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The boreal ecocline (ca 49°N) between the southern mixedwood (dominated by balsam fir) and the northern coniferous bioclimatic domain (dominated by black spruce) may be explained by a northward decrease of balsam fir regeneration, explaining the gradual shift to black spruce dominance. 7,010 sample plots, with absence of major disturbances, were provided by the Quebec Ministry of Forest, Fauna, and Parks. The regeneration (sapling abundance) of balsam fir and black spruce were compared within and between the two bioclimatic domains, accounting for parental trees, main soil type (clay and till) and climate conditions, reflected by summer growing degree-days above 5°C (GDD_5), total summer precipitation (May–August; PP_MA). Parental trees and soil type determined balsam fir and black spruce regeneration. Balsam fir and black spruce, respectively, showed higher regeneration in the mixedwood and the coniferous bioclimatic domains. Overall, higher regeneration was obtained on till for balsam fir, and on clay soils for black spruce. GDD_5 and PP_MA were beneficial for balsam fir regeneration on clay and till soils, respectively, while they were detrimental for black spruce regeneration. At a population level, balsam fir required at least 28% of parental tree basal area in the mixedwood, and 38% in the coniferous bioclimatic domains to maintain a regeneration at least equal to the mean regeneration of the whole study area. However, black spruce required 82% and 79% of parental trees basal area in the mixedwood and the coniferous domains, respectively. The northern limit of the mixedwood bioclimatic domain was attributed to a gradual decrease toward the north of balsam fir regeneration most likely due to cooler temperatures, shorter growing seasons, and decrease of the parental trees further north of this northern limit. However, balsam fir still persists above this northern limit, owing to a patchy occurrence of small parental trees populations, and good establishment subs

Aims: The aim of this study was to compare the leaf traits and branching patterns between Acacia tortilis, Balanites aegyptiaca and Ziziphus mauritiana seedlings, three species occurring in the same range in the Sahel, a semi-arid tropical area of Africa.Study Design: Seeds used in this experiment were collected from a semi-arid area (16°20’N, 15°25’W), from species growing in their natural range in the Sahel. Seeds were after transferred at the experimental site where they were germinated and grown in a common garden experiment, on an area of 1300 m².Place and Duration of Study: The experimental site was located at around 400 km south of the seed origin (17°33'N, 14°55'W). The experiments were carried out between October 2002 and October 2004.Methodology: The seedlings were watered three days a week from the beginning of the experiment until July 2004. Thereafter, seedlings were assessed at 11 months, 16 months old, and at 24 months after the seed germination date. In the assessment, we measured the length (LGU) and the number (NGU) of growth units, the number of nodes (Nnode), and leaves (Nleaf), and the single (Aleaf) and total (Afoliage) leaf areas.Results: Following the water stress, Nleaf and Aleaf only decreased in A. tortilis, to tightly control the transpiration. Aleaf increased in B. aegyptiaca and Z. mauritiana, due to persistent leaves flushed before the water stress. Afoliage, NGU and Nnode were generally consistent, while LGU decreased in B. aegyptiaca. The defoliation in dry season despite the watering suggested an endogen control, allowing the species to escape harsh conditions in their natural range.Conclusion: The study do not support the hypothesis according to which species naturally coexisting are likely to display similar trend in leaf traits and branching patterns in response to drought. The study has been limited to three weeks of water stress and deserves to be extending to provide more insight into traits pattern for a longer period of wate

En raison de la difficulté d'approvisionnement en bois de qualité que traverse le Québec, l'éclaircie commerciale a été préconisée comme un moyen sylvicole d'accroître le potentiel de croissance des espèces d'intérêt commercial. L'effet de l'éclaircie commerciale a été étudié chez le pin gris (Pinus banksiana Lamb.) et chez l'épinette noire (Picea mariana (Mill.) BSP). Notre hypothèse générale était que le pin gris, qui est intolérant à l'ombre, répondrait rapidement et intensément à l'éclaircie alors que l'épinette noire, modérément tolérante à l'ombre, répondrait plus faiblement. La thèse a été réalisée en Abitibi-Témiscamingue et aborde la réponse écophysiologique des deux essences à l'ouverture du couvert dans un contexte d'éclaircie commerciale à travers trois études prenant place à des échelles spatiales et temporelles différentes. Dans le cadre des deux premières études, des dispositifs expérimentaux distincts ont été mis en place pour chacune des deux essences. Dans chaque dispositif, un traitement témoin (sans intervention) et deux traitements d'éclaircie- des réductions relatives de surface terrière (G,) de 0,3 et 0,4 pour le pin gris, de 0,4 et 0,5 pour l'épinette noire (chapitre II) et de 0,4 et 0,5 pour les deux espèces (chapitre III), ont été assignées aléatoirement à des parcelles expérimentales dans deux blocs expérimentaux pour chacune des deux espèces (chapitre II) et dans cinq blocs expérimentaux pour le pin gris et quatre pour l'épinette noire (chapitre III). La première étude s'intéresse à la réponse des aiguilles et étudie la photosynthèse des deux essences sur une base journalière en même temps que les paramètres de la photosynthèse. La deuxième étude porte sur une période d'un à six ans après éclaircie et aborde la réponse des tiges individuelles mise en lumière à partir d'analyses de tiges effectuées sur 30 individus de pin gris et 24 d'épinette noire. Nous nous sommes intéressés à la distribution de croissance le long de la tige, aux modifi