Basis of General Vegetation Dynamics

Vegetational units are permanently subjected to changes in space and time. Each recording of vegetation is only a momentary event in a complex dynamic process. The temporal dynamics in the structure of species in space, and the corresponding changing patterns, are closely linked to the different demands of species, to their life cycle, mechanisms for reproduction and response to competitive pressures, and to the available resources, in the community of species. To understand the dynamics of vegetation, a knowledge of stress situations and responses to stress is important.

Vegetation dynamics aims to record and to explain similarities of vegetation patterns and processes with time and then to derive characteristics or typical responses and models. The causes of processes taking place may be found in disturbances of very different kinds and they are particularly important.

Discussions of vegetation dynamics are made difficult by a confusing multitude of terms, which is caused by insufficient consideration of spatial and temporal scales. Vegetation dynamics on a few square centimetres of bare rock must be described differently to that of square kilometres of large forests. For boreal coniferous forests a different terminology should probably

| Fig. 4.1.29. In the humid as well as in seasonally moist tropical regions, types of sustainable agricultural land use have been developed on all continents. A "Continuous" rice cultivation in south-east Asia, often combined with cultivation of different palm trees (Sulawesi, Indonesia). B In almost all tropical regions, small home gardens are multi-storied with different layers of cultivated plants, which 'copies' the species-rich tropical forests, e.g. trees such as oil palm (Elaeis guineensis), cashew nut (Anacardium occidentals) and banana (Musa paradisiaca) and annual vegetables below (Guinea-Bissau). C Sophisticated irrigation schemes on widespread terrace complexes have been in use for many centuries for cultivation of annual crops such as sorghum (Sorghum bicolor) and perennial shrubs such as coffee (Coffea arabica) and qat (Catha edulis) in the Yemen Arab Republic. (Photos K. Müller-Hohenstein)

Changes with intergiaciai climatic changes

Species migration

Primary succession; changes with soil development Evolution of populations and taxa; extinctions Secondary succession Changes with short-term climatic cycles Changes with annual climatic fluctuations Ontogenetic changes; regeneration cycles

Seasonal phenological changes -»I_I_I_I_I_I_I_I_I_I

0.1 1 10 100 1000 104 105 106 107 108

Time scale (years)

Fig. 4.1.30. Time scales for the dynamic changes in vegetation. (After Miles 1987)

be used to that for tropical rain forests. Temporal differences are probably even more important, reaching from the course of a day to centuries (Fig. 4.1.30). Ecology of vegetation must deal with extreme complexity, in the temporal sequence and the close spatial arrangement of species, populations and vegetation units.

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