FOREST PLANTING IN THE LAKE STATES 35 
passing through a plantation ; in any moderate depression the foliage 
on the tips of the twigs may be brown or withered from damage 
caused the previous spring, or a partial or total lack of living trees 
may testify to similar damage in previous years. Norway and north- 
ern white pine are particularly susceptible to frost damage. Jack 
pine is apparently able to withstand regularly frosts which would 
destroy the other two pines. The damage from frost rarely if ever 
occurs under the cover of natural tree growth or of other vegetation. 
Snow and ice breakage is infrequent and usually unimportant, but 
becomes serious in an exceptional storm like that of February 21, 
1922, which wrecked many hardwood trees in a zone through central 
Michigan and Wisconsin. Winter killing may be serious in the 
occasional year. It occurs when a period of unusually warm weather 
in the winter or early spring while the ground is frozen causes the 
leaves of the pines and spruce, which retain their foliage through 
the winter, to give off moisture which the roots can not replace. The 
result is a browning of the needles the following spring and possibly 
the death of many of the trees. 
In the plantations studied, the combined damage from snow and 
ice breakage, frost, and winter killing rarely affected more than 5 
per cent of the trees in any plantation, and evidence of such damage 
was only noted in 4 out of the 15 Norway spruce plantations, in 11 
per cent of the northern white pine plantations, in 7 per cent of the 
Norway pine, and in 6 per cent of the jack pine. 
ANIMALS 
The snowshoe rabbits in Minnesota and the adjacent part of north- 
ern Wisconsin during 1923, 1921, and 1925 caused excessive losses in 
forest plantations, and indeed to all young growth of forest trees. 
In Minnesota all of the northern white pine plantations examined 
were damaged, 78 per cent of the white spruce, and 55 per cent of the 
Norway pine. In a large number of plantations every tree which 
could be found had had the tip and branches cut back until only a 
stub was left. (PL 3, A.) More than half of the white pine and 
white spruce plantations had over 70 per cent of the trees cut back. 
The Norway pine plantations suffered less but apparently only be- 
cause they were more often set out on the dry open sandy areas where 
the rabbits are less abundant. 
The rabbit damage in most of Wisconsin and in Michigan is much 
less sever© and is usually confined to the planted areas near swamps 
and streams. Even so, however, rabbits are among the most serious 
enemies of forest planting in the region. Rabbit damage was noted 
in 53 per cent of the Norway spruce plantations in these States, 38 
per cent of the northern white pine, 8 per cent of the Norway pine, 
and 7 per cent of the jack pine plantations. Only rarely, however, 
had more than 30 per cent of the trees been injured. 
There is a general impression that the rabbit population fluctuates 
in cycles of about seven years; that is, after they have increased to a 
maximum number during a 7-year period the increase of their natural 
enemies or some epizootic ordinarily causes them to die off in such 
numbers that it is another six or seven years before they regain their 
former abundance. Recent years in which the rabbits have been 
most abundant are 1901, 1902, 1908, 1914, and 1923 to 1925. On the 
