6o THE PLANT SOCIETIES OF 



is a restless maze, advancing as a whole in one direction, but with indi- 

 vidual portions advancing in all directions. It shows all stages of dune 

 development and is forever changing. The windward slopes are gentle 

 and are furrowed by the wind, as it sweeps along; the lee slopes are 

 much steeper. The only plant that flourishes everywhere on the com- 

 plex is the succulent annual, Corispermum hyssopifolinm (Bugseed), 

 although Fopulus monilifera (Cottonwood) is frequent. The scanty flora 

 is not due to the lack of water in the soil, but to the instability of the 

 soil and to the xerophytic air. 



The influence of an encroaching dune upon a pre-existing flora 

 varies with the rate of advance, the height of the dune above the coun- 

 try on which it encroaches, and the nature of the vegetation. The burial 

 of forests is a common phenomenon. The dominant forest trees in 

 the path of advancing dunes are Pinus Banksiana (Scrub pine) and 

 Quercus coccinea tinctoria (Black oak). These trees are destroyed long 

 before they are completely buried. The dead trees may be uncovered 

 later, as the dune passes on beyond. A pine forest upon which a dune 

 is encroaching is shown \'^ fig- 35, while such a forest after the dune 

 has passed is shown \'a.fig- 36. 



In the Dune Park region there are a number of swamps upon 

 which dunes are advancing. While most of the vegetation is destroyed 

 at once, Salix glaucophylla, S. adenophylla, and Corniis stolonifera (Osier 

 dogwood) are able to adapt themselves to the new conditions, by elon- 

 gating their stems and sending out roots from the buried portions. 

 Thus hydrophytic shrubs are better able to meet the dune's advance 

 successfully than any other plants. The water relations of these plants, 

 however, are not rapidly altered in the new conditions. It may be, too, 

 that these shrubs have adapted themselves to an essentially xerophytic 

 life through living in undrained swamps. Again, it may be true that 

 inhabitants of undrained swamps are better able to withstand a partial 

 burial than are other plants. A swamp upon which a dune encroaches 

 is shown inyf^. 35. 



Vegetation appears to be unable to capture a rapidly moving dune. 

 While many plants can grow even on rapidly advancing slopes, they 

 do not succeed in stopping the dune. The movement of a dune is 

 checked chiefly by a decrease in the available wind energy, due to 

 increasing distance from the lake or to barriers. A slowly advancing 

 slope is soon captured by p.ants, because they have a power of verti- 

 cal growth greater than the vertical component of advance. Vege- 



