Seasonal dynamics of understory vegetation in four eastern Canadian forest types

Understory vegetation dynamics was monitored throughout the 1998 growing season in four eastern Canadian forest types: sugar maple-American beech (Acer saccharum Marsh.-Fagus grandifolia Ehrh.); sugar maple-yellow birch (Betula alleghaniensis Britton); balsam fir (Abies balsamea [L.] Mill.); and black spruce-jack pine (Picea mariana [Mill.] BSP-Pinus banksiana Lamb.). Significant differences in biomass were obtained among species groups (herbaceous, woody, and mosses) within each site. However, biomass did not vary significantly throughout the growing season, except in the sugar maple-American beech site. The four sites differed in total biomass, but these differences could be explained mainly by the presence of some key species (e.g., Lycopodium lucidulum Michx.) and the light regime under the canopy. Species richness varied throughout the growing season on each site. However, peaks in richness did not occur at the same time in each site: richness peaked later with increasing latitude, from late May in the southernmost site to late September in the northernmost site. Average richness did not vary across sites. Aboveground nutrient concentrations for herbaceous species were greater than nutrient concentrations for stems of woody species. However, nutrient concentrations for woody species leaves were comparable to those of herbaceous species. Our results highlighted the importance of understory species in the cycling of nutrients and their capacity to keep nutrients within a site. Specific leaf area (SLA) for herbaceous species was greater than SLA for woody species, which indicated a greater capacity to acclimate to different light conditions than woody species. Increase in leaf area ratio with decrease in height for sugar maple and beech suggests that both species decreased their efficiency to produce biomass at the ground level as they were overtopped by higher surrounding vegetation.