CONVERSION OF FOREST. 



157 



In America, the questions of the origin of the prairies, their derivation from 

 forest, and their present tendency to become forest, have produced a copious 

 literatiu-e, but the latter contains little or no conclusive evidence for one view- 

 er the other. 



Artificial conversion.— In spite of the almost total lack of direct proof, 

 there is so much observational evidence of the artificial conversion of forest 

 into scrub, heath, moor, or grassland as to create a strong presumption in 

 favor of this view, and to furnish the most promising working hypotheses for 

 intensive investigation. In the innumerable cases of the destruction of forest 

 by cutting, grazing, fire, or cultivation, and the estabUshment of a subclimax, 

 the feeling often amounts to positive conviction, which needs only experi- 

 mental proof to be final. Indeed, many ecologists would doubtless regard the 

 latter as altogether superfluous in most cases. In fact, one may well admit 

 that all the evidence in our possession confirms the frequent change of forest 

 to scrub or grassland where artificial agencies are at work. There is grave 

 doubt when we come to consider the effect of natural causes in producing such 

 changes. At present there is no incontestable proof of the conversion of 

 forests by natural causes, except of course where effective changes in climate 

 or physiography intervene. Graebner (1901 : 69, 97) has summarized the 

 results of his own studies, as well as those of other investigators, and has fur- 

 nished strong if not convincing evidence that forest may be replaced by heath 

 or moor. It is significant, however, that in the various processes described 

 by him, with one possible exception, the cutting of trees or an increase of 

 surface water is required to initiate the cha.nges which destroy the trees, and 

 permit the entrance of Calluna or Sphagnum. In short, conversion is typically 

 the consequence of destruction and subsequent progressive development, 

 often obscured by the fragmentary natvu-e of the areas concerned. 



Graebner's studies: Conversion of forest to heath. — Graebner's description 

 of the process is so detailed and so convincing that a full account of it is given 

 here. 



"Let us picture to ourselves the conversion to heath of a particular forest, 

 such as may have obtained on the Liineberg Heath with the disappearance of 

 the great forests. The calcareous pernutrient soil bears beech wood. The 

 latter is completely removed as a consequence of the great demand for wood. 

 While the ground remains bare and the forest slowly renews itself, the leaching- 

 out of the nutrients in the soil proceeds more intensively, since the water 

 formerly caught by leaves and mosses, and then evaporated, now soaks into 

 the soil. Finally the forest again becomes closed, and then mature, and is 

 again cut down. This may recur several times, during which the leaching-out 

 of the upper layers in particular progresses steadily. With the decrease of 

 nutrients in the upper layers, the growth of the herbs is made more and more 

 difficult, until finally these die out, since their roots are unable to reach into 

 the deeper unleached layers of soil. As a consequence, all herbs which demand 

 relatively large amounts of nutrients are excluded. The competition of plants 

 with low requirements and slow growth disappears, and leaves the field to 

 heath plants. 



"At first the heath plants colonize but sparsely beneath the trees. In such 

 a forest, one sees a few heath plants here and there, especially Calluna, which 

 have however a suppressed look because of the deep shade still found m most 

 places. The growth of tree seedUngs in the poor sandy soil becomes greatly 



