THE ECOLOGICAL RELATIONS OF THE VEGETATION 
ON THE SAND DUNES OF LAKE MICHIGAN. 
HENRY CHANDLER COWLES. 
[Contitaued from p. 202.| 
3. Encroachment on preexisting plant socteties. 
Txuos— who are at all familiar with wandering dunes are 
‘cquainted with their power to destroy vegetation in the path of 
their advance. This, indeed, is to many people the most con- 
spicuous feature of a sand dune area, because it often becomes a 
feature of the greatest economic importance. The effect of an 
acing dune upon the preexisting vegetation varies greatly as 
spaditions vary. The most important factors are the rate of 
aie the height of the advancing dune above the territory in 
a path, and the character of the vegetation that is encroached 
pon, 
The rate of advance is, of course, a decidedly variable factor, 
“aan rates, from nothing up to the maximum rate, may be 
the x ong nearly all advancing lee slopes. At a given point 
‘€ varies greatly during different seasons. An advancing 
Sta ie become checked and a checked portion may 
More a vee wind-sweeps are clogged up or opened ae 
affect th ¢ multiform changes on the complex, each and all, 
have bes jae = advance to a remarkable degree. ae 
Park, but a ae - vacuous the maxima of pare : area 
Satisfactor iis time has not elapsed as yet toa pee 
driven an ong In November 1897 a sta sag 
eight of i. asal edge of a rapidly advancing lee slope. ae 
Meter Sstake above the ground was a little more than | 
May 1898 the stake was nearly covered, and it 
ee Si ee 
NOOBS the ha; of advance amounted to a or 
899] Orizontal component, of course, was greate ' 
281 
n 
Could Not be 
