ABSTRACT
It can be said that in extreme biotopes a permanent situation of stress prevails, but we can see that this is debatable. Even in environments in which plant growth is exuberant, stresses are found that are not due to the biotope, but linked to the biocoenosis: e.g., stress due to high density, stress faced by plants of the undergrowth when the overhanging canopy captures all the available light, or stress caused by pathogens or predators that find favourable conditions. There are more or less permanent stresses or, at least, frequent stresses. There are also really exceptional situations that occur in environments that are "usually" quite favourable, for example "the flood of the century", fire caused by a storm, the "drought of the century", or even a very late spring thaw. These are the most typical stress situations in that they last a short time, occur suddenly, and are unpredictable. In the first section of this chapter, stress is defined and its major characteristics are described.
