To understand the processes taking place in the ecosystem the following literature is recommended:
• Brady and Weil: Elements of the nature and properties of soil, 2nd edn. Prentice-Hall, Englewood Cliffs, New Jersey, 2004
• Schlesinger: Biogeochemistry. Academic Press, London, 1997
• Schulze: Flux control in biological systems. Academic Press, San Diego, 1994
• Schulze: Carbon and nitrogen cycling in European forest ecosystems. Ecol Studies vol 142. Springer, Berlin Heidelberg New York, 2000.
In previous chapters the focus was on the responses of cellular processes (Chap. 1: Stress physiology) and the use of resources by individual plants (Chap. 2: Whole plant ecology). In the following, these responses will be related to conditions where plants:
• do not grow on their own but form stands, i.e. they compete with individual plants of the same species (perhaps even of the same rhizome) or with other species,
• are subject to use by other organisms (herbiv-ory, pathogens),
Experimental stand of beech trees "B1" in Soiling. The experimental site was important in the International Biological Program (IBP). It was the first time that all the fluxes in a terrestrial forest ecosystem were investigated. Methods developed in the years 1965-1975 at this site served as a model for the global work of the IBP (Ellenberg 1971; Ellenberg et al. 1986). Photo E.-D. Schulze
• rely on resources and conditions which arise from the activity of other species. This concerns particularly feedback supply of nutrients by microorganisms from dead organic matter into forms which can be used by plants.
The botanical analysis of these diverse interactions deals with the temporal and spatial dynamics of vegetation (history of vegetation, distribution of plant species, competition and succession). This will be discussed in Chapter 4: Sy-necology. However, there are also processes dealing with the turnover and the distribution of resources at certain sites. This "ecosystem aspect" will be discussed in this chapter.
By moving from the level of the individual plant to the level of material turnover at a site, the following conditions are changed:
• the reference system, and
• the availability of resources.
Reference system: Turnover no longer depends on the concentration (g g_1 or mol g_1) of material in certain organs or individual plants, but on the use of the habitat where the ground surface area is the only constant reference (g m"2ground or mol m~2ground) based on which air and ground surface area may be used by different organisms. The density of individuals and the biodiversity are just as important and variable parameters in considering turnover per ground surface area as is turnover in different partial systems which relate to the ground area and are interconnected. These partial systems range from the living plant cover, via litter, to soil organisms. They may be individually or as a whole connected laterally with neighbouring systems.
Availability of resources: New system characteristics become important with the focus on the ground area of the stand. The plant does not exist on its own and independently in the space, but is incorporated into a complicated structure where the resources are also turned over by neighbours and made available, within limits, by soil organisms. The limitations of space also limit the use of external resources (light, water, C02).
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