Sylvain Parent

Effects of overstory and understory vegetation on the understory light environment in mixed boreal forests

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 percentage 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 (R2 = 0.852). The proportion of sunflecks above 250 and 500 I¼mol 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.

Growth, biomass allocation, and adventitious roots of balsam fir seedlings growing in closed-canopy stands

Abstract Well-documented shade adaptations include physiological and morphological traits of shoots and leaves. Belowground shade adaptations (roots or other buried structures) are rarely mentioned in the literature and have never been studied in tree seedlings. This study evaluates the functional role of adventitious roots developed by most balsam fir (Abies balsamea) seedlings under a closed canopy. Our goal was to describe the relationships between growth, biomass allocation, and adventitious root development in balsam fir seedlings (5–26 cm tall, 9 to 33 y old) excavated under a closed canopy (< 8% full sunlight). Seedling height, branch number and crown width increased linearly with stem diameter measured at ground level. The dry mass of stems and roots increased exponentially and tree vigour (photosynthetic/non-photosynthetic biomass ratio) decreased exponentially as stem diameter increased. Results indicate that shade tolerance is limited by size and concurrent radial increment at the stem base. Results also demonstrate that the proportion of adventitious roots and the number of annual growth units buried in humus (age of the belowground section) increased as seedlings aged. Therefore, the reverse taper phenomenon caused by the formation of adventitious roots simultaneously limits the radial expansion of the stem at ground level and the maximum size attainable by seedlings. Balsam firs shade tolerance is attributable to root as well as shoot adaptations. The gradual burial of the stem in humus probably increases persistence in shade by limiting the plants respiration costs.