Branch surface area and its vertical distribution in coastal Douglas-fir

Wood area index (WAI; total surface area of branches and bole per unit of land area) is an important yet often neglected forest structural attribute. Branchwood surface area, in particular, has significant implications for many ecophysiological processes including total respiration and interception of radiation and rainfall. Branch surface area was estimated at the branch-, tree-, and stand-level for 33 Douglas-fir (Pseduotsuga menziesii [Mirb.] Franco) plantations in the Oregon Coast Range. Patterns in WAI, leaf area index (LAI; total surface area of needles per unit of land area), tree area index (TAI=WAI+LAI) and various ratios of these dimensions were then investigated. The main axes of primary branches (those attached to the main stem) comprised 82±13% of total branchwood surface area. Tree surface area (needles + woody tissue) increased with increasing tree size and crown length, and decreased with greater intensity of Swiss needle cast (SNC). At the stand-level, woody surface area increased with greater stand density and decreased with more severe SNC, but on average it constituted 29±12% of total tree surface area. Branchwood surface area and bole surface area contributed equally to WAI. The variation in WAI for a given LAI has important implications for radiation and rainfall attenuation in these stands and for accurate partitioning of intercepted radiation between photosynthetic and non-photosynthetic tissues.