Abstract. The main goal of this research was to determine the short-term effects of forest management practices (precommercial thinning and fertilization) on carbon efflux of a young black cottonwood (Populus trichocarpa) plantation in southern Iceland. Changes in leaf area index, tree growth, soil and air temperature were also monitored. Traditional precommercial thinning (50%) had little effect on soil temperature, but heavy thinning (80%) led to higher soil temperatures in spring and summer, but not much difference in winter. Thinning did not change foliage nutrient content or tree growth in the first treatment year. Effects on soil respiration were surprising; even if soil temperature was slightly higher and all organic material was left at site after precommercial thinning, soil respiration in the thinned stands was significantly lowered. Fertilization increased foliage nutrient content, but did not significantly change tree growth during the first growing season following spring application. It did however significantly increase soil respiration rate in the first treatment year. These results may be important when the effects of early forest management on carbon fluxes are modeled.