Ice Damage to Trees on the Virginia Tech Campus from Ice Storms

Northeastern Naturalist, 2007 by Rhoades, Richard W, Stipes, R Jay

Abstract -

The purpose of this study was to analyze ice damage to 228 trees of 9 species on the Virginia Tech campus. Damage was caused by three severe ice storms in February and March 1994. There were significant differences among species in amount of damage. Four ways of expressing percent damage were compared (% individuals damaged, % basal area of damaged trees, % crown damage, and an average of the three). The average method yielded the most significant comparisons, followed by percent crown damage. The species, ranked in four groups by mean total damage, are as follows: most damage (Acer saccharum, Chamaecyparis nooktakensis, Ulmus americana, and Acer nigrum: 29.7-26.4%); less damage (Quercus alba and Platanus occidentalis: 18.4-13.7%); lesser damage (Cornus florida: 10.2%); and least damage (Quercus rubra and Quercus palustris: 6.7-0%). The differences among groups were significant at P ≤ 0.05. Ice damage also caused a significant decrease in crown growth of four species. Comparisons with other studies revealed good correspondence, in general, with two or three exceptions. Our conclusions are that three factors were chiefly responsible for the relatively severe damage to trees: 1) the severity of the ice storms; 2) the open, exposed siting of all the trees, similar to trees growing at the edge of a forest; and 3) the high percentage of large trees with internal decay and asymmetric crowns.

Introduction

The effect of ice on trees has been studied since early in the 20lh century (Harshberger 1904). Ice storms occur with irregular frequency over a large part of the deciduous forest of eastern North America. In the southern Appalachians, very severe ice storms occur at a given site about once in twenty years (Abell 1934). Severe ice storms have occurred in western Virginia more often than that (R.W. Rhoades, pers. observ.: Dec. 1969, amount unknown; 1979, > 1 cm, see Whitney and Johnson [1984]; 1994, see text; Jan. 1997, 0.2 cm; Jan. 1998, 1.27 cm; Jan. 1999, 1.1 cm; Feb. 2000, 1.27 cm; Dec. 2000, 0.04 cm; Jan. 2002, 0.81 cm; Dec. 2002, 1.07 cm; Feb. 2003, 1.27 cm; and Dec. 2005,0.84 2.5 cm [data from 1997 to 2005 were recorded in a rain gauge at the residence of the senior author]).

An ice burden can cause large losses of biomass from forests (Boerner et al. 1988, Bruederle and Sterns 1985, Elstner and Ware 2001, Rhoades 1999). In addition to ice accumulation, factors such as wind, site, and topography contribute to damage during ice storms. While wind amplifies the effect of ice (Lemon 1961), it was not a factor in this study.

Various approaches have been used to study the effects of ice damage to trees. Three studies have been done in western Virginia. Whitney and Johnson (1984) compared damaged stands with undamaged stands on steep slopes. Rhoades (1999) compared density and basal area of a stand before and after the ice storms, and the effect of slope steepness. Warrillow and Mou (1999) analyzed the effect of topography on ice damage. One study was done in eastern Virginia (Elstner and Ware 2001). They described significant differences in ice damage between young and old stands and between small and large trees. All these studies of damaged areas have reported percent of individuals damaged, as did Siccama et al. (1976), Boerner et al. (1988), and Seischab et al. (1993).

Hauer et al. (1993) assessed damage in an urban area, and Lemon (1961) assessed damage and species resistance. De Steven et al. (1991) studied long-term changes in forest composition, due in part to disturbance by ice storms.

Four ice storms occurred in southwestern Virginia in 1994. The first was a large storm that spread over the southeastern US from February 9 to 13 (NOAA 1994a). Three storms occurred in March (NOAA 1994b). Ice deposits from the storms in Blacksburg were as follows: February 9-13, 2.5 cm; March 2-3, 2.9 cm; March 9, 2.65 cm; and March 18, 0.4 cm. Little or no wind was associated with any of these storms.

The purpose of this study was to assess ice damage to landscape trees on the Virginia Tech campus caused by these storms. We postulated that ice damage would be affected by several variables, namely, size of the tree (diameter at breast height [dbh] and crown diameter), presence of heart rot or disease, or asymmetry of the crown. One independent variable, percent of paved area beneath the crown, was found to affect growth in a prior study (Rhoades and Stipes 1999); so we also tested the effect of this factor on percent ice damage. We also evaluated the effectiveness of four methods of expressing ice damage. These were % individuals damaged, % basal area of damaged trees, % crown damage, and an average of the three. The overall objective of this study was to test for significant differences among species, and to test the effects of several variables on percent ice damage.

Methods

We measured dbh and crown diameter of 228 trees of 9 species on the Virginia Tech campus each year from 1993 to 1995 for a study of growth of trees (Rhoades and Stipes 1999). Crown diameter was the mean of two measurements at right angles through the crown at the drip line. The trees were selected to represent a range of diameters, and to be evenly divided between "poor" and "good" sites. Sites were judged to be poor if more than 10% of the area beneath the crown was paved over.

We also made notes on the condition of each tree, every year. These conditions included trunk or branch injury, presence of disease, evidence of heart rot, and crown structure, including degree of asymmetry. In 1995, we estimated percent of crown missing due to ice damage. We assigned each tree to one of four classes of crown loss: 1) light, 51% damage and tree removed. Degree of asymmetry was determined from a disparity between the two measurements of crown diameter. For most trees, the two diameters were similar, but in about 20% of trees the diameters were markedly different, and these were judged to be asymmetric. Percentage deviation from symmetry (larger diameter vs. smaller diameter) was light ( 10 to 50%), moderate (51 to 100 %), or maximal (101 to 200%).