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The effect of climate change on forest dynamics is likely to increase in importance in the forthcoming decades. For this reason, it is essential to predict the extent to which changes in temperature, precipitation, and atmospheric CO2 might affect the development of forest ecosystems and successional pathways. The gap model ZELIG-CFS was used to simulate the potential long-term effects of climate change on species-specific annual change in mean basal area and stand density under two scenarios of representative concentration pathways (RCP), 4.5 and 8.5, for the boreal forest region of Ontario, Canada, where mean temperature, precipitation, and atmospheric CO2 are expected to increase. Forest ecosystems in this boreal region included pure and mixed stands of black spruce (Picea mariana [Mill.] B.S.P.), paper birch (Betula papyrifera Marsh.), balsam fir (Abies balsamea [L.] Mill.), jack pine (Pinus banksiana Lamb.), trembling aspen (Populus tremuloides Michx.), white spruce (Picea glauca [Moench] Voss), northern white cedar (Thuja occidentalis L.), American larch (Larix laricina [Du Roi] K. Koch), and balsam poplar (Populus balsamifera L.). Simulation results under climate change generally predicted a decline in the basal area and stand density for black spruce, balsam fir, jack pine, and white spruce, but an increase for paper birch, trembling aspen, American larch, and balsam poplar. However, the extent of change differed regionally among species. Forest composition is expected to change over the long term. Simulation results indicated that shade-intolerant deciduous and conifer species will increase their dominance over the 100-year time horizon. This transition toward the increasing presence of deciduous forests is likely explained by more favorable temperature conditions for their growth and development. [ABSTRACT FROM AUTHOR]

Increasing the production of wood fibre from conifer species such as white spruce (Picea glauca (Moench) Voss) is one of many challenges in the management of boreal mixedwood forests. The effects of various competition measures on relative growth and relative growth rate variables were calculated for individual white spruce subject trees. Correlation analysis was used to explore relationships with competitor structural features, including the ratio of competitor basal area to subject tree basal area (CBAS), the ratio of competitor height to subject tree height (AHCS), and the proportion of softwood (FSW). Regression analysis was used to explore relationships with three distance-dependent competition indices. The ratio of subject tree height to diameter at breast height (DBH) (HDR), crown ratio (CR), and crown relative increment rate (CRIR) were significantly correlated with CBAS and AHCS. HDR, CR, CRIR, and DBH relative growth rate were all statistically significantly related to the competition indices. Results indicated that (i) relative growth and relative growth rate measures successfully captured a range of competition, (ii) crowns of trees with larger diameters used their horizontal growing space more efficiently to produce stemwood, and (iii) the proportion of softwood contributing to competition did not appear to influence subject tree production efficiency. Growth efficiency variables have the potential to improve our understanding of boreal mixedwood dynamics. Key words: growth efficiency, white spruce, trembling aspen, competition indices, boreal mixedwood forest. L'augmentation de la production de fibre ligneuse d'especes resineuses comme l'epinette blanche (Picea glauca (Moench) Voss) est un des nombreux defis de l'amenagement des forets boreales melangees. Les effets de differentes mesures de competition sur des variables de croissance relative et de taux relatif de croissance ont ete calcules pour des arbres sujets d'epinette blanche. L'analyse de correlation a ete utilisee pour explorer les relations avec les caracteristiques structurelles des competiteurs qui incluaient le ratio de la surface terriere des competiteurs et la surface terriere de l'arbre sujet (CBAS), le ratio de la hauteur des competiteurs et la hauteur de l'arbre sujet (AHCS) et la proportion de resineux (FSW). L'analyse de regression a ete utilisee pour explorer les relations avec trois indices de competition dependant de la distance. Le ratio de la hauteur de l'arbre sujet et le diametre a hauteur de poitrine (DHP) (HDR), le rapport de cime (CR) et le taux relatif d'accroissement des cimes (CRIR) ont ete significativement correles avec CBAS et AHCS. HDR, CR, CRIR et le taux relatif de croissance en DHP ont tous ete statistiquement lies aux indices de competition. Les resultats ont indique que (i) les mesures de croissance relative et de taux relatif de croissance ont bien saisi l'etendue de la competition, (ii) les cimes des arbres de plus grand diametre ont utilise leur espace horizontal de croissance de facon plus efficace que les plus petits arbres pour produire du bois de fut et (iii) la proportion des especes ligneuses qui contribuaient a la competition n'a pas semble influencer l'efficacite de production des arbres sujets. Les variables d'efficacite de croissance peuvent potentiellement ameliorer notre comprehension de la dynamique des forets boreales melangees. Mots-cles: efficacite de croissance, epinette blanche, peuplier faux-tremble, indices de competition, foret boreale melangee., Introduction In Canada, mixedwood forests of white spruce (Picea glauca (Moench) Voss) and trembling aspen (Populus tremuloides Michx.) have historically been managed for the white spruce component. During the past [...]

Competition in forest stands has long been of interest to researchers. However, much of the knowledge originates from empirical studies that examined the effects of competition. For instance, many studies were focused on the effects of the presence of herbaceous species on the development of tree seedlings or the decrease in individual tree growth with increases in stand density. Several models that incorporate competitive effects have been developed to predict tree and stand growth, but with simplified representations of competitive interactions. While these studies provided guidance useful for forest management, they contributed only partially to furthering our understanding of competitive mechanisms. Also, most competition studies were conducted in single-species stands. As competitive interactions occurring in mixed stands are characterized by a higher degree of complexity than those in single-species stands, a better understanding of these mechanisms can contribute to developing optimal management scenarios. The dynamics of forest stands with at least two species may be affected not only by competition, but also by facilitation or complementarity mechanisms. Thus, knowledge of the mechanisms may provide insight into the relative importance of intra- versus inter-specific competition and whether competition is symmetric or asymmetric. Special attention to the implementation of field experimental designs is warranted for mixed stands. While traditional spacing trials are appropriate for single-species stands, the examination of competitive interactions in mixed stands requires more complex experimental designs to examine the relative importance of species combinations. Forest productivity models allow resource managers to test different management scenarios, but again most of these models were developed for single- species stands. As competitive interactions are more complex in mixed stands, models developed to predict their dynamics will need to include more mechanistic representations of competition. Key words: competition, facilitation, complementarity, experimental designs, site resources, modelling. Resume : Les chercheurs s'interessent depuis longtemps a la competition dans les peuplements forestiers. Cependant, la majeure partie des connaissances provient d'etudes empiriques n'examinant que les effets de la competition. Par exemple, plusieurs etudes se sont concentrees sur les effets de la presence d'especes herbacees sur le developpement des plantules d'arbres ou sur la diminution de la croissance des arbres individuels avec l'augmentation de la densite. Plusieurs modeles incorporant les effets de la competition ont ete developpes pour predire la croissance des arbres et des peuplements, avec cependant des representations simplifiees des interactions competitives. Bien que ces etudes fournissent des pistes utiles pour l'amenagement forestier, elles contribuent seulement partiellement a ameliorer notre comprehension des mecanismes competitifs. De plus, la plupart des etudes sur la competition ont ete conduites dans des peuplements monospecifiques. Puisque les interactions competitives survenant dans les peuplements mixtes se caracterisent par un degre de complexite plus eleve que celles des peuplements monospecifiques, une meilleure comprehension de ces mecanismes pourra contribuer au developpement de scenarios d'amenagement optimises. La dynamique des peuplements comportant au moins deux essences peut se voir affectee non seulement par la competition, mais aussi par des mecanismes de facilitation ou de complementarite. Ainsi, la connaissance des mecanismes peut jeter un nouvel eclairage sur l'importance relative de la competition intra- vs inter-specifique et determiner s'il s'agit de competition symetrique ou asymetrique. Une attention speciale lors de la conception de plans experimentaux est essentielle pour les peuplements melanges. Alors que les essais d'espacement traditionnels conviennent pour les peuplements monospecifiques, l'examen des competitions interactives necessite des plans experimentaux plus complexes pour evaluer l'importance relative des combinaisons d'especes. Les modeles de productivite forestiere permettent a l'amenagiste des ressources de verifier divers scenarios d'amenagement, mais encore une fois la plupart de ces modeles ont ete developpes pour des peuplements monospecifiques. Les interactions competitives etant plus complexes dans les peuplements mixtes, les modeles developpes pour predire leur dynamique devront desormais inclure de plus fortes representations des mecanismes de competition. [Traduit par la Redaction] Mots-cles: competition, facilitation, complementarite, plans experimentaux, ressources des sites, modelisation., Introduction Although competition is a widespread phenomena [sic] exhibited by organisms from microbes to mammals, empirical evidence is elusive. Controlled laboratory experiments usually involve simplified competitive systems whose results cannot [...]

Abstract: As forest management intensifies, the conservation of forest biodiversity is a growing concern. Bryophytes are known to represent a considerable portion of plant diversity within northern forests. This is [...]

Abstract: Plantations have been claimed to be 'monocultures', or 'biological deserts'. We investigated these claims in the context of a long-term study on plant diversity within plantations with different indigenous [...]