The architecture of Picea sitchensis structural root systems on horizontal and sloping terrain

The coarse root systems of 24 Sitka spruce (Picea sitchensis (Bong.) Carr.) trees, from a 40-year-old plantation in west Scotland, were extracted, digitised in three dimensions, and root topology was recorded. Roots were from trees grown on a steep (ca. 30°) north-facing slope, and from an adjacent horizontal area with similar gleyed mineral soil. The prevailing wind was across-slope from the west. Analysis of below-ground parts of the trees in comparison with those above-ground revealed a positive linear relationship between coarse root volume and stem volume. Most non-directional characteristics of the root systems were similar between trees on the slope and on flat terrain. Allocation of root mass around trees was examined in relation to the slope and the prevailing wind direction. Trees on the horizontal area had more root mass in leeward sectors than other sectors, but trees on the slopes had more root mass in the windward sectors than other sectors. Centres of mass of the root systems from the horizontal part of the site were not significantly clustered in any direction, but root systems of trees on the slope had centres of mass significantly clustered across the slope in the windward direction. For trees on the slope, the mean direction of the largest sector without structural roots was 4° from north, i.e. downslope. The results are discussed in relation to soil characteristics and the biomechanical behaviour of trees on slopes.