CANOPY COVER EFFECTS ON SOIL-NITROGEN MINERALIZATION IN NORTHERN RED OAK (QUERCUS-RUBRA) STANDS IN NORTHERN LOWER MICHIGAN

It has been shown in many studies of nutrient cycling in forest ecosys tems that clearcutting creates increased nitrogen (N) mineralization i n the soils. However, little is known about the direction and rate of change of N mineralization following various levels of canopy removal in northern red oak (Quercus rubra L.) stands, despite the fact that a n understanding of these processes may have important implications for self-replacement in such stands. Accordingly, a series of experimenta l plots was manipulated to produce various levels of canopy cover, i.e . clearcut, 25% (50% during first sampling year), 75%, and uncut in no rthern red oak stands in northern Lower Michigan. Net N mineralization and nitrification in the top 15 cm of mineral soil were examined duri ng the first two growing seasons (1991-1992) following the treatments, using an in situ buried bag technique. Net N mineralization over the course of both growing seasons (May-October) ranged from 79 kg ha-l pe r growing season in the clearcut treatment to 22 kg ha(-1) per growing season in the uncut stand. The highest net N mineralization was obser ved in the clearcut with the highest increase of soil temperature. Net N mineralization was significantly higher in the partial canopy cover treatments compared to the uncut stand during the second growing seas on, but not in the first growing season. Nitrification was unaffected by factors such as increased soil pH, elevated NH4+ availability, and soil temperature after canopy removal. The results indicated that even small amounts of canopy removal (leaving 75% canopy cover) are suffic ient to cause substantial increases in the amount of N available for t he establishment and growth of understory oaks.