Long-Term Vegetation Dynamics. Beginnings of Succession Research

Medium- and long-term changes in plant cover over several decades and up to millennia are analysed in research on successions. Only some decades ago, the principles and models of these successions were derived from empirical data. However, from the beginning there was controversy in the scientific discussions between those with a holistic approach who consider the complete ecosystem and those with a reductionist approach who focus on individual functions and processes in succession. Both approaches had the same aim in trying to recognise the causes of vegetation dynamics by explaining the resulting effects and predicting them. There remains a belief that there are basic rules of dynamic sequences, but many processes are now seen as stochastic. This is demonstrated by the influence of extreme climatic events such as prolonged drought (Kreyling et al. 2011). The necessity of considering spatial and temporal scales in greater detail is generally accepted.

Beginnings of Succession Research.Systematic research on vegetation dynamics is connected with the names of the ecologists Clements and Gleason. Clements (1916) is regarded as the founder of the discipline of succes- sional research, starting from the idea that only undemanding species grow first on bare substrates. Once established, species change and influence the conditions and community of their site, which then facilitates invasion by others. Clear temporal periods are recognisable until the development of a uniform, final community, the so-called climax community, representing a relative equilibrium. Small fluctuations in community assemblages are possible, but a reversal of the clearly directed processes leading to this climax community is not.

Later Clements assumed three succession sequences. One of these sequences has its origin in a freshwater basin. During the processes of receding water and emerging land in temperate latitudes, deciduous forests develop as a climax via a boggy and pre-forest stage (hydroseries). Another sequence starts from a saline substrate (haloseries) and a third on bare rock (xeroseries), but all end in the same forest climax. Changes in the site always originate from plants themselves. Also, in larger areas with the same climatic conditions, the same final community will always emerge (monoclimax). This concept was well received, but there were critics who could not empirically confirm this theory. The reason for the discrepancy was that Clements concentrated on the dynamics of undisturbed vegetation (primary succession) and interferences of any kind, and developments arising from them (secondary succession), were disregarded.

Clements’ theory, which regarded plant communities as “organisms”, was rejected by Gleason (1926), who stressed that there was also an individualistic type of vegetation dynamics without clear steps and without formation of the different vegetation units. He started from the assumption that changes in species at a site depended on the species composition and the influx of diaspores to that site.

Today, the concepts of Clements and Gleason are no longer accepted in their pure form. In particular, the idea of a final monoclimax has been rejected in the scientific community, which now favours continuously dynamic or cyclical views. However, following this rejection, two important movements emerged and helped set the tone for successional research today:

- A relatively general interpretation of the broad spectrum of dynamic events in ecosystems
- A recognition of the enormous complexity of conditions and attempts to clarify individual aspects

Definitions of succession or vegetation dynamics are today broader and more general. These dynamics comprise changes in structure and species in vegetation units in space and time. Short-term, daily and seasonal changes are included. All forms may be described by the term patch dynamics (Pickett and White 1985). Another general definition underlines the notion that successions are medium- to long-term dynamic processes in a directed, temporal sequence of plant communities in an ecosystem. It is often stressed that these sequences are irre- versible—within limits—and predictable and may be triggered by endogenous or exogenous processes. During succession, self-organisation in the ecosystem (on the precondition that there is no disturbance) increases and a balance between primary production and mineralisation develops.

Odum (1980) lists a total of 24 different ecosystem characteristics that are able to change during a succession sequence. He regarded development as a well-ordered process resulting from changes caused by vegetation and ultimately leading to a stable ecosystem, where maximum functional interconnections are achieved. The hypothetical final state is understood as a complex community of organisms with complex links and interactions, in which once equilibrium is achieved, it can change again, for example, after a new disturbance.

However, successions are often also understood as stochastic processes, basically because the seed input in a biocoenosis at a site is random; the same applies to the germination and establishment rates or the availability of necessary safe sites and the occurrence of disturbance— their type, intensity and degree—which are generally not predictable.