466 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Secondary forest tends to be lower and to consist of trees of 

 smaller dimensions than does primary forest, the young growth being 

 often remarkably regular in including even-aged stands of one or a 

 few woody species, whereas later on the growth may be extremely 

 haphazard as already indicated. The early dominants are commonly 

 light-demanding and ' weedy ', as are the associated herbs. Such 

 forest may also be recognized by its floristic composition, which 

 usually differs markedly from that of the primary forest, even though 

 there are probably few if any species entirely restricted to the former. 

 Many of the components of the secondary forest are unusually wide- 

 spread, some often being introduced aliens, while its trees are mostly 

 quick-growing {e.g. 12 metres in three years), short-lived, and pos- 

 sessed of efficient means of seed-dispersal. Their leaves tend to be 

 of more various sizes and shapes than those of rain-forest trees. 

 The most shade-intolerant and quickly growing species are, as might 

 be expected, most characteristic of the early stages of the secondary 

 succession. 



The secondary forest at least when young is often dominated by 

 a single or small number of species, and usually has a much smaller 

 flora than the primary forest ; when very old, however, it may be 

 indistinguishable from virgin forest. On the other hand with long- 

 continued grazing, mowing, or recurrent burning, secondary savanna 

 or grassland is commonly formed, often characterized by species 

 of Lalang Grass (Imperata) ; alternatively, as in temperate grass- 

 lands, there mav be still further regression with overgrazing, or 

 invasion by Bracken [Pteridium). In general, however, secondary 

 successions appear to reproduce in their later stages the changes 

 characterizing natural regeneration of primary forest, in which gaps 

 formed bv the death of large old trees are first filled by the easily 

 dispersed and fast-growing dominants of the early stages of secondary 

 forest. The earlier stages in larger clearings commonly include 

 quick-growing Grasses and other weeds which are characteristic of 

 disturbed tropical areas, though they may be even less lasting than 

 their counterparts in cooler regions. 



While shrubs may form a stage in the successions occurring in 

 rain-forest areas, often thev are omitted, dominance passing directly 

 from herbaceous plants to trees. However, in drier regions shrubs 

 are apt to be important in the secondary successions, sometimes 

 remaining as more or less lasting dominants in what appear to have 

 been previously forested (though ' marginal ') areas. Indeed vegeta- 

 tional differences, such as frequently arise from differences in the 



