566 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



have, moreover, repeatedly seen how many plants become established 

 in an area only when suitable habitats have been prepared for them 

 by other species ; thus plant distributions, and consequently the 

 local vegetation, are often complicatedly dependent on the succes- 

 sional stage reached, or at least on the establishment of particular 

 plant communities. For conditions which preclude one species or 

 type will commonly favour some other. 



Plants vary greatly in the extent to which they can adjust them- 

 selves to varying environmental conditions, and the same is true 

 of the communities which they make up. Thus a patch of Fireweed 

 will bloom gloriously in the early years of a forest clearing but there- 

 after wane and soon disappear as the conditions are changed by 

 colonizing shrubs and saplings ; on the other hand, these woody 

 plants may enter at the same time as the Fireweed and persist right 

 through to the climax stage. All the above and numerous other 

 considerations lie behind the present-day distributions of plants and 

 consequently of vegetational types, as should be abundantly clear 

 from the present work. 



Manipulation of Vegetation 



Modern Man is the biotic superdominant of the world, and the 

 changes which he effects in vegetation are among the most striking 

 on earth. Such changes are apt to be of a more or less destructive 

 nature, examples being heavy lumbering and grazing, burning and 

 clearing, and the cultivation of lowly crops in place of lofty forests. 

 Yet many are constructive so far as Man's needs are concerned — 

 hence the whole vast industry of agricultural and related practices. 

 Some are even constructive from the points of view of both Nature 

 and mankind, an example being the stabilization of dunelands by 

 ' binding ' vegetation — particularly by planting such coarse sand- 

 binding grasses as Lyme-grass and Marram Grass. These are often 

 followed by leguminous plants and Heaths, and, in time, by bushes 

 and trees. Or afforestation may be effected by direct planting of 

 young trees in already vegetated areas. Thus stabilization can be 

 effectively carried out by Man's intervention, and frequently leads 

 to luxuriant growth and substantial productivity where there was 

 previously a mere barren waste. Man also in a sense manipulates 

 the vegetation over wide areas by perpetuating such features as fire 

 and grazing ' subclimaxes '. 



The above and some other aspects of what may be looked on 



