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Nitrogen availability and soil microclimate after clearcutting lodgepole pine Haskin, Catherine Marie


Long term management of lodgepole pine depends, in part, on the nutrient capital of the site. Harvesting has been shown to increase the availability of nitrogen and other nutrients for a limited period of time. This increase, or nutrient flush, following cutting has been attributed to several factors including changes in organic matter quantity and quality, soil moisture and temperature regimes, and competition for nutrients. Knowledge of the duration and the magnitude of the nutrient flush would be valuable for management decisions. A consequence of lodgepole pine clearcuts was studied for soil nitrogen availability and microclimate. Mineral soil and forest floor samples were anaerobically incubated and analyzed for ammonium-nitrogen (ppm). The mineral soil showed a flush lasting about 12 years, peaking by year 8, while the forest floor material showed no trend. The size of the increase from year 2 to year 8 was about 52%. Temperature (air and soil), soil moisture, solar radiation, and precipitation data were collected, and a soil water budget was calculated using a simple water balance model. The average calculated soil water deficit was 119 mm, but this may have been an over-estimate. Windrowing following cutting may have depleted the site nitrogen reserve which in turn reduced the amount that was mineralized during incubation. The interaction between the soil temperature and moisture may have further limited nitrogen availability to the lodgepole pine trees. It is concluded that the dynamics of nitrogen availability following clearcutting are important for the establishment and growth of lodgepole pine. The summer moisture deficit appears to limit the nitrogen mineralization rate as well as seedling growth.

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