Plant Species and Plant Communities: Proceedings of the International Symposium Held at Nijmegen, No

biological attributes of the three species according to the sequence in which they invade successively as a conse- Studies on sample plots in Halimione portulacoides com- munities show that environmental disturbances, either quence of environmental disturbance. It can therefore be concluded that for understanding this mechanism in the natural or induced by man, start a sequence of partly salt-marsh ecosystem a thorough study on the functioning overlapping density maxima in Suaeda maritima, Aster of these and other relevant biological attributes in the local tripolium and Puccinellia maritima successively, before the populations is essential. original Halimione community totally recovers. When Returning to the basic questions put in the introduction succession time before recovering is long enough, there are it is concluded that: tendencies in redundancy of this sequence stressing the unilinear character of the succession. Minor environmental a) There exist orderly and predictable succession patterns in the investigated salt-marsh vegetation. impacts induce a longer time-lag period of the Suaeda b) Following occasional disturbances, whatever these may density maximum, suggesting threshold values of these impacts for the species to maintain minimal population be, the vegetation recovers via a sequence of overlapping interim species populations showing a unilinear or obliga- densities or to become locally extinct. This sequence of tory (Hom 1976) succession pattern.