The effects of increased atmospheric carbon dioxide on growth and morphology of seedlings of silver birch (Betula pendula Roth.) and Sitka spruce (Picea sitchensis (Bong.) Carr.)

The concentration of CO2 is increasing at a rate of ca 1.8kmol mol-1 per year. If the current trend continues, atmospheric CO2 concentration will reach double the 1980's ambient concentration by about 2080. An increase in atmospheric CO2 concentration will have direct effects on plants as a result of the role CO2 plays in determining rate of photosynthesis and transpiration. It may also have secondary effects on plant growth. The objective of this study was to determine the effects of long-term exposure to elevated CO2 on the growth and morphology of seedlings of Sitka spruce and silver birch, with a focus on the effects of elevated CO2 on the expansion, surface characteristics, specific leaf area, anatomy and chlorophyll content of leaves of silver birch. Seedlings of Sitka spruce and silver birch were placed in pots in open top chambers receiving ambient or elevated CO2 . After one growing season there was a significant increase in biomass production in seedlings of Sitka spruce grown in elevated CO2 compared to those grown in ambient CO2, but after eighteen months this effect had disappeared. The initial increase in biomass was a result of a significant increase in root mass. At the end of the second growing season there was no effect of elevated CO2 on allocation of dry mass. There was no effect of elevated CO2 on total number of branches produced at the end of the second growing season but there was an increase in the rate of branch production. Nitrogen, phosphorus and potassium concentration of leaves was reduced in plants grown in elevated CO2. After one growing season biomass was increased in elevated CO2 plants by ca 20%, rates of photosynthesis were also higher in elevated CO2 plants. There was no effect of CO2 treatment on root: shoot ratio. There was a significant increase in branch production in plants grown in elevated CO2, but despite an increase in leaf area production early on in the growing season, there was no difference in total tree leaf area at the end of the growing season. Nitrogen, phosphorus and potassium concentration of leaves was reduced in plants grown in elevated CO2 and leaves on elevated CO2 plants senesced ca one week earlier than leaves of plants grown in ambient CO2. Leaf extension rates were higher in elevated CO2 plants during the morning but there were no consistent effects of CO2 treatment on leaf water potential or osmotic potential. Leaf thickness increased in elevated CO2 plants and was accompanied by an increase in the number of layers of palisade cells. Chlorophyll content was reduced in plants grown in elevated CO2. Stomatal conductance was reduced in plants grown and measured in elevated CO2 as a result of both a reduction in the width of apertures, and a reduction in stomatal density. There was no effect of elevated CO2 treatment on stomatal index and changes in stomatal density were a result of changes in leaf expansion. The implications of these results for the future functioning of Sitka spruce and silver birch, and the relevance of this study in relation to future predictions of tree growth in elevated CO2 are discussed.