The potential for use of a partial canopy for controlling growth of Calamagrostiscanadensis (Michx.)•Beauv., and Epilobiumangustifolium L. among regenerating Piceaglauca (Moench) Voss saplings was assessed in the understory of 24 established stands in the P. glauca–Viburnum–Rubuspubescens association of the lower boreal cordilleran ecoregion of Alberta. Stand overstories were dominated by Populustremuloides Michx., P. glauca, or were a mixture of these two species The composition, basal area, and light transmission of the overstory of each stand were measured. Hardwood-dominated overstories transmitted between 14 and 40% of incoming light while P. glauca canopies transmitted between 5 and 11 % of light. Cover and height of C. canadensis and E. angustifolium decreased with decreasing light transmission; at 40% light, both species were greatly reduced compared with open-grown conditions and both were virtually eliminated from stands with less than 10% light. The annual height increment of P. glauca saplings increased from 5 cm at 10% light to 25 cm at 40 % light; the latter increment was approximately equal to growth in 100% light conditions. The number of buds, the diameter of the current leader, and the height to diameter ratio of the tree also increased with light transmittance.
This paper reviews current information relating to the dynamics of light in northern and boreal forests and discusses factors affecting overstory light transmission, seasonality of light, sunflecks, canopy gaps, and understory development, particularly with regard to tree regeneration. Techniques for measurement of light in forests such as radiometers, photosensitive paper or chemicals, hemispherical canopy photographs, the plant canopy analyzer, or visual estimators of canopy density are each discussed in terms of their accuracy, costs, ease of use, and conditions required during measurement. Predictive models of light transmission based on canopy architecture are also described in terms of their assumptions, accuracy, and input data costs. Lastly the paper discusses the relationship among overstory and understory densities, ground-level light, and "windows of opportunity" for regeneration of trees in the understory following management interventions.Résumé : Cet article passe en revue les connaissances courantes sur la dynamique de la lumière dans les forêts nordiques et boréales. Les auteurs y discutent des facteurs qui affectent la transmission de la lumière dans le couvert, les variations saisonnières de la lumière, les trouées de lumière, les trouées du couvert et le développement du sousétage, particulièrement en relation avec la régénération des arbres. Les diverses techniques de mesure de la lumière, telles que l'utilisation de radiomètres, de produits chimiques ou de papiers photosensibles, de photographies panoramiques du couvert, d'analyseurs de la composition du couvert ou d'estimateurs visuels de la densité du couvert, sont analysées sur la base de leur précision, de leur coût, de leur facilité d'utilisation et des conditions requises pour effectuer les mesures. Les modèles de prédiction de la transmission de la lumière qui reposent sur l'architecture du couvert sont aussi décrits en fonction des hypothèses sur lesquelles ils reposent, de leur précision et du coût d'acquisition des données. Finalement, les auteurs discutent des relations entre la densité de l'étage dominant et celle du sous-étage, de la lumière au niveau du sol et des circonstances opportunes pour la régénération des arbres en sousétage suite à des interventions d'aménagement.[Traduit par la Rédaction] Review / Synthèse 811
-A series of conventional distance-independent and distance-dependent competition indices, a highly flexible distance-dependent crowding index, and two light resource estimation indices were compared to predict individual tree diameter growth of five species of mature trees from natural-origin boreal mixed forests. The crowding index was the superior index for most species and ecosites. However, distance-independent indices, such as basal area of competing trees, were also effective. Distance-dependent light estimation indices, which estimate the fraction of seasonal photosynthetically-active radiation available to each tree, ranked intermediate to low. Determining separate competition indices for each competitor species accounted for more variation than ignoring species or classifying by ecological groups. Species' competitive ability ranked (most competitive to least): paper birch ≈ white spruce ≈> trembling aspen > lodgepole pine > balsam poplar. Stratification by ecosite further improved model performance. However, the overall impact of competition on mature trees in these forests appears to be small. competition index / photosynthetically active radiation / distance dependence / growth model / boreal mixed forest Résumé -Évaluation de la compétition et indices d'éclairement pour la prédiction de la croissance radiale dans des forêts boréales mixtes adultes. Ce travail a évalué la capacité d'indices de compétition à prédire la croissance radiale individuelle d'arbres adultes de cinq espèces de forêts mixtes boréales. Ont ainsi été comparés : (1) une série d'indices conventionnels de compétition indépendants ou dépendants de la distance, (2) un indice très flexible d'encombrement dépendant de la distance et (3) deux indices d'estimation de l'éclairement. L'indice d'encombrement a été le plus efficace dans la plupart des stations et des espèces. Cependant, les indices indépendants de la distance tels que la surface terrière des arbres en compétition, ont été également efficaces. Les indices dépendants de la distance, d'estimation de l'éclairement, qui estiment la fraction saisonnière du rayonnement photosynthétiquement actif disponible pour chaque arbre, se sont classés en position intermédiaire. L'identification d'indices de compétition spécifiques de chaque espèce compétitrice a mieux rendu compte de la diversité des stations qu'un indice non spécifique ou qu'un classement des espèces par groupes écologiques. L'aptitude à la compétition des espèces a été classée de la manière suivante (de la plus à la moins compétitive) : Betula papyrifera, Picea glauca, Populus tremuloides, Pinus contorta, Populus balsamifera. La stratification par station améliore encore la performance du modèle. Cependant, l'impact général de la compétition sur les arbres adultes dans ces forêts semble être faible.indice de compétition / rayonnement photosynthétiquement actif / distance dépendante / modèle de croissance / forêt boréale mixte
Juvenile white spruce (Piceaglauca (Moench) Voss) under an aspen (Populustremuloides Michx.) overstory were studied in nine boreal mixedwood stands in west-central Alberta. In each stand, 50 understory white spruce were cut for stem analysis at ground level, 30, 70, 130 cm, and every 100 cm to tree height. In four stands, recruitment of these understory spruce occurred immediately after the disturbance, while in others the recruitment was delayed several decades. The period of recruitment was as short as 15–20 years or continued for decades, producing an uneven-aged understory. Trees initiated on rotten logs had a slightly lower initial annual diameter increment but did not differ in height growth compared with those initiated on normal forest floor. The annual height increment increased as the trees grew in height, presumably as they overtopped successive layers of shading vegetation. When seedlings were less than 30 cm tall they grew less than 10 cm per year, but attained growth rates of 30 cm per year or more when they were taller than 230 cm. Height growth rates for these understory trees were comparable to reported growth rates of white spruce of similar size and age from clearcut areas.
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