Root structure of western hemlock and western redcedar in single- and mixed-species stands

Root structure of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thuja plicata Donn. ex D. Don) was studied in single- and mixed-species stands of hemlock and western redcedar in Capilano River watershed, University of British Columbia Malcolm Knapp Research Forest, and Mission, southern coastal British Columbia. We sampled roots using a tree-to-tree trench excavation method. In each stand, roots were collected from three randomly located trenches using a 20 x 20 x 10 cm template along (i) a soil depth gradient: forest floor (FF), 0- to 10-cm (MS1), and 10- to 20-cm mineral soil layers (MS2) and (ii) a lateral soil gradient (tree-to-tree trenches). Hemlock-redcedar mixtures had the greatest root density, followed by pure hemlock and redcedar stands. In both pure hemlock stands and hemlock-redcedar mixtures, root density of all size classes (fine, medium, and coarse roots) decreased with increasing soil depth, whereas that of pure redcedar stands peaked at the MS1 layer. Density of roots along the lateral gradient varied among stand types and root size classes. Density of coarse roots was higher towards tree stems, while that of fine and medium roots was more evenly distributed. In the forest floor of the hemlock-redcedar mixtures, root density of all sizes except that of redcedar coarse roots was not related to the distance to stems, but fine root density of hemlock was greater than that of redcedar. The highest root density in the hemlock-redcedar mixtures may indicate the presence of overlapping and intermingling roots and intense competition for available soil resources.