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

Abstract

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were grown for two years in the ground in open-top chambers supplied with either an ambient or elevated (ambient + 400 μmol mol⁻¹) CO₂ concentration. Phenological observations and measurements of height and stem diameter growth, absolute and relative growth rates, starch and soluble carbohydrate concentrations of the needles, and crown structure and needle properties were made at frequent intervals throughout the two growing seasons. Elevated CO₂ significantly advanced the date of bud burst in both years. The increase in total needle area in response to elevated CO₂ was accounted for by longer shoots and an 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 tree height were enhanced more by the elevated CO₂ treatment in the first year than in the second, indicating a decreased effect of CO₂ on growth over time. This was confirmed by a study of absolute and relative growth rates of leader shoots. During the first growing season of CO₂ enrichment, mean weekly relative growth rates over the growing season (RGR_m) were significantly enhanced. During the second year, RGR_m in ambient CO₂ closely matched that in elevated CO₂.