204 CLIMAX FORMATIONS OF NORTH AMERICA, 



"There are all gradations between a simple moving dune and a moving 

 landscape; the latter may be called a dune-complex. The complex is a rest- 

 less maze, advancing as a whole in one direction, but with individual 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 complex is the succulent annual, Corispermum 

 hyssopifolium, although Populus monilifera 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 preexisting flora varies with 

 the rate of advance, the height of the dune above the country on which it 

 encroaches, and the nature of the vegetation. The burial of forests is a com- 

 mon phenomenon. The dominant forest trees in the path of advancing dunes 

 are Pinus banksiana, Qviercus cocdnea tinctoria, and Acer saccharinum. All 

 of these trees are destroyed long before they are completely buried. The dead 

 trees may be uncovered later, as the dune passes on beyond. 



" 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 Cornus stolonifera are able to adapt them- 

 selves to the new conditions, by elongating their stems and sending out roots 

 from the buried portions. Thus hydrophytic shrubs are better able to meet 

 the dime'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 xero- 

 phytic life through hving in undrained swamps. Again it may be true that 

 inhabitants of undrained swamps are better able to withstand a partial biu-ial 

 than are other plants. 



"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 de- 

 crease in the available wind energy, due to increasing distance from the lake or 

 to barriers. A slowly advancing slope is soon captured by plants, because they 

 have a power of vertical growth greater than the vertical component of 

 advance. Vegetation conunonly gets its first foothold at the base of lee slopes 

 about the outer margin of the complex, because of soil moisture and protection 

 from the wind. The plants tend to creep up the slopes by vegetative propa- 

 gation. Antecedent and subsequent vegetation work together toward the 

 common end. Where there is no antecedent vegetation, Ammophila and other 

 herbs first appear, and then a dense shrub growth of Cornus, Salix, Vitis cor- 

 difolia, and Prunus virginiana. Capture may also begin within the complex, 

 especially in protected depressions, where Salix longifolia is often abundant. 



"Tilia americana develops rapidly on the captured lee slopes, and the 

 thicket is transformed into a forest. The trees grow densely, and there is 

 little or no vegetation carpet. Associated with Tilia is a remarkable collection 

 of river bottom plants, so that the flora as a whole has a decided mesophytic 

 cast. These plants have developed xerophytic structures that are not present 

 in the river bottoms. Acer and Fagus succeed Tilia and represent the normal 

 climax type of the lake region, the deciduous forest. 



"On the established windward slopes the development is quite different 

 from that described above. There is a dominance of evergreens instead of 

 deciduous vegetation. The soil conditions are nearly alike on the two slopes, 

 but the air is more xerophytic on the windward slopes. The evergreen flora 

 starts as a heath formed of Arctostaphylus, Juniperus communis, and J. saMna 



