456 PLANT SUCCESSIONS 



Man produces most extensive changes in vegetation by such 

 operations as draining lakes and swamps, building levees, irrigat- 

 ing deserts and semi-deserts, clearing woodlands, and planting 

 treeless lands with the seeds of forest trees. 



433. Order of succession in special cases. Much study has 

 recently been given to the exact order in which assemblages of 

 plants follow each other in various kinds of succession. Only 

 a very few cases can here be mentioned. 



On the island of Krakatoa, which was completely laid waste 

 by a volcanic eruption in 1883, the first forms of plant life to 

 appear were microscopic blue-green algae (Sees. 207211). Three 

 years after the eruption the flora had come to contain many 

 ferns, with here and there a few seed plants, on the mountains 

 or the coast. 



In the mountains of Colorado the granite bowlders dislodged 

 from the faces of cliffs are covered first with incrusting lichens ; 

 then the gravel produced by the weathering of the granite gives 

 a footing to leaf -like lichens ; later the more weathered gravel 

 supports a growth of herbaceous seed plants ; afterward follow 

 thickets, then pine forests, and finally spruce forests (Plate XII). 



In the pine woods 1 of central Maine when the trees have 

 been cut away and the clearing (as is too often the case) burned 

 over, the most conspicuous plants which immediately succeed 

 the forest are fireweed, 2 raspberries, blackberries, wild cherries, 3 

 and aspens. 4 A deciduous forest of poplars and canoe birches 5 

 succeeds the thickets above-mentioned. This in turn would 

 doubtless, under natural conditions, after a long period, be dis- 

 placed by a pine forest. 



In eastern Maine the succession is very similar, except that 

 blackberries are not common in the burned clearings and the 

 tree growth which follows the thicket stage is usually of gray 

 birch. 6 



1 Pinus Strjbus. 4 Populus tremuloides. 



2 EpiloUum angustifolium. 5 Betula papyrifera. 



3 Prunus pennsylvanica. 6 B. populifolia. 



