Tree form, height growth, and susceptibility to wind damage in Acer saccharum

The influence of tree dimensions on efficiency of height growth was analyzed for Acer saccharum (sugar maple), an important shade-tolerant species of the northeastern United States. Support efficiency was analyzed in terms of the ratio of actual trunk diameter to the minimum required to keep the tree erect (the stability safety factor). Susceptibility of trees to wind was computed in terms of tree dimensions and was also estimated from measurements of catastrophic storm damage to sugar maple in northern Wisconsin. A trade-off was expected between height growth efficiency, which is maximized by a minimally designed trunk, and ability to resist storm winds. The analysis included canopy trees, suppressed saplings, and more rapidly growing saplings with apical dominance, so that the form of sugar maple could be evaluated throughout its life cycle. The lowest stability safety factors were observed in the saplings with apical dominance, which had a mean safety factor of 1.8, indicating an efficient form for height growth. The suppressed saplings were somewhat thicker trunked, while canopy maple had trunks ranging from 2 to 6 times the minimum diameter required to keep trees erect in the absence of wind. The stability safety factor increased with trunk diameter in canopy trees, but so did trunk breakage due to catastrophic winds. Calculations of wind-generated bending moments suggest that the greater vulnerability of large trees to wind is due to the loss of flexibility caused by increased girth, and to the expected increase in wind speed with height in the canopy. Comparison of these results for sugar maple with a similar analysis of Populus tremuloides (aspen), a shade-intolerant species with short-lived ramets, indicates that maple trees have greater stability safety factors than aspen and are more resistant to catastrophic wind. Maple saplings with apical dominance approached the minimal design of aspen, but were growing in more sheltered conditions beneath a maple canopy.

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