Primary successions

Abstract

Successions have traditionally been termed either primary or secondary, though it was noted earlier (Section 2.1.1) that the distinction was arbitrary, as certain successions could fit either category. The former occur on new, previously unvegetated bare areas, the latter where a previous vegetation cover has been disturbed or disrupted in some way, as by ploughing, fire or changing grazing pressures. It was also noted earlier (Sections 1.3.2 and 2.5) that Clements [2] greatly overemphasized the importance of site modification by plants as a driving force of succession (termed autogenic succession, as opposed to successions caused mainly by changes in extrinsic factors, like climate, or the rate of silt input into lakes, termed allogenic succession [224]). For example, Egler [50] stressed that vegetation changes during secondary successions on abandoned fields could largely be accounted for by differential growth, reproduction and survival of the species present initially, or colonizing shortly after abandonment (see Section 1.3.2). There seem to be no data that refute this [225]. There is very little evidence about primary successions from direct observations in time, in contrast with secondary successions. Nevertheless, it will be seen in this chapter that in general allogenic factors are probably more important in causing primary successions also, than are autogenic factors.