The extent of wildfires in tundra ecosystems has dramatically increased since the turn of the 21st century due to climate change and the resulting amplified Arctic warming. We simultaneously studied the recovery of vegetation, subsurface soil moisture, and active layer thickness (ALT) post-fire in the permafrost-underlain uplands of the Yukon-Kuskokwim Delta in southwestern Alaska to understand the interaction between these factors and their potential implications. We used a space-for-time substitution methodology with 2017 Landsat 8 imagery and synthetic aperture radar products, along with 2016 field data, to analyze tundra recovery trajectories in areas burned from 1953 to 2017. We found that spectral indices describing vegetation greenness and surface albedo in burned areas approached the unburned baseline within a decade post-fire, but ecological succession takes decades. ALT was higher in burned areas compared to unburned areas initially after the fire but negatively correlated with soil moisture. Soil moisture was significantly higher in burned areas than in unburned areas. Water table depth (WTD) was 10 cm shallower in burned areas, consistent with 10 cm of the surface organic layer burned off during fire. Soil moisture and WTD did not recover in the 46 years covered by this study and appear linked to the long recovery time of the organic layer.

In the context of global climate change, changes in unfrozen water content in permafrost significantly impact regional terrestrial plant ecology and engineering stability. Through Differential Scanning Calorimetry (DSC) experiments, this study analyzed the thermal characteristic indicators, including supercooling temperature, freezing temperature, thawing temperature, critical temperature, and phase-transition temperature ranges, for silt loam with varying starting moisture levels throughout the freezing and thawing cycles. With varying starting moisture levels throughout the freezing and thawing cycles, a model describing the connection between soil temperature and variations in unfrozen water content during freeze-thaw cycles was established and corroborated with experimental data. The findings suggest that while freezing, the freezing and supercooling temperatures of unsaturated clay increased with the soil's starting moisture level, while those of saturated clay were less affected by water content. During thawing, the initial thawing temperature of clay was generally below 0 degrees C, and the thawing temperature exhibited a power function relationship with total water content. Model analysis revealed hysteresis effects in the unfrozen water content curve during freeze-thaw cycles. Both the phase-transition temperature range and model parameters were sensitive to temperature changes, indicating that the processes of permafrost freezing and thawing are mainly controlled by ambient temperature changes. The study highlights the stability of the difference between freezing temperature and supercooling temperature in clay during freezing. These results offer a conceptual framework for comprehending the thawing mechanisms of permafrost and analyzing the variations in mechanical properties and terrestrial ecosystems caused by temperature-dependent moisture changes in permafrost.

Permafrost carbon could produce a positive climate feedback. Until now, the ecosystem carbon budgets in the permafrost regions remain uncertain. Moreover, the frequently used models have some limitations especially regarding to the freeze-thaw process. Herein, we improved the IBIS model by incorporating an unfrozen water scheme and by specifying the parameters to estimate the present and future carbon budget of different land cover types (desert steppe, steppe, meadow, and wet meadow) in the permafrost regions. Incorporating an unfrozen water scheme reduced the mean errors in the soil temperature and soil water content by 25.2%, and the specifying leaf area parameters reduced the errors in the net primary productivity (NPP) by 79.9%. Further, the simulation results showed that steppes are carbon sources (39.16 gC/m(2)/a) and the meadows are carbon sinks (-63.42 gC/m(2)/a ). Under the climate warming scenarios of RCP 2.6, RCP 6.0, and RCP 8.5, the desert steppe and alpine steppe would assimilated more carbon, while the meadow and wet meadow were projected to shift from carbon sinks to carbon sources in 2071-2100, implying that the land cover type plays an important role in simulating the source/sink effects of permafrost ecosystem carbon in the IBIS model. The results highlight the importance of unfrozen water to the soil hydrothermal regime and specific leaf area for the growth of alpine vegetation, and present new insights on the difference of the responses of various permafrost ecosystems to climate warming.

Atmospheric brown carbon (BrC), a short-lived climate forcer, absorbs solar radiation and is a substantial contributor to the warming of the Earth ' s atmosphere. BrC composition, its absorption properties, and their evolution are poorly represented in climate models, especially during atmospheric aqueous events such as fog and clouds. These aqueous events, especially fog, are quite prevalent during wintertime in Indo-Gangetic Plain (IGP) and involve several stages (e.g., activation, formation, and dissipation, etc.), resulting in a large variation of relative humidity (RH) in the atmosphere. The huge RH variability allowed us to examine the evolution of water-soluble brown carbon (WS-BrC) diurnally and as a function of aerosol liquid water content (ALWC) and RH in this study. We explored links between the evolution of WS-BrC mass absorption efficiency at 365 nm (MAE WS- BrC-365 ) and chemical characteristics, viz., low-volatility organics and water-soluble organic nitrogen (WSON) to water-soluble organic carbon (WSOC) ratio (org-N/C), in the field (at Kanpur in central IGP) for the first time worldwide. We observed that WSON formation governed enhancement in MAE WS-BrC-365 diurnally (except during the afternoon) in the IGP. During the afternoon, the WS-BrC photochemical bleaching dwarfed the absorption enhancement caused by WSON formation. Further, both MAE WS-BrC-365 and org-N/C ratio increased with a decrease in ALWC and RH in this study, signifying that evaporation of fog droplets or bulk aerosol particles accelerated the formation of nitrogen-containing organic chromophores, resulting in the enhancement of WS-BrC absorptivity. The direct radiative forcing of WS-BrC relative to that of elemental carbon (EC) was -19 % during wintertime in Kanpur, and - 40 % of this contribution was in the UV -region. These findings highlight the importance of further examining the links between the evolution of BrC absorption behavior and chemical composition in the field and incorporating it in the BrC framework of climate models to constrain the predictions.

Despite the extensive research conducted on plant-soil-water interactions, the understanding of the role of plant water sources in different plant successional stages remains limited. In this study, we employed a combination of water isotopes (delta 2H and delta 18O) and leaf delta 13C to investigate water use patterns and leaf water use efficiency (WUE) during the growing season (May to September 2021) in Hailuogou glacier forefronts in China. Our findings revealed that surface soil water and soil nutrient gradually increased during primary succession. Dominant plant species exhibited a preference for upper soil water uptake during the peak leaf out period (June to August), while they relied more on lower soil water sources during the post-leaf out period (May) or senescence (September to October). Furthermore, plants in late successional stages showed higher rates of water uptake from uppermost soil layers. Notably, there was a significant positive correlation between the percentage of water uptake by plants and available soil water content in middle and late stages. Additionally, our results indicated a gradual decrease in WUE with progression through succession, with shallow soil moisture utilization negatively impacting overall WUE across all succession stages. Path analysis further highlighted that surface soil moisture (0- 20 cm) and middle layer nutrient availability (20- 50 cm) played crucial roles in determining WUE. Overall, this researchemphasizes the critical influence of water source selection on plant succession dynamics while elucidating un- derlying mechanisms linking succession with plant water consumption.

Climate warming is causing significant changes in the Arctic, leading to increased temperatures and permafrost instability. The active layer has been shown to be affected by climate change, where warmer ground surface temperatures result in progressive permafrost thaw and a deepening active layer. This study assessed the effects of thermal modeling parameters on permafrost ground response to climate warming using the fifth phase of the Coupled Model Intercomparison Project (CMIP5) and TEMP/W software. We analyzed how variations in depth, water content, and soil type affect predictions of future active layer depths and settlement under various climate scenarios using the soil characteristics along Hudson Bay Railway corridor. The results indicate that, for finegrained soils, the depth of the model is a more significant parameter than for coarse-grained soils. The water content of all soil types is a critical factor in determining the time at which permafrost thaws and the depth at which the active layer is located, as higher water content leads to larger active layer changes and more settlement in most cases. Our findings have important implications for infrastructure and land use management in the Arctic region.

Permafrost degradation alters the flow rate, direction, and storage capacity of soil moisture, affecting ecohydrological effects and climate systems, and posing a potential threat to natural and human systems. The most widely distributed permafrost regions are coastal, high-latitudes and high-altitudes (mainly by the Qinghai-Tibet Plateau). Past studies have demonstrated that permafrost degradation in these regions lacks sorting out regional driving factors, assessing cascading effects on the hydrological environment and monitoring methods. To address this, we reviewed the historical research situation and major topics of permafrost degradation from 1990 to 2022. We analyzed the spatio-temporal dynamics and driving mechanism of permafrost degradation. Then, we comprehensively discussed the effects of permafrost degradation on the soil physical structure and hydraulic properties, soil microorganisms and local vegetation, soil evapotranspiration and stream runoff, and integrated ecohydrological effects. Permafrost field site data were then collected from existing findings and methods for direct or indirect monitoring of permafrost changes at different scales. These results revealed that the research on the hydrological effects of permafrost change was mainly centered on the soil. In addition, regional environmental factors driving permafrost degradation were inconsistent mainly in coastal regions influenced by sea level, high-latitude regions influenced by lightning and wildfire, and high-altitude regions influenced by topography. Permafrost degradation promoted horizontal and/or vertical hydrological connectivity, threatening the succession of high latitude vegetation communities and the transition from high altitude grassland to desert ecosystems, causing regional water imbalances would mitigate or amplify the ability of integrated ecohydrological benefits to cope with climate warming. The never-monitored permafrost area was 1.55x106 km2, but the limitations of using data for the same period remained a challenging task for soil moisture monitoring. Finally, future research should enhance the observation of driving factors at the monitoring site and combine remote sensing data, model simulations or numerical simulations, and isotope tracers to predict the future degradation state of deep permafrost effectively. It is expected that this review will guide further quantifying the driving mechanisms of permafrost degradation and the resulting cascading effects.

Soil water content (SWC) and soil temperature (ST) are important indicators of environmental change in permafrost regions. In this study, we conducted soil sampling at 89 locations in the Three Rivers Headwaters Region (TRHR) to investigate the individual and synergistic effects of environmental factors on SWC and ST. We used multivariable regression and random forest modelling to analyse the data. The results show that SWC and ST were higher in the southeast TRHR than in the northwest and higher in surface layers than deeper soil layers. The most important factors affecting SWC in the 0-20 cm and 20-40 cm soil layers were soil bulk density and precipitation, while bulk density was the most important factor in the 40-60 cm layer, and soil bulk density and steppe vegetation were the most important factors in the 60-80 cm layer. For ST, altitude, temperature and slope gradient were the drivers in the 0-20 cm surface layer, while altitude and temperature were the most critical drivers in the 20-40 cm, 40-60 cm and 60-80 cm layers. Overall, bulk density and altitude were the key environmental factors influencing SWC and ST values, respectively. The outcomes of this study provide valuable insights into the environmental factors that impact the SWC and ST in permafrost regions, which can guide decision-making processes for sustainable soil management in the context of climate change.

Although many studies have found that global warming has caused permafrost to thaw, we still lack understanding of the mechanism relating permafrost thawing and ecosystem carbon budgets. To compare the effects of freeze-thaw cycles on the grassland ecosystem carbon budget between a permafrost area (PA) and a non-permafrost area (NPA), we established two carbon dioxide flux towers since 2015 to monitor the net ecosystem exchange by eddy covariance (EC) systems at the site of Nalaikh in PA and Hustai in NPA. The gross primary production (GPP), respiration by ecosystems (Reco), and net ecosystem production (NEP) from 2016 to 2019 were estimated using EddyPro 7 and ToviTM. The result showed that, at the PA and NPA sites, the annual GPP was 686.3 and 654.9 g C m- 2 y-1, Reco was 611.5 and 699.6 g C m- 2 y-1, and NEP was 73.8 and -45.5 g C m-2 y- 1, respectively, which implies that the grassland ecosystem was a carbon sink in the PA but a carbon source in the NPA. Then, the effect of the freeze-thaw cycles on the carbon budget was also analyzed. The NEP in the PA (35.3 g C m-2) was significantly larger than in the NPA (0.3 g C m-2) during the thawing period and, similarly, the NEP in the PA (121.7 g C m-2) was also larger than in the NPA (72.1 g C m-2) during the thawed period, implying significantly larger carbon absorption in the PA than in the NPA during both the thawing and thawed periods. Finally, correlation analysis results revealed that the soil water content (SWC) plays an important role in maintaining the ecosystem carbon budget. The degradation of permafrost might accelerate soil thawing and promote the transfer of soil water, and thus greatly affect the carbon budget of grassland ecosystems in Mongolia.

Vegetation patch patterns, which are used as indicators of state, functionality, and catastrophic changes in the arid ecosystem, have received much attention. However, little is known about the controlling factors and indicators that underlie vegetation patch patterns in the alpine grassland ecosystem. Here, we firstly studied characteristics of vegetation patch patterns with aerial photography by using an unmanned aerial vehicle and evaluated the vegetation patch-size distribution with power law (PL) and truncated power law (TPL) models on the central part of the Qinghai-Tibetan Plateau (QTP). We then investigated the effects of environmental factors and biotic disturbances on vegetation patch patterns. The results showed that (1) there were four typical vegetation patch patterns, i.e. spot, stripe, sheet, and uniform patterns; (2) soil water content and air temperature were major environmental factors affecting vegetation patch patterns; (3) biotic disturbance of plateau pika (Ochotona curzoniae) affected vegetation patch patterns by changing the number, area, and connectivity of vegetation patches; and (4) vegetation patch-size distribution parameters were significantly related to soil hydrothermal variables (P < 0.01). We concluded that the development of alpine vegetation patch patterns was controlled by soil hydrothermal conditions and plateau pika's disturbance. We also proposed that gamma (TPL-PL) (difference between absolute values of slopes of TPL and PL curve fits) could serve as an effective indicator for monitoring alpine grassland conditions, and preventing patchiness was a critical task for the alpine ecosystem management and restoration.