HARPER: PLANT POPULATION OF MICHIGAN 33 
them has now a low scrubby growth of birch and aspen, which is 
burned too often for the white pine to reéstablish itself, though the 
red pine is making some headway. 
On steep bluffs and small islands, and in ravines, where fire is 
necessarily rare, we find some plants that are sensitive to fire but 
do not require as rich a soil as that of the hardwood forests, such 
as the few woody vines of the region. (Most of the existing white 
pines are found in such places, too.) The low sandy “‘ice ramparts”’ 
around the larger lakes are protected from fire on one side by the 
water and at the same time are too sterile to support vegetation 
dense enough to carry fire readily, so that certain fire-sensitive 
(or pyrophobic, if one may coin a new term) plants, such as A melan- 
chier sp., Prunus pumila,* Rosa sp., Rhus Toxicodendron, Arcto- 
staphylos Uva-ursi, Equisetum hyemale, Elymus sp., and Potentilla 
Anserina, are characteristic of such places. 
The normal frequency of fire in the jack-pine and spruce types 
of forest seems to be about once in the average lifetime of a tree. 
Pinus Banksiana is one of several pines whose cones remain closed 
and attached to the tree for many years, but open soon after a 
fire and discharge their seeds, thus re-stocking the forest. In the 
spruce bogs, as in the white-pine forests, one of the first effects of a. 
fire sweeping through the crowns of the trees is to liberate the 
potash and other mineral substances stored up in several years’ 
growth of leaves and twigs, which falls to the ground and acts as a 
high-grade fertilizer. Several quick-growing and short-lived trees 
and shrubs, such as Betula, Populus tremuloides, P. grandidentata, 
Prunus pennsylvanica, and a few characteristic herbs, known 
collectively as fire-weeds, soon invade the burned areas by means 
of seeds carried by wind or birds, and flourish until the surplus 
potash, etc., is exhausted. In the swamps one of the conifers 
itself, namely the deciduous one, Larix, acts as a sort of fireweed, 
being especially common in the first few decades after a fire; and 
its career is commonly terminated by saw-flies instead of fire. 
As it renews its whole crop of leaves every year it needs a larger 
food supply than the spruces, and it also extends farther into the 
of the writers cited in the bibliography, particularly Dr. Rusby. Dr. H. A. Gleason 
told me in 1912 that he did not know of a single acre of virgin white-pine forest in 
— Michigan 
bes Rises 18: 202. Sept. 1916. 
