CONVERSION OF FOREST. 161 



in a swamp very much as it does in forest. It is best in consequence not to 

 separate the consideration of the two, especially since the sections of moors 

 show that a layer of tree roots is very often found at the edge of the swamp or 

 heath moor layers. Such a moor was forested before it became covered with 

 heath moor." (96) 



Causes of conversion. — Graebner has described six processes by which forest 

 or swamp is converted into heath or heath-moor. In the first, forest is changed 

 into heath as a result of the removal of the trees in whole or in part, with 

 consequent leaching of the upper layer and the formation of "ortstein." 

 The need for destroying the reaction control of the trees, i. e., their shade, is 

 shown by his statement that Calluna has a suppressed appearance because of 

 the deep shade still found in many places. The artificial destruction of the 

 forest seems requisite. Graebner says that "the conversion to heath is natur- 

 ally hastened by the cutting and utilization of forest, though it must occur even 

 without this, through the operation of climatic factors upon soil." Our present 

 knowledge seems quite inadequate to confirm this statement. On theoretical 

 grounds, such conversion would seem quite impossible without the con- 

 tributing action of climatic variation, since a climate constantly like that under 

 which conversion occiu's would have prevented the development of the original 

 forest. The work of Douglass (1909; 1914), Hiunphreys (1913), and Hunting- 

 ton (1914) seems to indicate clearly that so-called changes of climate are but 

 the persistence for a time of variations such as occur from year to year. It 

 seems probable that the conversion of forest into heath as a result of the 

 formation of raw hiunus is a consequence of such climatic variations, and 

 that it is further aided by the influence of man and domesticated animals. 

 Graebner himself nowhere considers this matter of climatic oscillations, since 

 he is concerned primarily with the detailed changes in the soil. It is, however, 

 of the most vital importance in determining the real nature of secondary 

 development, since regression can be said to occm: only when the reactions 

 of the undisturbed vegetation produce an actual backward sequence of com- 

 munities. 



Of the four ways by which heath moor may arise from an existing swamp 

 or forest, one, the flooding of a swamp by enutrient water, is obviously a matter 

 of destruction and denudation. A careful analysis of the other cases likewise 

 shows that the process is here one of flooding and destruction. The essential 

 fact that the change is due to flooding is obscured by the intimate interrelation 

 between Sphagnum and water, and by the appearance of Sphagnum in many 

 separate spots. Ecologically, the water-soaked moss is the equivalent of the 

 direct flooding of an area by so much water, except that the Sphagnum water 

 has a much more marked effect, since most of it can not drain off, and since the 

 amount constantly increases. The Sphagnum is really a pioneer in a new if 

 minute water area, and differs only in degree from the thalli of algal pioneers, 

 such as Nostoc, which also absorb and retain water tenaciously. Graebner's 

 statements also support this view, for he says: 



"This is the point at which the change to heath moor begins. Sphagnum 

 colonizes the lower moister places and, in similar fashion as upon the moist 

 sandy soils, the cushions run together and first fill the hollows and ditches in 

 the swamp. The Sphagnum cushions now hold the water with great tenacity. 

 When the moss layer has reached a certain thickness, it forms a great reservoir 



