High Productivity in a Polestage Sitka Spruce Stand and its Relation to Canopy Structure

Abstract

The net annual above ground dry matter production of a 17 year plantation of Sitka spruce in Scotland was 26.7 t ha ⁻¹ y ⁻¹ . Total annual production which includes estimates for roots, was 35 t ha ⁻¹ y ⁻¹ , one of the highest values reported for coniferous forest in the temperate zone. When compared with other forests with high rates of net production this stand had the highest foliage and branch biomass and lowest rate of production per unit of foliage. Gross foliage increment to the canopy declined following the time of maximum stand basal area increment, which coincided with the onset of competition between individual trees. These changes in canopy and stand structure are discussed in relation to the decline in net production which has been observed in polestage conifer plantations. Foliage and branch production were greatest in the top 6 whorls of the canopy; over the period studied new foliage became concentrated nearer to the top of the trees. Significant branch wood increment ceased below the height where needle death balances needle production. New needles produced at increasing depth in a canopy weighed less per unit needle area. Generally needles lost weight as they aged. All needles survived for three years, the greatest mortality was of 5-year-old needles but some survived for 8 years. Needle area index was 10–11 at age 16, 7–8 at age 18. Branch area index was 3.6 and the ratio of main stem bark surface area to ground area was 0.4 at age 16.