Soil-Surface CO2 Flux and Growth in a Boreal Norway Spruce Stand Effects of soil warming and nutrition

Strömgren, M. 2001. Soil-Surface CO2 Flux and Growth in a Boreal Norway Spruce Stand. Effects of soil warming and nutrition. Acta Universitatis Agriculturae Sueciae, Silvestria 220. Doctor's dissertation. ISSN 1401-6230, ISBN 91-576-6304-1. Global warming is predicted to affect the carbon balance of forests. A change in the carbon balance would give a positive or negative feedback to the greenhouse effect, which would affect global warming. The effects of long-term soil warming on growth, nutrient and soil-surface CO2 flux (R) dynamics were studied in irrigated (I) and irrigated-fertilised (IL) stands of Norway spruce in northern Sweden. Soil temperature on heated plots (Ih and ILh) was maintained 5 C above that on unheated plots (Ic and ILc) from May to October, by heating cables. After six years’ soil warming, stemwood production increased by 100% and 50% in the I and IL treatment, respectively. The main production increase occurred at the beginning of the season, probably as an effect of the earlier increase in soil temperature. In the Ih treatment, however, the growth increase was evident during the entire season. The effect of increased nitrogen (N) mineralisation on annual growth appeared to be stronger than the direct effect of warming. From 1995−2000, the total amount of N stored in aboveground tree parts increased by 100 and 475 kg N ha on Ic and ILc plots, respectively. During the same period, 450 kg N fertiliser was added to the ILc plot. Soil warming increased the total amount of N stored in aboveground tree parts by 50 kg N ha, independently of nutrient treatment. Soil warming did not significantly increase R, except in early spring, when R was 30−50% higher on heated compared to unheated plots. The extended growing season, however, increased annual respiration (RA) by 12−30% throughout. RA losses were estimated to be 0.6−0.7 kg C ha a. Use of relationships between R and soil temperature, derived from unheated plots, overestimated RA on heated plots by 50−80%. These results suggest that acclimation of root or microbial respiration or both to temperature had occurred, but the exact process(es) and their relative contribution are still unclear. In conclusion, the study showed that soil warming stimulated tree growth, but resulted in only a minor increase of annual R, suggesting an increased carbon sink for boreal forests in a warmer climate.