Effect of Forest Structure on the Understory Light Environment and Growth Potential of Oak Seedlings in a Closed Canopy Riparian Forest

Abstract

This research was designed to examine the relationship between the residual structure following low intensity partial cutting regimes and the light characteristics found in a riparian hardwood forest. The relationships between these altered environments and oak seedling growth under the influence of a main canopy were also investigated. A better understanding of these relationships is needed to further the discussion of the feasibility of regeneration oak on mesic sites.

The model developed to describe the amount of photosynthetically active radiation (PAR) below the canopy indicated that a relationship exists between stem density, crown length, and crown closure. The presence of a midstory canopy significantly affects this relationship by increasing both stem density and overall canopy length. The models developed to describe light quality demonstrated that the R:FR ratio will depend on structure that impedes direct sunlight from reaching the forest floor. Thus, descriptions of vertical canopy density become important. The relative amount of blue light is dependent on diffuse light and is influenced most by the amount of visible blue sky below the canopy. Therefore, variables which describe canopy closure are significantly related to the blue light levels below the canopy.

The relationship between stand conditions and Nuttall oak (Quercus nuttallii Palmer) seedling growth indicate that, as individual variables, PAR transmittance and the presence of a midstory canopy accounted for roughly the same amount of variation (33-35%) in both diameter and height growth. Interestingly, the R:FR ratio also explained a considerable amount of variation in diameter and height growth in the low light levels examined (3-22% full sun). It is apparent that the only difference between the diameter and height growth models is the type of structure variable included. Canopy structure was more important for the diameter growth model, while basal area, a measure of stand density, accounted for more variation in the height growth model. The significance of this difference is difficult to determine. However, since minor changes in basal area may induce significant changes to the canopy, it is possible that seedling height growth is less sensitive to minor differences in canopy architecture than diameter growth.