Mosaic cycle concept

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The mosaic cycle concept is discussed in the areas of ecology and nature conservation . In addition to the technical-theoretical discussion of the development (succession) of ecosystems, it also serves to develop and implement nature conservation and use strategies (especially forest science ). The concept was u. a. Developed on the basis of studies of forest ecosystems ( primeval forest relics ).

The mosaic cycle concept is based on a recurring sequence ( cycles ) of different development stages ( succession stages ) of ecosystems . Within an ecosystem, all different stages of succession are to be found simultaneously on partial areas. Local and regional disruptions asynchronously reset the succession process on individual sub-areas, so that a heterogeneous mosaic of development stages results. The mosaic cycle theory is a special case of a dynamic consideration of ecosystems, which - contrary to what is often stated - is not identical to the “patch dynamics” or “gap dynamics” concept, insofar as these concepts do not know a climax stage, but they do Mosaic cycle theory.

The authors Hermann Remmert and Wolfgang Scherzinger described at the beginning of the 1990s that (forest) ecosystems rejuvenate cyclically and the succession within an ecosystem can take place out of phase and asynchronously. The causes for this can be both endogenous ( lying within the system ) and exogenous (lying outside the system). A nationwide uniform climax stage is never achieved according to this concept, contrary to older opinions (mono- and polyclimate theory). Rather, an ecosystem is developing that consists of a dynamic mosaic of different plant communities and ages.

Cycles and mosaics of succession

Like all natural systems, ecosystems are subject to natural dynamics. A forest can z. B. only obtained if individual trees die, but the resulting gaps are taken up again by new trees. This regeneration dynamics of a system has complex effects that may affect different spatial and temporal scales. The regeneration gaps in the forest in ideal locations for the forest in the ideal climate may not be larger than a single tree (if it dies and collapses from natural causes "while standing"), they can be several tree lengths (if a falling tree is accompanied by other trees tears). Factors such as fires, storms, insect calamities, etc. Stocks can also die off in large areas and form larger gaps.

A gap in the system can be filled in a timely manner by an individual of the same species. Often a faster growing "pioneer species" comes into play, which can only be displaced by the original species after a long time. It is also possible for several such pioneer types to follow one another. When observing such regeneration processes in natural ecosystems, researchers observed that such regeneration processes can take place in different systems according to certain patterns. So observed z. B. Sprugel and Bormann in the balsam spruce forests of North America, elongated strips of dead trees in the jungle, which belong to a regeneration cycle of approx. 80 years, which runs through the forest in a wave-like manner. Sousa summarizes various studies on North American forest ecosystems, which consist of patches of different sizes, each of which can be traced back to a forest fire. Stocks that were not created by fire do not exist in these forests.

The first scientist to describe such rejuvenation cycle patterns is believed to have been Andre Aubreville in 1938. If one observes the development of such a forest at a certain point over a longer period of time, one perceives a directed change, a succession . The regular sequence of such states is called a succession series. If one observes long enough, however, the last stage of the succession series changes back into the first, thereby closing it into a circle (from the Greek word: a cycle). The size of the surfaces on which this cycle is traversed synchronously is the size of the taper gap in the system. If the adjacent tapering gaps are not synchronized with one another, the cycles in each gap are staggered. When viewed spatially at any point in time, one sees a mosaic of quasi-homogeneous partial areas, each of which is in a certain state of the regeneration cycle.

This cyclical development of a forest unaffected by humans takes place in different forest types (for example subalpine , boreal etc.), i.e. depending on the structure of the systems, on sub-areas of different sizes. Seen from above, these different sub-areas of different stages of succession form a mosaic . The size of the "mosaic areas" is primarily influenced by the factors that control regeneration dynamics (large areas: e.g. forest fire, small areas: e.g. individual age death of the trees). The length of the cycle naturally depends on the lifespan of the species involved. However, as in the case of successive generations of pioneer species, it can under certain circumstances be considerably longer than the lifespan of the longest-lived species. In the case of forest fires, it may depend on factors such as productivity (replenishment of easily combustible dead wood) and is then independent of the lifespan of the species. It can also depend entirely on exogenous factors, e.g. B. on the average frequency of extreme storms (hurricanes, hurricanes). Usually a mixture of endogenous and exogenous factors seems to prevail.

The regeneration dynamics of primeval forests is naturally much more difficult to research in Central Europe with its millennia-old cultural landscapes than z. B. North America. Accordingly, the regeneration dynamics of the Central European primeval forests is highly controversial within research. Since primeval forests are only preserved on a small scale, mostly in a mountainous location in south-eastern Europe, and since humans determine the regeneration dynamics in commercial forests (including those that are managed in a natural way), the natural dynamics for almost all Central European forests can only be developed with difficulty by analogy. Of particular value here are the results of jungle research (e.g.)

Area size after damage (according to Scherzinger 1991)

Area sizes of the sections according to ecosystems (according to Jedicke 1994):

Phases (stages) of the mosaic cycle concept

Information based on the Remmert concept. Times and "dimensions" are only to be given approximately.

The mosaic cycle theory in forest management

In the case of commercial forests (forests), the ecosystem is disturbed by humans. Nevertheless, managed forests with a near-natural species composition take on substitute functions for an imagined, undisturbed ecosystem. The phases can be found in commercial forests, but they differ more or less significantly in the occurrence of dead wood and in the species composition. U. be viewed as substitute companies. The frequency and spatial extent of the stadiums is very different, depending on the management and interpretation of the theoretical doctrines.

The ruderal phase is avoided as far as possible in the commercial forest. Grazing acts as competition for forest plants and promotes damaging mouse populations. It usually delays the rapid development of the new stock and causes costs. In age-class forests, attempts are made to go through this phase as quickly as possible or to avoid it entirely by means of appropriate regeneration processes (clearing cut, Femel cut ). The ruderal phase is reached here especially after clear cuts . The ruderal flora and fauna roughly correspond to those of the clear cut. However, deforestation is increasingly banned or at least limited in space. In permanent forests ( e.g. plenter forest ), the ruderal phase has been completely switched off. It is strongly represented in low and middle forests , which today, however, only exist in relics. These are "put on the stick" every 20 to 30 years, which means that the ruderal phase is run through again in relatively short periods. The regeneration problem can be avoided in these forests, since the regeneration takes place by stick rash , which is more competitive than core growth in this phase . Many low and medium-sized forests are under nature protection, as the corresponding clear-cut flora is very species-rich and clear-cuts are becoming increasingly rare in today's economic forms as described above.

The pioneer forest phase corresponds to commercial forests closest to the cultures at the age class forest, in particular by natural regeneration (with respect to species composition) or in cultures to clear cutting (with respect to the influence of light and related occurrence of pioneer tree species). This age group phase is considered to be very susceptible to diseases, game browsing and small climatic influences, and attempts are made to keep it as short as possible. In the plenter forest this phase is switched off again. In low and middle forests, birch, willow and poplar have the opportunity to settle in the period from the first floor rash to the top of the crown . In the past, these pioneer tree species were often viewed as competition and thus as weeds and therefore removed during cultivation work. Today's view is different. The pioneer tree species help to bring about the crown closure more quickly and are to a certain extent accepted as nurturing tree species.

Thickening is also called the corresponding development stage in the high forest, which begins after the crown has closed and ends with the beginning of natural branch cleaning. In the coppice and middle forest, this phase takes up a large proportion of the rotation time. In the plenter forest it is not developed in the form described due to the strong mix of trees of different ages and diameters.

The final forest and optimal phase correspond to the forest development stages of poles and trees. The latter phase is no longer achieved in coppice forests, as the economic goal has already been achieved and the stocks have been put back on the hive; here the cycle is terminated prematurely and restarted. First yields can be achieved in the final forest phase, but the economic goal in today's silviculture is to reach the optimal phase with strong growth and high economic profit. The death of individual trees due to competitive pressure and the resulting dead wood is, however, greatly reduced in commercial forests, depending on the intensity of management. The selection of the trees to be removed is determined by the forester, and the fallen limbs (trunks) are removed from the stock and sold.

The plenter phase got its name from the plenter operation . However, the gaps in the commercial forest are not created by falling dead wood, but by targeted timber harvest. In the plenter forest, this phase is manifested in the long term through management. Other phases do not appear here in clear form. In the age group forest , the phase is reached with the beginning of the end use as soon as this is carried out, for example, as a clearing or Femelhau . The light falling into the holes prepares the soil for natural regeneration (heating accelerates humus degradation) and enables the first seedlings to germinate.

The decay phase and the collapse are passed through in the age class forest in the end use. However, the corresponding dead wood is missing and the process takes place in a very short time. The economic goal is to use the wood that is still standing and to introduce the new generation by illuminating or clearing the areas. This also corresponds to the end of the cycle.

In summary, it can be said that the structures of the mosaic cycle concept can also be found in commercial forests. However, the cycles are run through in significantly shorter periods of time. These can be less than 100 years on spruce farms and reach times of around 250 years on slow-growing oak farms. Some forms of management such as coppice forest or plenter forest systematically exclude individual phases (see above). The more and more sought- after principle of permanent forest management with the avoidance of bare areas also helps to displace the ruderal and pioneer forest phase. Another general difference in commercial forests is the lack of large amounts of deadwood, which is a defining factor in some phases. Many companies try to compensate for this shortcoming with deadwood programs. However, such measures are usually linked to a loss of income and are therefore often only implemented half-heartedly.

Consequences for ecology and nature conservation practice

The application of the mosaic cycle concept in nature conservation and forest management has led to the fact that the very great importance of temporal dynamics for the understanding of ecosystems is recognized much more comprehensively today. The earlier observation of the biotopes, based on the cultural landscape with its low dynamics, led to quite static concepts e.g. B. the forests, which were characterized by forest societies as plant-sociological units and related concepts such as climax vegetation or potential natural vegetation . Due to the patch dynamics and the jungle research, this is to be understood as a shortened view of the entire system. In nature conservation, the approach of process protection developed, which specifically includes the natural dynamics in the consideration.

Jedicke directs u. a. from this theory the demand for large protected areas to be secured in the long term in order to guarantee process protection of the ecosystem (forest fire, windthrow, etc.). In the event of a large windthrow in a forest that is under nature protection, the status of a nature reserve can be revoked because the protection purpose “preservation of the forest” no longer applies. Ultimately, the protective purpose would have to be extended to the processes of succession. The practical implementation and review of the concept will in future be made possible by the creation of wilderness development areas .

The practical applicability of the mosaic cycle concept in the narrower sense to the Central European natural forests, however, is largely viewed critically in the professional world today. The results of the jungle research in particular indicate that the regeneration cells (i.e. the mosaic areas) in the Central European deciduous forests are very small and rarely exceed the size of a single tree (approx. 2,000 m²). Furthermore, the climax forest is usually directly rejuvenated without going through ruderal or pioneer forest phases. Experimental findings on forest regeneration also point in this direction. If the size of the “mosaic” is no larger than a tree and the duration of the “cycle” corresponds to its lifespan, the concept becomes trivial. Vegetation experts such as Heinz Ellenberg (see lit.) therefore consider its application to be unnecessary.

Observations of sparrowhawks in primeval beech-oak forests in Iran south of the Caspian Sea, which in many respects correspond to the Central European forests in terms of species composition, are interesting. He observed small rejuvenation gaps in the midst of extensive "indoor forests" with powerful overhangs . At first, other types than the unusual appear in the individual gaps.

Link between the mosaic cycle and the influence of large grazing animals

With the megaherbivore hypothesis, there is another theory that emphasizes dynamic processes in natural forests and thus challenges the prevailing scientific view that single-stem regeneration is crucial for Central European natural forests. This theory is also highly controversial in its applicability to Central Europe. Research in semi-open pasture landscapes, particularly in the Netherlands, has revealed interesting correlations between the two hypotheses. A cycle driven by (especially cattle) grazing is assumed, in which young trees can only grow under the protection of thorny bushes and herbs. The growing trees shade their thorny "nurse bushes" as they grow up. This means that no young trees can grow under fully grown trees. If the trees die at some point, what remains is an open pasture in which thorny species (as willow weeds) arise. The cycle is now closed and can begin again. The result would be a brighter forest with clearings interspersed like a mosaic. The theory is based on observations in large Dutch protected areas with free-range grazing animals, but is still speculative and not generally accepted.

See also

literature

  • Eugene Odum: Principles of Ecology. Spectrum, Heidelberg 1991, ISBN 3-89330-712-5 .
  • Eckhard Jedicke: Biotope Network - Basics and Measures of a New Nature Conservation Strategy. 2nd Edition. Ulmer, Stuttgart 1994, ISBN 3-8001-3324-5 .
  • Heinz Ellenberg : Vegetation of Central Europe with the Alps from an ecological, dynamic and historical perspective. 5th, heavily changed and improved edition. Ulmer, Stuttgart 1996, ISBN 3-8001-2696-6 .

Individual evidence

  1. ↑ "The most obvious role that disturbance plays in ecosystems is in the deflection of a community from some otherwise predictable successional path. ... [W] e find that disturbance and environmental fluctuation prevent this path from being followed for any effective length of time." Pickett, STA & White, PS 1985: Patch dynamics: a synthesis. In: Pickett, STA & White PS (Eds.): The ecology of natural disturbance and patch dynamics. Academic Press, Orlando: 371-384, here 373; see. Pickett, STA 1980: Non-equilibrium coexistence of plants. Bulletin of the Torrey Botanical Club 107 (2): 238-248.
  2. ↑ "The final stage of natural vegetation, the climax, turns out to be a mosaic of different plant communities, each subject to its own cycle." (Remmert, H. 1985: What happens in the climax stage? Natural sciences 72: 505-512, here : 509)
  3. ↑ Hermann Remmert: The mosaic cycle concept and its significance for nature conservation - an overview. In: Laufener seminar papers. 5/1991, pp. 5-15.
  4. ↑ Wolfgang Scherzinger: The mosaic cycle concept from the point of view of zoological species protection. In: Laufener seminar papers. 5/1991, pp. 30-42.
  5. ↑ Wolfgang Scherzinger: Nature conservation in the forest - quality goals of dynamic forest development. Ulmer Verlag, Stuttgart 1996, ISBN 3-8001-3356-3 .
  6. ↑ Douglas G. Sprugel, FH Bormann: Natural Disturbance and the steady state in high-altitude balsam fir forests. In: Science. 211, no. 4480. (1981), pp. 390-393. (online) ( Memento from January 16, 2014 in the Internet Archive ) (PDF; 526 kB)
  7. ^ Wayne P. Sousa: The role of disturbance in natural communities. In: Annual review of Ecology and Systematics. 15 (1984), pp. 353-391 (online) ( Memento of the original from January 16, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 4.2 MB) @1@ 2Template: Webachiv / IABot / nature.berkeley.edu
  8. ^ A. Aubréville: La foret coloniale. In: Annales de l'Académie des Sciences Coloniales. tome IX, 1938, pp. 1-245.
  9. ↑ Stefan Korpel: The primeval forests of the Western Carpathians. Gustav Fischer Verlag, 1995, ISBN 3-437-30702-9 .
  10. ↑ Eckhard Jedicke: Space-time dynamics in ecosystems and landscapes. Knowledge of landscape ecology and formulation of a process protection definition. In: Nature conservation and landscape planning. 30 (1998), pp. 229-236.
  11. ↑ Wolfgang Schmidt: Dynamics of Central European Beech Forests. Critical comments on the mosaic cycle concept. In: Nature conservation and landscape planning. 30 (1998), pp. 242-249.
  12. ^ Georg Sperber: Mixed beech-oak forests and the megaherbivores. Forest travel impressions from Iran. In: Large animals as landscapers. LWF report 27 (2000). Published by the Bavarian State Institute for Forests and Forestry. (online) ( Memento from January 16, 2014 in the Internet Archive ) (PDF; 836 kB)
  13. ↑ FWM Vera: Grazing ecology and forest history. CABI publishers, New York 2000.
  14. ↑ H. Olff, FWM Vera, J. Bokdam, ES Bakker, JM Gleichman, K.de Maeyer, R. Smit (1999): Shifting Mosaics in Grazed Woodlands Driven by the Alternation of Plant Facilitation and Competition. In: Plant Biology. 1 1999, pp. 127-137.
  15. ↑ J. Bokdam: Nature conservation and grazing management. Free-ranging cattle as a driving force for cyclic vegetation succession. PhD thesis. Wageningen University, Wageningen, the Netherlands 2003. (online)