Trevor C. Lantz

Growing evidence suggests that the frequency and intensity of fire is increasing across the tundra biome. A change to tundra fire regimes is concerning because it will impact carbon emissions, permafrost conditions, and vegetation and habitat. In northern Canada little is known about the relative impact of climate and terrain factors on the risk of fire. In this study we used imagery from the Landsat satellite archive to identify and manually digitize the boundaries of tundra and northern taiga fires that burned in northcentral and northwestern Canada between 1985 and 2023. Our mapping shows that the area burned between 2005 and 2023 is higher than the period from 1985-2004, largely because the size of individual fires has increased. A random forests analysis using our fire inventory and spatial data on climate and terrain factors indicates that fuel conditions and mean summer temperature (June-September) are key determinants of fire occurrence. Geospatial modeling also suggests that ongoing climate change will increase the frequency of tundra fire and that future risk will be mediated by landscape characteristics.

Replicability of Digital Terrain Models (DTMs) and Canopy Height Models (CHMs) derived from drone photogrammetry is important to understand the extent to which time-series are exposed to methodological noise and conceal real environmental changes. Root mean square error (RMSE) distribution metrics (median/IQR) were used as indicators of replicability across seven drone survey setups, three dense matching scales, and 13 ground point filters in a challenging shrubland environment (total of 273 DTMs and CHMs). We conclude that methodological effects have considerable potential to negatively affect replicability. A power-law relationship between point cloud density and dense matching resolution suggested that important dense matching resolution thresholds exist beyond which replicability degrades considerably. For our Arctic study area, replicability of DTMs (median ± 0.1 m RMSE Vegetated Vertical Accuracy) and CHMs (within ±0.05 m of true site-level heights) is most likely when source imagery is collected with ≤1.5 cm spatial resolution and side-lap of >80%, and if classified point clouds are generated using full-scale dense matching and Triangular Irregular Network filtering. Negative biases for maximum shrub height estimates increased from 4–9% to 14–50% with coarser imagery. We advocate for increased attention to drone-derived model replicability to separate real environmental changes from noise during a period of rapid ecological and geomorphic change.

Arctic shrub productivity trends display variability at multiple spatial scales. Fine‐scale studies have generally observed the greatest shrub expansion in landscape positions that accumulate water and nutrients. While considerable work has focused on the mediating effect of these resources on growth responses to warming, less is known about the mechanisms constraining recruitment‐driven expansion. Given the low seed viability of many Arctic shrubs, spatial patterns of seed dispersal may play an important role in constraining fine‐scale variability of shrub recruitment. This variability may also be driven by ground cover suitability, though these relationships are understudied in undisturbed sites. Here, we developed models representing seed accumulation mechanisms around Alnus alnobetula (green alder) patches within the taiga‐tundra ecotone of the Northwest Territories and compared these with observations of seed and seedling density. We also investigated relationships between seedling abundance, topographic position, and ground cover. Observed patterns of recruitment were complex, with preferential expansion occurring beneath alder patches only on the steepest slopes. Seed accumulation models representing overland flow, wind, and source distance were important predictors of seedling recruitment. This provides indirect evidence of localized seed limitation around patches, suggesting future recruitment may not respond as expected to changing environmental conditions. Sphagnum cover also predicted recruitment, indicating the importance of seedbed conditions for establishment. We propose that developing models of shrub expansion that include both dispersal and environmental constraints may increase our ability to predict patterns and rates of expansion. Such predictions are necessary to understand future biosphere‐atmosphere interactions in a rapidly changing Arctic.

Arctic landscapes occupy a nexus of environmental change processes, globally significant soil carbon stores, wildlife populations, and subsistence-based human societies. In response to rapid climate warming, tundra ecosystems are experiencing widespread changes to vegetation and underlying permafrost, coupled with an array of ecological disturbances that are expected to intensify in the future. Declines in the extent of the cryosphere on land (permafrost and seasonal snow) and offshore (sea-ice) raise the question of whether and for how long warmer portions of the Low Arctic will fit established concepts of “what is Arctic,” given the influence the cryosphere has historically had on tundra ecosystem structure and function. The era of spaceborne observation of circumpolar tundra greenness, in the form of the Normalized Difference Vegetation Index (NDVI), has entered its fifth decade and provides foundational information concerning ecosystem conditions and responses to climatic trends, variability, ecological disturbance, and successional processes. Here we review the evolving story of Arctic greening, and synthesize long-term spaceborne records of NDVI, climatic data, field observations, and the knowledge base of Arctic residents to place the last four decades of Arctic environmental change in context, and establish expectations and research priorities for the coming decade. Greenness dynamics display high spatio-temporal variability, reflecting complex interactions of climatic warming and variability, landscape history, ecological disturbance, and other factors. Nonetheless, long-term increases in NDVI—commonly known as “the greening of the Arctic”—remain prominent across large areas in all available long-term spaceborne datasets and align with long-term shifts in vegetation structure documented in disparate Arctic regions. Common shifts reported from the Low Arctic, such as shrubification, generally portend declines in floristic diversity, and shifts in fauna that favor boreal forest species. Despite lingering uncertainties regarding trend attribution and sources of interannual variability, the sequence of record-high circumpolar tundra greenness values observed since 2020 provides strong evidence that Arctic tundra ecosystems have entered a state without historic precedent on timescales approaching a millennium.

Climate change is altering northern vegetation structure and below-ground carbon storage. Expanding forest and shrub cover has decreased soil organic carbon (SOC) storage in some parts of the forest-tundra ecotone. In this study, we linked measurements of SOC with terrain and vegetation structure derived from drone imagery across treelines underlain by continuous permafrost in the Northwest Territories, Canada. We classified sites into three treeline types representing differences in vegetation productivity and topography. Between treeline types, we observed differences in C:N ratios and organic matter depth related to the rate of soil carbon turnover and SOC storage. Overall, SOC showed small positive relationships with tree stem density and average canopy height. We did not find evidence that expanding tree- and shrublines would result in losses of SOC storage in our study area. Instead, topography and landscape drainage patterns, rather than vegetation structure may be more important predictors of SOC storage. We used medium resolution satellite data to extend predictions of treeline type across our study area. The majority of predicted treelines (82%) showed positive relationships between vegetation height and SOC storage. Our findings highlight the value of integrating vegetation structure and landscape features in understanding carbon dynamics in the forest-tundra ecotone.

Anthropogenic climate change has driven an increase in the frequency, size, and severity of fires at high latitudes. Recent research shows that increasing fire severity in the subarctic is altering the trajectories of forest succession, but to date, research on the effect of fire severity on tundra succession has been limited. In this study, we investigated short-term recovery of shrub tundra communities following fire in the Tuktoyaktuk Coastal Plain and Anderson River Plain ecoregions of the Northwest Territories. To understand the effects of fire severity, we documented vegetation and permafrost recovery within moderately burned, severely burned, and unburned portions of six tundra fires that burned in 2012. We found that vegetation structure at moderately and severely burnt sites recovered rapidly toward pre-fire levels, but that differences in community composition, characterized by a decrease in shrub and lichen cover as well as an increase in abundance of ruderals and graminoids, persisted at severely burned sites. The persistence of thermal changes and increased thaw depth indicate that while biotic recovery can occur promptly, severe fire may have long-term impacts on belowground conditions.

Coastal areas in the Western Canadian Arctic are predicted to experience increases in the frequency and intensity of storm surges as rapid climate change continues. Although storm surges have the potential to cause widespread and persistent vegetation loss, little information is available about the influence of decreasing disturbance intervals (between storms), expected timelines of recovery for vegetation, and how vegetation change alters habitat availability and (or) quality for local wildlife populations. We investigated how Arctic bird diversity is affected by heterogeneous vegetation recovery post-storm. Specifically, we employed field survey protocols from the Program for Regional and International Shorebird Monitoring to investigate how avian community assemblages were affected by heterogeneous re-vegetation 20 years following a record 1999 storm surge. Comparisons of these bird survey data with vegetation and habitat factors showed that the drier, post-storm vegetation barrens were preferred by ground-nesters and species that use open habitats such as Lapland longspur ( Calcarius lapponicus) and semipalmated plover ( Charadrius semipalmatus), whereas the wetter (usually with surface water) revegetated habitats were frequented by species of ducks ( Anas spp.), red-necked phalarope ( Phalaropus lobatus), and savannah sparrow ( Passerculus sandwichensis). Overall, this research shows that areas that have revegetated after the 1999 storm can be considered as functionally recovered in comparison to our Reference (i.e., unaffected) sites in terms of vegetation and bird communities, but that areas still exist ∼20 years post-storm that do not show evidence of recovery.