hares shunned the shoots but eagerly fed 
on the twigs. He then assayed the nutri- 
tional content of the twigs and shoots 
and found that the shoots had higher 
tissue concentrations of nutritional 
chemicals. Their apparent low payabil- 
ity, Klein hypothesized, must be a result 
of their toxicity. 
This startling experiment suggested 
to me that the browsing caused a defen- 
sive response in the hares’ preferred 
browse species. Observations I had 
made about the relationship between 
Alaska paper birch (Betula papyrifera 
spp. humilis) and snowshoe hares sup- 
ported this hypothesis. Alaska paper 
birch is a member of a group of birches 
characterized by an abundance of toxic 
resin glands in the one-year-old twigs of 
adventitious shoots and an absence of 
resin glands in the one-year-old twigs of 
mature plants. The foliar buds of both 
kinds of twigs contain high concentra- 
tions of resins. When hares feed upon 
the mature-plant twigs, they reject only 
the buds; when feeding upon the stump- 
sprout twigs, they reject the entire twig. 
Hares are the only herbivores 1 know of 
that exhibit such feeding behavior. I 
suggested that resins known to protect 
birch foliar buds against insects are ex- 
tended to the twigs of adventitious 
shoots when the risk of hare attack is 
high. Subsequent experiments have 
shown that such an extension of chemi- 
cal defenses is also characteristic of the 
highly preferred poplars and the much 
less palatable alders. 
Many juvenile woody plants produce 
thorns or large quantities of toxic sec- 
ondary compounds, which appear to de- 
ter herbivory. The highly complex con- 
trols over the progression from juvenility 
to maturity are far from clearly under- 
stood, but plant physiologists do know 
that in deciduous woody plants, the pro- 
gression is reversible: severe pruning can 
cause a mature plant to revert to a 
juvenile form. As a part of the reversion, 
the plant will increase its production of 
defensive structures and substances. 
Since the hares’ intense browsing of 
their preferred plants at the peak phase 
of the cycle is nothing more than severe 
pruning, reversion to juvenility may pro- 
vide the physiological explanation for 
the defensive response turned up in 
Klein’s experiments. 
Such an induced defense appears to 
fit well into the overall adaptive strategy 
of early successional, disturbance-toler- 
ant trees and shrubs. These plants store 
relatively large carbon and nutrient re- 
serves in underground parts, such as 
roots, from which they can regenerate 
ground herbivory and that the plants 
have limited ability to replace parts 
eaten by herbivores. Real or simulated 
hare browsing of late successional 
boreal forest trees and shrubs greatly 
reduces their reproductive output and 
results in high mortality levels. That 
these plants have evolved chemical 
defenses against herbivores is therefore 
not surprising. 
Late successional trees and shrubs are 
not the only woody plants of the boreal 
forest that are toxic to snowshoe hares. 
Plant chemical defenses also play a cru- 
cial role in the interactions between 
early successional woody plants and 
hares but in a very different way. This 
difference may provide part of the ex- 
planation of the hare cycle. 
During the winter of 1972-73, hare 
numbers around Fairbanks peaked and 
started to decline. David Klein of the 
Alaska Cooperative Wildlife Research 
Unit noticed that hares fed heavily that 
winter on mature early successional 
trees and shrubs but appeared to avoid 
the adventitious shoots, or stump 
sprouts, being regenerated by these pre- 
ferred browse species. Furthermore, the 
avoidance seemed to be even stronger 
than the avoidance of black spruce 
twigs, which at this point in the cycle 
made up much of the diet of the declin- 
ing hare population. Klein speculated 
that adventitious shoots were avoided 
either because they contained few nutri- 
ents or because they were toxic. To test 
these alternatives, he first confirmed 
experimentally that when equal quanti- 
ties of both adventitious shoots and 
small-diameter, mature twigs of willows, 
poplars, and birches were available, the 
51 
