• Energy Research

The frequency and intensity of droughts have increased over the decades, leading to increased forest decline. The response of forest to drought can be evaluated by both its sensitivity to drought (resistance) and its post-drought recovery rate (resilience). However, it remains uncertain how drought resistance and resilience of forests change over time under climate change. We assessed the spatiotemporal dynamics of forest resistance and resilience to drought over the past century (1901-2015) with global tree ring data records from 2,935 sites, in conjunction with plant trait data. We found that gymnosperms and angiosperms showed different spatial patterns of drought resistance and resilience, driven by variations in eco-physiological traits. Resistance and resilience also varied with drought seasonal timing. Surprisingly, we found that the trade-off between resistance and resilience for gymnosperms, previously reported only spatially, also occurred at the temporal scale. In particular, drought resilience markedly increased, but resistance decreased, for gymnosperms between 1950-1969 and 1990-2009, indicating that previous model simulations assuming invariant resistance may have underestimated the impacts of drought on gymnosperm-dominated forests under future climate change.

AbstractThe global monsoon is characterised by transitions between pronounced dry and wet seasons, affecting food security for two-thirds of the world’s population. Rising atmospheric CO2 influences the terrestrial hydrological cycle through climate-radiative and vegetation-physiological forcings. How these two forcings affect the seasonal intensity and characteristics of monsoonal precipitation and runoff is poorly understood. Here we use four Earth System Models to show that in a CO2-enriched climate, radiative forcing changes drive annual precipitation increases for most monsoon regions. Further, vegetation feedbacks substantially affect annual precipitation in North and South America and Australia monsoon regions. In the dry season, runoff increases over most monsoon regions, due to stomatal closure-driven evapotranspiration reductions and associated atmospheric circulation change. Our results imply that flood risks may amplify in the wet season. However, the lengthening of the monsoon rainfall season and reduced evapotranspiration will shorten the water resources scarcity period for most monsoon regions.

Surface-water availability, defined as precipitation minus evapotranspiration, can be affected by changes in vegetation. These impacts can be local, due to the modification of evapotranspiration and precipitation, or non-local, due to changes in atmospheric moisture transport. However, the teleconnections of vegetation changes on water availability in downwind regions remain poorly constrained by observations. By linking measurements of local precipitation to a new hydrologically weighted leaf area index that accounts for both local and upwind vegetation contributions, we demonstrate that vegetation changes have increased global water availability at a rate of 0.26 mm yr−2 for the 2001–2018 period. Critically, this increase has attenuated about 15% of the recently observed decline in global water availability. The water availability increase is due to a greater rise in precipitation relative to evapotranspiration for over 53% of the global land surface. We also quantify the potential hydrological impacts of regional vegetation increases at any given location across global land areas. We find that enhanced vegetation is beneficial to both local and downwind water availability for ~45% of the land surface, whereas it is adverse elsewhere, primarily in water-limited or high-elevation regions. Our results highlight the potential strong effects of deliberate vegetation changes, such as afforestation programmes, on water resources beyond local and regional scales.

AbstractVegetation autumn phenology is critical in regulating the ecosystem carbon cycle and regional climate. However, the dominant drivers of autumn senescence and their temporal shifts under climate change remain poorly understood. Here, we conducted a multi‐factor analysis considering both direct climatic controls and biological carryover effects from start‐of‐season (SOS) and seasonal peak vegetation activities on the end‐of‐season (EOS) to fill these knowledge gaps. Combining satellite and ground observations across the northern hemisphere, we found that carryover effects from early‐to‐peak vegetation activities exerted greater influence on EOS than the direct climatic controls on nearly half of the vegetated land. Unexpectedly, the carryover effects from SOS on EOS have significantly weakened over recent decades, accompanied by strengthened climatic controls. Such results indicate the weakened constraint of leaf longevity on senescence due to prolonged growing season in response to climate change. These findings underscore the important role of biological carryover effects in regulating vegetation autumn senescence under climate change, which should be incorporated into the formulation and enhancement of phenology modules utilized in land surface models.

AbstractPlant phenology, the annually recurring sequence of plant developmental stages, is important for plant functioning and ecosystem services and their biophysical and biogeochemical feedbacks to the climate system. Plant phenology depends on temperature, and the current rapid climate change has revived interest in understanding and modeling the responses of plant phenology to the warming trend and the consequences thereof for ecosystems. Here, we review recent progresses in plant phenology and its interactions with climate change. Focusing on the start (leaf unfolding) and end (leaf coloring) of plant growing seasons, we show that the recent rapid expansion in ground‐ and remote sensing‐ based phenology data acquisition has been highly beneficial and has supported major advances in plant phenology research. Studies using multiple data sources and methods generally agree on the trends of advanced leaf unfolding and delayed leaf coloring due to climate change, yet these trends appear to have decelerated or even reversed in recent years. Our understanding of the mechanisms underlying the plant phenology responses to climate warming is still limited. The interactions between multiple drivers complicate the modeling and prediction of plant phenology changes. Furthermore, changes in plant phenology have important implications for ecosystem carbon cycles and ecosystem feedbacks to climate, yet the quantification of such impacts remains challenging. We suggest that future studies should primarily focus on using new observation tools to improve the understanding of tropical plant phenology, on improving process‐based phenology modeling, and on the scaling of phenology from species to landscape‐level.

AbstractWhile climate warming reduces the occurrence of frost events, the warming-induced lengthening of the growing season of plants in the Northern Hemisphere may actually induce more frequent frost days during the growing season (GSFDs, days with minimum temperature < 0 °C). Direct evidence of this hypothesis, however, is limited. Here we investigate the change in the number of GSFDs at latitudes greater than 30° N using remotely-sensed and in situ phenological records and three minimum temperature (Tmin) data sets from 1982 to 2012. While decreased GSFDs are found in northern Siberia, the Tibetan Plateau, and northwestern North America (mainly in autumn), ~43% of the hemisphere, especially in Europe, experienced a significant increase in GSFDs between 1982 and 2012 (mainly during spring). Overall, regions with larger increases in growing season length exhibit larger increases in GSFDs. Climate warming thus reduces the total number of frost days per year, but GSFDs nonetheless increase in many areas.