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Montane ecosystems are crucial for maintaining global biodiversity and function that sustain life on our planet. Yet, these ecosystems are highly vulnerable to changing temperatures and may undergo critical transitions under ongoing climate change. What we do not know is to what extent montane biodiversity and ecosystem services will respond to local temperature variations in a gradual versus abrupt manner across global environments. To fill this knowledge gap, we conducted a global synthesis, including 4,462 observations from 290 elevation gradients, to investigate how biodiversity (spanning animals and plants) and ecosystem services (including plant production, soil carbon, and fertility) respond to local temperature variations along elevation gradients. We found that nearly one-third of these gradients exhibited abrupt shifts in multiple biodiversity and ecosystem services in response to local variations in temperature along elevation gradients. More specifically, we showed that once a particular local temperature level (~10 °C for mean annual temperature) was reached, even small increases in temperature resulted in dramatic variations in biodiversity and ecosystem services. We further showed that those abrupt shifts in response to local temperature increases were commonly positive for plant and animal diversity, as well as plant production, while soil carbon and fertility more commonly exhibit negative abrupt trends. Our work, based on the most comprehensive empirical evidence available so far, reveals the pervasive abrupt responses of biodiversity and ecosystem services to local temperature variations in montane ecosystems worldwide, highlighting the highly sensitive nature of montane ecosystems in the context of climate change. 

Societal Impact StatementForest ecosystems absorb and store about 25% of global carbon dioxide emissions annually and are increasingly shaped by human land use and management. Climate change interacts with land use and forest dynamics to influence observed carbon stocks and the strength of the land carbon sink. We show that climate change effects on modeled forest land carbon stocks are strongest in tropical wildlands that have limited human influence. Global forest carbon stocks and carbon sink strength may decline as climate change and anthropogenic influences intensify, with wildland tropical forests, especially in Amazonia, likely being especially vulnerable. SummaryHuman effects on ecosystems date back thousands of years, and anthropogenic biomes—anthromes—broadly incorporate the effects of human population density and land use on ecosystems. Forests are integral to the global carbon cycle, containing large biomass carbon stocks, yet their responses to land use and climate change are uncertain but critical to informing climate change mitigation strategies, ecosystem management, and Earth system modeling.Using an anthromes perspective and the site locations from the Global Forest Carbon (ForC) Database, we compare intensively used, cultured, and wildland forest lands in tropical and extratropical regions. We summarize recent past (1900‐present) patterns of land use intensification, and we use a feedback analysis of Earth system models from the Coupled Model Intercomparison Project Phase 6 to estimate the sensitivity of forest carbon stocks to CO₂and temperature change for different anthromes among regions.Modeled global forest carbon stock responses are positive for CO₂increase but neutral to negative for temperature increase. Across anthromes (intensively used, cultured, and wildland forest areas), modeled forest carbon stock responses of temperate and boreal forests are less variable than those of tropical forests. Tropical wildland forest areas appear especially sensitive to CO₂and temperature change, with the negative temperature response highlighting the potential vulnerability of the globally significant carbon stock in tropical forests.The net effect of anthropogenic activities—including land‐use intensification and environmental change and their interactions with natural forest dynamics—will shape future forest carbon stock changes. These interactive effects will likely be strongest in tropical wildlands. 

Abstract The determinants of fire-driven changes in soil organic carbon (SOC) across broad environmental gradients remains unclear, especially in global drylands. Here we combined datasets and field sampling of fire-manipulation experiments to evaluate where and why fire changes SOC and compared our statistical model to simulations from ecosystem models. Drier ecosystems experienced larger relative changes in SOC than humid ecosystems—in some cases exceeding losses from plant biomass pools—primarily explained by high fire-driven declines in tree biomass inputs in dry ecosystems. Many ecosystem models underestimated the SOC changes in drier ecosystems. Upscaling our statistical model predicted that soils in savannah–grassland regions may have gained 0.64 PgC due to net-declines in burned area over the past approximately two decades. Consequently, ongoing declines in fire frequencies have probably created an extensive carbon sink in the soils of global drylands that may have been underestimated by ecosystem models.