Allometry and size structure of trees in two ancient snow forests in coastal British Columbia

Abstract

Shade-tolerant trees that start in the understory but ultimately reach the canopy persist through different microhabitat conditions during their ontogeny. We evaluate how the height to diameter ratio and the crown ratio (crown length/tree height) change during development and differ among four coniferous species (Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes, Chamaecyparis nootkatensis (D. Don) Spach, Tsuga mertensiana (Bong.) Carrière, and Tsuga heterophylla (Ref.) Sarg.) in ancient forests. At two sites, we measured diameter, height, and height to the base of live crown for trees 4.0 cm diameter at breast height in four 0.25 ha plots. For each species, we constructed models of height based on diameter and crown length using part of the data for model development and part as the test data set. Models predicting tree height were effective for all species, and adding crown length considerably improved the prediction over diameter alone. The height to diameter ratio increased until the subcanopy and then decreased for two shade-tolerant species (A. amabilis and T. heterophylla) but decreased linearly throughout ontogeny for C. nootkatensis. Crown ratio increased as trees became larger except for C. nootkatensis, where the reverse occurred. Differences in allometric relationship among species reflect patterns of crown development and also suggest different approaches to a common structural constraint in these forests: heavy snow loads.Résumé : Les arbres tolérants à l'ombre qui poussent dans le sous-bois avant d'atteindre finalement le couvert, persistent dans différentes conditions de microsite pendant leur ontogénie. Nous évaluons comment le rapport hauteur/diamètre et le rapport de cime (longueur de la cime/hauteur de l'arbre) changent pendant leur développement et diffèrent parmi quatre espèces de conifères (Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes, Chamaecyparis nootkatensis (D. Don) Spach, Tsuga mertensiana (Bong.) Carrière, and Tsuga heterophylla (Ref.) Sarg.) dans les forêts anciennes. Dans deux stations, nous avons mesuré le diamètre, la hauteur et la hauteur à la base de la cime vivante pour les arbres dont le diamètre à la hauteur de poitrine était 4,0 cm dans quatre placettes de 0,25 ha. Pour chaque essence, nous avons construit des modèles de hauteur basés sur le diamètre à la hauteur de poitrine et la longueur de la cime, en utilisant une partie des données pour la calibration et l'autre partie pour la validation. Les modèles de prédiction de la hauteur des arbres étaient efficaces pour toutes les essences et l'ajout de la longueur de la cime a considérablement amélioré la prédiction par rapport à l'utilisation seule du diamètre à la hauteur de poitrine. Pour les deux espèces tolérantes à l'ombre (A. amabilis et T. heterophylla), le rapport hauteur/diamètre a augmenté jusqu'à l'étage des codominants pour ensuite diminuer. Mais, pour le C. nootkatensis, ce rapport a diminué linéairement durant toute l'ontogénie. Le rapport de cime a augmenté à mesure que les arbres sont devenus plus g...

Individual variability in tree allometry determines light resource allocation in forest ecosystems: a hierarchical Bayesian approach

“…Tree species' differences in crown size and allometry (Aarssen 1995;Bohlman and O'Brien 2006;Poorter et al 2006;Rees et al 2001) are often highlighted as key characteristics determining species' strategies for light interception and successional dynamics in the forest (Canham et al 1994;Grubb 1998;King 1996;Parish et al 2008;Shukla and Ramakrishnan 1986;Yokozawa et al 1996). Dominant species may be those capable of large size, thereby increasing their light exposure, casting shade on competitors, and increasing reproductive or dispersal organs (Aarssen 1995).…”

“…Dominant species may be those capable of large size, thereby increasing their light exposure, casting shade on competitors, and increasing reproductive or dispersal organs (Aarssen 1995). Late-successional shade-tolerant species may reach greater heights and develop deeper and larger crowns than early successional shade-intolerant species (Canham 1988;Pacala et al 1996;Parish et al 2008). Since they cast more shade, late-successional species promote the regeneration of shade-tolerant species, progressively excluding early successional shade-intolerant species (Canham 1988;Deutschman et al 1997;Grubb 1998).…”

“…Indeed it is well known that soil nutrients and water availability (Deleuze et al 1996) and climatic conditions such as temperature and air humidity (Geburek et al 2008) affect the potential maximum size of a tree species and its crown size and shape. Biotic interactions in the stand (e.g., competition between tree species, effects of pathogens and herbivores) modulate this potential (Oldeman 1990;Parish et al 2008). Parish et al (2008) underlined that damage consisting of branch and leader breakage associated with external disturbances (e.g., windthrows, snowfalls) also explained a large part of the intraspecific variability.…”

“…Biotic interactions in the stand (e.g., competition between tree species, effects of pathogens and herbivores) modulate this potential (Oldeman 1990;Parish et al 2008). Parish et al (2008) underlined that damage consisting of branch and leader breakage associated with external disturbances (e.g., windthrows, snowfalls) also explained a large part of the intraspecific variability. Multiple external environmental factors combine with intraspecific genetic variability to determine individual growth trajectories (Aarssen 1992), resulting in random individual crown sizes and shapes.…”

“…Multiple external environmental factors combine with intraspecific genetic variability to determine individual growth trajectories (Aarssen 1992), resulting in random individual crown sizes and shapes. Paradoxically, the ecological consequences of the overall individual variability in crown size on light interception and successional dynamics have been poorly studied (Parish et al 2008).…”