Ecosystems at the southern edge of the permafrost distribution are highly sensitive to global warming. Changes in soil freeze-thaw cycles can influence vegetation growth in permafrost regions. Extant studies mainly focused on analyzing the differences of vegetation dynamics in different permafrost regions. However, the intrinsic drivers of permafrost degradation on vegetation growth remain elusive yet. Based on the top temperature of permafrost (TTOP) model, we simulated the spatial distribution of permafrost in Northeast China (NEC) from 2001 to 2020. Using the data of the vegetation Net Primary Productivity (NPP), vegetation phenology, climate and permafrost phenology, and analytical methods including partial correlation, multiple linear regression, and path analysis, we explored the response of vegetation growth and phenology to soil freeze-thaw changes and climate change under different degrees of permafrost degradation. Overall, the start date of the growing season (SOS) was very sensitive to the start date of soil thaw (SOT) changes, and multiple regression analyses showed that SOT was the main factor influencing SOS in 41.8% of the NEC region. Climatic factors remain the main factors affecting vegetation NPP in NEC, and the results of partial correlation analysis showed that only 9.7% of the regional duration of soil thaw (DOT) had a strong correlation with vegetation NPP. Therefore, we determined the mechanism responsible for the soil freeze-thaw changes and vegetation growth relationship using the path analysis. The results indicated that there is a potential inhibitory effect of persistent permafrost degradation on vegetation growth. Our findings would contribute to the improvement of process-based models of forest dynamics in the boreal region, which would help to plan sustainable development and conservation strategies in permafrost areas.

The dendroecology of larch (Larix gmelinii Rupr.) in the world's northernmost forest provided insight into the complex relationship of tree growth, forest stand establishment, and changing eco-climatic factors. The Ary-Mas forest in the northern Siberia (72 & DEG; + NL) is an ecological island, surrounded by tundra. We hypothesized that the environmental constraints that limit larch growth in this harsh habitat include soil moisture and winter winds as well as low air temperature. We constructed and analyzed the larch growth index (GI) chronology from the eighteenth century until 2019. We found that the larch GI depended on the air temperature, soil moisture anomalies, and winter wind speed, and that dependence was significantly different before and after the 2000s. Larch GI responded to the onset of climatic warming in the 1970s by a minor GI increase followed by a GI decrease until the end of 1990. Increased air temperature early in the growing season favored increased GI, whereas elevated winter wind speed negatively influenced larch growth. After warming in the 2000s, the length of the growing season increased by 15 days, and larch GI was sensitive to air temperature both early and late in the growing season. The adverse influence of winter winds has gradually decreased since the 1970s, becoming a minor factor in the 2000s. Soil moisture in wet, cold soils negatively influenced larch growth. Meanwhile, decreased soil moisture in the northern lowlands favored increased larch growth. We found that larch growth increases were strongly correlated with GPP and NPP (gross and net primary productivity) within the Ary-Mas site and for the central Siberian Arctic. We infer that this Arctic region continues to be a carbon sink.

The Tibetan Plateau has the largest expanse of high-elevation permafrost in the world, and it is experiencing climate warming that may jeopardize the functioning of its alpine ecosystems. Many studies have focused on the effects of climate warming on vegetation production and diversity on the Plateau, but their disparate results have hindered a comprehensive, regional understanding. From a synthesis of twelve warming experiments across the Plateau, we found that warming increased aboveground net primary production (ANPP) and vegetation height at sites with permafrost, but ANPP decreased with warming at non-permafrost sites. Aboveground net primary production responded more negatively to warming under drier conditions, due to both annual drought conditions and warming-induced soil moisture loss. Decreases in species diversity with warming were also larger at sites with permafrost. These results support the emerging understanding that water plays a central role in the functioning of cold environments and suggest that as ecosystems cross a threshold from permafrost to non-permafrost systems, ANPP will decrease across a greater proportion of the Tibetan Plateau. This study also highlights the future convergence of challenges from permafrost degradation and grassland desertification, requiring new collaborations among these currently distinct research and stakeholder groups.

The ongoing and projected warming in the northern high latitudes (NHL; poleward of 60 degrees N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO2 concentration stimulate vegetation productivity, taking up CO2. On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 +/- 0.3 Pg C yr-1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 +/- 20 Pg C over 1901 levels, of which 17 +/- 8 Pg C comes from vegetation (43%) and 21 +/- 16 Pg C from the soil (8%). Both CO2 fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO2 fertilization effect, permafrost, and the role of soil moisture. Unlike the CO2 fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid-latitudes. These effects are further enhanced as a result of positive carbon cycle-climate feedbacks due to additional CO2 and warming.

This study deals with changes in the plant cover and its net carbon sequestration over 30 years on a subarctic Sphagnum-mire with permafrost near Abisko, northernmost Sweden, in relation to climatic variations during the same period. Aerial colour infrared images from 1970 and 2000 were compared to reveal changes in surface structure and vegetation over the whole mire, while the plant populations were studied within a smaller, mainly ombrotrophic part. The results demonstrated two processes, namely (1) that wet sites dominated by graminoids expanded while hummock sites dominated by dwarf shrubs receded, and (2) that on the hummocks lichens expanded while evergreen dwarf shrubs and mosses decreased, both processes creating an instability in the surface structure. A successive degradation of the permafrost is the likely reason for the increase in wet areas, while the changes in the hummock vegetation might have resulted from higher spring temperatures giving rise to an intensified snow melt, exposing the vegetation to frost drought. Because of the vegetation changes, the annual litter input of carbon to the mire has increased slightly, by 4 g m(-2) a(-1) (7.3%), over these years while an increased erosion has resulted in a loss of 40-80 Mg carbon or 7-17 g m(-2) a(-1) for the entire mire over the same period. As the recalcitrant proportion of the litter has decreased, the decay rate in the acrotelm might be expected to increase in the future.