In Northeast China, permafrost is controlled by a combination of biotic, climatic, physiographic, and anthropogenic factors. Due to the complexity of these governing or influencing factors, it is challenging to exactly describe the features of the Xing'an permafrost in Northeast China. By integrating remote sensing (RS) and geographic information system (GIS) technologies, we have quantified these influencing factors of permafrost changes as an important approach to understanding the nature of latitudinal and mountain permafrost in Northeast China at the mid-latitudes in the Northern Hemisphere. In this study, we combine Geographical Detector (Geodetector) model, trend analysis, and multi-source RS data to quantify the controlling or influencing factors of permafrost thermal state and of permafrost changes, and explain the interactions among permafrost, environment, and climate. The results indicate that, at the regional scale, changes in the thermal state of permafrost are primarily governed or influenced by mean annual land surface temperature (MALST), precipitation, and snow cover duration (SCD). Topographic factors also affect the spatial patterns of permafrost development. Additionally, in the context of climate warming, the insulation effect of snow cover on the permafrost is weakened, or has been weakening. Moreover, the interactive effects among various factors significantly enhance their explanatory power for changes in the thermal state of permafrost. The study emphasizes the complexity of the interactions among permafrost, climate, and the environment, and highlights the significance of understanding these interactions for regional socio-economic development, ecological management, carbon pool stabilization, and research on future climate change in Northeast China.

The accelerated or decelerated freezing-thawing processes of the active layer in Xing'an permafrost regions are crucial for the protection of permafrost. To better understand the freezing-thawing processes of the active layer and its driving factors, according to the observation from 2017 to 2020 of soil temperature and water content in the active layer of forest and peatland in two representative hemiboreal ecosystems in the Da Xing'anling Mountains, Northeast China, the study explored in detail the effects of climatic conditions and local factors on the hydrothermal and freezing-thawing processes of active layer soils. The results showed that during the freezing-thawing cycles of 2017-2020, freezing and thawing start times in the peatland and forest ecosystems soils were generally delayed, and it took longer for the active layer soil to completely thaw than to freeze. The annual average soil temperature in the peatland's active layer (5-80 cm) was 0.7-2.0 degrees C lower than that in the forest, and the annual average soil moisture content on the peatland was 5.5%-26.7% higher than that in the forest. Compared with the forest ecosystem soils, the ground surface freezing time of the peatland was delayed by 3-10 d, and the freezing rate decreased by 1.1-1.5 cm d-1, while the beginning time of thawing was advanced by 22-27 d, and the thawing rate decreased by 1.3-1.4 cm d-1. In the process of decreasing soil temperature and increasing soil moisture content, the freezing and thawing rate of the active layer would be reduced, decelerating the freezing-thawing processes of the active layer in the process of decreasing soil temperature and increasing soil moisture content. The results provide the key original data for studying the formation and evolution of active layer and permafrost in the Xing'an permafrost regions in Northeast China and can be used to validate the prediction of ecosystem succession under the combined influences of climate change and permafrost degradation.

Boreal forest and wetland have important influences on the development and protection of the ecosystem-dominated Xing'an permafrost. However, the responses of different ecosystems to climate change and the impacts on the underlying permafrost are still unclear. Here, based on the multi-period land use/land cover (LULC) data and long-time series of air temperature, combined with the ordinary least squares (OLS) and ordinary kriging (OK) methods, the effects of land use and cover change (LUCC) on the distribution of mean annual air temperature (MAAT) and permafrost in Northeast China were analyzed. From 1980s to 2010s, MAAT showed an upward trend (0.025 degrees C per yr) and extents of permafrost showed a decreasing trend (-3668 km(2)yr(-1)) in Northeast China. Permafrost degradation mainly occurred in forested land and grassland, with areal reductions of 4.0106 x 10(4) and 3.8754 x 10(4) km(2), respectively. The transformation of LULC aggravates the degradation of permafrost. The conversions of forested land and grassland to cultivated land and forested land to grassland resulted in the shrinkage of permafrost extent by 6233 km(2) from 1980s to 2010s . Our results confirm the significant impacts of LUCC on the Xing'an permafrost resulting in its degradation. Additionally, they can provide a scientific basis for ecological environment protection and restoration and sustainable development of boreal forest and wetland ecosystems in permafrost regions of Northeast China.

In the northern Da Xing'anling Mountains in Northeast China on the southern margin of the Eastern Asia permafrost body, the ground thermal state and boreal ecological environment are sensitive to climate change and human activities. Since the 1980s, the Hola Basin here has been continuously and extensively developed. In particular, open pits and later backfilling in strip coal mining alters land-atmospheric hydrothermal exchanges in permafrost regions, leading to serious damages to the permafrost environment and boreal forest. After mining, pits need to be backfilled timely and properly for hydrothermal recovery of Xing'an permafrost and the boreal ecological environment. In this study, based on the comparative analysis of monitored ground temperatures in backfilled and undisturbed areas, influencing factors of thermal recovery after backfilling were analyzed through numerical simulations. Results show that the thermal recovery of permafrost in the backfilled area is closely related to temperature, depth, material, and soil moisture content of backfill. The warmer, finer, and thicker the backfill soils, the longer the permafrost recovery. Thermal recovery of permafrost also depends on the moisture content of backfill; the shortest recovery occurs at 15-25% in the backfilled soil moisture content. Based on numerical simulations and combined with enlightenments from features of the ecosystem-protected Xing'an permafrost in Northeast China, a composite configuration of organic soil, crush-rock layer, and proper re-vegetation measures is advised. Based on prudent regulation of heat transfer modes, this composite backfilling method can effectively cool the backfilled ground and can even possibly offset the climate warming.

Permafrost is a key element of the cryosphere and sensitive to climate change. High-resolution permafrost map is important to environmental assessment, climate modeling, and engineering application. In this study, to estimate high-resolution Xing'an permafrost map (up to 1 km(2)), we employed the surface frost number (SFN) model and ground temperature at the top of permafrost (TTOP) model for the 2001-2018 period, driven by remote sensing data sets (land surface temperature and land cover). Based on the comparison of the modeling results, it was found that there was no significant difference between the two models. The performances of the SFN model and TTOP model were evaluated by using a published permafrost map. Based on statistical analysis, both the SFN model and TTOP model efficiently estimated the permafrost distribution in Northeast China. The extent of Xing'an permafrost distribution simulated by the SFN model and TTOP model were 6.88 x 10(5) km(2) and 6.81 x 10(5) km(2), respectively. Ground-surface characteristics were introduced into the permafrost models to improve the performance of models. The results provided a basic reference for permafrost distribution research at the regional scale.