tinue to keep the hare population down 
once the vegetation begins to recover? 
To answer this question, my associates 
and I have been investigating the inter- 
actions between snowshoe hares and the 
woody plants of the boreal forests in 
which they live. Snowshoe hares, like all 
subarctic browsing mammals, reach 
high densities only in habitats that con- 
tain patches of early successional vege- 
tation. While they do inhabit late suc- 
cessional black spruce ( Picea mariana) 
forests and alder ( Alnus spp.) thickets, 
hares are most abundant where both 
dense late successional vegetation and 
early successional plants are found. 
Within this mosaic environment, the 
hares use late successional vegetation 
primarily for shelter from severe winter 
storms and predators. They move once 
at dusk and once at dawn into the adja- 
cent patches of early successional vege- 
tation to feed on willows, birches, pop- 
lars, and jack pines, their preferred 
winter food. Unless they are starving, 
they feed lightly, if at all, on black 
spruce and green alder (A. crispa). 
In controlled feeding experiments, 
hares can be maintained indefinitely un- 
der winter conditions if fed a diverse, 
highly palatable diet of the mature parts 
of early successional browse species. By 
contrast, they survive only a few days 
when fed nothing but alder and other, 
less palatable late successional woody 
plant species. Malnutrition is an un- 
likely cause of the hares’ rapid deterio- 
ration. Alder is a nitrogen-fixing plant 
whose winter dormant twigs contain 
very high concentrations of nutritious 
proteins and soluble carbohydrates. Its 
chemical content would appear to make 
alder the most nutritious food available, 
yet it is one of the least-browsed sub- 
arctic woody plants. Why? 
Knowing that black spruce and alder 
are noted for their high tissue concentra- 
tions of toxic secondary metabolites, I 
decided to test the possibility that 
snowshoe hares avoid feeding on late 
successional trees and shrubs because of 
the plants’ toxicity. I extracted terpene 
and phenolic resins from both green 
alder and black spruce and incorporated 
In winter, snowshoe hares seek 
protection from storms and predators 
beneath snow-laden, late successional 
trees such as spruce. For feeding, they 
prefer early successional species, 
including willow and birch. 
Mario Grassi, AlaskaPhoto 
50 
the extracts into commercial hare food, 
which I then offered to both free-rang- 
ing and captive snowshoe hares. In all 
the feeding experiments, hares avoided 
the resin-treated food but eagerly ate 
the unadulterated control food. More- 
over, captive hares that were offered 
only resin-treated food refused to eat for 
several days, and when hunger finally 
forced them to feed on the treated food, 
they lost weight even more rapidly than 
when they were eating nothing. 
Subsequent experiments have begun 
to show why ingestion of these resins is 
so stressful to hares. Green alder resin 
contains relatively large quantities of 
pinosylvin methyl ether, a phenolic sub- 
stance that appears to be repellent to 
hares and is known to be repellent and 
toxic to a variety of herbivores, from 
termites to rabbits. When wood that is 
normally heavily fed upon by tropical 
termites is treated with this substance — 
in quantities as low as one-tenth of one 
percent of the dry weight of the treated 
wood — the termites will not even ap- 
proach the wood for at least two years. 
The toxicity of these plants is partly a 
consequence of the conditions of ex- 
treme nutritional stress under which 
they grow. Many trees and shrubs 
adapted to growth on nutrient-deficient 
soils have slow turnover rates of twigs 
and leaves. Consequently, they experi- 
ence less of the nutrient loss that accom- 
panies the frequent shedding of leaves 
and twigs characteristic of woody plants 
adapted to growth on nutrient-rich soils. 
This nutritional conservatism, however, 
also means that a large fraction of the 
nutrient and carbon reserves of these 
plants is exposed year-round to above- 
