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 substrates.

To explain the transition between the southern mixedwood and the northern coniferous bioclimatic domains respectively dominated by balsam fir and black spruce, 59 sample plots were selected throughout the two bioclimatic domains. The regeneration (seedling abundance) and mortality (difference between seedlings and saplings) of balsam fir seedlings were compared within and between the two bioclimatic domains. We also determined the soil types (clay and till), summer growing degree-days above 5°C (GDD_5), and total summer precipitation (PP_MA). Balsam fir regeneration was strongly linked with parental trees and was higher in the mixedwood than in the coniferous bioclimatic domain, with higher regeneration occurring on till than on clay soils. Also, the mortality was higher on till than on clay soils, due to increasing competition on tills where the regeneration is higher. Unlike PP_MA, GDD_5 positively influenced balsam fir regeneration. The dynamic of balsam fir stands in the coniferous bioclimatic domain was attributed to a decrease of balsam fir regeneration and increase of its mortality most likely due to a lack of good establishment substrates (till soils), and cooler temperatures. However, balsam fir stands persist above this northern limit prospected, owing to patchy occurrences of good establishment substrates where parental trees are found.

A decrease in the average diameter of commercially harvested tree species in the Eastern boreal forest of Canada has led to a decrease in availability of quality wood for the forest industry. Commercial thinning has been proposed as a means to increase stem diameter growth. However, little is known about physiological responses underlying species responses to thinning. We assessed the effect of canopy opening on the photosynthetic response of mature jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill.) BSP) trees. Two years after thinning and for each species, light response curves and the diurnal course of photosynthesis were characterized from measurements taken in a completely randomized block experiment on current-year and one-year-old needles of 12 trees from stands subjected to different levels of canopy opening. Soil water content, air and soil temperatures, and needle N concentration were not affected by thinning for either species. However, light availability increased with basal area removed and could explain the significantly positive relationship between thinning intensity and diurnal course of photosynthesis for one-year-old needles of jack pine. Black spruce photosynthesis did not respond to increases in light. Light-saturated rate of net photosynthesis (A_max), photosynthetic efficiency (α), light compensation point (LCP), and diurnal respiration (R_d) did not vary with thinning for either of the species. Jack pine and black spruce responses to thinning should be interpreted in light of species autecology.

En forêt boréale, les effets bénéfiques du passage du feu sur le substrat de germination ont souvent été liés à la présence de charbon et à son effet adsorbant sur les composés phénoliques qui inhibent la germination et la croissance des semis. L'objectif de la présente étude visait à mettre en évidence l'influence directe du charbon actif sur la germination et la croissance primaire des plantules de pin gris (Pinus banksiana Lamb.), une espèce très bien adaptée au feu. Quatre concentrations de charbon actif (0, 50, 100 et 150 % par rapport à la masse des semences) combinées à deux températures, respectivement 26 à 27 °C et 18 à 20 °C, ont été testées in vitro à une humidité constante dans des boîtes de Pétri. Les résultats montrent que l'ajout de charbon actif dans le milieu n'a pas eu d'effet significatif sur la germination des semences de pin gris. Pour ce qui est de la croissance des plantules cependant, le charbon actif exerce un effet inhibiteur, lorsque sa concentration dans le milieu est supérieure à 100 %. Une augmentation significative de la croissance a été uniquement observée seulement avec 50 % de charbon combiné à la température plus élevée (26–27 °C). Nous avons noté l'apparition de malformations à l'apex de la radicule particulièrement aux températures basses associées à des concentrations élevées de charbon actif dans le milieu.

In the boreal forest, the beneficial effect of wildfire on germination substrates has often been linked to the adsorption by charcoal of phenolic compounds detrimental for seedling germination and growth. Our goal was to show that active charcoal has direct positive effects on germination and early growth of jack pine (Pinus banksiana Lamb.) plantlets, a species well adapted to fire. Four concentrations of active charcoal (0%, 50%, 100%, and 150% seed mass) along with two temperatures (26–27 °C and 18–20 °C) were tested in vitro, in Petri dishes, under constant moisture conditions. Results showed that adding active charcoal to the substrate had no significant effect on jack pine seed germination but inhibited plantlet growth at concentrations over 100%. Significant growth increase was observed only with 50% charcoal at the highest temperature (26–27 °C). Deformities were noted at the apex of radicles, particularly with low temperatures and high levels of active charcoal in the substrate. ©2006 NRC Canada