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In montane areas, climate change can shift tree species distributions upslope over time which can affect forest ecosystem structure and functioning. Seedlings of low-elevation temperate broadleaf trees establishing beyond their ranges at high elevations need to overcome both herbivore pressure and soil nutrient limitations. To assess the influence of these two stressors, we quantified leaf and stem herbivory, soil and foliar nutrient status, and two-year survival of experimentally grown seedlings of two hardwood species, sugar maple (Acer saccharum) and American beech (Fagus grandifolia) along elevation gradients in the Green Mountains of Vermont, USA. While insect foliar herbivory was reduced on maple seedlings growing beyond range boundaries, suggesting enemy escape, the opposite pattern was observed for beech. Mammalian browsing increased with elevation for both species. In general, foliar nutrient concentrations and resource utilization (that is, the relationship between soil nutrient availability and foliar nutrient status) declined with elevation for both species (although more so for maple, especially calcium), while toxic foliar aluminum concentrations increased for maple. Survival decreased with elevation for both species, but especially for sugar maple, linked more to declining foliar nutrient status than herbivory at higher elevations. Thus, the effects of herbivory and nutrient utilization on seedling survival can be critical for shaping tree species range shifts and, ultimately, species composition and forest ecosystem functioning. 

Abstract Global warming has been shifting climatic envelopes of many tree species to higher latitudes and elevations across the globe; however, unsuitable soil biota may inhibit tree migrations into these areas of suitable climate. Specifically, the role of mycorrhizal fungi in facilitating tree seedling establishment beyond natural species range limits has not been fully explored within forest ecosystems. We used three experiments to isolate and quantify the effects of mycorrhizal colonization and common mycorrhizal networks (CMN) on tree seedling survival and growth across (within and beyond) the elevational ranges of two dominant tree species in northeastern North America, which were associated with either arbuscular mycorrhiza (AMF,Acer saccharum) or ectomycorrhiza (EMF,Fagus grandifolia). In order to quantify the influence of mycorrhiza on seedling establishment independent of soil chemistry and climate, we grew seedlings in soils from within and beyond our study species ranges in a greenhouse experiment (GE) as well as in the field using a soil translocation experiment (STE) and another field experiment manipulating seedling connections to potential CMNs (CMNE). Root length colonized, seedling survival and growth, foliar nutrients, and the presence of potential root pathogens were examined as metrics influencing plant performance across species' ranges. Mycorrhizal inoculum from within species ranges, but not from outside, increased seedling survival and growth in a greenhouse setting; however, only seedling survival, and not growth, was significantly improved in field studies. Sustained potential connectivity to AMF networks increased seedling survival across the entire elevational range ofA. saccharum. Although seedlings disconnected from a potential CMN did not suffer decreased foliar nutrient levels compared with connected seedlings, disconnected AM seedlings, but not EM seedlings, had significantly higher aluminum concentrations and more potential pathogens present. Our results indicate that mycorrhizal fungi may facilitate tree seedling establishment beyond species range boundaries in this forested ecosystem and that the magnitude of this effect is modulated by the dominant mycorrhizal type present (i.e., AM vs. EM). Thus, despite changing climate conditions beyond species ranges, a lack of suitable mutualists can still limit successful seedling establishment and stall adaptive climate‐induced shifts in tree species distributions. 

Abstract One Health's interdisciplinary approach has been effective at the nexus of human and animal health but often overlooks environmental health, including wildland fire. Fire management seeks to suppress dangerous fires and to manage others for resource benefit, which inherently pits human health against the health of fire-dependent biodiversity. One Health may better address past failures to achieve fire management goals by providing a more comprehensive and cohesive framework explicitly recognizing interconnectedness of plant, animal, human, and environmental health. Although there are exceptions, we suggest novel health solutions and maximal health benefits will most often result from pyrohealth synchrony: actively aligning contemporary fire management with historic fire-regimes (i.e., long-term patterns in fire frequency, intensity, severity, seasonality, and spatial extent). We also present a process for making fire-integrated One Health an applied reality capable of galvanizing stakeholders around interventions that better interconnect fire to the health of plants, animals, people, and environments. 

Aim: As climate warms, contemporary tree regeneration patterns will affect future forest composition. Yet the factors influencing forest regeneration across environmental gradients remain difficult to disentangle. We studied how conspecific canopy tree dominance (as a proxy for seed availability) interacts with microsite factors along elevational climatic gradients to affect regeneration patterns and recruitment limitations across temperate deciduous and montane boreal forests. Location: Adirondack and Appalachian Mountains (NY, VT, NH, and ME), USA. Taxa: Abies balsamea, Acer saccharum, Betula alleghaniensis, B. papyrifera var. cordifolia, Fagus grandifolia, and Picea rubens. Methods: We measured overstory tree dominance (via species-specific basal area), understory seedling abundance, and microsite factors (e.g., understory vegetation cover, canopy openness) across elevation on ten mountains. Generalised linear regression models were fit to quantify the importance of conspecific tree basal area (representing seed availability), microsite factors, and their interaction on recruitment for each species along elevational climatic gradients. Results: Seedling abundance increased with greater conspecific adult basal area (i.e., overstory-understory relationships were positive) for all species except the shade-intolerant and wind-dispersed B. papyrifera var. cordifolia. Sparse understory vegetation amplified positive overstory-understory relationships for A. saccharum and B. alleghaniensis, while closed canopies dampened them for the shade-intolerant B. alleghaniensis and B. papyrifera var. cordifolia. The positive overstory-understory relationship for F. grandifolia was weaker in warmer microclimates, while that for the shade-tolerant A. balsamea was greater under closed canopies, likely owing to seedling banks. Main Conclusions: Nearly ubiquitous positive overstory-understory relationships indicated that local conspecific adult tree dominance drives recruitment (likely through seed dispersal) as expected. However, microsite conditions interacted with overstory basal area and modified these overstory-understory relationships differently for each species to either inhibit or facilitate tree recruitment. This framework integrates broad-scale density-dependent processes (i.e., overstory-understory conspecific relationships) with fine-scale drivers to better understand tree species recruitment across biome boundaries. 

A century of beech bark disease (BBD) in North America has transformed hardwood forests by reducing the canopy biomass of American beech (Fagus grandifolia), even as beech has come to dominate the sapling layer of many forests. We do not understand the extent to which environmental change drivers such as climate, acidic atmospheric deposition (and its legacy of acidified soils), and invasive disease (BBD) may have contributed to this transformation. We investigated how BBD effects and tree community composition varied along a well-documented soil acidity gradient in the northeastern United States. We surveyed overstory and sapling layer tree species composition, BBD effects, and soil chemistry on 30 watersheds in forests codominated by beech and sugar maple (Acer saccharum). We analyzed potential drivers of community composition, BBD, and beech sapling density using linear models and non-metric multidimensional scaling. Predictors accounted for soil chemistry, climate, overstory beech (importance value, IV), mortality, and BBD defect. Overall overstory species composition varied most along the acidity gradient, while beech and BBD severity varied along their own distinct environmental gradient. Species composition of the overstory and sapling layers diverged significantly, with the latter dominated by beech. Beech sapling density was positively related to the proportion of standing dead overstory beech and soil exchangeable aluminum, but was unrelated to the overall proportion of overstory beech or their BBD severity. The dominance of sapling layers by beech may have resulted from a gradual accumulation of canopy-opening events precipitated by BBD and sugar maple decline, the latter driven by stressors such as acidification and climate change. 

Alpine treeline ecotones are influenced by environmental drivers and are anticipated to shift their locations in response to changing climate. Our goal was to determine the extent of recent climate‐induced treeline advance in the northeastern United States, and we hypothesized that treelines have advanced upslope in complex ways depending on treeline structure and environmental conditions. Location: White Mountain National Forest (New Hampshire) and Baxter State Park (Maine), USA. Taxon: High‐elevation tree species—Abies balsamea, Picea mariana and Betula cordata. Methods: We compared current and historical high‐resolution aerial imagery to quantify the advance of treelines over the last four decades, and link treeline changes to treeline form (demography) and environmental drivers. Spatial analyses of the aerial images were coupled with ground surveys of forest vegetation and topographical features to ground‐truth treeline classification and provide information on treeline demography and additional potential drivers of treeline locations. We used multiple linear regression models to examine the importance of both topographic and climatic variables on treeline advance. Results: Regional treelines have significantly shifted upslope over the past several decades (on average by 3 m/decade). Gradual diffuse treelines (characterized by declining tree density) showed significantly greater upslope shifts (5 m/decade) compared to other treeline forms, suggesting that both climate warming and treeline demography are important correlates of treeline shifts. Topographical features (slope, aspect) as well as climate (accumulated growing degree days, AGDD) explained significant variation in the magnitude of treeline advance (R2 > = 0.32). Main Conclusions: The observed advance of treelines is consistent with the hypothesis that climate warming induces upslope treeline shifts. Overall, our findings suggest that gradual diffuse treelines at high elevations may be indicative of climate warming more than other alpine treeline ecotones and thus they can inform us about past and ongoing climatic changes. 

The persistence of future forests depends on the success of tree seedlings which are experiencing increasing physiological stress from changing climate and air pollution. Although the moss layer can serve as an important substrate for tree seedlings, its potential for reducing environmental stress and enhancing the establishment of seedlings remains poorly understood. We tested if the moss layer decreased environmental stress and increased the abundance of balsam fir seedlings dominant in high-elevation forests of northeastern United States that are sensitive to changing climate and mercury deposition. We surveyed balsam fir seedling density by substrate (moss, litter, other) on 120 quadrats (1 × 1 m) in two contrasting canopy environments (in gaps and under canopies), measured seedling stress, and quantified mercury content in seedlings and substrates. We observed that, in both canopy environments, tree seedlings established on moss exhibited (i) increased density, (ii) decreased physiological stress, and (iii) higher potential to recruit into larger size classes, compared to seedlings established in litter. Regardless of canopy environment, seedling foliar mercury levels did not correspond to substrate mercury despite large differences in substrate mercury concentrations (relative to moss, litter concentrations were ~ 4-times greater and soil concentrations were ~ 6-times greater), likely reflecting the dominance of foliar over root uptake of mercury. Because the moss layer appeared to mitigate seedling drought stress, and to increase seedling establishment and recruitment compared to other substrates, these microsite effects should be considered in models predicting forest regeneration and dynamics under increased drought stress associated with the ongoing climate warming. 

Canopy openness is an important forest characteristic related to understory light environment and productivity. Although many methods exist to estimate canopy openness, comparisons of their performance tend to focus on relatively narrow ranges of canopy conditions and forest types. To address this gap, we compared two popular approaches for estimating canopy openness, traditional spherical densiometer and modern smartphone hemispherical photography, across a large range of canopy conditions (from closed canopy to large gaps) and forest types (from low-elevation broadleaf to high-elevation conifer forests) across four states in the northeastern United States. We took 988 field canopy openness measurements (494 per instrument) and compared them across canopy conditions using linear regression and t-tests. The extensive replication allowed us to quantify differences between the methods that may otherwise go unnoticed. Relative to the densiometer, smartphone photography overestimated low canopy openness (<10%) but it underestimated higher canopy openness (>10%), regardless of forest type.