Intensifying wildfire activity and climate change can drive rapid forest compositional shifts. In boreal North America, black spruce shapes forest flammability and depends on fire for regeneration. This relationship has helped black spruce maintain its dominance through much of the Holocene. However, with climate change and more frequent and severe fires, shifts away from black spruce dominance to broadleaf or pine species are emerging, with implications for ecosystem functions including carbon sequestration, water and energy fluxes, and wildlife habitat. Here, we predict that such reductions in black spruce after fire may already be widespread given current trends in climate and fire. To test this, we synthesize data from 1,538 field sites across boreal North America to evaluate compositional changes in tree species following 58 recent fires (1989 to 2014). While black spruce was resilient following most fires (62%), loss of resilience was common, and spruce regeneration failed completely in 18% of 1,140 black spruce sites. In contrast, postfire regeneration never failed in forests dominated by jack pine, which also possesses an aerial seed bank, or broad-leaved trees. More complete combustion of the soil organic layer, which often occurs in better-drained landscape positions and in dryer duff, promoted compositional changes throughout boreal North America. Forests in western North America, however, were more vulnerable to change due to greater long-term climate moisture deficits. While we find considerable remaining resilience in black spruce forests, predicted increases in climate moisture deficits and fire activity will erode this resilience, pushing the system toward a tipping point that has not been crossed in several thousand years.

Questions: Popular methods to analyse community-trait-environment relationships constrain community patterns by trait and environment relationships. What if some traits are strongly associated with community composition but unrelated to environmental variables and vice versa? We take a different approach, unconstrained by this assumption using non-parametric methods. We applied this technique to lichen (fungal/algal and/or cyanobacterial symbioses) communities across environmental and fire age gradients by measuring richness and cover of four important functional traits: energy generation (type of photosynthetic symbiont), water relations (inferred from growth form), dispersal capability (from vegetative propagules) and microsite specificity (measured by substrate affinity).Location: Denali National Park and Preserve, Alaska, USA. Methods:We ordinated plots in species space and regressed trait and environmental variables against ordination axes, resulting in one-or two-dimensional trait and environment surfaces. We then superimposed these surfaces on the ordination to create a new visual display, the 'hilltop plot', which enabled simultaneous measurement and display of one-and two-dimensional, non-linear community-trait-environment associations.Results: Most traits examined show non-linear relationships with community structure. Fire favoured simple cladoniiform lichens, species with higher vegetative dispersal capacity and specificity to grow on wood, but excluded the 'reindeer' lichens, which had lower cover even more than 20 yrs after fire. Forests had more sorediate lichens than non-forested habitats, whereas high elevation, rocky areas had more green algal and fruticose lichens. Cyanobacterial lichen richness was positively related to shrub cover, while tripartite (cyanobacteria and green algae in a single lichen) and foliose lichen richness was highest in areas with higher moss cover.Conclusions: Different combinations of lichen functional traits peaked along environmental and disturbance gradients, which we interpreted as balancing energy generation, water relations, vegetative dispersal and habitat specificity. Our method of trait-environment-community analysis revealed numerous one-and two-dimensional, non-linear relationships between community composition and functional traits, environmental variables and fire age gradients, which informed mechanisms behind community assembly. Our results indicate non-parametric and non-linear methods of trait-environment-community analysis have the potential to detect patterns that would have been missed using current popular techniques.

The expansion of shrubs and trees across high‐latitude ecosystems is one of the most dramatic ecological manifestations of climate change. Most of the work quantifying these changes has been done in small areas and over relatively recent time scales. These land‐cover transitions are highly spatially variable, and we have limited understanding of the factors underlying this variation. We use repeat photography to generate a data set of land‐cover changes in Denali National Park and Preserve, Alaska, stretching back a century and spanning a range of edaphic, topographic, and climatic conditions. Most land‐cover classes were quite stable, with low probabilities of transitioning to other land‐cover types. The advance of woody vegetation into low‐stature tundra, and the spread of conifer trees into shrub‐dominated areas, were both more likely at low elevations and in areas without permafrost. Permafrost also reduced the likelihood of herbaceous vegetation transitioning to woody cover. Exceptions to the general trend of relative stability included nearly all (96%) of the broadleaf forest–dominated areas being invaded by conifers, an expected successional trajectory, and many open gravel river bars (17.8%) transitioning to thick shrubs. These floodplain areas were distinctly not at equilibrium, as only 0.1% of shrub‐dominated areas converted to gravel. Warming temperatures in coming decades and concomitant declines in the extent of permafrost are predicted to enhance the spread of woody vegetation in Denali further, but only by ~3%. Land‐cover transitions, notably the rapid advance of trees and shrubs observed in other studies, could be less likely and more spatially heterogeneous here than in other high‐latitude systems.

. 2016. Proliferating poplars: the leading edge of landscape change in an Alaskan subalpine chronosequence. Ecosphere 7(7):e01398. 10. 1002/ecs2.1398 Abstract. We remeasured a classic chronosequence study in the subalpine zone of the Alaska Range to evaluate how plant community attributes have changed across a set of different-aged terraces over a 54-yr period (1958-2012). Our work focused on whether the tempo and trajectory of successional development described in the original study have changed over this period during which summer temperatures warmed by approximately 2°C. Our work revealed a rapid increase in the distribution, stature, and abundance of balsam poplar trees that was unanticipated in the original successional model alongside evidence that established late-successional plant communities have changed relatively little over the same time period. The spatial distribution of poplar expansion was both directional and highly variable, with greater expansion occurring in sites that were young surfaces in 1958, or else were disturbed during the intervening period. We present evidence that early successional environments in this region may be particularly susceptible to rapid alteration stimulated by climate warming that has allowed tree establishment and growth in subalpine areas. Sparsely vegetated sites allow for invasion or expansion of some species to be quickly realized because there is less resistance from competition with established vegetation, including mosses that insulate and paludify the soil. We suggest that established vegetation communities may have physical characteristics (such as cold and/or acidic soil profiles) that are inimical to the establishment of balsam poplar and may also be a source of competitive inertia, conferring a measure of resistance to directional changes in the landscape mosaic. However, when an early successional species has traits that allow it to persist and fundamentally alter the vegetation mosaic over time, as is the case with balsam poplar, it may serve as the leading edge of compositional changes with profound consequences. Our results highlight the capacity of a single species to catalyze the changes that may eventually lead to the altering of an entire landscape mosaic.

Abstract. Climate warming has initiated changes to vegetation across subarctic North America with potential to dramatically alter the distribution of biodiversity of vascular plants, mosses, and macrolichens. However, landscape-scale studies of the patterns and drivers of species richness in this region are scarce, raising the possibility that dramatic changes to biodiversity could occur undetected over a wide area with serious consequences for ecosystem integrity and conservation. We used a hierarchical, systematic design to compile a uniquely large and comprehensive diversity dataset for our study area in subarctic North America. We utilized a unified sampling frame at the landscape scale to record diversity of vascular plants, mosses, and macrolichens as the three primary components of vegetation species richness. We applied Bayesian hierarchical modeling techniques to identify site attributes associated with richness of the three functional groups. We also examined whether richness of groups was positively or negatively intercorrelated across multiple spatial scales. Our goal was to quantify the fundamental relationships of species richness with site attributes across this landscape to better understand the possible responses of species richness to habitat changes forecast to occur in the subarctic as a result of a warming climate and other stressors. Moss, vascular plant, and terricolous macrolichen species richness were strongly inter-correlated, due to an underlying marked positive association of each with increasing elevation into the alpine zone across multiple spatial scales. Our results further reveal varying influences of factors such as soil pH, disturbance, and plant canopy cover on diversity in these physiologically different functional groups. Taken together, the patterns revealed by our work provide a new framework to consider how predicted habitat changes wrought by warming climates in interior Alaska may affect fundamental diversity patterns of primary producers in the future, with important implications for ecosystem function and conservation.