IMPACT OF MOOSE BARK STRIPPING ON MOUNTAIN ASH IN VERMONT
Moose (Alces alces) have always been present in Vermont, but their population began to increase from approximately 25 in 1960 to the present when their population is estimated to be approximately 2,500. The increase in moose numbers began in the northeastern part of the state and has gradually spread south. State wildlife biologists have noticed the loss of American mountain ash (Sorbus americana) in some stands due to bark stripping by moose. Mountain ash fruit is a valuable soft mast resource for other wildlife, notably black bears (Ursus americanus). We assessed bark stripping throughout Vermont, examining 1,535 mountain ash stems at 42 sites. Trees wounded by moose accounted for 27% of the total. There was no bark stripping in the southern part of the state where the moose population is still low, but 1/3 if all trees in northern regions were wounded. A conservative estimate of mortality associated with bark stripping was 7% of all stems >5 cm within a total mortality of 23%. More trees will die in time as the result of pathogen introduction and/or continued girdling. Once girdling reached 3/4 of total tree circumference, the incidence of mortality was higher than expected. Bark stripping on a local scale could be severe; at some sites 3/4 of the total circumference of all stems was girdled. Mortality was significantly correlated with amount of girdling (y = 49.759x + 12.555, r2 = 0.51, P<0.0001). Girdling was significantly correlated with moose activity as measured by pellet group counts (y = 0.0195x + 0.1756, r2 = 0.37, P<0.0001). Amount of girdling or mortality was not correlated with aspect, altitude, tree size, or tree density. Trees on gentler slopes sustained more damage than those on steeper slopes (χ2 = 9.509, P<0.049). There was significantly more moose activity (Z = -2.797, P<0.0052) near logging operations as well as more girdling (Z = -2.018, P<0.044). Mountain ash are protected from moose on steep rocky slopes, and near ski areas (n = 5 sites). However, regeneration may not be able to keep pace with loss due to bark stripping, and an overall decline of the species is expected as the moose population in Vermont continues to increase. Some local stands of mountain ash have already been eradicated.
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