The age structure of balsam fir (Abies balsamea (L.) Mill.) regeneration is frequently used to investigate boreal forest dynamics of North America. Tree ages are usually estimated by counting annual growth rings at the shoot-root interface located above or close to the root system. Inaccurately locating the shoot-root interface could lead to imprecise age determination. In this study, balsam fir seedlings (<2 m height) were harvested in whole from closed forest stands located in the province of Quebec, Canada. Seedling age was determined by (i) counting the number of annual growth rings at the presumed shoot-root interface, and (ii) counting all terminal bud scars from the apex to the base of the hypocotyl (true collar). For all seedlings with adventitious roots, the number of terminal bud scars on the entire trunk was higher than the number of growth rings at the shoot-root interface. The formation of adventitious roots on the belowground trunk was accompanied by a reverse taper phenomenon, i.e., the number of annual growth rings decreased from the presumed shoot-root interface to the true collar. Counting annual growth rings at any level on the trunk of balsam fir seedlings that form adventitious root systems would not be reliable, underestimation's exceeding 20 years are possible and the resulting age structures could lead to erroneous interpretations of regeneration dynamics.

La structure d'âge de la régénération de sapin baumier (Abies balsamea (L.) Mill.) est souvent utilisée pour étudier la dynamique de la forêt boréale en Amérique du Nord. L'âge des arbres est habituellement déterminé en comptant les cernes annuels à la jonction entre la tige et les racines qui est située au-dessus ou près du système racinaire. La détermination de l'âge peut manquer de précision si la jonction entre la tige et les racines n'est pas localisée avec exactitude. Dans cette étude, des semis de sapin baumier (<2 m de haut) ont été récoltés en entier dans des peuplements forestiers fermés situés dans la province de Québec, au Canada. L'âge des semis a été déterminé (i) en comptant le nombre de cernes annuels à la jonction présumée entre la tige et les racines et (ii) en comptant les cicatrices des écailles de tous les bourgeons terminaux de l'apex à la base de l'hypocotyle (collet véritable). Chez tous les semis avec des racines adventives, le nombre de cicatrices des écailles des bourgeons terminaux sur toute la tige était plus élevé que le nombre de cernes annuels à la jonction entre la tige et les racines. La formation de racines adventives sur la portion hypogée de la tige était accompagnée d'un phénomène de défilement inversé, c'est-à-dire que le nombre de cernes annuels diminuait de la jonction présumée entre la tige et les racines jusqu'au collet véritable. Le décompte des cernes annuels à n'importe quel niveau sur la tige des semis de sapin baumier qui forment un système de racines adventives n'est pas fiable; on peut sous-estimer l'âge par plus de 20 ans et les structures d'âge qui en résultent peuvent conduire à des interprétations erronées de la dynamique de la régénération. ©2000 NRC Canada

The percentage of above-canopy Photosynthetic Photon Flux Density (%PPFD) was measured at 0, 50 and 100 cm above the forest floor and above the main understory vegetation in stands of (1) pure Betula papyrifera (White birch), (2) pure Populus tremuloides (Trembling aspen), (3) mixed broad-leaf-conifer, (4) shade-tolerant conifer and (5) pure Pinus banksiana (Jack pine) occurring on both clay and till soil types. %PPFD was measured instantaneously under overcast sky conditions (nine locations within each of 29 stands) and continuously for a full day under clear sky conditions (five locations within each of eight stands). The percent age cover of the understory layer was estimated at the same locations as light measurements.

Mean %PPFD varied from 2 % at the forest floor under Populus forests to 15 % above the understory vegetation cover under Betula forests. Percent PPFD above the understory vegetation cover was significantly higher under shade intolerant tree species such as Populus, Betula and Pinus than under shade tolerant conifers. No significant differences were found in %PPFD above the understory vegetation cover under similar tree species between clay and till soil types. The coefficient of variation in %PPFD measured in the nine locations within each stand was significantly lower under deciduous dominated forests (mean of 19 %) than under coniferous dominated forests (mean of 40 %). %PPFD measured at the forest floor was positively correlated with %PPFD measured above the understory vegetation and negatively correlated with cumulative total percent cover of the understory vegetation (R-2 = 0.852). The proportion of sunflecks above 250 and 500 mu mol m^-2 s^-1 was much lower and %PPFD in shade much higher under Populus and Betula forests than under the other forests. Differences in the mean, variability and nature of the light environment found among forest and soil types are discussed in relation to their possible influences on tree succession.