratio should be consistent independent of environmental conditions. More importantly, the findings of this study have fundamental significance for our understanding of ongoing global climatic change. In regions and time periods experiencing drought or increased temperatures, plant ecosystems would suffer a higher ecosystem respiration cost and their net productivity would shrink. [ABSTRACT FROM AUTHOR]

Aim To test two predictions of the phylogenetic niche conservatism hypothesis for the latitudinal diversity gradient: (1) species in colder regions tend to be more phylogenetically related to each other (i.e. greater phylogenetic clustering) than those in warmer regions, and (2) clades are younger in colder regions. Location North America. Methods Correlation analysis was used to relate richness, mean clade age and phylogenetic relatedness (measured as phylogenetic species variability and net relatedness index) of angiosperm trees in 1175 regional assemblages (each with 12100 km2) to latitude and minimum temperature. The analysis was conducted for North America north of Mexico as a whole as well as for each of the three longitudinal bands (eastern, central and western) of North America. Results Species richness and mean clade age are negatively correlated with latitude and positively correlated with minimum temperature. Tree species in regional assemblages tend to be more phylogenetically related (clustered) in regions at higher latitudes with lower temperatures. Main conclusions The results of this study support two of the major predictions of the phylogenetic niche conservatism hypothesis for the latitudinal diversity gradient: species tend to be more phylogenetically clustered and ages of clades tend to be younger in colder regions, compared with those in warmer regions. [ABSTRACT FROM AUTHOR]

Aim To understand: (1) how temperature, precipitation and stand age control the above-ground biomass carbon density ( BCDa) of mature forests and its macroecology patterns across latitudes; (2) the age threshold for old-growth forests at a global scale. Location Global forests. Methods We compiled a database (897 sites) of mature forests between 80 and 1200 years old. The site data include latitude, longitude, mean annual temperature, mean annual precipitation, forest type, stand age, BCDa, living biomass (above- and below-ground biomass) carbon density and total (living plus dead) biomass carbon density. Based on the site data, we performed regression analyses to show how BCDa changes with climate and forest stand age. Results At a global scale, the highest BCDa of mature forests occurred mainly in the mid-latitude regions where mean annual temperatures were 8-10 °C and mean annual precipitation was between 1000 and 2500 mm. The average BCDa of forests in the stand age class of 450-500 years was higher than those in the other stand age classes. For forests between 80 and 450 years old, which form the majority of mature forests, carbon accumulation was faster in dead biomass than in living biomass. Main conclusions The highest BCDa of mature forests is located in mid-latitude regions with cool temperatures and moderate precipitation. The age threshold for old-growth forests at a global scale should be 450-500 years, which is much older than the previously documented age of 100-200 years. This older age threshold for old-growth forests is probably one of the primary reasons why recent works have concluded that old-growth forests are still carbon sinks. [ABSTRACT FROM AUTHOR]