In the Arctic, the spatial distribution of boreal forest cover and soil profile transition characterizing the taiga-tundra ecological transition zone (TTE) is experiencing an alarming transformation. The SIBBORK-TTE model provides a unique opportunity to predict the spatiotemporal distribution patterns of vegetation heterogeneity, forest structure change, arctic-boreal forest interactions, and ecosystem transitions with high resolution scaling across broad domains. Within the TTE, evolving climatological and biogeochemical dynamics facilitate moisture signaling and nutrient cycle disruption, i.e. permafrost thaw and nutrient decomposition, thereby catalyzing land cover change and ecosystem instability. To demonstrate these trends, in situ ground measurements for active layer depth were collected to cross-validate below-ground-enhanced modeled simulations from 1980-2017. Shifting trends in permafrost variability (i.e. active layer depth) and seasonality were derived from model results and compared statistically to the in situ data. The SIBBORK-TTE model was then run to project future below-ground conditions utilizing CMIP6 scenarios. Upon visualization and curve-integrated analysis of the simulated freeze-thaw dynamics, the calculated performance metric associated with annual active layer depth rate of change yielded 76.19%. Future climatic conditions indicate an increase in active layer depth and shifting seasonality across the TTE. With this novel approach, spatiotemporal variation of active layer depth provides an opportunity for identifying climate and topographic drivers and forecasting permafrost variability and earth system feedback mechanisms.

Over the past several decades significant changes in forest productivity have been observed in the northern boreal region. However, the magnitude of these changes is not uniform across all regions. Some of this variation can be explained by disturbances, such as wildfire, or by increased climate variability. However, in northern regions underlain by permafrost, the interactions between climate, disturbance, and plant productivity may be more complex. Permafrost is a major component of northern ecosystems that, where present, strongly influences plant community development and structure, and is itself vulnerable to disturbances and climate change. There is strong evidence that changes in permafrost can directly alter surface conditions and plant community composition, but an evaluation of the role of changing permafrost conditions on boreal forest productivity is lacking. Long-term (1984-2019) permafrost monitoring sites established by the Geological Survey of Canada offer a unique opportunity to directly evaluate how changes in permafrost conditions have impacted plant productivity and surface conditions in the Northwest Territories, Canada. We used Landsat satellite data to determine the normalized difference vegetation index (NDVI) as a measure of plant productivity, ground thermal data to characterize active layer thickness and permafrost conditions, and categorical information on ground ice content to reflect subsidence potential. On average, we found that peak growing season NDVI has increased over the 35-year study period. Our preliminary results point to greater positive trends in plant productivity in northern areas compared to southern areas. We are currently exploring correlations between these trends, ground ice content, and interannual variability in ground thermal regime. Our results will help evaluate the impacts of climate change on permafrost conditions and plant productivity in northern regions, which northern communities, governments, and industries can use for adaptation planning.

Boreal landscapes are composed of diverse ecosystem types that form a highly patchy mosaic driven by numerous biotic and geomorphic factors. As part of a U.S. Department of Defense Strategic Environmental Research and Development Program funded study to project how ecotypes will change in central Alaska over the next century, we took high-resolution photographs to illustrate many of the 65 ecotypes documented across riverine, lowland, upland, subalpine, and alpine landscapes on military lands in central Alaska. We also provide photographic examples of drivers of change, including fire, post-fire early and late succession, thermokarst, permafrost aggradation, channel erosion and deposition, paludification, landslides, glaciation, beaver ponding, and human disturbances, such as roads, trails, and clearings. Results of this study will be used to help manage natural resources on military lands that are expected to undergo large changes in response to climate change, and to better communicate to public the vulnerability of boreal ecosystems.

Atmospheric CO2 concentration (Ca) and increasing air temperature can profoundly affect intrinsic water use efficiency (iWUE), which will become a potential impact factor of tree growth. At the same time, the degradation of permafrost in high latitudes caused by climate warming will also affect the iWUE and growth of trees. However, the extent to which global warming at high latitudes and permafrost degradation affect the iWUE and radial growth of trees is poorly constrained by prior research. In this study, tree ring width and stable 13C data were used to estimate the growth and iWUE changes of Dahurian larch (Larix gmelinii) under slope conditions (with deeper active layers or no permafrost) and gully conditions (with a shallower active layer) from 1900 to 2015 above three different permafrost degraded regions (severely generally and mildly) in the Greater Khingan Mountains, China. Our result indicated that from 1976 to 2015 all the nature stands significantly increased their iWUE. Generally, radial growth and leaf intercellular CO2 concentration（Ci）were intensively constrained by temperature and moisture in the region with severe and general levels of permafrost degradation, meanwhile the BAI experienced a significant decline. Compared to the gully conditions, tree growth of slope conditions declined more seriously. The increase of temperature and iWUE showed a negative effect on the growth of Dahurian larch. Radial growth and Ci/Ca from the mildly degraded regions showed a weak response to temperature and moisture conditions, meanwhile accompanied by an increasing radial growth. In different permafrost regions and under different permafrost conditions, the radial growth and iWUE of Dahurian larch are different in response to warming and permafrost degradation. In contrast to mildly degraded areas, the rising temperature limits the stomatal conductance of Dahurian larch in the severely and generally degraded areas, radial growth cannot be benefit from climate warming. Dahurian larch may face more severe drought stress in the southern margin of the high latitude permafrost region as a result of the warming intensification.

The eastern part of Eurasian boreal forest extends much south than its western part. The hemiboreal forests, which is the southernmost distribution of the boreal forest, meets the southernmost distribution of permafrost in this region. Climate warming has led to permafrost degradation, which further exhibit effects of the hemiboreal forest. In our study, we associated spatio-temporal patterns of vegetation and permafrost in this region. Based on the investigation of plant communities and the thickness of permafrost active layer in 211 sample plots under different topographical conditions, it is found that plant species composition and community structure can indicate the active layer thickness (ALT) of permafrost. The ALT is negatively correlated with moss thickness and shrub coverage, which may play a buffer role in regional climate warming. Sporopollen assemblages spanning the medieval warm period, the little ice age and the modern rapid warming period show that, the change of proportion of broadleaf trees corresponds to the change of temperature. However, rapid warming after 1950 caused the relationship between vegetation and climate to be broken, and the proportion of conifers increased markedly, probably due to the thawing of permafrost. This effect is most obvious in low-lying areas. Because the ALT in low-lying area is small, the increase of ALT caused by temperature rise provides more root space for shallow rooted conifers, and melt water provides water for their growth. As the southern boundary of permafrost moves northward, the low-lying areas will experience dramatic vegetation changes. With a monitoring of surface soil temperature at 10 sites across the hemiboreal forest from central Siberia to northeast China since 2008, we found that the amplitude and duration of freeze-thaw cycle of grassland soil were significantly higher than that of forest soil. The freezing time of grassland and forest soil was similar, but the thawing time of deciduous broad-leaved forest, evergreen coniferous forest and deciduous coniferous forest was 14, 19 and 25 days later than that of grassland, respectively. The difference of snow cover caused by different vegetation structure is the main reason for the difference of thawing time.