The Effects of Canopy Structure on Stemflow, Throughfall and Interception Loss in a Young Sitka Spruce Plantation

(1) The partitioning of incident precipitation into throughfall, stemflow and interception loss was assessed in a 14-year-old Picea sitchensis plantation in South Scotland. The tree crowns overlapped to form a dense canopy with 25, 44, 28 and 3% of ground area covered respectively by foliage from one, two, three and four trees. All main branches sloped down inwards to the tree trunk. (2) Over a single calendar year the incident precipitation was 1639 mm. Stemflow accounted for 27% of this, throughfall 43% and interception loss, estimated by difference, accounted for 30%. Percentage stemflow was consistent throughout the year but, during the winter period (January-March), throughfall increased to 57% and interception loss decreased to 15%. (3) The amount of stemflow for individual trees was positively correlated (P = 0.05) with the projected area of tree crowns. (4) The amount of throughfall was greatest close to the tree trunks and between trees in the same row, particularly for intermediate levels of rainfall (20-40 mm week$^{-1}$). (5) The spatial pattern of throughfall parallels the distribution of fine roots in the soil.