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Regionally fitted allometric equations for individual trees and root-to-shoot ratio values are normally used to estimate local aboveground and belowground forest biomass, respectively. However, uncertainties arising from such applications are poorly understood. We developed equations for both aboveground and belowground biomass using destructive sampling for three dominant upland boreal tree species in northwestern Alberta, Canada. Compared with our equations, the diameter-based national equations derived for use across Canada underestimated aboveground biomass for Picea glauca (Moench) Voss but gave reasonable estimates for Populus balsamifera L. and Populus tremuloides Michx. The national equations based on both tree diameter and height overestimated aboveground biomass for the Populus species but underestimated it for Picea glauca in our study area. The approach of root-to-shoot ratio proposed by the Intergovernmental Panel on Climate Change (IPCC) overestimated belowground biomass by 16%-41%, depending on forest cover type, in comparison with our values estimated directly on site, with the greatest bias in deciduous-dominated stands. When the general allometric equations for aboveground biomass and the root-toshoot ratio for belowground biomass were combined to estimate stand biomass, overestimation could be as high as 18% in our study area. The results of our study support the development of improved regional allometric equations for more accurate local-scale estimations. Incorporating intraspecific variation of important traits such as tree taper may be especially helpful. Key words: EMEND (Ecosystem Management Emulating Natural Disturbance), intraspecific trait variation, root-to-shoot ratio, tree taper, wood density. On utilise generalement des equations allometriques ajustees a l'echelle regionale pour les arbres individuels et des valeurs du rapport racines-tiges pour estimer respectivement la biomasse forestiere locale, aerienne et souterraine. Cependant, les incertitudes decoulant de telles applications sont mal comprises. Nous avons developpe des equations de biomasse aerienne et souterraine en utilisant un echantillonnage destructif pour trois especes d'arbres boreales dominantes dans les hautes terres du nord-ouest de l'Alberta, au Canada. Comparativement a nos equations, les equations nationales basees sur le diametre developpees pour etre utilisees partout au Canada sous-estimaient la biomasse aerienne de Picea glauca (Moench) Voss, mais donnaient des estimations raisonnables pour Populus balsamifera L. et Populus tremuloides Michx. Les equations nationales basees sur le diametre et la hauteur des arbres surestimaient la biomasse aerienne des especes de Populus L., mais la sous-estimaient pour Picea glauca dans notre zone d'etude. L'approche du rapport racines-tiges, proposee parle Groupe d'experts intergouvernemental sur l'evolution du climat (GIEC), a surestime la biomasse souterraine de 16 a 41% selon le type de couvert forestier par rapport a nos valeurs estimees directement sur le site, le biais le plus important etant observe dans les peuplements domines par les feuillus. En combinant les equations allometriques generales pour la biomasse aerienne et le rapport racines-tiges pour la biomasse souterraine pour estimer la biomasse du peuplement, la surestimation pouvait atteindre 18% dans notre zone d'etude. Les resultats de notre etude justifient le developpement d'equations allometriques regionales ameliorees pour des estimations plus precises a l'echelle locale. L'incorporation des variations intraspecifiques des attributs importants, tels que le defilement de la tige des arbres, peut etre particulierement utile. [Traduit par la Redaction] Mots-cles : amenagement ecosystemique, variations intraspecifiques des attributs, rapport racines-tiges, defilement de la tige, densite du bois., Introduction Increasing pressure to reduce net carbon dioxide (CO2) emissions in light of climate change is spurring development of global strategies such as carbon markets that encourage carbon offsets through [...]

Survival and growth of planted white spruce (Picea glauca (Moench) Voss) were assessed at year 15 in boreal mixedwood stands of northern Alberta, Canada, in stands that were deciduous-dominated prior to logging or were conifer-dominated. Three overstory retention levels (0%, 50%, and 75% retention) and four site preparation treatments (mound, high speed mix, scalp, and no treatment) were evaluated. In deciduous-dominated stands, planted spruce performed best in the 50% retention; here, stem volume was at least double that of any other retention treatment after 15 years. In contrast, spruce had reduced growth in coniferous-dominated stands in both 50% and 75% retention treatments compared with the 0% retention. Survival of planted spruce was unaffected by level of retention, but survival was lower in coniferous-dominated stands than in deciduous- dominated stands; in the coniferous-dominated stands, survival was better with mounding and mixing and lowest with scalp treatments. All height variables tended to be greater in the mix and mound site preparation treatments. Finally, the best estimates of future total growth (regenerated spruce and deciduous combined) in the coniferous-dominated stands were in the clearcut treatment. In terms of regenerated spruce growth, the best estimates occurred in the deciduous-dominated - 50% retention stand planted with soil mixing-mounding treatments, where projected growth of spruce was comparable with that of open-grown and tended stands in Alberta's boreal forests. Key words: boreal mixedwood, shelterwood, site preparation, survival, partial harvest, planted, variable retention, white spruce. La survie et la croissance de plants d'epinette blanche (Picea glauca (Moench) Voss) ont ete evaluees a l'age de 15 ans dans des peuplements mixtes boreaux du nord de l'Alberta, au Canada, sur des stations dominees avant la coupe par des feuillus ou par des coniferes. Nous avons evalue trois niveaux de retention du couvert dominant (0, 50 et 75 % de retention) et quatre traitements de preparation de terrain (monticules, melange du sol agrande vitesse, scalpage et temoin non traite). Dans les peuplements domines par des feuillus, les plants d'epinette croissaient mieux a 50 % de retention du couvert dominant. Dans ce cas, le volume des tiges etait au moins le double de celui observe dans les autres traitements de retention apres 15 ans. En revanche, une croissance reduite de l'epinette a ete observee dans les peuplements domines par des coniferes avec 50 et 75 % de retention comparativement a 0 % de retention. La survie des plants d'epinette n'a pas ete influencee par le niveau de retention, mais elle etait plus faible dans les peuplements domines par des coniferes que dans ceux domines par des feuillus. Dans les peuplements domines par des coniferes, la survie etait meilleure dans les traitements par monticules et par melange et etait la plus faible dans le traitement de scalpage. Toutes les variables de hauteur avaient tendance a etre superieures dans les traitements de preparation de terrain par melange et par monticules. Finalement, la meilleure estimation de la croissance totale future (regeneration d'epinette et de feuillus combinee) des peuplements domines par des coniferes a ete observee dans la coupe a blanc. En ce qui concerne la croissance de la regeneration d'epinette, la meilleure croissance a ete observee avec 50 % de retention du couvert feuillu et les preparations de terrain par melange du sol et par monticules, oU la croissance projetee de l'epinette etait comparable a celle de peuplements libres de croitre et eduques de la foret boreale de l'Alberta. [Traduit par la Redaction] Mots-cles : peuplement mixte boreal, coupe progressive, preparation de terrain, survie, coupe partielle, plants, retention variable, epinette blanche., Introduction Mixedwood forests of aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench.) Voss) cover extensive areas of the boreal region of Canada, ranging from the Yukon to western [...]

: In an operational-scale experiment, we examined natural regeneration of white spruce (Picea glauca (Moench) Voss) under various degrees of variable retention harvesting across different overstory canopy compositions. We sampled four types of overstory canopy compositions (ranging from deciduous dominated to conifer dominated) and six rates of retention (2%, 10%, 20%, 50%, 75% and 100%). Spruce seedling densities, maximum height, and stocking rates were assessed in 432 transects and were found to be lowest in the deciduous stands and with 100% canopy retention (control). Regression tree analysis indicated that densities, maximum height, and stocking of spruce seedlings were greatest with higher availability of seed trees (>30*[ha.sup.-1]) and on machine corridors; in these circumstances stocking reached 74%. By contrast, stocking was less than 14% on retention strips with no machine traffic, when seed tree density was less than 11 seed trees*[ha.sup.-1]. However, stocking also declined with higher density of residual trees. Stocking in relation to density of seed trees was highest within clearcuts and lowest within the 75% retention intensity where low light transmission may have limited regeneration success. Passive soil disturbance from skidding appeared to be more important for the establishment of spruce seedlings than was the abundance of competing vegetation. Resume: A l'aide d'une experience a l'echelle operationnelle, nous avons etudie la regeneration naturelle de l'epinette blanche (Picea glauca (Moench) Voss) en fonction de differents degres de coupe a retention variable et de differentes compositions du couvert dominant. Nous avons echantillonne quatre types de composition du couvert dominant (allant d'un couvert domine par des feuillus a un couvert domine par des coniferes) et six taux de retention (2, 10, 20, 50, 75 et 100 %). La densite de semis d'epinette, la hauteur maximale et le coefficient de distribution de la regeneration (CDR) ont ete estimes le long de 432 transects. La densite de semis d'epinette, la hauteur maximale et le CDR avaient les plus basses valeurs dans les peuplements feuillus et dans le traitement a retention de 100 % (temoin). Une analyse par arbre de regression a indique qua la densite, la hauteur maximale et le CDR des semis d'epinette atteignaient les valeurs les plus elevees lorsque la disponibilite des arbres semenciers etait elevee (>30 [ha.sup.-1]) et dans les sentiers de debardage. Dans ces circonstances, le CDR atteignait 74 %. Au contraire, le CDR etait inferieur a 14 % dans les bandes de retention ou la machinerie ne circulait pas lorsque la densite des arbres semenciers etait inferieure a 11 [[ha.sup.-1]. Cependant, le CDR diminuait aussi dans les traitements avec les plus fortes densites d'arbres residuels. En fonction de la densite des arbres semenciers, le CDR etait le plus eleve dans les coupes a blanc et le plus bas dans les coupes a retention de 75 % ou la faible transmission de la lumie re peut avoir limite le succes de la regeneration. Une perturbation passive du sol due au debardage a semble plus importante pour l'etablissement des semis d'epinette que l'abondance de la vegetation concurrente. [Traduit par la Redaction], Introduction Over the last decade, forest managers and silviculturists have increasingly used variable retention harvesting systems as an approach to sustainable forest management that attempts to emulate natural disturbances (Mitchell [...]

Temperate tree species are expected to expand their distribution into the boreal forest in response to climate change. However, it is becoming increasingly clear that many species will experience significant setbacks in capacity to migrate due to a series of unfavourable conditions impacting their recruitment success, and thus their ability to colonize new locations.We quantify the relative influence of a series of factors important for tree seedling recruitment at range margins: propagule dispersal, substrate favourability and the influence of the local hetero‐specific species canopy composition. We hypothesized that boreal trees are responsible for priority effects that influence the establishment of temperate tree species seedlings. To do so, we analysed two recruitment stages (first‐year seedlings and older seedlings) for seven tree species: Abies balsamea (ABBA), Acer rubrum (ACRU), Acer saccharum (ACSA), Betula papyrifera (BEPA), Betula alleghaniensis (BEAL), Populus tremuloides (POTR) and Fagus grandifolia (FAGR) commonly found within the temperate‐boreal ecotone forests of northeastern North America.Overall, we found that boreal canopy trees influence the distribution of substrates, more specifically the occurrence of needle cover and decayed wood in recruitment plots. Interestingly, decayed wood was a poor substrate in almost all cases. This association between canopy and substrate led to highly unfavourable substrates that affected the seedling densities of all temperate tree species. In addition, we found that seedling dispersion was highly localized, where the mean dispersal distance of all trees occurred in close proximity of parent trees.Synthesis. Unfavourable substrates and limited mean dispersal distance of trees due to resident boreal trees generate (strong) priority effects within the temperate‐boreal ecotone. Together, these conditions promise to cause significant lags in temperate tree species migration into the boreal forest in the future. [ABSTRACT FROM AUTHOR]

Accurately estimating plant water uptake and use is essential to improving model prediction(s) and our understanding of current and future water fluxes within the forested landscapes. Water-use of mature trees is estimated from extrapolating physiological measurements taken at the leaf and branch scale in parallel with the increasing use of in-situ methods such as sap flow systems; estimating water volumes moving through the stem. However, temporal lags and discrepancies between canopy measurements and xylem sap-flow measurements have been reported, highlighting the importance of water storage along the stem in trees. Properly assessing the partitioning between flux and storage requires accurate sap flow measurements. Although largely tested on young and small trees, the accuracy of the Heat Ratio Method (HRM) on large diameter trees has yet to be directly tested and remains a significant knowledge gap to our understanding of sap flow measurements in mature trees. Using a modified weighing lysimeter on a large mature boreal aspen (Populus tremuloides) equipped with 20 sap flow sensors, we tested the accuracy of the HRM sap flow measurements and assessed water storage dynamics along the stem.Over 10 days, hourly sap-flow rates measured with the sensors showed a very good agreement with water uptake flow measured by the scale (R2 > 0.9) for the basal sensors, while cumulative water uptake was best estimated by sensors placed on the north and east sides of the trunk. Hourly upper-stem sap flow rates were significantly lower than basal-stem rates, suggesting substantial stem water storage. The partitioning between the transpiration stream and water storage in the sap flow rates varied with cardinal orientation across the stem. As a result, the strength of the correlation between hourly stem sap flow rates and climatic conditions changed with distance from the canopy.Although the HRM sap flow sensors were shown to provide reliable estimates of total tree water use in this large boreal tree, the results show a substantial variation between cardinal orientation in sap flow measurements. We highlight the importance of measuring and separating the transpiration stream and stem water storage in the sap flow measurements, especially when correlating to climatic conditions and modelling water fluxes at both the individual tree and forested landscape scales. [ABSTRACT FROM AUTHOR]