Part IV

In the chapters of Part IV, another level of ecological hierarchy is discussed, namely plant communities and their biological diversity. To fully understand the many aspects of the ecology of single plants, of plant communities and of ecosystems and their processes it is necessary to complement investigations under strictly controlled conditions in a laboratory by field studies. It is clear that no plant lives on its own in a natural environment; it can only exist in complex communities composed of plants, animals, fungi and microbes and their numerous interactions, that is in a “biocoenosis”.

Along with the increasing complexity of processes to be considered at the community and ecosystem level comes a decreasing ability to make accurate predictions about plant community development in time and space. Thus, community ecology is mainly based on observations, comparisons and experiments, with an always changing spatio-temporal context. Many experiments in community ecology are carried out by nature itself, including all anthropogenic influences, which increases the complexity of the system considerably. The task is to recognise and to describe patterns of plant communities in time and space and to explain the underlying mechanisms and principles of plant community assembly. For this, quantitative data are needed but are sometimes difficult to obtain, and qualitative assessments gain importance.

In analysing ecosystems (Chap. 16), it already became obvious that two aspects are connected with the step from the laboratory into the field, namely the importance of space and time. Plant communities develop over the course of time and may display directed or cyclic dynamics. Their history of development must be known in order to understand their actual structure. Here, the influences of human activities are particularly important. The historical development of plants and of plant communities is discussed in Chap. 17. During their development over time, plants and plant communities conquer habitats and expand into new areas. However, the present-day global and regional differentiation of the plant cover can only be understood by taking into account the direct and indirect, intended or unintentional influences of humans.

The basis of the spatial distribution of plant communities is discussed in Chap. 18. During their development over time, plants and plant communities colonise new habitats and expand in space. Different vectors, such as wind, water and animals, are particularly responsible for the dispersal of propagules. The resulting final establishment and pattern of expansion depend furthermore on environmental conditions. A hierarchical order from species area types to plant kingdoms is the basis for plant geographical research. To illustrate, we focus on examples from Europe and Africa.

In Chap. 19, biotic interactions among plants and between plants, microorganisms and animals are considered. Plants and plant communities are strongly regulated by the environment, but they also influence their environment, resulting in feedbacks to the microclimatic conditions or soil conditions. Intra- and interspecific interactions, including competition or facilitation, structure plant communities and determine the coexistence of species. Finally, plants are embedded in a complex network of interactions with animals and microorganisms, some of which are essential for their reproductive success (pollination, dispersal), while others are primarily considered detrimental for plants (herbivory).

Chapter 20 introduces the concept of biodiversity and how plant diversity is controlled by environmental factors. It also describes the relationships between plant diversity and ecosystem functioning, i.e. raising the question about the role of plants and their diversity in regulating and maintaining ecosystem processes. Here, a trait-based approach to plant ecology is needed. This topic not only reveals the fundamental principles of community ecology but also has profound implications for ecosystem management and nature conservation.