322 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Drastic microhabitat development may take place at different levels 

 or other situations in a forest, or on different sides of a hillock or 

 even pebble. Thus the conditions under which an Alga or Moss 

 lives on the bole of a tree are substantially different from those of 

 an epiphyte in a crutch or on a branch high up in the crown, or of 

 course from those of a herb on the sheltered forest floor. And again, 

 the shelter from wind and sun given by even a minor projection from 

 the ground, may enable a delicate plant or small community to grow 

 there which could not exist in the exposed surrounding areas. The 

 effect may even extend to the soils and their biota, and affect the 

 plant habitat through them. Especially striking are the differences 

 of temperature (and consequently of important dependent factors) 

 on the north- and south-facing sides of tussocks at high latitudes in 

 summer, which may vary by more than 20° C. within a few centi- 

 metres, and allow active growth to take place in one spot when 

 adjacent areas are frozen solid. Such effects may be the key not only 

 to the micro-distributions of plants but also to their ranges over wide 

 areas. Consequently it is important that we recognize the concept 

 of microhabitat, for it is a very real and indeed fundamental one. 



Finally it should be recalled that not only different (phylogenetic) 

 strains and even differently treated individuals may respond dif- 

 ferently to microhabitat or other vagaries, but that different stages 

 in the (ontogenetic) development of an individual may have critically 

 different reactions, young seedlings being in general relatively feeble. 

 Similarly, the tender young parts of older plants often differ greatly, 

 in their resistance, from the remaining portions of the same plants — 

 hence the familiar ' killing back ' of shoots by frosts in temperate 

 regions, the rest of the plant being commonly unharmed. 



Main Successions 



When dealing in the last chapter with environmental factors we 

 referred briefly to the competitive and other ' internal ' ones en- 

 gendered by the plants themselves. Thus weeds compete for space 

 and nutrients, some of those introduced to inhabited regions creating 

 major nuisances by choking waterways, destroying the habitats of 

 wildlife, or colonizing and rendering practically useless whole areas 

 of agricultural land. The shade cast by dominant species, and the 

 shelter they give, affect all the plants within the community ; also 

 affected are the local atmospheric humidity and, often, soil structure 

 and development as well as composition. 



