Aspen (Populus tremuloides Michx.) is a clonal tree species that commonly regenerates via root suckering after disturbance. This paper reviews the literature and identifies critical gaps in our understanding of the dynamics of aspen root suckering. The role of plant growth regulators (e.g., hormones, carbohydrates), environmental conditions (e.g., soil moisture, temperature, nutrient availability), overstory disturbance (e.g., harvesting, wildfire), ground disturbance (e.g., soil compaction, wounding or severing of roots), vegetation competition, predisturbance stand condition, and clonal (genetic) differences are discussed as they relate to sucker initiation, sucker growth, and (or) patterns of site establishment. The paper presents a series of conceptual figures summarizing our knowledge of the factors controlling suckering dynamics and identifies areas of future research.

The effects of gradients in light levels and tree height on growth and crown attributes of six conifer species were studied in eastern and western Canada. Three conifers were studied in British Columbia (Abies lasiocarpa (Hook.) Nutt., Picea glauca (Moench) Voss × Picea engelmannii Parry ex Engelm., and Pinus contorta Dougl. ex Loud. var. latifolia Engelm.), and three in Quebec (Abies balsamea (L.) Mill., Picea glauca, and Pinus banksiana Lamb.). For several growth and morphological parameters, conifers reacted strongly to both an increase in light and tree height. Significant or nearly significant interactions between light classes and height were found for height and diameter growth of most species as well as for many crown attributes for both Abies and Picea. These interactions usually indicated that growth or morphological changes occurred with increasing height from a certain light level. Within a single genus, both eastern and western tree species showed the same overall acclimation to light and height. As generally reported, Pinus species showed less variation in growth and morphological responses to light than Abies and Picea species. Résumé: Les effets de gradients d'intensité lumineuse et de hauteur des arbres sur la croissance et les attributs du houppier de six espèces de conifères ont été étudiés dans l'est et l'ouest du Canada. Trois espèces de conifères ont été étudiées en Colombie-Britannique (Abies lasiocarpa (Hook.) Nutt., Picea glauca (Moench) Voss × Picea engelmannii Parry ex Engelm. et Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) et trois autres au Québec (Abies balsamea (L.) Mill., Picea glauca et Pinus banksiana Lamb.). Pour plusieurs paramètres liés à la croissance et à la morphologie, les conifères réagissent fortement à la fois à une augmentation de la lumière et de la hauteur des arbres. Des interactions significatives ou presque significatives entre les classes de lumière et de hauteur ont été trouvées pour la croissance en hauteur et en diamètre de la plupart des espèces ainsi que pour de nombreux attributs du houppier dans le cas d'Abies et de Picea. Ces interactions signifient habituellement que les changements dans la croissance ou la morphologie apparaissent avec une augmentation de la hauteur à partir d'un certain niveau de lumière. À l'intérieur d'un genre, les espèces de l'Ouest et de l'Est montrent la même acclimatation globale à la lumière et à la hauteur. Tel que rapporté généralement, les espèces de Pinus montrent moins de variation dans leurs réponses à la lumière, dans le cas de la croissance et de la morphologie, que les espèces d'Abies et de Picea.[Traduit par la Rédaction] Claveau et al. 468

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

Five methods for estimating the mean growing season percent photosynthetic photon¯ux density (PPFD) were compared to continuous measurements of PPFD throughout the growing season within a young bigleaf maple stand on Vancouver Island (Canada). Measured PPFD was recorded continuously as 10-min averages over the growing season (May 18±October 14, 1996) using 52 gallium arsenide phosphide photodiodes in the understory and a LI-COR quantum sensor in the open. Photodiodes were randomly located on a systematic grid of points and represented a wide range of above canopy openings which were classi®ed into three different types of light environments: closed canopy, gaps of various sizes, and open canopy. Objectives of this study were to compare different methods for estimating the growing season %PPFD and to determine the ef®ciency of these methods in the three light environments. At each photodiode location, instantaneous light measurements using a Ceptometer on sunny days around noon and a LAI-2000 Plant Canopy Analyzer were made and hemispherical canopy photographs were taken. 10-min averages recorded by the photodiodes during completely overcast sky conditions were used as surrogate values for a method that uses instantaneous measurements on overcast days. Finally, a new light model (LITE) developed to estimate growing season %PPFD in a deciduous canopy was tested. All these ®ve methods provided estimates of growing season %PPFD and are much less time consuming than continuous measurements of %PPFD using photodiodes. The three most accurate (r 2 >0.89) methods to estimate the growing season %PPFD were the 10 min averages on overcast days, the diffuse non-interceptance calculated using the LAI-2000, and the gap light index (GLI) calculated from the hemispherical canopy photographs. These three methods performed similarly in each type of light environment. Although the relationship between the LITE model and the growing season %PPFD was good (r 2 0.79), the model systematically underestimated light transmission. The instantaneous sunny days around noon method was the least ef®cient method (r 2 0.68) for estimating the growing season %PPFD, although replacing instantaneous measures with the mean of two 10-min averages improved r 2 to 0.84. Estimates on sunny days tended to be low in low light and high in high light. Practical considerations such as equipment availability, cost, sampling and processing time, sky conditions, and the number of microsites to be sampled should be taken into account in the selection of the suitable method for a particular study. #

The distribution of tree biomass and the allocation of production was measured in four stands of lodgepole pine (Pinuscontorta var. latifolia Engelm.), two growing on sites with xeric soil moisture regimes and two on sites with mesic soil moisture regimes. At the time of sampling the stands were 70–78 years old. Aboveground biomass ranged from 116.5 Mg•ha−1 on one xeric site to 313.1 Mg•ha−1 on one mesic site. Stem biomass represented 68 and 73% of total tree biomass on the xeric and mesic sites, respectively. Total root biomass represented between 20 and 28% of total lodgepole pine biomass. Fine and small roots (<5 mm diameter) represented 4 and 1.5% of total tree biomass on the xeric and mesic sites, respectively. Total net primary production ranged from 7.9 Mg•ha−1•year−1 on the xeric sites to 11.9 Mg•ha−1•year−1 on the mesic sites. Stemwood production represented 20 and 27% of total net primary production on the xeric sites and 35% on both mesic sites. Belowground production represented 38 and 46% of total net primary production on the two mesic sites (4.5 and 5.5 Mg•ha−1•year−1, respectively) and 55 and 66% on the two xeric sites (4.3 and 6.3 Mg•ha−1•year−1, respectively). Fine and small roots represented 82–94% of belowground production. Production allocation was in the following order: fine and small roots > stems > foliage > coarse roots > branches, for all but the wettest site, where stem production exceeded fine and small root production.