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Fig. 4.1.45. Polycormophyte succession of woody species in grassland: / with establishment of low-growing woody species; 2 followed by tall-growing woody species; 3 with woody, tall trees. (After Hard 1975, from Dierschke 1994)

boreal forests the pieces of the mosaic are several square kilometres because of fires; in contrast, in tropical rain forests they are hardly ever larger than 100 m2 (treefall gaps). The duration of cycles also differs. For the well-analysed forests of the boreal zone several centuries are assumed, in the North American sequoia forests cycles last several thousands of years. Processes of vegetation dynamics similar to mosaic cycles have also been described for North Atlantic heath and bog complexes. For tropical communities (rain forests, savannahs, mangroves, coral reefs) this type of change occurs over time. This does not mean that the cycles must always be uniform. Certain phases may be left out.

Disturbance is again seen as the driving force for cyclic vegetation dynamics. The life span of "key organisms" is important, and competition following the death of key organisms for light and nutrients. Mechanical influences (wind breakage, fire) play an important role. Changed conditions in the site result from disturbance. Species which are able to obtain the necessary resources from the site after the disturbances can establish and their regeneration strategy enables them to start a new cycle.

Constant conditions are not to be expected in an pristine forest, it is a mosaic of asynchronous phases of cyclic vegetation dynamics. During the individual phases, structural characteristics of forests as well as the diversity change. This means that large sections of a forest contain all phases and possess normal population distribution of the dominant species. As a consequence, all phases in such vegetation dynamics

Fig. 4.1.45. Polycormophyte succession of woody species in grassland: / with establishment of low-growing woody species; 2 followed by tall-growing woody species; 3 with woody, tall trees. (After Hard 1975, from Dierschke 1994)

Fig. 4.1.46. Mosaic cycle in a tropical rainforest. From Richter (1997). Open tree crown Pioneer species; thick tree crown canopy species; medium tree crown shade-tolerant species

Phases:

1. Destruction 2. Regrowth 3. Competition 4. Death of pioneer species 5. Homeostasis

Phases:

1. Destruction 2. Regrowth 3. Competition 4. Death of pioneer species 5. Homeostasis

Fig. 4.1.46. Mosaic cycle in a tropical rainforest. From Richter (1997). Open tree crown Pioneer species; thick tree crown canopy species; medium tree crown shade-tolerant species

Carousel dynamics (almost) same species composition

Fig. 4.1.47. Developmental phases of different types of plants during the changes in vegetation. (After Richter 1997)

Carousel dynamics (almost) same species composition

Fig. 4.1.47. Developmental phases of different types of plants during the changes in vegetation. (After Richter 1997)

must be regarded as a (dynamic) ecosystem with all biotic partners and the also changing abiotic conditions, as occurs in sigma sociology (Schwabe 1991). The term climax is also regarded in this mosaic concept as the sum of different stages of the affected system over a large area (Jax 1994).

Today, further types of cyclic vegetation dynamics are distinguished, based on questions of scale and preferential occurrence in different biomes. Formation of mosaics over large areas in Hawaii as a consequence of "death of cohorts" is described by Muller-Dombois (1995). Demographically unfavourable situations caused and maintained by fire and storm damage are the driving force. In this context, the endogenous mass-dying of bamboo-like grasses in all tropical regions should also be included; so far this has not been convincingly explained.

In "cohort dynamics", as in the mosaic cycle, the species composition may change. However, in the "carousel dynamics", the pattern formation in grasslands, occurring over small areas, is not included according to Van der Maarel and Sykes (1993). The basic idea of this change over time is that species are able to recycle in the short term and on smallest areas. A "regeneration gap" is sufficient free space for seeds and establishment of germinating plants. These spaces become available through endogenous death of individual plants, and are conquered in a sort of "guerrilla strategy" or even according to the lottery principle of species in the community in which all have occupied the same regeneration niche. As these species are short lived, the carousel model describes vegetation dynamics at the smallest space and of the shortest duration. Populations of a species are always available, but are very mobile. The species composition remains constant. Figure 4.1.47 compares different spatial and temporal scales of cohort, gap and carousel dynamics schematically.

Aspects of Applied Succession Research The dynamic nature of plant communities has often been recognised via bio-indicators. It is assumed that the response of a plant community to disturbance is specific, and perhaps quantifiable. The vitality of species of the plant community and shifts in the species spectrum are measured. Knowledge about the formation of plant communities and their regulation is gained by empirical findings of many succession studies. For instance pioneer plants with uniform seeds are selected for greening of open spaces, or to stabilise open slopes after road building (Fig. 4.1.48 A, B). It was believed, in many cases, that vegetation dynamics could be predicted.

Today, we know about asynchronous vegetation cycles and have doubts about the term "ecological equilibrium" and therefore realise the limits to practical application of current knowl

| Fig. 4.1.48. Pioneer plant species are able to start the succession on areas which require protection, such as those subject to erosion. Pennisetum setaceum (A) is a tussock grass with extensive roots which is suitable for stabilisation of steep slopes (mountains in Yemen Arab Republic). B As part of a project to protect against severe degradation of vegetation and soils in the Central Atlas Mountains (Morocco), a mixture of suitable plant species, such as dwarf and brushwood scrubs, dwarf palms and Opuntia cactus, was used to achieve the initial stages in regeneration of the area. (Photos K. Müller-Hohenstein)

| Fig. 4.1.48. Pioneer plant species are able to start the succession on areas which require protection, such as those subject to erosion. Pennisetum setaceum (A) is a tussock grass with extensive roots which is suitable for stabilisation of steep slopes (mountains in Yemen Arab Republic). B As part of a project to protect against severe degradation of vegetation and soils in the Central Atlas Mountains (Morocco), a mixture of suitable plant species, such as dwarf and brushwood scrubs, dwarf palms and Opuntia cactus, was used to achieve the initial stages in regeneration of the area. (Photos K. Müller-Hohenstein)

edge of vegetation dynamics. Measures for maintenance also need to be rethought, as well as minimum spatial dimensions of protected areas or the concepts of a biotope (Baierlein 1991). Without question, current knowledge of vegetation dynamics helps in the selection of replacement areas, where neighbouring areas have been lost because of disturbance and new communities are to develop (Beierkuhnlein 1998).

Biomonitoring was developed in order to understand syndynamic processes, to indicate damaging influences so that they could be quickly counteracted. Examples are air pollution, as well as eutrophication of water. Vegetation changes resulting from interventions at certain sites have also been recorded with biomonitoring. Knowledge of vegetation dynamics is important for cultivation and maintenance of plant communities, formed during the development of our landscape, which might get lost without traditional forms of management. Grasslands without any fertilisation on not very fertile soils and grasslands on very shallow dry soils are examples of this.

Human influences may also degrade the vegetation and this can also be monitored. Multiple scales on hemeroby of spaces for vegetation have been devised (see Chap. 4.1.4.6). Red List species, loss of species, proportion of therophytes and neophytes are important values for structuring vegetation units according to the degree of human intervention. Ultimately, knowledge about vegetation dynamics is applied in attempts to improve areas to be used for agriculture or forestry and to control such attempts. Thus, for example, application of Ca-containing fertilisers in order to combat acidification of soils in forests and the effects of herbicide application in vineyards were assessed in this way.

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