Urban warming advances spring phenology but reduces the response of phenology to temperature in the conterminous United States.

Urbanization has caused environmental changes, such as urban heat islands (UHIs), that affect terrestrial ecosystems. However, how and to what extent urbanization affects plant phenology remains relatively unexplored. Here, we investigated the changes in the satellite-derived start of season (SOS) and the covariation between SOS and temperature (RT ) in 85 large cities across the conterminous United States for the period 2001-2014. We found that 1) the SOS came significantly earlier (6.1 ± 6.3 d) in 74 cities and RT was significantly weaker (0.03 ± 0.07) in 43 cities when compared with their surrounding rural areas (P < 0.05); 2) the decreased magnitude in RT mainly occurred in cities in relatively cold regions with an annual mean temperature <17.3 °C (e.g., Minnesota, Michigan, and Pennsylvania); and 3) the magnitude of urban-rural difference in both SOS and RT was primarily correlated with the intensity of UHI. Simulations of two phenology models further suggested that more and faster heat accumulation contributed to the earlier SOS, while a decrease in required chilling led to a decline in RT magnitude in urban areas. These findings provide observational evidence of a reduced covariation between temperature and SOS in major US cities, implying the response of spring phenology to warming conditions in nonurban environments may decline in the warming future.

Warming enhances dominance of vascular plants over cryptogams across northern wetlands.

Climate warming causes profound effects on structure and function of wetland ecosystem, thus affecting regional and global hydrological cycles and carbon budgets. However, how wetland plants respond to warming is still poorly understood. Here, we synthesized observations from 273 independent sites to explore responses of northern wetland plants to warming. Our results show that warming enhances biomass accumulation for vascular plants including shrubs and graminoids, whereas it reduces biomass accumulation for cryptogams including moss and lichen. This divergent response of vascular plants and cryptogams is particularly pronounced in the high latitudes where permafrost prevails. As warming continues, this divergent response is amplified, however, the reduction in cryptogams is more drastic. Warming leads to declined surface soil moisture and lowered water table, thereby shifting wetlands from a wet system dominated by cryptogams to a drier system with increased cover of vascular plants. Under a high-emission scenario of Shared Socioeconomic Pathways (SSP5), a 4.7-5.1°C mean global temperature rise will cause more than fivefold loss of cryptogams compared with current climate. As cryptogams are largely concentrated at northern high latitudes, where warming will likely be greater than the projected global mean, modification in wetland plant composition and major reduction in cryptogams are expected to occur even much earlier than 2100.

Scale matters in understanding the complexity of Amazon fires: A response to the Editor.

The Amazon Basin is experiencing climate change, altered hydrological cycles, and forest loss. The processes causing increased fires are complex, and therefore cannot be attributed to climate change or human-induced deforestation alone. Here, we show why the Amazon fires must be understood across spatial scales within the regional coupled system.

Much stronger tundra methane emissions during autumn freeze than spring thaw.

Warming in the Arctic has been more apparent in the non-growing season than in the typical growing season. In this context, methane (CH4 ) emissions in the non-growing season, particularly in the shoulder seasons, account for a substantial proportion of the annual budget. However, CH4 emissions in spring and autumn shoulders are often underestimated by land models and measurements due to limited data availability and unknown mechanisms. This study investigates CH4 emissions during spring thaw and autumn freeze using eddy covariance CH4 measurements from three Arctic sites with multi-year observations. We find that the shoulder seasons contribute to about a quarter (25.6 ± 2.3%, mean ± SD) of annual total CH4 emissions. Our study highlights the three to four times higher contribution of autumn freeze CH4 emission to total annual emission than that of spring thaw. Autumn freeze exhibits significantly higher CH4 flux (0.88 ± 0.03 mg m-2  hr-1 ) than spring thaw (0.48 ± 0.04 mg m-2  hr-1 ). The mean duration of autumn freeze (58.94 ± 26.39 days) is significantly longer than that of spring thaw (20.94 ± 7.79 days), which predominates the much higher cumulative CH4 emission during autumn freeze (1,212.31 ± 280.39 mg m-2  year-1 ) than that during spring thaw (307.39 ± 46.11 mg m-2  year-1 ). Near-surface soil temperatures cannot completely reflect the freeze-thaw processes in deeper soil layers and appears to have a hysteresis effect on CH4 emissions from early spring thaw to late autumn freeze. Therefore, it is necessary to consider commonalities and differences in CH4 emissions during spring thaw versus autumn freeze to accurately estimate CH4 source from tundra ecosystems for evaluating carbon-climate feedback in Arctic.

Wetland Heterogeneity Determines Methane Emissions: A Pan-Arctic Synthesis.

Methane (CH4) emissions from pan-Arctic wetlands provide a potential positive feedback to global warming. However, the differences in CH4 emissions across wetland types in these regions have not been well understood. We synthesized approximately 9000 static chamber CH4 measurements during the growing season from 83 sites across pan-Arctic regions. We highlighted spatial variations of CH4 emissions corresponding to environmental heterogeneity across wetland types. CH4 emission is the highest in fens, followed by marshes, bogs, and the lowest in swamps. This gradient is controlled by the water table, soil temperature, and dominant plant functional types and their interactions. The water table position for maximum CH4 emission is below, close to, and above the ground surface in bogs, marshes/fens, and swamps, respectively. The temperature sensitivity (Q10) of CH4 emissions varied among different wetland types, ranging from the lowest in swamps to the highest in fens. The interactive impact of temperature and the water table positions on CH4 emissions are regulated with dominant plant functional types. CH4 emissions from wetlands dominated by vascular plants rely more on species composition than that dominated by non-vascular plants. Wetlands with greater abundance of graminoids (e.g., fens) have higher CH4 emissions than tree-dominated wetlands (e.g., swamps). This synthesis emphasizes the role of wetland heterogeneity in determining the strength of CH4 emissions.

Climate regime shift and forest loss amplify fire in Amazonian forests.

Frequent Amazonian fires over the last decade have raised the alarm about the fate of the Earth's most biodiverse forest. The increased fire frequency has been attributed to altered hydrological cycles. However, observations over the past few decades have demonstrated hydrological changes that may have opposing impacts on fire, including higher basin-wide precipitation and increased drought frequency and severity. Here, we use multiple satellite observations and climate reanalysis datasets to demonstrate compelling evidence of increased fire susceptibility in response to climate regime shifts across Amazonia. We show that accumulated forest loss since 2000 warmed and dried the lower atmosphere, which reduced moisture recycling and resulted in increased drought extent and severity, and subsequent fire. Extremely dry and wet events accompanied with hot days have been more frequent in Amazonia due to climate shift and forest loss. Simultaneously, intensified water vapor transport from the tropical Pacific and Atlantic increased high-altitude atmospheric humidity and heavy rainfall events, but those events did not alleviate severe and long-lasting droughts. Amazonia fire risk is most significant in the southeastern region where tropical savannas undergo long seasonally dry periods. We also find that fires have been expanding through the wet-dry transition season and northward to savanna-forest transition and tropical seasonal forest regions in response to increased forest loss at the "Arc of Deforestation." Tropical forests, which have adapted to historically moist conditions, are less resilient and easily tip into an alternative state. Our results imply forest conservation and fire protection options to reduce the stress from positive feedback between forest loss, climate change, and fire.

Climate change-induced vegetation shifts lead to more ecological droughts despite projected rainfall increases in many global temperate drylands.

Drylands occur worldwide and are particularly vulnerable to climate change because dryland ecosystems depend directly on soil water availability that may become increasingly limited as temperatures rise. Climate change will both directly impact soil water availability and change plant biomass, with resulting indirect feedbacks on soil moisture. Thus, the net impact of direct and indirect climate change effects on soil moisture requires better understanding. We used the ecohydrological simulation model SOILWAT at sites from temperate dryland ecosystems around the globe to disentangle the contributions of direct climate change effects and of additional indirect, climate change-induced changes in vegetation on soil water availability. We simulated current and future climate conditions projected by 16 GCMs under RCP 4.5 and RCP 8.5 for the end of the century. We determined shifts in water availability due to climate change alone and due to combined changes of climate and the growth form and biomass of vegetation. Vegetation change will mostly exacerbate low soil water availability in regions already expected to suffer from negative direct impacts of climate change (with the two RCP scenarios giving us qualitatively similar effects). By contrast, in regions that will likely experience increased water availability due to climate change alone, vegetation changes will counteract these increases due to increased water losses by interception. In only a small minority of locations, climate change-induced vegetation changes may lead to a net increase in water availability. These results suggest that changes in vegetation in response to climate change may exacerbate drought conditions and may dampen the effects of increased precipitation, that is, leading to more ecological droughts despite higher precipitation in some regions. Our results underscore the value of considering indirect effects of climate change on vegetation when assessing future soil moisture conditions in water-limited ecosystems.

Accelerated rise in wildfire carbon emissions from Arctic continuous permafrost.

Wildfires over permafrost put perennially frozen carbon at risk. However, wildfire emissions from biomass burning over the diverse range of permafrost regions and their share in global wildfire emissions have not been revealed. The results showed a dramatic increase in wildfire carbon emissions from permafrost regions over the period 1997-2021. The share of permafrost in global wildfire CO2 emissions increased from 2.42% in 1997 to 20.86% in 2021. Accelerating wildfire emissions from continuous permafrost region is the single largest contributor to increased emissions in northern permafrost regions. Fire-induced emissions from 2019 to 2021 alone accounted for approximately 40% of the 25-year total CO2 emissions from continuous permafrost regions. The rise in wildfire emissions from continuous permafrost regions is explained by desiccation within a 5-10 cm soil depth, where wildfires combust belowground fuel. These findings highlight the acceleration of fire-induced carbon emissions from continuous permafrost regions, which disturb the organic carbon stock and accelerate the positive feedback between permafrost degradation and climate warming, thus stimulating permafrost towards a climatic tipping point.

Divergent effects of intensified precipitation on primary production in global drylands.

Amplification of hydrological cycle under warming climate is anticipated to result in intensified precipitation characterized by fewer, more intense events and correspondingly longer dry intervals between events, even without major changes in annual total precipitation. Vegetation gross primary production (GPP) in drylands is highly responsive to intensified precipitation, however, how intensified precipitation influences GPP in global drylands is not well understood. Based on multiple satellite datasets from 2001 to 2020 and in-situ measurements, we investigated the effects of intensified precipitation on global drylands GPP under diverse annual total precipitation along the bioclimate gradient. Dry, normal, and wet years were identified as years with annual precipitation anomalies below, within, and above the range of one standard deviation. Intensified precipitation led to increases or decreases of GPP during dry or normal years, respectively. However, such effects were largely weakened during wet years. The responses of GPP to intensified precipitation were mirrored by soil water availability, as intensified precipitation enhanced root zone soil moisture, and thus vegetation transpiration and precipitation use efficiency during dry years. During wet years, root zone soil moisture showed less response to changed precipitation intensity. Land cover types and soil texture regulated the magnitude of the effects along the bioclimate gradient. Under intensified precipitation, shrubland and grassland distributed in drier region with coarse soil texture showed greater increases of GPP during dry years. As global precipitation will likely further intensify, the impacts of intensified precipitation on dryland carbon uptake capacity will be highly diverse along the bioclimate gradients.

Delayed Antarctic melt season reduces albedo feedback.

Antarctica's response to climate change varies greatly both spatially and temporally. Surface melting impacts mass balance and also lowers surface albedo. We use a 43-year record (from 1978 to 2020) of Antarctic snow melt seasons from space-borne microwave radiometers with a machine-learning algorithm to show that both the onset and the end of the melt season are being delayed. Granger-causality analysis shows that melt end is delayed due to increased heat flux from the ocean to the atmosphere at minimum sea-ice extent from warming oceans. Melt onset is Granger-caused primarily by the turbulent heat flux from ocean to atmosphere that is in turn driven by sea-ice variability. Delayed snowmelt season leads to a net decrease in the absorption of solar irradiance, as a delayed summer means that higher albedo occurs after the period of maximum solar radiation, which changes Antarctica's radiation balance more than sea-ice cover.

Protected areas provide thermal buffer against climate change.

Climate change is pushing temperatures beyond the thermal tolerance of many species. Whether protected areas (PAs) can serve as climate change refugia for biodiversity has not yet been explored. We find that PAs of natural (seminatural) vegetation effectively cool the land surface temperature, particularly the daily maximum temperature in the tropics, and reduce diurnal and seasonal temperature ranges in boreal and temperate regions, as compared to nonprotected areas that are often disturbed or converted to various land uses. Moreover, protected forests slow the rate of warming more at higher latitudes. The warming rate in protected boreal forests is up to 20% lower than in their surroundings, which is particularly important for species in the boreal where warming is more pronounced. The fact that nonprotected areas with the same type of vegetation as PAs show reduced warming buffer capacity highlights the importance of conservation to stabilize the local climate and safeguard biodiversity.

Irrigation plays significantly different roles in influencing hydrological processes in two breadbasket regions.

Agriculture is a major water user, especially in dry and drought-prone areas that rely on irrigation to support agricultural production. In recent years, the over-extraction of groundwater, exacerbated by climate change, population growth, and intensive agricultural irrigation, has led to a drop in water levels and influenced the hydrological cycle. Understanding changes in hydrological processes is essential for pursuing water sustainability. This study aims to estimate the amount and impact of irrigation on hydrological processes in two breadbasket regions, Jing-Jin-Ji (JJJ), China, and northern Texas (NTX), US. We used the Soil and Water Assessment Tool (SWAT) to explore spatiotemporal variations of irrigation from 2008 to 2013 and compared changes in hydrological processes caused by irrigation. The results indicated that deficit irrigation is more common in JJJ than in NTX and can reduce approximately 50 % of irrigation water use in areas with intensively irrigated cropland. The applied irrigation varies less over time in NTX but fluctuates in JJJ. Compared with NTX, the higher irrigation intensity in JJJ results in a more significant change in downstream peak streamflow of around 6 m3/s. Moreover, the difference in crop growing seasons can lead to different impacts of irrigation on hydrological processes. For example, the percentage change of surface runoff under real-world relative to the no-irrigation scenario was the greatest, around 40 %, in JJJ and NTX. However, the peak change occurred at different times, with the nearing maturity of winter wheat in May in JJJ and corn in August in NTX. The great potential to reduce groundwater extraction by adopting water conservation irrigation techniques calls for policies and regulations to help farmers shift towards more sustainable water management practices.

Visual range trends in the Yangtze River Delta Region of China, 1981-2005.

Visual range (VR) data from 1981 to 2005 were examined for 20 meteorological monitoring sites in the Yangtze River Delta Region of China. Cumulative percentile analysis was used to construct VR trend. The 25-yr average domain-average 50% VR was approximately 21.9 +/- 1.9 km. Domain-average 50% VR decreased from 1981 to 2005 with a trend of -2.41 km/decade. The worst 20% and 50% and best 20% VR and variation rates for the 20 sites were analyzed. The 50% VR of the town, county-level city, and prefecture-level city sites were 24.1, 21.5, and 19.4 km, respectively. The best 20% VR decreased fastest with a rate of -3.5 km/decade. Regional median VR decreased from the coastal sites to the inland sites. Ridit analysis and cumulative percentile were adopted to study the VR variation properties between economically developed areas (e.g., Nanjing and Hangzhou) and remote areas (e.g., Lvsi). The two analyses show that VR decreased in Nanjing and Hangzhou but remained constant in Lvsi from 1981 to 2005.

Vegetation greening in the Canadian Arctic related to decadal warming.

This study is presented within the context that climate warming and sea-ice decline has been occurring throughout much of the Arctic over the past several decades, and that terrestrial ecosystems at high latitudes are sensitive to the resultant alterations in surface temperatures. Results are from analyzing interannual satellite records of vegetation greenness across a bioclimate gradient of the Canadian Arctic over the period of 1982-2006. Here, we combine multi-scale sub-pixel analysis and remote sensing time-series analysis to investigate recent decadal changes in vegetation greenness along spatial gradients of summer temperature and vegetation. Linear autoregression temporal analysis of vegetation greenness was performed with relatively "pure" vegetation pixels of Advanced Very High Resolution Radiometer (AVHRR) data, spanning Low Arctic, High Arctic and polar desert ecosystems. Vegetation greenness generally increased over tundra ecosystems in the past two decades. Peak annual greenness increased 0.49-0.79%/yr over the High Arctic where prostrate dwarf shrubs, forbs, mosses and lichens dominate and 0.46-0.67%/yr over the Low Arctic where erect dwarf shrubs and graminoids dominate. However, magnitudes of vegetation greenness differ with length of time series and periods considered, indicating a nonlinear response of terrestrial ecosystems to climate change. The decadal increases of greenness reflect increasing vegetation production during the peak of the growing season, and were likely driven by the recent warming.

Seasonal and interannual variations in carbon fluxes in East Asia semi-arid grasslands.

Semiarid regions have substantial interannual variation in carbon exchange between terrestrial ecosystem and atmosphere but the diverse responses of carbon fluxes to climate variability are poorly known. We compared carbon exchange processes and the responses to environmental factors in a meadow steppe at Tongyu (TY) and a typical steppe at Maodeng (MD) using long-term continuous eddy covariance measurements. TY precipitation was 25% greater than MD. Both grasslands had interannual fluctuations of carbon sink and source and acted as weak carbon sinks averagely (-22.9 ±â€¯41.0 gCm-2 yr-1 for TY and - 11.8 ±â€¯45.0 gCm-2 yr-1 for MD). The seasonal dynamics of carbon fluxes were significantly related to water availability at MD but more strongly related to air temperature at TY. During dry years, the controlling effect of water availability on carbon fluxes increased. Summer precipitation and soil moisture played key roles in the interannual variations in carbon fluxes in both grasslands. At MD, net carbon uptake was negatively related to summer air temperature likely because warming induced water deficit decreased photosynthesis. Greenness index derived from PhenoCam images captured key phenological phases and diverse magnitude of canopy dynamics. The index was correlated with seasonal and annual variations in carbon fluxes at both grasslands, suggesting the potential of PhenoCam for monitoring the spatial and temporal variations in canopy dynamics in different semiarid grasslands.

ENSO elicits opposing responses of semi-arid vegetation between Hemispheres.

Semi-arid ecosystems are key contributors to the global carbon cycle and may even dominate the inter-annual variability (IAV) and trends of the land carbon sink, driven largely by the El Niño-Southern Oscillation (ENSO). The linkages between dynamics of semi-arid ecosystems and climate at the hemispheric scale however are not well known. Here, we use satellite data and climate observations from 2000 to 2014 to explore the impacts of ENSO on variability of semi-arid ecosystems, using the Ensemble Empirical Mode Decomposition method. We show that the responses of semi-arid vegetation to ENSO occur in opposite directions, resulting from opposing controls of ENSO on precipitation between the Northern Hemisphere (positively correlated to ENSO) and the Southern Hemisphere (negatively correlated to ENSO). Also, the Southern Hemisphere, with a robust negative coupling of temperature and precipitation anomalies, exhibits stronger and faster responses of semi-arid ecosystems to ENSO than the Northern Hemisphere. Our findings suggest that natural coherent variability in semi-arid ecosystem productivity responded to ENSO in opposite ways between two hemispheres, which may imply potential prediction of global semi-arid ecosystem variability, particularly based on variability in tropical Pacific Sea Surface Temperatures.

Future soil moisture and temperature extremes imply expanding suitability for rainfed agriculture in temperate drylands.

The distribution of rainfed agriculture, which accounts for approximately ¾ of global croplands, is expected to respond to climate change and human population growth and these responses may be especially pronounced in water limited areas. Because the environmental conditions that support rainfed agriculture are determined by climate, weather, and soil conditions that affect overall and transient water availability, predicting this response has proven difficult, especially in temperate regions that support much of the world's agriculture. Here, we show that suitability to support rainfed agriculture in temperate dryland climates can be effectively represented by just two daily environmental variables: moist soils with warm conditions increase suitability while extreme high temperatures decrease suitability. 21st century projections based on daily ecohydrological modeling of downscaled climate forecasts indicate overall increases in the area suitable for rainfed agriculture in temperate dryland regions, especially at high latitudes. The regional exception to this trend was Europe, where suitability in temperate dryland portions will decline substantially. These results clarify how rising temperatures interact with other key drivers of moisture availability to determine the sustainability of rainfed agriculture and help policymakers, resource managers, and the agriculture industry anticipate shifts in areas suitable for rainfed cultivation.

Climate change reduces extent of temperate drylands and intensifies drought in deep soils.

Drylands cover 40% of the global terrestrial surface and provide important ecosystem services. While drylands as a whole are expected to increase in extent and aridity in coming decades, temperature and precipitation forecasts vary by latitude and geographic region suggesting different trajectories for tropical, subtropical, and temperate drylands. Uncertainty in the future of tropical and subtropical drylands is well constrained, whereas soil moisture and ecological droughts, which drive vegetation productivity and composition, remain poorly understood in temperate drylands. Here we show that, over the twenty first century, temperate drylands may contract by a third, primarily converting to subtropical drylands, and that deep soil layers could be increasingly dry during the growing season. These changes imply major shifts in vegetation and ecosystem service delivery. Our results illustrate the importance of appropriate drought measures and, as a global study that focuses on temperate drylands, highlight a distinct fate for these highly populated areas.

Evaluating spatial-temporal dynamics of net primary productivity of different forest types in northeastern China based on improved FORCCHN.

An improved individual-based forest ecosystem carbon budget model for China (FORCCHN) was applied to investigate the spatial-temporal dynamics of net primary productivity of different forest types in northeastern China. In this study, the forests of northeastern China were categorized into four ecological types according to their habitats and generic characteristics (evergreen broadleaf forest, deciduous broadleaf forest, evergreen needleleaf forest and deciduous needleleaf forest). The results showed that distribution and change of forest NPP in northeastern China were related to the different forest types. From 1981 to 2002, among the forest types in northeastern China, per unit area NPP and total NPP of deciduous broadleaf forest were the highest, with the values of 729.4 gC/(m(2)•yr) and 106.0 TgC/yr, respectively, followed by mixed broadleaf- needleleaf forest, deciduous needleleaf forest and evergreen needleleaf forest. From 1981 to 2002, per unit area NPP and total NPP of different forest types in northeastern China exhibited significant trends of interannual increase, and rapid increase was found between the 1980s and 1990s. The contribution of the different forest type's NPP to total NPP in northeastern China was clearly different. The greatest was deciduous broadleaf forest, followed by mixed broadleaf- needleleaf forest and deciduous needleleaf forest. The smallest was evergreen needleleaf forest. Spatial difference in NPP between different forest types was remarkable. High NPP values of deciduous needleleaf forest, mixed broadleaf- needleleaf forest and deciduous broadleaf forest were found in the Daxing'anling region, the southeastern of Xiaoxing'anling and Jilin province, and the Changbai Mountain, respectively. However, no regional differences were found for evergreen needleleaf NPP. This study provided not only an estimation NPP of different forest types in northeastern China but also a useful methodology for estimating forest carbon storage at regional and global levels.

[Simulation of carbon stocks of forest ecosystems in Northeast China from 1981 to 2002].

Based on the forest carbon budget model for China FORCCHN, which had been improved through adding variables and modules of precipitation (rainfall and snowfall) intercepted by tree crown and of understory and litter, the spatiotemporal distribution pattern of carbon stocks of forest ecosystems in Northeast China from 1981 to 2002 were simulated. The results showed that from 1981 to 2002, the forest ecosystems in Northeast China played a role of carbon sink, and the total carbon stock was about 12.37 Pg C x a(-1), of which, the stock of vegetation and soil was 4.01 Pg C x a(-1) and 8.36 Pg C x a(-1), respectively. During the study period, the carbon stock of both vegetation and soil all had an increasing trend, and the increase of air temperature contributed more than the change of precipitation. Spatially, the carbon density of vegetation had the characteristics of high in southeast part and low in northwest part, with an average of 10.45 kg C x m(-2). Most of the forest ecosystems in Northeast China had a higher soil carbon density, with a mean value of 21.78 kg C x m(-2) and the maximum in part regions of Daxing'anling, Xiaoxing'anling, and Changbai mountains. The forest area in Northeast China accounted for 31.4% of that in the whole country, possessing an important position in China forest carbon pool. The carbon stock of vegetation and soil in the forests of Northeast China occupied 74.23% and 63.88%, and the carbon density of vegetation and soil was 2.70 and 1.22 times of that in the forests of the whole country, respectively.