Photosynthetic and morphological responses of white birch, balsam poplar, and trembling aspen to freezing and artificial defoliation

Comparative stress resistance of 1-year-old white birch (Betula papyrifera Marsh.), balsam poplar (Populus balsamifera L.), and trembling aspen (Populus tremuloides Michx.) seedlings was evaluated after exposure to freezing or defoliation. Photosynthesis in leaves surviving freezing (-5 °C) declined immediately after treatment, but nearly fully recovered within 3 weeks. Defoliation did not significantly increase photosynthesis in the remaining leaves. Refoliation occurred after freezing that killed terminal shoots and released current buds from apical dominance, while new leaves of larger size were produced through continuous growth of terminal shoots in 50% or 100% defoliation. Freezing and complete defoliation significantly reduced diameter and height growth in all species, whereas 50% defoliation did not affect growth. These results indicate some of the physiological and morphological responses to foliage loss in broadleaved boreal species that can help to maintain growth and productivity under a warming climate, which may result in more frequent damaging spring frosts and insect defoliation. Key words: frost damage, refoliation, leaf morphology. Les auteurs ont compare la resistance au stress chez des plantules agees d'une annee, de bouleau a papier (Betula payrifera Michx.), de peuplier baumier (Populus balsamifera L.) et de peuplier faux-tremble (Populus tremuloides Michx.) apres exposition au gel et a la defoliation. Chez les feuilles ayant survecu au gel (-5 [degre]C), la photosynthese a immediatement decline apres le traitement, mais s'est retablie completement en moins de 3 semaines. La defoliation n'augmenta pas significativement la photosynthese dans les autres feuilles. La refoliation survient apres un gel ayant tue les tiges terminales et supprime l'effet de la dominance apicale sur les bourgeons actifs, alors que de nouvelles feuilles plus grandes se forment par croissance continue des tiges terminales, apres 50 % et 100 % de defoliation. Le gel et la defoliation totale reduisent la croissance en diametre et en hauteurs chez toutes les especes, alors qu'une defoliation a 50 % n'affecte pas la croissance. Ces resultats illustrent certaines reactions physiologiques et morphologiques en reaction de la perte de feuillage chez des especes boreales a larges feuilles pouvant aider a maintenir la croissance et la productivite sous un climat en rechauffement, lequel pourra conduire a des gels printaniers et des defoliations par les insectes plus frequents. [Traduit par la Redaction] Mots-cles : dommage par le froid, refoliation, morphologie foliaire.
Introduction Although most studies of North American boreal forests have focused on the more economically important conifer species, broadleaved trees are nevertheless important ecologically and as potential sources of fibre. [...]