12] VEGETATIONAL TYPES OF TEMPERATE LANDS 375 



and various if rather ephemeral communities, examples of which 

 are all too familiar to every farmer, gardener, and estate-owner. 

 Moreover, owing to almost universal introduction bv Man and to 

 the similar ' openness ' of the habitats mvolved, these colonies of 

 weeds tend to be remarkably alike in similar situations throughout 

 the temperate and allied regions, often involving the selfsame 

 species in both Old and New Worlds, and in both Northern and 

 Southern Hemispheres. But in spite of the common luxuriance of 

 such weed communities, the full abandonment of agricultural or 

 other waste areas usually allows succession to proceed so rapidly 

 towards the local climax that within a very few years the weeds are 

 liable to have disappeared entirely. These areas being usuallv in 

 tracts that had been cleared of former forests, the weeds are com- 

 monlv superseded in the first few years by scrub or such weedv 

 trees as Birches or Poplars, before the return of anything like the 

 climax forest. 



Some Physiographic Effects 



Many physiographic effects, such as differences in exposure and 

 water-conditions due to ridges and depressions, have already been 

 dealt with, and the vegetation of uplands above the limit of arbor- 

 escent growth is treated in the next chapter. Outstanding, however, 

 are ditTerences due to aspect and particularly altitude below the 

 tree-line, which can lead to marked differences in conditions and 

 local vegetation. This we have already mentioned and illustrated in 

 Figs. 83, A, and 83, B, but must consider further here. Such differ- 

 ences are largely due to differences in local climate as explained in 

 Chapter X. The communities involved usually lie within the orbit 

 of those described elsewhere and so need not be treated in any 

 detail, though a few examples of the effects of (i) aspect and (2) 

 altitude in temperate and allied regions may be given with advantage. 



The most widespread and commonly obvious aspect effect is that 

 due to orientation with regard to the sun's rays. For example, in 

 the ]\Iediterranean region some ridges lying east and west may bear 

 almost entirely different vegetation on their north- and south-facing 

 slopes, in extreme instances having not a single ecologicallyimportant 

 species in common. Thus the south-facing slopes, exposed to the 

 full glare of the midday sun, tend to be occupied by the highly 

 xerophilous maquis or garigue of more or less sparse shrubs and 

 herbs. The ower parts of any steep north-facing slope, however, 



