Charlie Ott, Photo Researchers 
aboveground parts destroyed by disturb- 
ance. Hare browsing can be viewed as a 
form of disturbance, so why, one might 
ask, do the plants respond to intense 
browsing by becoming toxic? The an- 
swer lies in another aspect of adaptation 
to growth on recently disturbed sites. A 
severe disturbance to a late successional 
ecosystem — wildfire, for example — 
opens up the forest canopy and releases 
large quantities of nutrients from the 
living and the dead vegetation. Early 
successional trees and shrubs then com- 
pete for the nutrients and the light to 
gain dominance in the opening and 
reach the forest canopy. 
At this stage in their development, 
these juvenile browse species need to 
grow upward quickly, so they allocate 
much of their carbon to vertical growth 
and chemical defense against hare 
browsing; once out of reach of hares, 
they allocate more carbon to lateral de- 
velopment of the crown and production 
of seeds ahd less to chemical defense 
and vertical growth. In the event of 
intense browsing, however, plants can — 
by reverting to juvenile form — increase 
the allocation of carbon to defense. 
Without this ability, repeated regenera- 
tion of leaves and twigs at times of 
heavy browsing might stress the under- 
ground carbon reserves so severely that 
the plant could no longer respond defen- 
sively to browsing or compete success- 
fully with other plants. 
To the hare, the defensive response of 
its preferred browse species means a 
serious deterioration in the nutritional 
quality of its winter food supply. My 
experiments strongly suggest that this 
deterioration lasts for two to three 
years — the time lag predicted by the 
Hutchinson-May model of the hare cy- 
cle — even though only the one-year-old 
twigs of adventitious shoots appear to be 
highly repellent to hares. 
The first year after heavy browsing, 
all the new twigs produced by these 
early successional woody plants are 
chemically defended and of little nutri- 
tive value to hares. Adventitious shoots 
grow rapidly, and in the second year of 
growth after browsing, the two-year-old 
twigs are no longer toxic, but they are 
usually more than 4 mm in diameter 
and have very high wood/bark ratios. 
Since most of the nutrients of the twig 
are stored in the inner bark, a high 
wood/ bark ratio is associated with low 
nutrition irrespective of toxicity; these 
twigs are also essentially useless to the 
hares. The only small-diameter twigs 
available during this second year are one 
Canada lynx and other predators can 
take a heavy toll of snowshoe hares. 
The hares’ capacity to increase in 
number is enormous, however, and 
predation alone cannot cause the 
cyclic crash of the hare population. 
year old and thus chemically defended. 
In the third year of growth, however, a 
few lateral branches support a modest 
number of small-diameter, two-year-old 
twigs that hares can feed on efficiently. 
After three years, this usable biomass 
increases rapidly, and the hare popula- 
tion can begin its recovery. 
In the end, then, the cycle may be a 
consequence of the adaptation of boreal 
trees and shrubs to disturbance, and 
particularly to fire, but the fire-succes- 
sion-hare connection seems to be tied 
more closely to the behavior of the 
plants themselves than was previously 
realized. The plants’ ability to produce 
toxins — an ability so advantageous to 
them and so devastating to the hares — 
may occupy a pivotal place in the almost 
legendary ten-year population cycle of 
the snowshoe hare. □ 
Ralph H Williams. Bruce Coleman 
52 
