Geomorphology, as a subfield of geosciences, deals with terrestrial landforms and related processes. Its aim is to reveal the complex interrelations between the origin of landforms and the dynamics of their alteration caused by erosion, mass movements, or tectonics. In this regard, the dating and reconstruction of past geomorphic processes, the determination of recurrence intervals, and their quantification is of special interest to reveal landform development and to predict future changes in light of global change. The most accurate dating technique in geosciences is dendrochronology, in which the outermost ring of a living tree is used for dating. Environmental changes and geomorphic processes cause growth changes in trees, which are datable to the year and even the season.
The response of trees to environmental change has been the subject of research in botany and plant physiology since the middle of the nineteenth century. Research on trees at this time focused on descriptive analysis of the structure and variability of tree rings in an ecological context and even the effects of external mechanical impacts were discussed (Schweingruber, 1996). This rather holistic approach was eclipsed when Andrew Ellicott Douglass invented the revolutionary crossdating technique for tree-ring research (Parker et al., 1984) (see Dendrochronology). For the first time, this method enabled the accurate dating of tree rings to the year and is regarded as the backbone of tree-ring research in all fields of its application (Schweingruber, 1996). In subsequent decades, crossdating formed the basis for the widespread use of tree rings, mainly in archeology (see Dendroarchaeology) and climate reconstructions (see Dendroclimatology) (Fritts, 1976). Nevertheless, the first attempts at reconstructing glacial fluctuations were made by Lawrence (1950). Since then new ecological aspects of tree-ring research have been integrated, especially in geosciences. Fundamental research studied the effect of environmental factors such as wind on the growth of trees (Jacobs, 1954). In the early stages, this research focused on growth reactions on a macroscopic level, although single attempts have been done on a microscopic level. In the 1960s, studies on erosion rates on slopes (LaMarche, 1968) and streams (LaMarche, 1966) were also conducted using annual growth rings in roots. In the following years, tree-ring analysis was established as a standard methodology in geosciences. Alestalo (1971) was the first to write a monograph on the use of tree rings in dating geomorphic processes and invented the term dendrogeomorphology (Alestalo, 1971, p. 7). Since then, in parallel to the development of computers, various types of dendroecological techniques have been developed in all subfields of the geosciences (Fig. 1). These techniques facilitated the dating of land surfaces, reconstruction of variable hydrological conditions in soils (e.g., water table changes), and the dating of volcanic eruptions or widespread effects of earthquakes. Additionally, in geomorphology, special focus was placed on reconstructing the frequencies of geomorphic processes such as rock fall, landslides, debris flows, or creeping slopes on permafrost. In this context, various species-specific reaction mechanisms in stems and even roots and branches have been linked to mechanical stresses or environmental changes. These reactions, used either independently or in combination with only small modifications over the last decades, are presented in this article, as well as new applications and trends.