The boreal forest is expected to experience the greatest warming of all forest biomes. The extent of the boreal forest, the large amount of carbon contained in the soil, and the expected climate warming, make the boreal forest a key biome to understand and represent correctly in global carbon models. It has been suggested that an increase in temperature could stimulate the release of CO2 caused by an increased decomposition rate, more than biomass production, which could convert current carbon sinks into carbon sources. Most boreal forests are currently carbon sinks, but it is unclear for how long in the future the carbon sink capacity of the boreal forest is likely to be maintained. The impact of soil warming on stem volume growth was studied during 6 years, in irrigated (I) and irrigated-fertilized (IL) stands of 40-year-old Norway spruce in Northern Sweden. From May to October heating cables were used to maintain the soil temperature on heated-irrigated plots (Ih and ILh) 5 °C above that on unheated control plots (Ic and ILc). After six seasons' warming, stem volume production (m3 ha−1 a−1) was 115% higher on Ih than on unheated (Ic) plots, and on heated and irrigated-fertilized plots (ILh) it was 57% higher than on unheated plots (ILc). The results indicate that in a future warmer climate, an increased availability of nitrogen, combined with a longer growing season, may increase biomass production substantially, on both low- and high-fertility sites. It is, however, too early to decide whether the observed responses are transitory or long lasting. It is therefore crucial to gain a better understanding of the responses of boreal forest ecosystems to climate change, and to provide data to test and validate models used in predicting the impact of climate change.
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