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Succession on fallow fields

Expansion and settlement

Succession on fallow fields

Biological attributes

Expansion and settlement

Biological attributes

Fig. 4.1.43. Successions on fallowed agricultural fields: influencing factors and interrelations. (Schmidt 1993)

Secondary succession on fallow pasture is difficult because of the existing dense grassland community Schreiber (1995) shows, from observations over 20 years, that establishment of woody plants occurs in several ways. Occasionally, well-known basic processes in the succession of different types of plants from annuals to woody species occur. However, pastures, initially relatively homogeneous, often show formation of dominance patterns with few species that cover the area well after just a few years. With good nutrient supply herbaceous plants dominate because biomass is not removed (auteutrophica-tion), but with poor nutrient supply grasses dominate. The temporal sequence of individual stages differs considerably. On some areas, 1015 m high pre-forest vegetation occurs; on other potential forest sites neither trees nor shrubs grow. The difference in appearance and establishment of woody plants does not fit any succession model so far described; a prediction on changes of life forms and species is not yet possible, even if occasionally such phases in vegetation dynamics may be clearly seen.

Secondary successions not only develop on fallow or abandoned land, but also in plant communities affected by disturbance such as from fire or avalanches. Here processes directed towards a re-establishment of the original status occur and fit into a cyclic regeneration scheme.

Examples originate from managed forests where man is the driving force of such cycles. Pignatti and Pignatti (1984) analysed such regeneration cycles for Mediterranean forests (Fig. 4.1.44) and showed two variants - one for regeneration after clearing, and one, more regressive, succession after several fires. After clearing various weeds appear which are excluded by oaks. Repeated fires lead to a permanent stage, with regeneration occurring - if at all - only after a long period.

A special case of secondary succession is the polycormophyte succession. Vegetative side shoots of plants form shoot colonies, able to expand in closely covered herbaceous plant communities faster than via seeds. This type of succession usually starts with a pioneer woody species with defence against herbivores, e.g. Prunus spinosa. Such plants are able to survive over large areas for decades. Finally, higher growing unprotected shrubs and trees establish in the centres of such areas, so that forest islands and, ultimately, closed forest areas may form (Hard 1975). This development may be interpreted as an autogeneous type of succession, regulated by the vegetation itself. Study of this form of succession allows strategies of competition in woody species to be recognised (Fig. 4.1.45).

Arable weeds Acidophilic species Forest weeds

Arable weeds Acidophilic species Forest weeds

Larqe abundance of therophytes

— Nitrophilic weeds —Acidophilic species from Mediterranean shrub communities (macchie) — Nitrophilic forest species

Larqe abundance of therophytes

— Nitrophilic weeds —Acidophilic species from Mediterranean shrub communities (macchie) — Nitrophilic forest species

Degraded permanent communities

Fig, 4.1.44. Succession in a Mediterranean oak forest. A Cycles following clear felling. B Regression after multiple fires. (After Pignatti and Pignatti 1984, from Dierschke 1994)

Degraded permanent communities

Fig, 4.1.44. Succession in a Mediterranean oak forest. A Cycles following clear felling. B Regression after multiple fires. (After Pignatti and Pignatti 1984, from Dierschke 1994)

The concept of successions as a directed, more or less deterministic process, and therefore also predictable, must be corrected according to empirical results obtained in recent decades.

These changes over time are very complex, there are many variables influencing development, e.g. site, biological reproduction, space and time. Particularly important is the type, intensity and duration of disturbance. All these factors make it difficult to regulate and predict successions.

Cyclic Vegetation Dynamics Cyclic processes in the development of vegetation are also important, in addition to the directed - progressive or regressive - dynamic processes. Glavac (1996) interprets the final phase of secondary succession as a sort of self-preservation cycle. In a cyclic succession, different ages of plants enter and replace each other in the same vegetational community. Structural changes with the same or different species at one site are important. Remmert (1985) in particular - stimulated by ideas on cyclic successions from Aubreville (1949) - built on this interpretation and confirmed it theoretically, as well as practically. Further concepts about cyclic vegetation dynamics originated from Watt (1947) who introduced the term "gaps" into the discussion, i.e. sites where these processes take place. In each case an important observation was that the climax stage (mature stage) does not extend over enormous areas in natural forests, but that various phases of development are spread like a mosaic over the whole area. Displaced phases occur side by side, in small areas at all stages. For central European forest areas three phases are distinguished:

• an optimal phase corresponding to a forest of uniform age with few species; a closed stand with little undergrowth;

• ageing and decaying phases when the tree layer is disrupted over large areas and species requiring light and nutrient are able to invade;

• a rejuvenating and juvenile phase, with young plants of the same or different tree species, with the previous phases occurring again. Often light-demanding trees invade first, followed by shade-tolerant trees.

As explained in Chapter 3.2.1 these phases may be regarded as a continuous process of self-thinning.

Figure 4.1.46 shows schematically the cyclic dynamics for a tropical rain forest. The size of the pieces of the mosaic differs, depending on the diversity of species. In nemoral deciduous forests an average size of 1-2 ha is assumed, in

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