• Energy Research

ABSTRACTRapid warming in northern lands has led to increased ecosystem carbon uptake. It remains unclear, however, whether and how the beneficial effects of warming on carbon uptake will continue with climate change. Moreover, the role played by water stress in temperature control on ecosystem carbon uptake remains highly uncertain. Here, we systematically explored the trend in the temperature control on gross primary production (measured by “SGPP‐TAS”) across northern lands (> 15°N) using a standardized multiple regression approach by controlling other covarying factors. We estimated SGPP‐TAS using three types of GPP datasets: four satellite‐derived GPP datasets, FLUXNET tower observed GPP datasets, and GPP outputs from nine CMIP6 models. Our analysis revealed a significant positive‐to‐negative transition around the year 2000 in the trend of SGPP‐TAS. This transition was primarily driven by synchronized changes in soil water content over time and space. The SGPP‐TAS trend transition covered about 32% of northern lands, especially in grasslands and coniferous forests where leaf water mediation and structural overshoot accelerated the drought‐induced transition, respectively. In the future, widespread negative SGPP‐TAS trends are projected in northern lands corresponding with decreasing soil water availability. These findings highlight the shrinking temperature control on northern land carbon uptake in a warmer and drier climate.

Biodiversity contributes to the ecological and climatic stability of the Amazon Basin1,2, but is increasingly threatened by deforestation and fire3,4. Here we quantify these impacts over the past two decades using remote-sensing estimates of fire and deforestation and comprehensive range estimates of 11,514 plant species and 3,079 vertebrate species in the Amazon. Deforestation has led to large amounts of habitat loss, and fires further exacerbate this already substantial impact on Amazonian biodiversity. Since 2001, 103,079-189,755 km2 of Amazon rainforest has been impacted by fires, potentially impacting the ranges of 77.3-85.2% of species that are listed as threatened in this region5. The impacts of fire on the ranges of species in Amazonia could be as high as 64%, and greater impacts are typically associated with species that have restricted ranges. We find close associations between forest policy, fire-impacted forest area and their potential impacts on biodiversity. In Brazil, forest policies that were initiated in the mid-2000s corresponded to reduced rates of burning. However, relaxed enforcement of these policies in 2019 has seemingly begun to reverse this trend: approximately 4,253-10,343 km2 of forest has been impacted by fire, leading to some of the most severe potential impacts on biodiversity since 2009. These results highlight the critical role of policy enforcement in the preservation of biodiversity in the Amazon.

Abstract The northern hemisphere has experienced regional cooling, especially during the global warming hiatus (1998–2012) due to ocean energy redistribution. However, the lack of studies about the natural cooling effects hampers our understanding of vegetation responses to climate change. Using 15,125 ground phenological time series at 3,620 sites since the 1950s and 31-year satellite greenness observations (1982–2012) covering the warming hiatus period, we show a stronger response of leaf onset date (LOD) to natural cooling than to warming, i.e. the delay of LOD caused by 1°C cooling is larger than the advance of LOD with 1°C warming. This might be because cooling leads to larger chilling accumulation and heating requirements for leaf onset, but this non-symmetric LOD response is partially offset by warming-related drying. Moreover, spring greening magnitude, in terms of satellite-based greenness and productivity, is more sensitive to LOD changes in the warming area than in the cooling. These results highlight the importance of considering non-symmetric responses of spring greening to warming and cooling when predicting vegetation-climate feedbacks.

AbstractUnderstanding the influence of climate variability and fire characteristics in shaping postfire vegetation recovery will help to predict future ecosystem trajectories in boreal forests. In this study, I asked: (1) which remotely-sensed vegetation index (VI) is a good proxy for vegetation recovery? and (2) what are the relative influences of climate and fire in controlling postfire vegetation recovery in a Siberian larch forest, a globally important but poorly understood ecosystem type? Analysis showed that the shortwave infrared (SWIR) VI is a good indicator of postfire vegetation recovery in boreal larch forests. A boosted regression tree analysis showed that postfire recovery was collectively controlled by processes that controlled seed availability, as well as by site conditions and climate variability. Fire severity and its spatial variability played a dominant role in determining vegetation recovery, indicating seed availability as the primary mechanism affecting postfire forest resilience. Environmental and immediate postfire climatic conditions appear to be less important, but interact strongly with fire severity to influence postfire recovery. If future warming and fire regimes manifest as expected in this region, seed limitation and climate-induced regeneration failure will become more prevalent and severe, which may cause forests to shift to alternative stable states.

Fire regime refers to the statistical characteristics of fire events within specific spatio-temporal contexts, shaped by interactions among climatic conditions, vegetation types and natural or anthropogenic ignitions. Under the dual pressures of intensified global climate changes and human activities, fire regimes worldwide are undergoing unprecedented transformations, marked by increasing frequency of large and intense wildfires in some regions, yet declining fire activity in others. These fire regime changes (FRC) may drive responses in ecosystem structure and function across spatio-temporal scales, posing significant challenges to socio-economic adaptation and mitigation capacities. To date, research on the patterns and mechanisms of global FRC has rapidly expanded, with investigations into driving factors revealing complex interactions. This review synthesizes research advancements in FRC by analysing 17 articles from this special issue and 249 additional publications retrieved from the Web of Science. We systematically outline the key characteristics of FRC, geographical hotspots of fire regime transformation, critical fire-prone vegetation types, primary climatic and anthropogenic drivers and ecosystem adaptations and feedbacks. Finally, we highlight research frontiers and identify key approaches to advance this field and emphasize an interdisciplinary perspective in understanding and adapting to FRC. This article is part of the theme issue ‘Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks’.

Increasingly frequent and severe forest fires, exacerbated by warmer and drier conditions, significantly affect forest ecosystems. Understanding the dynamics of post-fire forest recovery is crucial for assessing forest resilience and guiding forest management. However, most post-fire recovery studies focus primarily on spatial variation, while recovery changes over time are relatively less studied. In this study, we examined the patterns, trends and drivers of spectral recovery from forest fires that burned between 2002 and 2018 in boreal and temperate forests. We used relative recovery indicators (RRIs) developed from three spectral indices—the normalized burn ratio, normalized difference vegetation index and near-infrared reflectance of vegetation—to capture post-fire spectral recovery. Our results showed that post-fire spectral recovery rates in temperate forests are faster than those in boreal forests, with quicker recovery in regions with higher percentages of broad-leaved species, less severe fires, higher temperature and precipitation. The decline in spectral forest recovery rates of boreal forests indicates that boreal forest post-fire recovery is becoming increasingly challenging. Our work provides valuable insights into forest management and conservation in the face of increasing fire frequency and intensity. This article is part of the theme issue ‘Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks’.

AbstractSatellite remote sensing observations show an increased greenness trend over land in recent decades. While greenness observations can indicate increased productivity, estimation of total annual productivity is highly dependent on vegetation response to climate and environmental conditions. Models have been struggling to determine how much carbon is taken up by plants as a result of increased atmospheric CO2 fertilization. Current remote sensing light use efficiency (LUE) models contain considerable uncertainty due to the lack of spatial and temporal variability in maximum LUE parameter and climate sensitivity defined for global plant functional types (PFTs). We used the optimum LUE (LUEopt) previously derived from the global FLUXNET network to improve estimation of global gross primary productivity (GPP) for the period 1982–2016. Our results indicate increasing GPP in northern latitudes owing to reduced cold temperature constraints on plant growth, thereby suggesting increasing negative carbon‐climate feedback in high latitudes. In the tropics, by contrast, our results indicate an emerging positive climate feedback, mainly due to increasing atmospheric vapor pressure deficit (VPD). Further pervasive VPD increase is likely to continue to reduce global GPP and amplify carbon emissions.