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

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-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.

This study examines light competition between aspen and spruce during the sequence of aspen development. Leaf area index and light transmission were measured or estimated for aspen stands from 2 to 125 years old. Light transmission was lowest at 15-25 years, and in some stands, transmission was less than 5% of above-canopy light. Hypothetical aspen stands with various stem configurations and heights were developed, and positions were identified that would meet or fail Alberta free-to-grow (FTG) standards. Light transmission was estimated at each position with the MIXLIGHT forest light simulator. Positions in canopy gaps or at the northern sides of canopy gaps had higher light. In general, however, there was little difference in available light between positions that met or failed FTG criteria. Stand density and size of aspen trees appears to be a better index to predict light transmission and spruce success in juvenile aspen stands than current FTG criteria.Key words: competition, free to grow, hardwood, spruce, light Cette étude porte sur la compétition pour la lumière entre le peuplier faux-tremble et l'épinette au cours de la période de développe-ment du peuplier faux-tremble. L'indice de la surface foliaire et la transmission de la lumière ont été mesurés ou estimés dans des peupleraies âgées de 2 à 125 ans. La transmission de la lumière a été la plus faible dans les peuplements de 15 à 25 ans, et dans certains peuplements, la transmission a été de moins de 5% par rapport à la lumière au-dessus des cimes. Des peupleraies hypothétiques représen-tant diverses configurations de tige et de hauteur ont été élaborées, et des positions ont été identifiées qui correspondraient ou non aux normes de tige libre de croître de l'Alberta. La transmission de la lumière a été estimée pour chaque position au moyen du simulateur de lumière sous couvert forestier MIXLIGHT. Les positions retrouvées dans les ouvertures du couvert ou du côté nord des ouvertures dans le couvert recevaient plus de lumière. De façon générale, cependant, il existe peu de différence au niveau de la lumière disponible entre les positions qui répondaient ou non aux normes de croissance sans concurrence. La densité du peuplement et la taille des tiges de peuplier semblent être un meilleur indice de prédiction de la transmission de la lumière et du succès de l'épinette dans les jeunes peupleraies que ne le sont les normes actuelles de croissance sans concurrence.

Insects, diseases, fire and drought and other disturbances associated with global climate change contribute to forest decline and mortality in many parts of the world. Forest decline and mortality related to drought or insect outbreaks have been observed in North American aspen forests. However, little research has been done to partition and estimate their relative contributions to growth declines. In this study, we combined tree-ring width and basal area increment series from 40 trembling aspen (Populus tremuloides Michx.) sites along a latitudinal gradient (from 52° to 58°N) in western Canada and attempted to investigate the effect of drought and insect outbreaks on growth decline, and simultaneously partition and quantify their relative contributions. Results indicated that the influence of drought on forest decline was stronger than insect outbreaks, although both had significant effects. Furthermore, the influence of drought and insect outbreaks showed spatiotemporal variability. In addition, our data suggest that insect outbreaks could be triggered by warmer early spring temperature instead of drought, implicating that potentially increased insect outbreaks are expected with continued warming springs, which may further exacerbate growth decline and death in North America aspen mixed forests.

The toxic effects of aluminum, cadmium, copper, manganese, nickel, and zinc stress on root growth of Triticum aestivum were modelled using a modified version of the Weibull frequency distribution. The modified Weibull function is a continuous, nonlinear mathematical function that provides direct estimates of several important biological parameters, including maximum growth, minimum growth, growth response, toxicity threshold, and maximum unit toxicity. The function was fit to experimental data using standard nonlinear regression techniques. Despite variation among metals in the characteristics of observed dose responses, the Weibull function provided a close fit for each of the metals tested (0.96 ≤ R2 ≤ 0.99). Estimates of maximum unit toxicities indicated that cadmium was most toxic to T. aestivum (152% growth reduction (GR)/(μmol∙L)), followed by copper (19.0% GR/(μmol∙L)), nickel (3.9% GR/(μmol∙L)), aluminum (1.3% GR/(μmol∙L)), zinc (0.5% GR/(μmol∙L)), and manganese (0.2% GR/((μmol∙L)). Estimated toxicity thresholds also indicated that cadmium was most toxic. Cadmium showed the lowest threshold for metal injury (0.02 μM), followed by copper (3.4 μM), nickel (11 μM), aluminum (18 μM), manganese (37 μM), and zinc (45 μM). The shift in the relative position of Mn and Zn suggests that unit toxicity and the toxicity threshold may be independent measures of the response of plants to metal stress. These experiments clearly demonstrate the utility of the Weibull function in modelling the response of plants to metal stress. Furthermore, the ability of the function to model primary growth data directly provides a suitable technique for investigation of potential interactions between phytotoxic metals. Key words: dose response, modelling, metal stress, toxicity threshold, unit toxicity.