Effects of nitrogen availability on northern red oak seedling growth in oak and pine stands in northern Lower Michigan

The nitrogen (N) status and dry weight increment of northern red oak (Quercus rubra L.) seedlings in relation to N availability were examined at various levels of canopy cover and understory removal treatments under field conditions in red oak and red pine (Pinus resinosa Ait.) stands. Net N mineralization rates over two growing seasons following canopy cover treatments were determined by the in situ buried bag technique. Canopy removal increased N mineralization in both oak and pine stands. Net N mineralization rates were approximately 1.2-2.2 times higher in the clearcut and the other partial canopy cover treatments than in the uncut stands. Net N mineralization in the same canopy cover treatments was approximately 2-3 times higher in red oak stands than in red pine stands. However, red oak seedlings from the same canopy cover treatments in both stand types had similar dry weight, N concentrations, N content, and N-use efficiency despite differences in soil N availability. The only exception was approximately 2 times greater seedling dry weight and N content in the red oak clearcuts compared with the red pine clearcuts. The similarity in seedling performance within partial canopy removal or uncut stands may have been due to limiting factors other than N in the red oak stands. Red oak seedlings from litter removal treatments within the clearcuts had significantly higher N-use efficiency than those from the herb and shrub removal treatments. These results suggest that (i) canopy manipulation increases soil N availability; (ii) increases in dry weight and N uptake by red oak seedlings when forest canopies are completely removed are due, in part, to increased available soil N; (iii) red oak seedling response to soil N availability resulting from no or only partial canopy removal may be the same in different stand types (e.g., red oak vs. red pine) because factors other than N, such as light, are limiting.