Effects of simulated insect damage on early growth of nursery-grown hybrid poplars in northern Wisconsin

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Abstract
Insect damage on hybrid poplars grown from rooted tip cuttings was simulated to assess potential growth impact under nursery conditions. Four experiments were conducted at Rhinelander, Wisconsin, from June 1975 through August 1977. Partial defoliations in several patterns were tested both early and late in the growing season. Basal injury was also investigated.Whereas defoliations of 40% caused negligible growth impact, defoliations of 75 to 80% reduced growth by about 20%. The 75% defoliations did not differ significantly according to distribution in the crown. Timing of defoliations was not significant as a main effect. Faster growing clones suffered greater growth reduction in one experiment where cultural regime was not so intensive. Basal injury did not produce significant impacts unless it was severe enough to weaken the stem almost to the point of collapse.Results indicate that young and vigorous rooted tip cuttings have substantial reserve photosynthetic capacity. Even heavy defoliations do not counterbalance gains that can be realized by genetic selection for rapid growth.

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