Cambial electrical resistance does not assess vitality of individual sweetgum trees

Cambial electrical resistance (CER) was as- sessed over a year-long period, using the Shigometer, for 109 sweetgum trees (Liquidambarstyraciflua) planted along streets in Seattle, Washington. CER decreased through the spring, reached a minimum in summer, increased through the fall and was maximal in mid-winter. The seasonal maximum was 200- 600% greater than the summer minimum. CER measurements could not be related to growth parameters such as seasonal shoot increment and diameter growth. Management of urban forests often focuses on the performance of individual trees. This contrasts with traditional forest management, where the aggregate development of trees within a stand is more commonly the concern. Urban trees fre- quently endure environmental stresses that impair development and shorten life-span. An individual tree can represent either a substantial asset or potential liability. Thus, urban tree managers need a rapid, easy-to-use method to assess tree vigor and predict the pattern of future development. Arborists have relied upon visual examination to assess crown density, shoot elongation and other measures of vegetative growth as indicators of vigor. Cambial electrical resistance (CER) has been used in forestry to monitor tree vigor. Since CER measures the resistance to electrical flow between two electrodes, it is a function of the number of cells and solute content in a radial file of cambium. The most vigorous trees would be those with thick cambial zones and high cation concentrations. Such trees would have low CER readings. CER has been used to assess the vigor of trees defo- liated by gypsy moth (13) and damaged by spruce budworm (2,4). It has also been used to monitor release of BetulapaperiferaioWow'mg thinning (12),