Effects of elevated atmospheric CO2 on phenology, growth and crown structure of Scots pine (Pinus sylvestris) seedlings after two years of exposure in the field

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were grown for tw o years in the ground in open-top chambers supplied with either an ambient or elevated (ambient + 400 mu mol mol(-1)) CO2 concentration. Phenological observations and measurements of height and stem diameter growth, absolute and relative growth rates, starch and soluble carbohydrate concentrations o f the needles, and crown structure and needle properties were made at frequ ent intervals throughout the two growing seasons. Elevated CO2 significantl y advanced the date of bud burst in both years. The increase in total needl e area in response to elevated CO2 was accounted for by longer shoots and a n increase in individual needle area in the first year, and by an increase in the number and length of shoots in the second year. Stem diameter and tr ee height were enhanced more by the elevated CO2 treatment in the first yea r than in the second, indicating a decreased effect of CO2 on growth over t ime. This was confirmed by a study of absolute and relative growth rates of leader shoots. During the first growing season of CO2 enrichment, mean wee kly relative growth rates over the growing season (RGR(m)) were significant ly enhanced. During the second year, RGR(m) in ambient CO2 closely matched that in elevated CO2.