© 1996 Heron Publishing—Victoria, Canada
Adaptive growth of tree root systems in response to wind action and site conditions
Bruce C. Nicoll and Duncan Ray
Forestry Commission Research Division, Northern Research Station, Roslin, Midlothian EH25 9SY, Scotland, U.K. / Received October 25, 1995
Summary
Soil-root plate dimensions and structural root architecture were examined on 46-year-old Sitka spruce (Picea sitchensis (Bong.) Carr.) trees that had been mechanically uprooted. Rooting depth was restricted by a water table, and root system
morphology had adapted to resist the wind movement associated with shallow rooting. The spread of the root system and the
ratio of root mass to shoot mass (root/shoot ratio) were both negatively related to soil-root plate depth. Root systems had
more structural root mass on the leeward side than the windward side of the tree relative to the prevailing wind direction.
Cross sections of structural roots were obtained at distances of 0.5, 0.75, 1.0, and 1.25 m from the tree center. Buttressed
parts of roots had greater lateral and vertical secondary thickening above rather than below the biological center. This uneven
growth, which produced a shape similar in cross section to a T-beam, was greater on the leeward side of the tree, and was
greatest at 0.5 m from the tree center of shallow rooted trees. Further from the tree, particularly on the windward side,
many roots developed eccentric cross-sectional shapes comparable to I-beams, which would efficiently resist vertical flexing.
Roots became more ovoid in shape with increasing distance from the tree, especially on deep rooted trees where lateral roots
tapered rapidly to a small diameter.
We conclude that these forms of adaptive growth in response to wind movement improve the rigidity of the soil-root plate and
counteract the increasing vulnerability to windthrow as the tree grows.
Keywords:
buttress development, I-beam roots, root morphology, structural roots, thigmomorphogenesis, tree stability, windthrow.
