32 HARPER: PLANT POPULATION OF MICHIGAN 
evidence that these will ever grow on top of deep sour peat. 
Furthermore, the swamps are colder at night than the adjoining 
slopes, and lack some of the characteristic soil fauna of clayey 
uplands, and these fundamental differences can hardly be oblit- 
erated by succession. 
It has been assumed also that the pine forests on the sandy 
uplands would likewise be succeeded by hardwood forests, if fire 
and lumbering did not prevent the accumulation of humus. 
Although the sand is undoubtedly poorer than the clay, one might 
suppose that in the course of centuries the pines could bring up 
enough mineral matter from a depth of several feet, and deposit it 
on the surface in decaying leaves, to make a rich soil that would 
support trees that make a complete new crop of leaves every year, 
and that such a forest when once established would be self-per- 
petuating, as it would return to the soil every fall what it took up 
in the spring and summer. But the difficulty is that in sandy soils 
leaching probably goes on fast enough to prevent the accumula- 
tion of any considerable amount of plant food on the surface of 
the ground. (On clayey slopes there is less leaching, and erosion 
is constantly exposing fresh layers of soil, and thus maintaining 
its fertility indefinitely.) It seems that in many if not most cases 
the proportion of deciduous trees in a forest does not increase with 
succession, but depends on fundamental soil characters.* 
Another type of succession is a periodic one resulting from fire. 
In a state of nature the hardwood forests were rarely visited by 
fire, and one might say figuratively that the plants in such habi- 
tats made no provision for such an occurrence. The pine forests 
have been so nearly destroyed that it is difficult to determine what 
the normal frequency of fire in them may have been. As Pinus 
resinosa is not very sensitive, it may have been subject to ground- 
fires every few years, like some of the southeastern pines; while 
in the white-pine forests fire may have been almost as rare as in 
the hardwoods. Be that asit may, the “‘ pernicious activities” of 
the lumbermen a few decades ago removed the greater part of 
these two valuable pines,t and the ground formerly occupied by 
* See H 
. W. Wiley, Science II. 17: 794-795. May ts, 1903; Harper, Bull. 
. Torrey Club 41: 209-220. 1914; B. Moore, Bot. Gaz - 61: 59-66. Jan. 1916. 
T See Pop. Sci. Monthly 85: 343. IQI4. 
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