| Summary: | In boreal forests, the main factors limiting biomass production are the harsh climate, which combines a short growing season and low annual levels of incoming solar energy, and the limited availability of nitrogen. These limitations will be directly affected by climate change, and may in turn substantially affect the carbon budget of the boreal forests, the production of wood and biofuel, biodiversity and other ecosystem services. This thesis addresses the effects of climate change on the rate of carbon assimilation by boreal Norway spruce, Picea abies (L.) Karst. The study focussed on examining how the mechanisms regulating uptake of CO2 in mature, field-grown trees are affected by exposure to elevated concentrations of atmospheric carbon dioxide [CO2] and air temperature. The experiment was conducted at the Flakaliden research site in northern Sweden. Twelve whole-tree chambers (WTCs) were used to impose combinations of [CO2] and temperature treatments as predicted for the region in the year 2100. Shoot CO2 gas exchange was measured continuously within the chambers, using shoot cuvettes. The effect of the climate change treatments on developing shoots was studied during their first growing season; the effect of the treatments on spring recovery and annual photosynthetic performance in 1-year old shoots was also examined. The elevated temperature induced an earlier start and completion of the structural development of the current year’s shoots, as well as an earlier shift from negative to positive net carbon assimilation rate (NAR) by one to three weeks. The elevated CO2 increased photosynthetic performance by 30% in high season. Consequently, the current year’s shoots had assimilated their own mass in carbon 20-30 days earlier under the climate change conditions than under the current climatic conditions. For the 1-year old shoots, an increase in the maximum photosynthetic rate of ~50% was recorded, and the spring recovery of photosynthetic capacity was completed three to four weeks earlier than under the ...
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