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We examine the influence of (i) the spatial distribution and abundance of parent trees (as seed sources) and (ii) the abundance and favourability of seedbed substrates, on seedling recruitment for the major tree species in northwestern interior cedar-hemlock forests of British Columbia, under four levels of canopy openness (full canopy, partial canopy, large gap, and clearcut). Substrate distribution varied with canopy openness, and substrate favourability was a function of both canopy openness and seedling species. Lack of suitable substrates was the predominant factor limiting seedling density under full canopies. Partial canopy and gap sites provided a broad range of favourable substrates in close proximity to parent trees, resulting in the highest observed seedling densities. There was much higher effective dispersion of seedlings away from parent trees in gaps than in the partially cut stands. Seedling dispersion to clearcut sites was poor with seedlings being tightly restricted to a narrow band along the forest edge. Thus, seedling recruitment in these forests was a reflection of the interaction between the abundance of seed and substrate favourability, and the relative importance of these factors varied significantly with canopy structure. Résumé : Les auteurs ont examiné l'influence (i) de la répartition spatiale et de l'abondance d'arbres parentaux consi-dérés comme sources de semences et (ii) de l'abondance et de la convenance du substrat des lits de germination, sur le recrutement des semis des principales espèces d'arbres dans les forêts de thuya et de pruche de la partie nord-ouest de l'intérieur de la Colombie-Britannique, en fonction de quatre niveaux d'ouverture du couvert (couvert complet, couvert partiel, grande ouverture et coupe à blanc). La répartition du substrat variait avec le degré d'ouverture du couvert et la convenance du substrat était fonction à la fois de l'ouverture du couvert et de l'espèce du semis. Le manque de subs-trats appropriés était le principal facteur qui limitait la densité des semis sous un couvert complet. Les plus hautes den-sités de semis ont été observées sous couvert partiel et dans les ouvertures qui procuraient un large éventail de substrats appropriés dans l'environnement immédiat des arbres parentaux. La dispersion des semis loin des arbres pa-rentaux était beaucoup plus efficace dans les ouvertures que dans les peuplements partiellement coupés. La dispersion des semis dans les sites coupés à blanc était faible, les semis étant limités à une bande étroite située le long de la li-sière de la forêt. Ainsi, le recrutement des semis dans ces forêts était le résultat de l'interaction entre l'abondance des semences et la convenance du substrat, et l'importance relative de ces facteurs variait de façon significative selon la structure de la canopée. [Traduit par la Rédaction] LePage et al. 427

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Abstract:We have characterized overstory light transmission, understory light levels, and plant communities in mixedwood boreal forests of northwestern Quebec with the objective of understanding how overstory light transmission interacts with composition and time since disturbance to influence the diversity and composition of understory vegetation, and, in turn, the further attenuation of light to the forest floor by the understory. Overstory light transmission differed among three forest types (aspen, mixed deciduous-conifer, and old cedar-dominated), with old forests having higher proportions of high light levels than aspen and mixed forests, which were characterized by intermediate light levels. The composition of the understory plant communities in old forests showed the weakest correlation to overstory light transmission, although those forests had the largest range of light transmission. The strongest correlation between characteristics of overstory light transmission and understory communities was found in aspen forests. Species diversity indices were consistently higher in aspen forests but showed weak relationships with overstory light transmission. Light attenuation by the understory vegetation and total height of the understory vegetation were strongly and positively related to overstory light transmission but not forest type. Therefore, light transmission through the overstory influenced the structure and function of understory plants more than their diversity and composition. This is likely due to the strong effect of the upper understory layers, which tend to homogenize light levels at the forest floor regardless of forest type. The understory plant community acts as a filter, thereby reducing light levels at the forest floor to uniformly low levels.Résumé : Nous avons caractérisé la transmission de la lumière par le couvert dominant, les niveaux de lumière en sous-étage et les communautés végétales dans les forêts boréales mixtes du nord-ouest du Québec. L'objectif de l'étude était de comprendre comment la transmission de la lumière par le couvert dominant interagit avec la composition et le temps écoulé depuis la dernière perturbation pour influencer la diversité et la composition de la végétation en sousétage et, par la suite, l'atténuation supplémentaire de la lumière au sol causée par ce sous-étage. La transmission de la lumière par le couvert dominant était différente entre les trois types forestiers (peuplements dominés par le peuplier, par un mélange de conifères et de feuillus et par de vieux thuyas), les vieilles forêts ayant de plus fortes proportions de niveaux élevés de lumière que les forêts de peuplier et mixtes qui étaient caractérisées par des niveaux intermédiai-res de lumière. La composition des communautés végétales du sous-étage des vieilles forêts était la plus faiblement corrélée à la transmission de la lumière par le couvert dominant, même si ces forêts montraient la plus grande variation de transmission de la lumière. La plus forte corrélation entre les caractéristiques de ...

We have characterized canopy geometry and light transmission by the nine dominant conifer and broad-leaved tree species of the interior cedar-hemlock (ICH) forests of northern British Columbia. Our field data were used to parameterize a spatially explicit model of light transmission through mixed-species forests. That model, a component of the forest dynamics simulator SORTIE, was developed for eastern deciduous forests, and this paper presents a test of that model in a very different ecosystem. Our results show that individual crowns of the ICH forests intercepted much more light than species of eastern deciduous forests but that the canopy as a whole allowed greater light penetration, largely because of openings between the relatively narrow, conical crowns of the western conifers. Light transmission by individual crowns was correlated with shade tolerance among the conifers (as in eastern deciduous species), but crown depth was not (in contrast with eastern species). Despite the fundamental differences in the nature of light transmission in the two ecosystems, the SORTIE light model developed for eastern deciduous forests was effective at predicting spatial variation in understory light levels in these western coniferous forests. The goodness of fit of such a simple model suggests that the most important factors regulating spatial variation in understory light levels in these forests are simply the sizes and distribution of nearby trees, and the local sky brightness distribution. Discrete canopy gaps represent a special case in which a region of the canopy is not occupied by crowns.