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Aims Respiration of sugar maple ( Acer saccharum ) surface fine roots has been shown to partially acclimate to experimentally increased soil temperature. In this study, we assessed how larger roots and roots at deeper depths responded to experimental warming. Methods We quantified specific root respiration and root biomass for three different diameter classes (<1, 1–2, and 2–10 mm) from three soil depths (0–10, 10–30, and 30–50 cm) in a sugar maple forest that had received a factorial combination of increased soil temperature (4 to 5 °C above ambient) and supplemental precipitation for three growing seasons. Results Partial temperature acclimation occurred for respiration of fine-roots (<1 mm) at 0–10 cm, limiting the increase to 30% above that for roots in the control treatment. In contrast, there was no evidence for acclimation of fine-roots at deeper depths, where soil warming caused respiration to more than double. There was evidence of acclimation for 1–2 mm roots at the 0–10 cm depth (20% reduction in respiration at an 18 °C reference temperature) but not for the larger diameter roots at any of the three soil depths. Root biomass was not altered by soil warming or moisture addition. Conclusions Despite partial thermal acclimation in surface fine-root respiration, soil warming caused an overall 41% increase in the C flux to the atmosphere from respiration of roots in the upper 30 cm of soil, from 21.3 to 30.1 μmol m −2  s −1 , potentially reducing C availability for biomass production.

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.

Effect of geraniin extracted from sugar maple (Acer saccharum) leaves on the viability of the phytopathogen Xanthomonas campestris pv. vitians was evaluated with the SYTOX Green nucleic acid stain, penetrating only compromised membranes, and plate counts. In parallel, structural changes of treated bacteria were examined in transmission electron microscopy (TEM). Based on SYTOX Green and plate counts, geraniin at the minimum bactericidal concentration (3.125 mg/ml) increased mortality after 45 min by 37% and 62%, respectively, when compared with controls. According to observations in TEM, geraniin caused morphological alterations of these rod-shaped bacteria, including degradation of their envelopes, as also suggested by the incorporation of SYTOX. These alterations were often accompanied by cytoplasm leakage and the formation of more pronounced whitish areas in the cytoplasm similar to vacuolization. Moreover, multi-membranous and/or – wall systems were at times formed in the treated bacteria. The presence of some extracellular electron-dense material was frequently noted around the treated bacteria. The matrix surrounding control bacteria tended to disappear after geraniin treatment. This study highlights for the first time the effect of geraniin on bacterial ultrastructure, thus contributing to a better understanding of the mechanism by which this molecule exerts antibacterial activity.

Most studies of forest litter dynamics examine the biochemical characteristics and decomposition of leaf litter, but fine roots are also a large source of litter in forests. We quantified the concentrations of eight biochemical fractions and nitrogen (N) in leaf litter and fine roots at four sugar maple (Acer saccharum)-dominated hardwood forests in the north-central United States. We combined these results with litter production data to estimate ecosystem biochemical fluxes to soil. We also compared how leaf litter and fine root biochemistry responded to long-term simulated N deposition. Compared with leaf litter, fine roots contained 2.9-fold higher acid-insoluble fraction (AIF) and 2.3-fold more condensed tannins; both are relatively difficult to decompose. Comparatively, leaf litter had greater quantities of more labile components: nonstructural carbohydrates, cellulose and soluble phenolics. At an ecosystem scale, fine roots contributed over two-thirds of the fluxes of AIF and condensed tannins to soil. Fine root biochemistry was also less responsive than leaf litter to long-term simulated N deposition. Fine roots were the dominant source of difficult-to-decompose plant carbon fractions entering the soil at our four study sites. Based on our synthesis of the literature, this pattern appears to be widespread in boreal and temperate forests.

Phyllosphere bacterial diversity is shaped through interactions between hosts and microbes. Most studies having focused on pairwise associations between host taxa and their symbionts, little is yet understood about the influence of the host community as a whole in shaping these interactions. Envisioning phyllosphere bacterial communities as a spatially structured network of communities linked by dispersal (i.e., metacommunities) can help us better understand the relative importance of species sorting among host populations and species versus dispersal from the neighboring host community for bacterial community assembly in forest ecosystems. Here we investigate drivers of metacommunity structure of epiphytic bacteria of the phyllosphere among 33 tree host species distributed across a large-scale transition from deciduous to boreal forest. We expect the identity and traits of hosts to play an important role in determining phyllosphere bacterial composition. We further hypothesize that bacterial dispersal from neighboring host species will modulate the match between a focal host species and its microbiota, and shape opportunities for host specialization of phyllosphere bacteria at local and regional scales. We defined specialization as the level of phylogenetic similarity among hosts that a bacterial symbiont associates with. We found that host taxonomic identity and traits were important drivers of bacterial community turnover and variation in host specialization across the landscape. Dispersal from neighboring communities further played a role in homogenizing bacterial communities. The microbiota of focal hosts such as sugar maple was thus increasingly similar to that of neighboring host species along the transition from deciduous to boreal forest. Specialization of bacterial taxa on sugar maple was further positively correlated with the relative abundance of this host in the landscape, revealing a role for the host community context in shaping evolutionary relationships between phyllosphere bacteria and their tree hosts. These results overall suggest that the dispersal of phyllosphere bacteria from the dominant tree community members may be constraining the match between tree species and their symbionts, particularly at their range limits. We also demonstrate that considering host-associated microbial communities as part of metacommunities within the host landscape is a promising tool for improving our understanding of host-symbiont matching.

Climate warming is expected to cause the poleward and upward elevational expansion of temperate plant species, but non‐climatic factors such as soils could constrain this range expansion. However, the extent to which edaphic constraints on range expansion have an abiotic (e.g. soil chemistry) or biotic (e.g. micro‐organisms) origin remains undetermined. We conducted greenhouse experiments to test if the survival and growth of a major North American temperate tree species, Acer saccharum (sugar maple), is independently or jointly constrained by abiotic and biotic properties of field‐collected soils from within and beyond the species' elevational range. Abiotic factors, particularly low base cation concentrations, were major constraints to seedling establishment in boreal forest soils (beyond the range edge), but insufficient arbuscular mycorrhizal fungal inoculum (biotic factor) also strongly reduced seedling performance in these soils. Synthesis. Our results suggest that forecasting future changes in forest composition under climate warming requires consideration of soil properties as well as the mycorrhizal status of tree species. Plant establishment in a new habitat can be mediated by soil‐related abiotic and biotic factors. Our study shows that unfavourable soil chemistry and lack of beneficial mycorrhizal fungi in boreal forests greatly diminished the potential survival and growth of sugar maple seedlings. Predicting future changes in forest composition under climate warming will require consideration of soil properties as well as the mycorrhizal status of tree species. Translated Résumé Le réchauffement climatique devrait entraîner l'expansion des espèces tempérées vers les pôles et en altitude, mais des facteurs non‐climatiques pourraient limiter cette expansion. Les sols en particulier pourraient potentiellement contraindre l'expansion des essences tempérées dans les forêts boréales à cause de propriétés édaphiques biotiques (micro‐organismes p. ex.) et/ou abiotiques (propriétés chimiques p. ex.). Dans cette étude, nous avons mené des expériences en serre utilisant des manipulations et traitements de sols frais récoltés sur le terrain. Nous avons ainsi testé si l'établissement, dans la forêt boréale, d'une espèce d'arbre dominante en forêt tempérée d'Amérique du Nord (Acer saccharum) est limitée par les propriétés édaphiques. Nous avons évalué le rôle joué par les micro‐organismes et la chimie du sol sur la survie, la biomasse et la performance globale des semis. Les facteurs abiotiques tels que de faibles concentrations en cations pourraient constituer des contraintes édaphiques importantes à l'établissement d'Acer saccharum au‐delà de son aire de répartition, mais aussi les facteurs biotiques tel que le faible potentiel d'inoculum de champignons mycorhiziens arbusculaires dans les sols boréaux. Nos résultats suggèrent que la prévision des changements futurs dans la composition de la forêt sous l'effet du réchauffement climatique nécessite de prendre en compte les propriétés édaphiques, ainsi que les traits souterrains des arbres tels que leur stratégie mycorhizienne.

Acidic deposition has been well studied for its impacts on forest soils and plants and is thought to detrimentally affect sugar maple (Acer saccharum) in northeastern North America. The physiological mechanism driving reduced tree growth on acid impaired sites is not well understood, but has been suggested to involve increased autotrophic respiration rates. We measured foliar respiration and leaf elemental composition on two species with contrasting acid tolerances (sugar maple and American beech, Fagus grandifolia) across a naturally occurring soil base saturation (BS) gradient in the Adirondack Mountains of New York. Foliar chemistry varied strongly as anticipated across the BS gradient, with decreases in base cations and increases in phytotoxic metals (e.g., Mn) on sites with highly acidified soils. However, foliar respiration rates were not correlated with most measures of acidity (e.g., BS, foliar concentrations of Ca, Mg, Al content). Respiration rates did correlate with other leaf traits (N content, leaf mass per unit area) reflective of leaf morphological variation in response to variable light availability. After accounting for multivariate trait covariation, mass-based respiration rates were associated with foliar Mn content, suggesting a positive relationship between the concentration of this phytotoxic metal and foliar respiration rates. Soluble foliar Ca, Mg, and Mn were closely correlated with total foliar concentrations of these elements. Overall these results demonstrate that soil acidification and loss of base cations is largely unrelated to foliar respiration rates, although the accumulation of foliar Mn on acidified soils may contribute to the metabolic burden of both sugar maple and American beech trees.

Global warming is affecting plant phenology, with potential consequences on the dynamics of growth reactivation of sugar maple and the timings of maple syrup production. In this study, we assess the temperatures inducing the daily reactivation or cessation of sap production. We selected 19 sugarbushes across Quebec, Canada, using a tapping method associated with the tubing system, we recorded the daily timings of onset and ending of sap production during winter and spring 2018, and we associated the hourly temperatures at each site. Sap production occurred from mid-February to the end of April, starting on average between 10 and 11 AM, and ending from 6 to 8 PM. We observed a seasonal pattern in the onset and ending of sap production during spring, with the onset showing a greater change than the ending. Onset and ending of sap production occurred mostly under temperatures ranging between –2 and 2 °C. The production of sap in maple is closely related to circadian freeze–thaw cycles and occurs under nighttime and daytime temperatures fluctuating below and above 0 °C. The daily lengthening of the duration of sap production mirrors the changes in the timings of freeze and thaw events and can be explained by the physical properties of the water and the physiological processes occurring during growth reactivation. The ongoing warming will result in earlier and warmer springs, which may anticipate the cycles of freeze and thaw and advance sap production in sugar maple.

Climate change is prompting plants to migrate and establish novel interactions in new habitats. Because of the pivotal roles that microbes have on plant health and function, it is important to understand the ecological consequences of these shifts in host–microbe interactions with range expansion. Here we examine how the diversity of plant-associated microbes varies along the host’s current range and extended range according with climate change predictions, and assess the relative influence of host genotype (seed provenance) and environment in structuring the host microbiome. We collected sugar maple seeds from across the species current range, then planted them in temperate and mixedwood/transitional forests (current range) and in the boreal region (beyond range but predicted future range in response to climate change). We used amplicon sequencing to quantify bacterial, fungal, and mycorrhizal communities from seedling leaves and roots. Variation among sites and regions were the main drivers of the differences in host microbial communities, whereas seed provenance did not play a large role. No unifying pattern was observed for microbial community richness, diversity, or specialization, demonstrating the complexity of responses of different taxa on above- and belowground plant compartments. Along the latitudinal gradient, we (1) observed reductions in mycorrhizal diversity that can negatively impact maple establishment; (2) and revealed reductions in fungal leaf pathogens that can have opposite effects. Our results highlight the need for an integrated approach including the examination of various microbial taxa on different plant compartments to improve our understanding of plant range shifts and plant— microbe interactions.

AimsClimate change is expected to shift climatic envelopes of temperate tree species into boreal forests where unsuitable soils may limit range expansion. We studied several edaphic thresholds (mycorrhizae, soil chemistry) that can limit seedling establishment of two major temperate tree species, sugar maple (arbuscular mycorrhizal, AM) and American beech (ectomycorrhizal, EM).MethodsWe integrate two field surveys of tree seedling density, mycorrhizal colonization, and soil chemistry in montane forests of the Adirondack and Green Mountains (Mtns) in the northeastern United States. We conducted correlation and linear breakpoint analyses to detect soil abiotic and biotic thresholds in seedling distributions across edaphic gradients.ResultsIn the Green Mtns, sugar maple seedling importance (an index of species relative density and frequency, IV) declined sharply with low pH (< 3.74 in mineral soil) and low mycorrhizal colonization (< 27.5% root length colonized). Sugar maple importance was highly correlated with multiple aspects of soil chemistry, while beech was somewhat sensitive to pH only; beech mycorrhizal colonization did not differ across elevation. Mycorrhizal colonization of sugar maple was positively correlated with soil pH and conspecific overstory basal area. In the Adirondacks, sugar maple importance, but not beech, plateaued above thresholds in soil calcium (~ 2 meq/100 g) and magnesium (~ 0.3 meq/100 g).ConclusionsThe establishment of sugar maple, but not beech, was impeded by both biotic and abiotic soil components in montane conifer forests and by soil acidity in temperate deciduous forests. These differences in species sensitivity to edaphic thresholds will likely affect species success and future shifts in forest composition.