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Phyllosphere microbial communities have received considerable attention given their important influence on their plant hosts and on ecosystem functioning. In a context where climate change threatens the sustainability of ecosystems, it is important to understand how phyllosphere microbes will respond to changes in their environment. We used 16S rRNA gene amplicon sequencing to quantify phyllosphere bacterial communities of black spruce (Picea mariana (Mill.) B.S.P.) exposed to nitrogen canopy enrichment and soil warming in the boreal forest of Quebec, Canada. The treatments were applied from April to September 2015 and the sampling was done in September. Neither treatment influenced the overall community structure and diversity of black spruce phyllosphere bacterial communities. However, some bacterial taxa and inferred microbial functions did differ among treatments, revealing in particular a stronger response of some bacteria to soil warming rather than nitrogen enrichment. Our results suggest that soil warming could potentially induce more changes in phyllosphere bacterial taxa abundances and functions than could nitrogen addition, with potential consequences for microbial diversity and boreal forest ecosystem function under likely climate change scenarios. Our study suggests avenues for further research to integrate a more mechanistic understanding of the importance of phyllosphere microbes for black spruce and boreal forest ecosystems. Key words: phyllosphere, nitrogen, soil warming, black spruce, climate changes. Les communautes microbiennes de la phyllosphere ont suscite un interet considerable compte tenu de l'influence significative qu'elles exercent sur les plantes hotes et sur le fonctionnement de l'ecosysteme. Dans un contexte oU le changement climatique menace la durabilite des ecosystemes, il est important de comprendre de quelle facon les microbes de la phyllosphere vont reagir aux changements dans leur environnement. Nous avons utilise le sequencage du produit de l'amplification du gene codant pour l'ARNr 16S pour quantifier les communautes bacteriennes de la phyllosphere de l'epicea noir (Picea mariana (Mill.) B.S.P.) en presence d'un apport d'azote dans le couvert forestier et d'un rechauffement du sol dans la foret boreale du Quebec, au Canada. Les traitements ont ete appliques du mois d'avril au mois de septembre 2015 et l'echantillonnage a ete realise en septembre. Aucun des traitements n'a globalement influence la structure ni la diversite des communautes bacteriennes de la phyllosphere de l'epicea noir. Cependant, certains taxons bacteriens et par consequent certaines fonctions bacteriennes differaient selon le traitement, revelant en particulier que certaines bacteries reagissaient plus fortement au rechauffement du sol qu'a l'apport d'azote. Nos resultats indiquent que le rechauffement du sol pourrait induire plus de changements dans les fonctions et l'abondance des taxons bacteriens de la phyllosphere que l'apport d'azote, et possiblement avoir des consequences sur la diversite microbienne et la fonction de l'ecosysteme de la foret boreale compte tenu des scenarios probables de changement climatique. Notre etude suggere des avenues futures de recherche pour integrer une comprehension plus mecaniste de l'importance des microbes de la phyllosphere pour l'epicea noir et les ecosystemes de la foret boreale. [Traduit par la Redaction] Mots-cles : phyllosphere, azote, rechauffement du sol, epicea noir, changements climatiques., Introduction Phyllosphere microbial communities, the microorganisms living on aerial plant surfaces including leaves (Lindow and Brandl 2003), have recently received considerable attention due to the various impacts they can have [...]

The consequences of forest fragmentation and edge effects on soil nutrient availability and nutrition of sugar maple (Acer saccharum Marsh.) forests remain understudied. We assessed soil chemistry (bulk pH, total carbon (C) and nitrogen (N), extractable phosphorus (P), exchangeable cations, and mineralizable N) and foliar nutrition (N, P, potassium (K), calcium (Ca), and magnesium (Mg)) of mature sugar maple forests along an edge-interior gradient (10, 20, 30,50,70, and 120 m from the forest edges) in seven sugar maple forest stands (three on sandstone and four on dolomite bedrock) that are embedded within an agricultural landscape in southern Quebec, Canada. We hypothesized that foliar nutritional imbalances of sugar maple forests would decline along this gradient. Foliar nutrition was analyzed using the diagnosis and recommendation integrated system (DRIS) and the compositional nutrient diagnosis with isometric log-ratio (CND- ilr) method. At the sandstone sites, rates of N mineralization and nitrification increased with increasing distance from the forest edge. Other soil chemical properties and all sugar maple foliar indices of nutritional diagnostics varied weakly along the edge-interior gradient. Assessment of sugar maple forest nutritional status through different nutritional indices revealed K and P deficiencies in all stands that were sampled and at all distances from the forest edge. Overall, we found weak forest-edge effects on soil chemical properties and sugar maple forest nutritional status. Key words: edge effects, foliar nutrition, phosphorus limitation, soil chemistry, sugar maple. Les consequences de la fragmentation de la foret et des effets de bordure sur la disponibilite des nutriments dans le sol et la nutrition des forets d'erable a sucre (Acer saccharum Marsh.) n'ont pas ete etudiees suffisamment. Nous avons evalue les caracteristiques chimiques du sol (pH global, carbone (C) et azote (N) totaux, phosphore (P) extractible, cations echangeables et N mineralisable) et la nutrition foliaire (N, P, potassium (K), calcium (Ca) et magnesium (Mg)) de forets matures d'erable a sucre le long d'un gradient allant de la bordure vers l'interieur de la foret (10, 20, 30,50,70 et 120 m de la bordure de la foret) dans sept peuplements forestiers d'erable a sucre (trois sur un substrat rocheux de gres et quatre sur un substrat rocheux de dolomite) imbriques dans un paysage agricole dans le sud du Quebec, au Canada. Nous avons fait l'hypothese que les desequilibres nutritionnels dans le feuillage des forets d'erable a sucre diminueraient le long de ce gradient. La nutrition foliaire a ete analysee a l'aide du Systeme integre de diagnostic et de recommandation (DRIS) et des methodes de diagnostic compositionnel de la nutrition avec transformation log ratio isometrique (CND-ilr). Dans les stations etablies sur du gres, les taux de mineralisation et de nitrification de N augmentaient avec la distance depuis la bordure de la foret. Les autres proprietes chimiques du sol et tous les indices de diagnostic nutritionnel du feuillage de l'erable a sucre variaient faiblement le long du gradient de la bordure vers l'interieur de la foret. L'evaluation du statut nutritionnel de la foret d'erable a sucre au moyen de differents indices nutritionnels a revele des deficiences en K et P dans tous les peuplements qui ont ete echantillonnes et a toutes les distances depuis la bordure de la foret. Dans l'ensemble, nous avons observe de faibles effets de bordure sur les proprietes chimiques du sol et le statut nutritionnel de la foret d'erable a sucre. [Traduit par la Redaction] Mots-cles: effets de bordure, nutrition foliaire, deficience en phosphore, chimie du sol, erable a sucre., Introduction It is estimated that 37% of Earth's land surface is currently under agricultural production (World Bank 2018). Yet, the intensification and acceleration of land-use conversion to agriculture remains a [...]

It has recently become clear that the regeneration density of serotinous species within a burned area declines with local fire intensity. It is assumed that this occurs because variation in local fire intensity leads to variation in incident heat fluxes and, ultimately, seed necrosis. We argue here that this same relationship between incident heat flux and seed necrosis is important at the scale of individual plant crowns. Using Picea mariana (Mill.) B.S.P. (black spruce), we show that postfire seed viability increases with crown height, depth into the crown, and angle from wind direction (with the windward side enjoying greater viability). All three effects are what one would expect given the physics of buoyant plumes, interactions of moving fire lines with wake flow around cylinders, and heat transfer in porous bodies such as a tree crown. We conclude by discussing the broader consequences of cone cluster size and global change on regeneration in serotinous species. Key words: Picea mariana, fire, seed viability, cone clustering, abscission, establishment. Il est recemment devenu clair que la densite de la regeneration des especes serotineuses dans une zone brulee diminue avec l'intensite locale du feu. On assume que cela se produit parce qu'une variation de l'intensite locale du feu entraine une variation des flux de chaleur incidente et, ultimement, la necrose des graines. Nous pretendons ici que cette meme relation entre le flux de chaleur incidente et la necrose des graines est importante a l'echelle de la cime des individus. A partir de l'etude de Picea mariana (Mill.) B.S.P. (epinette noire), nous montrons que la viabilite des graines apres feu augmente avec la hauteur de la cime, la profondeur dans la cime et l'angle forme avec la direction du vent (le cote expose au vent profitant d'une plus grande viabilite). Les trois effets correspondent a ce a quoi on s'attendrait etant donne les caracteristiques physiques des panaches flottants, les interactions des lignes de feu mobiles avec le sillage de l'ecoulement autour de cylindres et le transfert de chaleur dans les corps poreux tels que la cime des arbres. Nous concluons en discutant des consequences plus larges de la taille des grappes de cones et des changements globaux sur la regeneration des especes serotineuses. [Traduit par la Redaction] Mots-cles : Picea mariana, feu, viabilite des graines, regroupement des cones, abscission, etablissement., Introduction In biomes where the prevailing natural disturbance regime is dominated by large, stand-replacing fires (Doerr and Santin 2016), serotiny is a primary adaptation for postfire sexual reproduction of plants [...]

We measured organic-layer (OL) recovery and carbon stocks in dead woody debris a decade after wildfire in black spruce (Piceamariana (Mill.) B.S.P.) forests of interior Alaska. Previous study at these research plots has shown the strong role that landscape position plays in governing the proportion of OL consumed during fire and revegetation after fire. Here, we show that landscape position likely influences fire dynamics in these stands through changes in mineral soil texture. The content of fine-textured materials in underlying mineral soils was positively related to OL depths measured 1 and 10 years after fire, and there was an interaction between soil texture and elevation in governing OL consumption and OL recovery a decade following fire. OL depths 10 years after fire were 2 cm greater than 1 year after fire, with a range of 19 cm of accumulation to 9 cm of subsidence. Subsidence was inversely related to the percentage of fine textures within the parent material. The most influential factor determining the accumulation of OL carbon stocks a decade following wildfire was the interaction between landscape position and the presence of fine-textured soil. As such, parent material texture interacted with biological processes to govern the recovery of soil organic layers. Key words: Picea mariana, parent material, clay, loess, wild fire. Nous avons mesure le retablissement de la couche organique (CO) et les reserves de carbone dans les debris ligneux 10 ans apres un feu d'origine naturelle survenu dans des forets d'epinette noire (Picea mariana (Mill.) B.S.P.) de l'interieur de l'Alaska. Une etude anterieure realisee dans ces placettes de recherche a montre que la position dans le paysage a un effet determinant sur la proportion de la CO consumee pendant le feu et la revegetalisation apres le feu. Dans la presente etude, nous montrons que la position dans le paysage influence probablement la dynamique du feu dans ces peuplements via des changements de texture du sol mineral. Le contenu en materiaux de texture fine dans le sol mineral sous-jacent a ete positivement relie a la profondeur de la CO mesuree 1 et 10 ans apres le feu, et il y avait une interaction entre la texture du sol et l'altitude concernant la consumation et le retablissement de la CO 10 ans apres le feu. Dix ans apres le feu, la CO etait 2 cm plus profonde qu'un an apres le feu, avec une etendue des observations de 19 cm d'accumulation a 9 cm de subsidence. La subsidence etait inversement reliee au pourcentage de materiaux de texture fine dans le materiau parental. Le facteur qui influencait le plus l'accumulation des reserves de carbone dans la couche organique 10 ans apres le feu etait l'interaction entre la position dans le paysage et la presence de sol a texture fine. Ainsi, la texture du materiau parental interagit avec les processus biologiques pour determiner le retablissement des couches organiques du sol. [Traduit par la Redaction] Mots-cles: Picea mariana, materiau parental, argile, lress, feu d'origine naturelle., Introduction Boreal forests currently harbor about a third of the Earth's rapidly cycling soil organic carbon (C) (McGuire et al. 1995; Pan et al. 2013), which is a pool size [...]

We undertook a gap analysis of how protected areas represent the tree-species diversity within the Canadian boreal forest, as measured from Forest Resource Inventory (FRI) maps. We used a new compilation of Forest Resource Inventory designed to support ecological analyses over large areas and across jurisdictional boundaries. The analysis was stratified into four analytical regions determined by terrestrial ecozones. We calculated the relative abundance of species within regions, developed rarity criteria, and evaluated the relative abundances and prevalence of rare species. We characterized representation gaps when the abundance of a tree species in protected areas within an analytical region differed markedly (by more than a factor of 2) from the expectation, calculated as the product of regional abundance and the proportional area protected. Most species were well represented in the most speciesdiverse region (n = 33), the Boreal Shield in eastern Canada, due apparently to a large number of relatively small protected areas in the southern part of the region. Some marked gaps existed in the more species-depauperate western zones, notably for montane conifers in the Boreal Plains. As is common for species abundance distributions, as few as five species accounted for 90% of total abundance in each zone. Relatively rare species were more numerous. Mostly associated with southern temperate or hemiboreal forests, these reached their highest prevalence and abundance in the managed forests of the Boreal Shield. Our work identified some gaps in representation in the protected areas network of Canada in western Canada, substantiates the use of species distribution mapping based on FRI data to inform conservation planning--including the identification of high conservation biodiversity elements for forest certification--and demonstrates the need for improved vegetation mapping in National Parks. Key words: gap analysis, boreal forest, forest inventory data, species distributions, tree species mapping. Nous avons entrepris une analyse de carence pour determiner dans quelle mesure les aires protegees sont representatives de la diversite des especes d'arbres dans la foret boreale canadienne a partir des cartes de l'Inventaire des ressources forestieres (IRF). Nous avons utilise une nouvelle compilation des cartes de l'IRF realisee dans le but de faciliter les analyses ecologiques sur de vastes superficies qui chevauchent les limites de juridiction. L'analyse a ete stratifiee en quatre regions d'analyse definies par des ecozones terrestres. Nous avons calcule l'abondance relative des especes a l'interieur des regions, elabore des criteres de rarete et evalue l'abondance relative et la prevalence des especes rares. Nous avons caracterise les carences dans la representation lorsque l'abondance d'une espece d'arbre dans des zones protegees situees dans une region visee par l'analyse differe de fa^on marquee (plus du double) de la valeur esperee, calculee comme le produit de l'abondance regionale et de la proportion en zone protegee. La plupart des especes sont bien representees dans la region la plus riche en especes (n = 33), l'ecozone du Bouclier boreal de l'est du Canada, apparemment en raison d'un grand nombre de petites aires protegees situees dans la partie sud de cette region. Des carences marquees existent dans les regions pauvres en especes de l'Ouest, notamment chez les coniferes montagnards de l'ecozone de la Plaine boreale. Comme c'est generalement le cas des distributions d'abondance d'especes, aussi peu que cinq especes representaient 90 % de l'abondance totale dans chaque zone. Les especes relativement rares etaient plus nombreuses. Ces especes, generalement associees aux forets temperees meridionales ou aux forets hemiboreales, atteignaient leur plus grande prevalence et abondance dans les forets amenagees du Bouclier boreal. Notre etude identifie certaines carences dans la representative du reseau des aires protegees du Canada dans l'ouest du pays, justifie l'utilisation de cartes de distributions d'especes faites a partir des donnees de l'IRF pour appuyer la planification de la conservation, incluant l'identification des elements a haute valeur de conservation de la biodiversite pour la certification forestiere, et demontre la necessite d'ameliorer la cartographie de la vegetation dans les parcs nationaux. [Traduit par la Redaction] Mots-cles: analyse de carence, foret boreale, donnees d'inventaire forestier, distributions d'especes, cartographie des especes arborescentes., Introduction Species richness of many taxa decreases at higher latitudes (Hillebrand 2004), and this is particularly true for trees (Currie and Paquin 1987). The circumpolar boreal biome (Shugart et al. [...]

Base cations (Ca, Mg, and K) are essential nutrients for forest growth. Many studies have reported important de-creases in the soils of several forests in eastern North America, partly because of atmospheric acid deposition and forest harvesting. To quantify the impacts of these perturbations on forest base cations, accurate estimation of tree biomass and nutrient content is needed. However, most of tree nutrient contents are calculated with general allometric equations, leading to inaccurate estimates. We thus calculated tree biomass and base cation content for three common forest types in eastern Canada using site-specific allometric equations and compared them with those calculated with general allometric equations and nutrient concentrations of tree compartments taken from the literature. General allometric equations resulted in above-ground tree biomass estimates in the same range as ours ([+.or.-]15%), but the use of nutrient concentrations taken from the literature resulted mainly in overestimation of above-ground tree nutrient content (-63% to +81%), leading to inaccurate wood requirement estimates (-63% to +86%). Therefore, the development of site-specific equations to estimate above-ground tree base cation content is recommended. Resume: Les cations basiques (Ca, Mg and K) sont essentiels a la croissance des arbres. Plusieurs etudes ont rapporte one diminution important de captions bisques dams le sol de forets du Nord-Est americain, due en partie aux depots atmospheriques z et a la recolte forestiere. Quantifier l'impact de ces perturbations sur le contenu en cations basiques des forets necessite une evaluation exacte du contenu en cations basiques des arbres. Toutefois, la mineralomasse des arbres est generalement calculee avec des equations allometriques generales, ce qui produit des estimes inexacts. Nous avons donc calcule la biomasse et la mineralomasse des arbres de trois types forestiers communs de l'Est du Canada avec des equations allometriques specifiques aux sites, et les avons comparees a celles calculees avec des equations generales et des concentrations en elements nutritifs provenant de la litterature. Utiliser des equations allometriques generales produisait des estimes de la bio-masse de la partie aerienne des arbres du meme ordre de grandeur que les notres ([+.or.-]15%). Toutefois, utiliser des concentrations en elements nutritifs provenant de la litterature entrainait surtout des surestimations de la mineralomasse de la partie aerienne des arbres (-13% a +81%), produisant des estimes inexacts de l'accumulation des cations basiques dans le bois (-63% a +86%). Il est donc recommande de developper des equations de mineralomasse en cations basiques specifiques aux sites., Introduction Base cations (Ca, Mg, and K) are essential nutrients for trees. Their decrease in the soils of several forest ecosystems in eastern Canada and the northeastern United States is [...]

A shadow fraction method was developed previously for mapping forest attributes of northern black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb.) forests. This paper evaluates application of the method for (i) balsam fir stands (Abies balsamea (L.) Mill.), (ii) stands with higher volume and biomass than those of previous studies, and (iii) stands with a higher composition of deciduous trees and steeper slopes. Models developed for new test sites in (i) central Labrador and (ii) western Newfoundland were not statistically different from previous models for biomass, volume, and basal area. Rela-tive root mean square errors ([RMSE.sub.r]) for central Labrador were slightly lower than those found in other test sites ([RMSE.sub.r]: 24%-29%) but higher for western Newfoundland ([RMSE.sub.r] = 37%-43%), attributed to the higher upper limit of measured at-tributes and increased presence of deciduous trees. Results suggest that reasonable estimates can be generated for conifer forests of northeastern Canada; however, an alternative solution is needed where mixed and deciduous stands are prevalent. Measurement of ground plots over a wider range of species composition and forest structure is recommended for broader application to northern boreal forests and to further assess the potential role of the shadow fraction method in national-scale inventory programs. Resume: Une methode basee sur la fraction d'ombre a ete developpee precedemment pour cartographier les attributs des peuplements nordiques d'epinette noire (Picea mariana (Mill.) Britton, Sterns & Poggenb.). Cet article evalue l'application de la methode pour (i) les peuplements de sapin baumier (Abies balsamea (L.) Mill.), (ii) les peuplements dont le volume et la biomasse sont plus eleves que ceux des etudes precedentes et (iii) les peuplements situes sur des pentes plus abruptes et dont la proportion d'essences feuillues est plus elevee. Les modeles developpes pour les nouveaux sites d'essai dans (i) le centre du Labrador et (ii) l'ouest de Terre-Neuve n'etaient pas statistiquement differents des modeles precedents dans le cas de la biomasse, du volume et de la surface terriere. Les erreurs quadratiques moyennes relatives pour le centre du Labrador etaient legerement inferieures a celles qui ont ete calculees dans d'autres sites d'essai (24%-29%), mais elles etaient plus elevees pour l'ouest de Terre-Neuve (37%-43%). Cela serait du a la limite superieure plus elevee des attributs qui ont ete mesures et a la presence accrue de feuillus. Les resultats indiquent que des estimations raisonnables peuvent etre generees pour les forets de coniferes du nord-est du Canada, mais une solution alternative est necessaire la ou les peuplements mixtes et feuillus sont repandus. La mesure de placettes terrestres representant une large gamme de composition d'especes et de structure forestiere est recommandee pour permettre une application plus large aux forets boreales nordiques et continuer a evaluer le role potentiel de la methode proposee dans les programmes nationaux d'inventaire. [Traduit par la Redaction], Introduction Forest biomass, volume, and related attributes are important for their role in supporting forest management planning and improving understanding of the global carbon balance (Houghton 2005). In conventional forest [...]

Since plant growth in the boreal forest is often considered to be limited by low temperatures and low N availability and these variables are projected to increase due to climate warming and increased anthropogenic activities, it is important to understand whether and to what extent these disturbances may affect the growth of boreal trees. In this study, the hypotheses that wood phenology and anatomy were affected by increased soil temperatures and N depositions have been tested in two mature black spruce (Picea mariana (Mill.) BSP) stands at different altitudes in Quebec, Canada. For 3 years, soil temperature was increased by 4 ° C during the first part of the growing season and precipitations containing three times the current N concentration were added in the field by frequent canopy applications. Soil warming resulted in earlier onsets of xylogenesis and interacted with N addition producing longer durations of xylogenesis for the treated trees. The effect of warming was especially marked in the phenology of roots, while wood production, in terms of number of tracheids, was not affected by the treatment. Xylem anatomy and soil and needle chemistry showed no effect of the treatments, except for an increase of cell wall thickness in earlywood of treated trees. This short-term experiment with black spruce suggested that previous fertilization studies that used large and unrealistic rates of N addition may have overestimated the impact of N depositions on boreal forest productivity. Resume: Comme on considere souvent que les basses temperatures et la faible disponibilite du N limitent la croissance des plantes dans la foret boreale, et qu'une augmentation de ces variables est prevue a cause du rechauffement du climat et de l'augmentation des activites anthropiques, il est important de comprendre si, et dans quelle mesure, ces perturbations pour-raient influencer la croissance des arbres boreaux. Dans cette etude, les hypotheses que les augmentations de la temperature du sol et des depots de N ont un effet sur l'anatomie et la phenologie du bois ont ete testees dans deux peuplements matures d'epinette noire (Picea mariana (Mill.) BSP) situes a differentes altitudes au Quebec, Canada. Pendant 3 ans, la temperature du sol a ete elevee de 4 ° C durant la premiere partie de la saison de croissance et des precipitations contenant trois fois la concentration actuelle de N ont ete ajoutees au champ par de frequentes applications aeriennes. Le rechauffement du sol a entraine le declenchement plus hatif de la xylogenese et a interagi avec l'addition de N pour allonger la duree de la xylogenese chez les arbres traites. L'effet du rechauffement s'est fait particulierement sentir sur la phenologie des racines tandis que la production de bois, en termes de nombre de tracheides, n'a pas ete influencee par le traitement. Les traitements n'ont eu aucun effet sur l'anatomie du xyleme ni sur les caracteristiques chimiques du sol et des aiguilles, a l'exception d'une augmentation de l'epaisseur de la paroi cellulaire dans le bois initial des arbres traites. Cette experience a court terme avec l'epinette noire indique que les etudes precedentes de fertilisation, dans lesquels des taux eleves et irrealistes d'apport en N ont ete utilises, ont pu surestimer l'impact des depots de N sur la productivite de la foret boreale. [Traduit par la Redaction], Introduction The boreal forest is expected to experience a greater increase in surface temperature than any other forest ecosystem in the world (Stromgren and Linder 2002). For eastern Canada, recent [...]

Observed modifications of ectomycorrhizal (ECM) communities have been connected to the increased N depositions of the 20th century. Because of their narrow niche width, small disturbances of soil conditions can produce greater effects on the fungal species than on their host trees. This study investigated the ECM community in a black spruce (Picea mariana (Mill.) BSP) stand subjected to long-term additions of 9 and 30 kg [[N*ha.sup.-1]*year.sup.-1] of ammonium nitrate, representing 3 and 10 times the atmospheric N deposition at the site, respectively. Root tip vitality and ECM presence were detected on samples collected from the organic horizon and ECM were classified into morphotypes according to their morphological and anatomical characters. In the control, 80.6% of the root tips were vital, 76.5% of them showing ECM colonization. Higher root tip vitality and mycorrhization were observed in the treated plots. Forty-one morphotypes were identified, most of them detected at the higher N inputs. Results diverging from the expectations of a reduction in ECM presence and diversity could be related to a higher growth rate of the trees following fertilization. The repeated application of small N doses could have been a better imitation of natural inputs from atmospheric deposition and could have provided more reliable responses of ECM to treatment. Resume: Les modifications qui ont ete observees chez les communautes ectomycorhiziennes (ECM) ont ete reliees a l'augmentation des depets de N au cours du [20.sup.e] siecle. A cause de leur niche etroite, de faibles perturbations des conditions de sol peuvent avoir plus d'effets sur les especes de champignons que sur les arbres qui leur servent d'hetes. Cette etude a porte sur la communaute ECM dans un peuplement d'epinette noire (Picea mariana (Mill.) BSP) soumis a des apports a long terme de 9 et 30 kg [[N*ha.sup.-1]*an.sup.-1] de nitrate d'ammonium representant respectivement 3 a 10 fois les depets de N atmos-pherique sur le site. La vitalite des apex racinaires et la presence d'ECM ont ete detectees sur des echantillons collectes dans l'horizon organique et les ECM ont ete classees par morphotypes selon leurs caracteristiques anatomiques et morpholo-giques. Dans le traitement temoin, 80,6 % des apex racinaires etaient pleins de vitalite; 76,5 % etaient colonises par des ECM. Une meilleure mycorhization et une plus grande vitalite des apex racinaires ont ete observees dans les parcelles traitees. Quarante et un morphotypes ont ete identifies dont la plupart ont ete detectes dans les endroits ou les apports de N etaient les plus eleves. Le fait que les resultats ne correspondent pas aux attentes, soit une reduction de la presence et de la diversite des ECM, pourrait etre relie a un taux de croissance plus eleve des arbres a la suite de la fertilisation. L'application repetee de faibles doses de N aurait peut-etre mieux imite les apports naturels provenant des depets atmospheriques et aurait pu fournir des reactions plus fiables des ECM au traitement. [Traduit par la Redaction], Introduction Because of low evapotranspiration and decomposition, the boreal forest is characterized by slow nutrient cycles and a gradual accumulation of organic matter either as raw humus or as peat [...]

A critical component of assessing the impacts of climate change on forest ecosystems involves understanding associated changes in the biogeochemical cycling of elements. Evidence from research on northeastern North American forests shows that direct effects of climate change will evoke changes in biogeochemical cycling by altering plant physiology, forest productivity, and soil physical, chemical, and biological processes. Indirect effects, largely mediated by changes in species composition, length of growing season, and hydrology, will also be important. The case study presented here uses the quantitative biogeochemical model PnET-BGC to test assumptions about the direct and indirect effects of climate change on a northern hardwood forest ecosystem. Modeling results indicate an overall increase in net primary production due to a longer growing season, an increase in N[O.sub.3.sup.-] leaching due to large increases in net mineralisation and nitrification, and slight declines in mineral weathering due to a reduction in soil moisture. Future research should focus on uncertainties, including the effects of (1) multiple simultaneous interactions of stressors (e.g., climate change, ozone, acidic deposition); (2) long-term atmospheric C[O.sub.2] enrichment on vegetation; (3) changes in forest species composition; (4) extreme climatic events and other disturbances (e.g., ice storms, fire, invasive species); and (5) feedback mechanisms that increase or decrease change. Une composante critique de l'estimation de l'impact des changements climatiques sur les ecosystemes forestiers implique la comprehension des changements associes au cycle biogeochimique des elements. Une recherche conduite dans les forets du nord-est de f Amerique du Nord montre clairement que les effets directs des changements climatiques entraineront des changements dans le cycle biogeochimique en alterant la physiologie des plantes, la productivite forestiere et les processus impliques dans la physique, la chimie et la biologie des sols. Les effets indirects engendres surtout par des changements dans la composition en especes, la longueur de la saison de croissance et le regime hydrique seront aussi importants. Une etude de cas est presentee ici a f aide du modele quantitatif biogeochimique PnET-BGC pour tester des hypotheses au sujet des effets directs et indirects des changements climatiques sur un ecosysteme forestier de feuillus nordiques. Les resultats des simulations permettent d' anticiper une augmentation generale de la production primaire nette causee par l'allongement de la saison de croissance, une augmentation du lessivage de N[O.sub.3.sup.-] due a de fortes augmentations de la mineralisation et de la nitrification nettes ainsi qu'une legere diminution de l'alteration des mineraux due a une reduction de l'humidite du sol. Les recherches futures devraient etre concentrees sur les incertitudes, lesquelles incluent les effets (1) des interactions multiples et simultanees des facteurs de stress (p. ex. : les changements climatiques, l'ozone, les depots acides), (2) de (enrichissement a long terme en C[O.sub.2] atmospherique sur la vegetation, (3) des changements de la composition en especes forestieres, (4) des evenements climatiques extremes et des autres perturbations (p. ex. : les verglas, le feu, les especes invasives) et (5) des mecanismes de retroaction qui augmentent ou diminuent ces changements. [Traduit par la Redaction], Introduction Human activity has increased atmospheric greenhouse gas concentrations since preindustrial times, which, in turn, has led to a positive radiative forcing of climate. Observed changes in climate include a [...]

Abstract: Increases in the emission of greenhouse gases, particularly during the second half of the 20th century, have been associated with climate warming at the global scale. High latitude areas [...]

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From 1989 to 1996, ion deposition in precipitation, throughfall, andstemflow were measured under a deciduous and a coniferous stand, located in the Lake Clair Watershed, during the growing and the dormant seasons. During the growing season, throughfall deposition under bothstands was significantly depleted in H+ and NH4+ compared with wet deposition, and a significant uptake of NO3- was observed under the coniferous canopy. Deposition of Ca2+, Mg2+, K+, Na+, Cl-, and SO42- was significantly higher in the throughfall than in the wet precipitation. During the growing season, the coniferous stand was more efficient in retaining nitrogen (NH4+ and NO3-), while H+ was more intensively retained in the deciduous stand. Significant interactions between precipitation and forest canopies were also observed during the dormant season: throughfall depositions of Ca2+, Mg2+, K+, Na+, and Cl- were significantly higher than wet precipitation under both canopies, while throughfall SO42- was significantly enriched only under the coniferous stand.Using a Na+ ratio method, foliar leaching was found mostly responsible for the throughfall enrichment on a full-year basis in both stands, with values averaging 61, 73, and 96% of the total throughfall fluxes for Ca2+, Mg2+, and K+, respectively. Under both stands, net canopy exchange (NCE) of base cations, expressed on a monthly basis, were correlated to water volume and to H+ and SO42- deposition. Multiple regression models including wet SO42- deposition and an estimate of dry S deposition, explained up to 88% (Ca2+ in the coniferous stand) of the variance in base cationNCE [ABSTRACT FROM AUTHOR]

Variability of throughfall volume and ion deposition were measured under a mixed hardwood forest using 36 or 72 collectors from 1988 to 1994 during the growing period (May to October) for 163 events. The ions showing the highest variability in results were (in decreasing order) NH4+, H+, Mg2+, K+, Ca2+, Cl-, Na+, NO3-, and SO42-, based on the median coefficient of variation of deposition. The variability observed was related to the reactivity of the ion in the canopy, with the more reactive ions showing higher spatial variability. The number of collectors required to obtain estimates of throughfall ion depositions within a 10% error and a confidence interval of 95% varied from 21 (SO42-) to 165 (NH4+) on a weekly basis based on events of median variability. The error associated with deposition measurements was decreased considerably from weekly, to monthly, to seasonal deposition (3 months). The number of collectors necessary to measure throughfall deposition depends on the data quality objectives of the study (i.e., error and confidence interval chosen as acceptable), the variability specific to a given ion, and thetime period needed to study a given process with adequate temporal resolution. A figure is proposed that may be used as a guideline to determine the number of collectors needed in mixed hardwood forests based on the latter information. [ABSTRACT FROM AUTHOR]

We performed a nearest-neighbor analysis to determine the populationdispersion pattern and the association between males and females in a Juniperus communis L. var. depressa Pursh population occupying a continental dune in subarctic Quebec, Canada. The overall dispersion pattern was contagious, and males (or females) were proportionately as likely to have a male as they were to have a female nearest neighbor.Crown size was positively related to nearest-neighbor distance for the male-male comparison only, suggesting a somewhat stronger intrasexcompetition between males. Nearest-neighbor distance increased with crown size (significantly related to age) suggesting a change in the intensity of aggregation with age possibly related to self-thinning. Higher mortality as a result of stronger male-male competition could explain the female-biased sex ratio and the absence of spatial segregation between sexes. The overall contagious dispersion pattern in thepopulation may be related to the fact that most seed cones fall directly underneath the mother plant. Birds can eat the cones of J. communis and thus disperse seeds. However, these seeds are deposited in clumps, a process that may also explain contagion within the population. [ABSTRACT FROM AUTHOR]