Wood properties of northern forest trees grown under elevated CO2, O3 and temperature

The aim of this study was to investigate the effects of climate change on the radial growth, wood structure and chemistry of silver birch (Betula pendula Roth), trembling aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh.), sugar maple (Acer saccharum Marsh.) and Norway spruce (Picea abies (L.) Karst.). The materials for this study were obtained from climate change studies carried out in Finland, USA and Sweden. Elevated CO2 concentration, O3 concentration and temperature affected wood properties of the forest trees. In silver birch, elevated CO2 increased ring width and the concentrations of extractives and starch, while the concentrations of cellulose and gravimetric lignin were decreased. The responses to elevated O3 depended on the clone: vessel percentage and nitrogen concentration decreased, while cell wall percentage increased in one clone. In vessel percentage, elevated CO2 ameliorated the O3-induced decrease. Responses of wood properties to elevated CO2 and O3 differed between 3-year-old and 5-year-old aspen and paper birch. In 3-year-old aspen clones and birch, lignin concentration increased under elevated O3. However, elevated CO2 ameliorated the effect, and the lignin response was no longer found with 5-year-old trees. In aspen, elevated CO2 decreased uronic acids and elicited clone-dependent increases in concentrations of extractives and starch. Elevated O3 reduced stem radial growth and vessel lumen diameter, while it increased cell wall thickness in aspen. In 5-year-old paper birch, elevated CO2 increased extractives and decreased starch, while elevated O3 increased both of them. Three-year-old maple was the least responsive of the tree species to both elevated CO2 and O3. In 40-year-old Norway spruce, elevated CO2 decreased nitrogen concentration, earlywood cell wall thickness and tracheid lumen diameter, while ring width in nonfertilised trees and latewood tracheid lumen diameter were increased. Elevated temperature decreased the concentrations of extractives and soluble sugars, and increased cell wall thickness and wood density. The data from these exposure studies shows that wood properties may change as a result of climate change, but the responses to increasing concentrations of CO2 and O3 and to increasing temperature may depend on species and the age of trees. Since trees are longliving organisms, further data on long-term exposure studies are still needed before wood characteristics and material properties for different end-use purposes can be predicted for a future climate.