Above-and Below-ground Characteristics Associated with Wind Toppling in a Young Populus Plantation Key Words Wind Damage @bullet Root Morphology @bullet Stem Form @bullet Thigmomorphogenesis @bullet Clonal Deployment

Damage from a dormant-season windstorm in a 3-year-old Populus research trial differed among four clones and three spacings and between monoclonal and polyclonal plots. Clonal differences in susceptibility to toppling (or leaning) were associated with both above-and below-ground characteristics. Susceptible clones had less taper in the lower stem and more weight in branches on the upper stem. The most susceptible clone also had the most above-ground biomass per unit of cross-sectional root area. The other susceptible clone had the least root system development in the windward quadrants. Wind toppling was least at the closest spacing. Apparently, mutual support was more important than individual tree characteristics from which the most damage would be expected at the closest spacing. Differences between paired trees of the same clone and spacing which did or did not topple were primarily associated with distribution of root systems by compass quadrant or depth. At the closest spacing where crown sway would have been minimized, trees which did not topple had greater cross-sectional root area in the windward direction than trees which did topple. At the widest spacing where crown sway would have been greatest , windfirm trees had greater cross-sectional root area than non-windfirm trees in both the windward and leeward directions. Toppling was reduced in polyclonal plots; this reduction may have been the result of more rapid stand differentiation in the polyclonal plots or reduction in the "domino effect" by inclusion of more windfirm clones in the mixture.