Light extinction coefficients specific to the understory vegetation of the southern boreal forest, Quebec

Abstract

This study was conducted in six different forest types in Abitibi, Que., (i) to identify the factors that most influence understory light transmission in the southern boreal forest and (ii) to develop light extinction coefficients (k), which could be used to simulate light transmission in the understory. Light availability and understory vegetation (cover, composition, vertical distribution, and leaf area index) were characterized within three strata (0.05-5 m) in a total of 180 quadrats. Calculated k values were based on measured light availability and leaf area index. These values varied among forest types, strata, understory vegetation types, and cover in the upper stratum. The highest k values were generally associated with a dense stratum of Acer spicatum Lam. We developed five sets of k values based on the factors that most affected light transmission. Measured transmission (T m ) was compared with transmission predicted (T p ) from each set of k values. Light transmission predicted using a single k value (mean k = 0.54) underestimated T m . More accurate predictions were obtained when we used the other four sets of k values. Our results indicate that, in the southern boreal forest, the understory vegetation can be quite heterogeneous and patterns of light transmission cannot be accurately simulated using a unique k value. However, the various sets of k values developed in this study could be used in prediction models of forest dynamics to obtain relatively good predictions of understory light extinction in forest types similar to the ones studied here.Résumé : Cette étude a été effectuée dans six différents types forestiers en Abitibi (Québec) afin (i) d'identifier les principaux facteurs influençant la transmission de la lumière en sous-bois dans la forêt boréale méridionale et (ii) de développer des coefficients d'extinction de la lumière (k) qui pourraient être utilisés pour simuler la transmission de la lumière en sous-bois. La disponibilité en lumière et la végétation en sous-bois (recouvrement, composition en espèces, distribution verticale et indice de surface foliaire) ont été caractérisées au sein de trois strates (0,05-5 m) dans un total de 180 quadrats. Des valeurs de k ont été calculées à partir des mesures de disponibilité en lumière et de l'indice de surface foliaire. Ces valeurs de k variaient en fonction du type forestier, de la strate de végétation, du type de végéta-tion de sous-bois et du recouvrement dans la strate supérieure de végétation. Les valeurs de k les plus élevées étaient généralement associées à une strate dense d'Acer spicatum Lam. Nous avons développé cinq ensembles de valeurs de k en nous basant sur les facteurs qui influençaient le plus la transmission de la lumière. La transmission mesurée (T m ) a été comparée à la transmission prédite (T p ) à partir de chacun de ces cinq ensembles de k. La transmission prédite à partir d'une valeur unique de k (k moyen = 0,54) sous-estimait la T m . Des prédictions plus justes ont été obtenues à partir des quatre autres ens...

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