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Species composition of temperate forests vary with successional age (i.e., years after a major disturbance) and seems likely to change in response to significant global climate change. Because photosynthesis rates in co-occurring tree species can differ in their sensitivity to environmental conditions, these changes in species composition are likely to alter the carbon dynamics of temperate forests. To help improve our understanding of such atmosphere-biosphere interactions, we explored changes in leaf-level photosynthesis in a 60-70 yr old temperate mixed-deciduous forest in Petersham, Massachusetts (USA). Diurnally and seasonally varying environmental conditions differentially influenced in situ leaf-level photosynthesis rates in the canopies of four mature temperature deciduous tree species: red oak (Quercus rubra), red maple (Acer rubrum), white birch (Betula papyrifera), and yellow birch (Betula alleghaniensis). We measured in situ photosynthesis at two heights within the canopies (top of the canopy at ∼ 20 m, and in the sub-canopy of the same individual trees at ∼ 14-16 m) through a diurnal time course on 7 d over two growing seasons. We simultaneously measured a suite of environmental conditions surrounding the leaf at the time of each measurement. We used path analysis to examine the influence of environmental factors on in situ photosynthesis in the tree canopies. Overall, red oak had the highest photosynthesis rates, followed by white birch, yellow birch, and red maple. There was little evidence for a substantial midday depression in photosynthesis. Instead, photosynthesis declined throughout the day, particularly after 1600. Diurnal patterns of light reaching canopy leaves, leaf and air temperature, and vapor pressure deficit (VPD) contributed to diurnally varying photosynthesis rates. Large differences in these parameters through the growing season partly led to the seasonal differences observed in photosynthesis rates. Path analyses helped to identify the relative contribution of various environmental factors on photosynthesis and further revealed that species-specific sensitivities to various environmental conditions shifted through the season. Red oak photosynthesis was particularly sensitive to air temperatures late in the season when air temperatures were low. Further, red maple photosynthesis was particularly sensitive to high VPDs through the growing season. Incorporating data on the physiological differences among tree species into forest carbon models will greatly improve our ability to predict alterations to the forest carbon budgets under various environmental scenarios such as global climate change, or with differing species composition.

1. Climate change is expected to result in a reorganization of the continental distribution of tree species. Recent shifts in distribution patterns have been reported, but it is not always clear how climate change influences these patterns locally, especially in relation to other disturbances. 2. We investigated latitudinal shifts of four ecologically important tree species between 1970 and 2014 within a study area that encompasses their northernmost range limit in northeastern North America (Quebec, Canada; ~761,000 km²). Changes in latitudinal limits were defined in relation to changes in tree saplings' occurrence patterns within forest plots resampled over two time periods (1970-1977 and 2003-2014). By examining changes in the frequency of occurrences in different portions of the study area along a latitudinal gradient, we were able to identify spatially explicit patterns of loss or gain (sapling recruitment) resulting in the shifts observed. We then estimated the probability of observing a recruitment event in response to changes in climate, disturbance and their interaction, using a multimodel selection approach. 3. Latitudinal limits of all four species shifted northward, but these shifts resulted from different patterns of plot occurrence changes, depending on the species and the location examined. Greater recruitment at northern locations than at southern ones drove shifts for Acer saccharum Marsh., Fagus grandifolia Ehrh. and Acer rubrum L., but less so for Betula alleghaniensis Britt. Climate variables indicating changes in early or late growing season conditions were most often selected in models. Warming tended to reduce recruitment probability in the south but increase it in the north, leading to divergent responses for a given species across the study area. Disturbance effects were generally less important than climate change effects, as was their interaction. 4. Synthesis. Spatially explicit and divergent responses to climate change and disturbance drive recruitment patterns underlying latitudinal shifts of tree species. The importance of early- or late-season climate variables points towards biological processes being affected at critical stages of the life cycle. Understanding the factors that influence species' migration capacity in a changing climate is crucial to inform adaptive management and conservation practices.

Foliar traits can reflect fitness responses to environmental changes, such as changes in nutrient availability. Species may respond differently to these changes due to differences in traits and their plasticity. Traits and community composition together can influence forest nutrient cycling. We compared five traits—foliar N, foliar P, specific leaf area (SLA), leaf dry matter content (LDMC), and leaf carbon isotope ratio (δ13C)—in six northern hardwood tree species (Acer rubrum, Acer saccharum, Betula alleghaniensis, Betula papyrifera, Fagus grandifolia, and Prunus pensylvanica) in a nitrogen (N) and phosphorus (P) fertilization study across 10 mid- and late-successional forest stands in New Hampshire, USA. We also analyzed the response of tree growth to N and P addition. Nutrient addition shifted trait values towards the “acquisitive” side of the spectrum for all traits except δ13C, reflecting a tradeoff between water-use efficiency and nutrient-use efficiency. Treatment responses in relative basal area increment revealed that the Betula species were N-limited, but traits of all species responded to either or both N and P addition in ways that suggest N and P co-limitation. Two species displayed lower foliar P under N addition, and three species displayed lower foliar N under P addition, which also suggests co-limitation. These indications of co-limitation were reflected at the community level. Specific leaf area, LDMC, and δ13C differed with stand age within several species. Examining trait responses of tree species and communities to nutrient availability increases our understanding of biological mechanisms underlying the complex effects of nutrient availability on forests.

Climate change is projected to dramatically increase boreal wildfire activity, with broad ecological and socioeconomic consequences. As global temperatures rise, periods with elevated fire weather are expected to increase in frequency and duration, which would be expected to increase the number and size of fires. Statistical forecasts or simulations of future fire activity often account for direct climatic effects only, neglecting other controls of importance, such as biotic feedbacks. This could result in overestimating the effects of climate change on fire activity, if the future distribution of vegetation or fuels were to change. We incorporated sensitivity to climate or fire weather and vegetation in a fire simulation model and represented explicitly two key biotic feedbacks linked to succession and regeneration processes. We used this model to forecast annual fire activity from 2011 to 2099 over a large region of boreal forest in Québec, Canada, dominated by balsam fir (Abies balsamea (L.) Mill) and yellow birch (Betula alleghaniensis Britt.) or paper birch (Betula papyrifera Marsh.), with and without the biotic feedbacks. Our simulations show that vegetation changes triggered by fire disturbance altered future fire activity and may even be as important a driver as climate change itself. Indeed, over the course of the century, vegetation changes were projected to offset much of the increase in fire activity that would be expected due to global warming as such. It follows that if biotic feedbacks are not included in statistical or simulation-based forecasts, the resultant projections of future fire activity could be biased upward to a very considerable degree. For the case of end-ofcentury mean annual burn rate, we estimated this positive bias to be as high as 400%. Accounting for biotic feedbacks in simulation models is therefore necessary for accurate projection of future wildfire activity and associated vegetation changes. Purely statistical forecasts based on current vegetation cannot be relied upon, in the presence of biotic feedbacks. Our results further suggest that vegetation management could reduce fire risk in some systems by altering the abundance and distribution of the most highly flammable fuels and thus mitigate the impact of climate change on fire activity.

Forests in the northeastern US are experiencing shifts in community composition due to the northward migration of warm-adapted tree species and certain species’ declines (for example, white ash and eastern hemlock) due to invasive insects. Changes in belowground fungal communities and associated functions will inevitably follow. Therefore, we sought to investigate the relative importance of two important tree characteristics—mycorrhizal type [ectomycorrhizal (EcM) or arbuscular mycorrhizal (AM)] and leaf habit (deciduous or evergreen) on soil fungal community composition and organic matter cycling. We sampled soil in the organic and mineral horizons beneath two AM-associated (Fraxinus americana and Thuja occidentalis) and two ECM-associated tree species (Betula alleghaniensis and Tsuga canadensis), with an evergreen and deciduous species in each mycorrhizal group. To characterize fungal communities and organic matter decomposition beneath each tree species, we sequenced the ITS1 region of fungal DNA and measured the potential activity of carbon- and nitrogen-targeting extracellular enzymes. Each tree species harbored distinct fungal communities, supporting the need to consider both mycorrhizal type and leaf habit. However, between tree characteristics, mycorrhizal type better predicted fungal communities. Across fungal guilds, saprotrophic fungi were the most important group in shaping fungal community differences in soils beneath all tree species. The effect of leaf habit on carbon- and nitrogen-targeting hydrolytic enzymes depended on tree mycorrhizal association in the organic horizon, while oxidative enzyme activities were higher beneath EcM-associated trees across both soil horizons and leaf habits.

The composition of forests in the northeastern United States and the ecosystem services they provide to future generations will depend on several factors. In this paper, we isolate the effects of two environmental drivers, nitrogen (N) deposition and climate (temperature and precipitation) change, through an analysis of a single cohort of 24 dominant tree species. We assembled a tree database using data from U.S. Forest Service Forest Inventory and Analysis monitoring plots. Applying observed species-specific growth and survival responses, we simulated how forest stands in a 19-state study area would change from 2005 to 2100 under 12 different future N deposition–climate scenarios. We then estimated implications for three selected forest ecosystem services: merchantable timber, aboveground carbon sequestration, and tree diversity. Total tree biomass (for 24 species combined) was positively associated with both increased N deposition and temperatures; however, due to differences in the direction and magnitude of species-specific responses, forest composition varied across scenarios. For example, red maple (Acer rubrum) trees gained biomass under scenarios with more N deposition and more climate change, whereas biomass of yellow birch (Betula alleghaniensis) and red pine (Pinus resinosa) was negatively affected. Projections for ecosystem services also varied across scenarios. Carbon sequestration, which is positively associated with biomass accumulation, increased with N deposition and increasing climate change. Total timber values also increased with overall biomass; however, scenarios with increasing climate change tended to favor species with lower merchantable value, whereas more N deposition favored species with higher merchantable value. Tree species diversity was projected to decrease with greater changes in climate (warmer temperatures), especially in the northwestern, central, and southeastern portions of the study area. In contrast, the effects of N deposition on diversity varied greatly in magnitude and direction across the study area. This study highlights species-specific and regional effects of N deposition and climate change in northeastern U.S. forests, which can inform management decision for air quality and forests in the region, as well as climate policy. It also provides a foundation for future studies that may incorporate other important factors such as multiple cohorts, sulfur deposition, insects, and diseases.

Tree-derived dissolved organic matter (DOM) comprises a significant carbon flux within forested watersheds. Few studies have assessed the optical properties of tree-derived DOM. To increase understanding of the factors controlling tree-derived DOM quality, we measured DOM optical properties, dissolved organic carbon (DOC) and calcium concentrations in throughfall and stemflow for 17 individual rain events during summer and fall in a temperate deciduous forest in Vermont, United States. DOC and calcium fluxes in throughfall and stemflow were enriched on average 4 to 70 times incident fluxes in rain. A multiway model was developed using absorbance and fluorescence spectroscopy to further characterize DOM optical properties. Throughfall contained a higher percentage of protein-like DOM fluorescence than stemflow while stemflow was characterized by a higher percentage of humic-like DOM fluorescence. DOM absorbance spectral slopes in yellow birch (Betula alleghaniensis) stemflow were significantly higher than in sugar maple (Acer saccharum) stemflow. DOM optical metrics were not influenced by rainfall volume, but percent protein-like fluorescence increased in throughfall during autumn when leaves senesced. Given the potential influence of tree-derived DOM fluxes on receiving soils and downstream ecosystems, future modeling of DOM transport and soil biogeochemistry should represent the influence of differing DOM quality in throughfall and stemflow across tree species and seasons.

Filling wood cells with polymeric materials offers great opportunities to improve wood strength. Some applications, such as flooring, may require densification on one side of the material only. Yellow birch (Betula alleghaniensis Britt.), sugar maple (Acer saccharum Marsh.) and red oak (Quercus rubra L.) were surface densified through lateral chemical impregnation of monomers under vacuum followed by in situ electron beam polymerization. Lateral impregnation led to low mean chemical retention of 4% for sugar maple, 11% for red oak and 12% for yellow birch. X-ray densitometry and microtomography revealed an asymmetric density profile comparable to mechanically surface densified wood due to polymer-filled vessels. Scanning electron microscopy images showed presence of polymer-filled fibers beneath the surface. Brinell hardness of all species increased significantly compared to untreated wood. Low chemical retention significantly improved hardness due to localized polymer beneath the surface. Density profile characteristics were extracted from densitometry curves and correlated to hardness. Deep monomer penetration was more favorable to hardness than high surface density peak.

With climate change, climatic optima are shifting poleward more rapidly than tree migration processes, resulting in a mismatch between species distributions and bioclimatic envelopes. Temperate hardwood tree species may take advantage of the release of climate constraints and forest management to migrate into the boreal forest. Here, we use the SORTIE‐ND forest simulation model to determine the potential for the persistence of three temperate species (sugar maple, red maple and yellow birch) when introduced at seedling stage in typical balsam fir–paper birch (BF–PB) bioclimatic domain stands of eastern Canada, quantifying the consequences on the native species composition. SORTIE‐ND is a spatially explicit, individual‐based forest stand model that simulates tree growth, regeneration and mortality. We performed a novel parameterization of the SORTIE‐ND tree growth equation allowing for the inclusion of climate modifiers on tree growth. After validating our model with data from permanent forest inventory plots, we modeled the dynamics of unharvested stands at different successional stages, as well as post‐harvest stands, after the addition of sugar maple, red maple and yellow birch seedlings at different densities. Our results show that current BF–PB domain climate conditions do not limit growth and survival of temperate species in boreal stands. Of the temperate species introduced, sugar maple had the lowest ability to grow and survive by the end of the simulation. Species assemblages of host stands were impacted by the presence of temperate species when the addition of seedlings was above 5000 temperate seedlings per hectare at the beginning of the simulation. For stands that were recently clear cut, temperate seedlings were unable to grow due to intense competition from aspen regeneration. Our results suggest that both current climate and competitive interactions between temperate species and boreal species should not impede the ability of temperate species to grow and survive in the BF–PB domain.

Abstract Background and Aims Understanding root traits and their trade-off with other plant processes is important for understanding plant functioning in natural ecosystems as well as agricultural systems. The aim of the present study was to determine the relationship between root morphology and the hydraulic characteristics of several orders of fine roots (<2 mm) for species differing in shade tolerance (low, moderate and high). Methods The morphological, anatomical and hydraulic traits across five distal root orders were measured in species with different levels of shade tolerance and life history strategies. The species studied were Acer negundo, Acer rubrum, Acer saccharum, Betula alleghaniensis, Betula lenta, Quercus alba, Quercus rubra, Pinus strobus and Pinus virginiana. Key Results Compared with shade-tolerant species, shade-intolerant species produced thinner absorptive roots with smaller xylem lumen diameters and underwent secondary development less frequently, suggesting that they had shorter life spans. Shade-tolerant species had greater root specific hydraulic conductance among these roots due to having larger diameter xylems, although these roots had a lower calculated critical tension for conduit collapse. In addition, shade-intolerant species exhibited greater variation in hydraulic conductance across different root growth rings in woody transport roots of the same root order as compared with shade-tolerant species. Conclusions Plant growth strategies were extended to include root hydraulic properties. It was found that shade intolerance in trees was associated with conservative root hydraulics but greater plasticity in number of xylem conduits and hydraulic conductance. Root traits of shade-intolerant species were consistent with the ability to proliferate roots quickly for rapid water uptake needed to support rapid shoot growth, while minimizing risk in uncertain environments.