Increasing fire and the decline of fire adapted black spruce in the boreal forest

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

Data from: Regional variation in interior Alaskan boreal forests is driven by fire disturbance, topography, and climate

High latitude regions are warming rapidly with important ecological and societal consequences. Utilizing two landscape-scale datasets from interior Alaska, we compared patterns in forest structure in two regions with sharply differing fire disturbance, topography, and climate. Our goal was to evaluate a set of hypotheses concerning possible warming-driven changes in forest structure suggested by recent literature. We found essentially consistent habitat associations for the tree flora across two disparate study areas concomitant with considerable differences in observed patterns of forest structure and composition. Our results confirmed expected increases in broadleaved species occupancy and abundance in the warmer, more fire-affected study region along with considerably higher tree occupancy and abundance in high elevation areas there. However, contrary to our predictions, we found no evidence of expected reductions in conifer occupancy or increases in non-fire related tree mortality. Instead, both individual and combined tree species occupancy, density, abundance, and richness were considerably higher in the warmer, more fire-influenced region, except in the warmest, driest areas (steep and south-facing slopes at low elevation). Our comparison of two landscape-scale datasets suggests that changes in tree distribution and forest structure in interior Alaska will proceed unevenly, governed by a mosaic of site-dependent influences wherein forest community composition and species dominance will shift along different trajectories and at different rates according to variation in underlying landscape attributes. Although there were clear differences in forest structure between the two areas that were likely attributable to differences in growing season warmth and fire disturbance, we found scant support for the concept of an incipient, ongoing biome shift in interior Alaska resulting from impending diminution of boreal forest cover over the short to medium term. Indeed, we suggest that (depending on severity of disturbance dynamics and the rapidity of future warming) cooler areas of interior Alaska’s forest may reasonably be expected to sustain marginal increases in forest cover with additional warming, at least in certain topographic positions (such as poorly drained basins and cool treeline sites) and/or geographic regions, prior to any landscape-scale diminution of forest cover due to warming

NelsonPeterBotanyPlantPathNonParametricMethodsReveal.pdf

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.Keywords: Disturbance, Denali, Lichen, Alaska, Photobiont, Community assembly, Functional traits, Fire, Vegetative dispersal, Growth formKeywords: Disturbance, Denali, Lichen, Alaska, Photobiont, Community assembly, Functional traits, Fire, Vegetative dispersal, Growth for

High-elevation ground-layer plant community composition across environmental gradients in spruce-fir forests

We examined the influence of vegetation structure and soil chemistry on post-adelgid, ground-layer plant communities in high-elevation forests of the southern Appalachian Mountains. Specifically, we hypothesized that post-disturbance community composition and diversity would vary along a gradient of soil acidity and other soil characteristics influenced by acid deposition. Ground-layer vegetation and soils were sampled on 60 randomly located nested vegetation plots in the spruce-fir (Picea abies) zone of Great Smoky Mountains National Park, TN and NC, USA. To capture a range of deposition levels, plot placement was stratified based on modeled acid deposition classes. Ordination and multiple regression results showed that ground-layer composition and diversity were negatively associated with acidity of the A horizon and the presence of ericaceous shrubs (i. e., Rhododendron spp.). A strong correspondence between soil acidity and ericaceous shrub cover was also observed, suggesting that soil acidity may be, in conjunction with overstory disturbance resulting from chronic acid deposition and adelgid induced mortality, an important driver of ericaceous shrub thicket expansion. Slow-decaying, acidic ericaceous litter may also induce a positive feedback resulting in enhanced acidification. © 2011 The Ecological Society of Japan

Regeneration responses to exogenous disturbance gradients in southern Appalachian Picea-Abies forests

Because of the devastation caused by the combined impacts of the balsam woolly adelgid (Adelges piceae; BWA, a non-native insect) and chronic acid deposition, Picea-Abies (spruce-fir) forests are one of the most threatened vegetation communities in North America. Endemic Abies fraseri (Fraser fir), the dominant overstory species in these forests, has experienced near complete overstory mortality as result of the adelgid. Observed forest regeneration patterns suggest high spatial variability, with dense patches of Rubus spp. (blackberry), A. fraseri, and deciduous regeneration repeating across the landscape. To quantify the spatial variability and density of A. fraseri, Picea rubens (red spruce), and deciduous regeneration in these forests, we sampled 60 randomly selected plots within Picea-Abies forests of Great Smoky Mountains National Park (GSMNP). As a measure of local variability in regeneration, we used regeneration density within 30 1. ×. 1. m subplots per plot to calculate a coefficient of within-plot variation for each species. Despite the impacts of the BWA and chronic acid deposition, A. fraseri remained an important component of southern Appalachian Picea-Abies forests at elevations above 1750. m. Density of A. fraseri in all size classes was associated with time since disturbance by BWA infestation. On plots where A. fraseri regeneration occurred, its local variability (among 30 subplots) was significantly greater than that of P. rubens or deciduous species. Regression models attributed this local-scale variability in A. fraseri regeneration to the influence of elevation, Rubus spp. cover, B-horizon nitrogen concentration, and O-horizon calcium to aluminum ratio. We propose that co-occurring gradients of BWA-induced mortality and acid deposition have created patches of increased light and nitrogen availability, which have increased competition from ruderal species such as Rubus spp. Additionally, our results suggest that high soil aluminum content relative to calcium may exclude A. fraseri from certain locations, further contributing to the local variability of A. fraseri regeneration. © 2012 Elsevier B.V

Elevational gradients of bryophyte diversity, life forms, and community assemblage in the southern appalachian mountains

We investigated the influence of fine-scale elevational gradients and overstory disturbance on bryophyte distribution, diversity, and community composition. Bryophyte species cover and richness were sampled across 60 randomly selected plots within high-elevation spruce-fir (Picea-Abies) forests of Great Smoky Mountains National Park. Ordination and regression analyses revealed a fine-scale elevation gradient (700 m) in bryophyte community composition. Observed changes in bryophyte diversity and community composition were also associated with variation in deciduous basal area and thus litter composition, the prevalence of herbaceous plants, and the degree of canopy openness resulting from balsam woolly adelgid (Adelges piceae Ratz.) infestation. Although overstory disturbances, such as those caused by the adelgid, create suitable substrate for bryophyte colonization, the corresponding increase in light availability and deciduous basal area may alter bryophyte diversity and community assemblages

Ground-layer bryophyte communities of post-adelgid picea-abies forests

Spruce-fir forests of the southern Appalachians are threatened by the widespread death of Abies fraseri (Fraser Fir) caused by the exotic Adelges piceae (Balsam Woolly Adelgid). Subsequent canopy opening, due to decimation of the fir population, has likely affected ground-layer dynamics and diversity. We sampled bryophytes on 60 randomly selected plots within the spruce-fir zone of Great Smoky Mountains National Park (GSMNP) using the line-intercept method (total sampling distance of 1800 m). Our sampling revealed 97 bryophyte species (64 mosses and 33 liverworts) comprising 32 families and 60 genera on ground-layer substrates in spruce-fir forests. Our results suggest that upwards of 20% of the bryoflora of GSMNP can be found on ground-level substrates in the spruce-fir zone

Borealization and its discontents: drivers of regional variation in plant diversity across scales in interior Alaska

Abstract Quantitative studies of regional variation in plant diversity across eastern Beringia (northern Alaska and adjacent areas) are lacking due to an absence of datasets of sufficient scale and scope. We interrogated a landscape‐scale plant diversity dataset collected across two regions of interior Alaska with different disturbance, topographic, and climate attributes to investigate hypotheses regarding drivers of plant species richness. Our approach integrated a multi‐scale sampling design with an analytical framework focused on quantifying how components of plant diversity (growth forms, biogeographic groups, and dominant species) respond to site factors that vary along landscape gradients. Our results revealed essential similarities in both the composition of the overall floras and the influences on local and meso‐scale species richness across both regions. However, these continuities at smaller scales contrasted with differences in landscape‐level distribution of plant diversity patterns along elevation gradients. Our findings suggest that local drivers of richness and occupancy interacted with differing macro‐scale attributes (e.g., relative continentality) to produce distinctive landscape‐level diversity patterns. Our results confirm that high levels of local and meso‐scale plant richness in interior Alaska depend on conditions that foster richness of herbaceous and northerly distributed species groups. However, we found that important differences in landscape‐level richness patterns were driven by regional differences in climate, topoedaphic variables, and disturbance. In the warmer region, woody species and boreal plant communities extended to higher elevations and common species occupancy showed marginally greater influence of fire. Overall richness was relatively low in alpine areas of the warmer region but heterogeneous edaphic and topographic circumstances stimulated higher species turnover in lower elevations there, increasing landscape‐level richness. In contrast, in the cooler region, woody species showed restricted distribution across the elevation gradient while site attributes associated with increased species richness aligned with the elevation gradient and thus peak richness occurred in the alpine zone. Our results show how total and growth‐form richness, as well as community composition, vary regionally in relation to important drivers (including growing season warmth) across interior Alaska. Consequently, our study provides new insights into the potential trajectories of future change in biodiversity patterns in this rapidly warming region