Potential natural vegetation
As a potential natural vegetation ( PNV ) is called the final state of vegetation , one would expect without human intervention in the respective area. The term is mainly used in connection with vegetation analysis and reconstruction. In the PNV concept, unlike in the current potential natural vegetation (hpnV) concept, irreversible / permanent human changes in site conditions that occurred in the past are not taken into account.
The concept of potential natural vegetation was developed by the plant sociologist Reinhold Tüxen as an alternative to the controversial climax theory . However, an exact determination of the respective potential natural vegetation is considered problematic today, above all because methodologically this can often be challenged and ecosystems are increasingly viewed not as static, but as dynamic systems .
Reinhold Tüxen the potential natural vegetation is a concept for analyzing syndynamischer relationships between plant communities ( successional series and replacement companies ). In today's potential natural vegetation as the subject of vegetation mapping from 1956, he defines it as the hypothetical state of vegetation that would prevail or arise for a certain area under today's environmental conditions if humans no longer intervene.
The potential natural vegetation is a time-bound construction. It may have to be updated if climatic changes affect the vegetation. In the definition, Tüxen expressly excluded the influence of climate change . The term was later further refined by Westhoff and van der Maarel, and defined as that vegetation that would ultimately develop after the end of any human activity, if the end stage were reached immediately.
The potential natural vegetation differs from the vegetation of the primeval landscape as it existed before human intervention and also from the reconstructed natural vegetation that would have arisen under the current climate if humans had never intervened. Nor is it necessarily a question of the vegetation that would appear at the end of the succession series as a permanent final community ( climax vegetation ), because the succession processes can change the location. For example, the potential natural vegetation of a eutrophic shallow lake is a community of aquatic plants , not a reed bed or an alder forest, as would eventually arise in the course of the (natural) silting up of the lake after a long time.
The key word for the concept of potential natural vegetation is the location potential , i.e. the specific characteristics of the location factors such as B. Soil moisture, nutrient content, base content of the soil as well as summer and winter temperatures, frost and drought periods, length of the vegetation period as climatic factors. Since the potential natural vegetation is intended to map the site potential, changes in the site caused by humans are included. This is particularly evident in the case of landfills or excavations where the soil no longer corresponds to the original conditions. According to the more recent view, pollution loads, urban heat islands and comparable factors are to be mapped in the potential natural vegetation, because otherwise this would no longer correspond to the actual location potential and thus would have no prognostic or planning value. In the same way, the potential natural vegetation of a diked flood plain, drained by drainage ditches, e.g. B. an oak-hornbeam forest, not - as in the natural landscape or with a hypothetical disappearance of mankind - an oak-elm-hardwood alluvial forest. In the same way, agriophytes and also neophytes that have penetrated the near-natural vegetation must be taken into account when constructing the potential natural vegetation. This can go so far that in Ireland bushes of the overgrown Pontic Azalea ( Rhododendron ponticum ) are now regarded as part of the potential natural vegetation.
Classification and mapping
The potential natural vegetation depends heavily on the climatic conditions of the respective region, but is not identical to the concept of vegetation zones . If one takes other ecological factors (soil, water balance, fauna) into consideration, one arrives at the concept of biomes . Opposite these biomes are the so-called anthromes . These are forms of vegetation that were created by humans, such as arable land or pasture land.
Maps of potential vegetation are typically at scales below 1: 25,000. More detailed maps can hardly be justified due to the hypothetical demarcation between individual vegetation units. For large parts of Central Europe, for example, the beech forest is assumed to be a potential natural vegetation . However, a detailed map of the potential natural vegetation can be created from local or regional vegetation maps for planning purposes.
From the potential natural vegetation, statements can be made about the favor of the location and the range of substitute societies that arise under different anthropogenic influences at a place of growth. In this respect, the concept of potential natural vegetation was first developed in the 1950s as a plant-sociological aid for agricultural and forestry decisions and was only taken up by nature conservation later (around the 1970s). Since the 1990s, the conservation distances in the Urlandschaftsforschung ( wilderness concept, process protection , Megaherbivorenhypothese ) again from the potential natural vegetation as a "natural Ideal".
When the concept of potential natural vegetation is applied in planning practice, there are regularly a number of misinterpretations that have brought its application into disrepute among experts. The most common mistake is that the potential natural vegetation is not constructed from the specific location factors of the treated area, as required in the procedure, but is simply taken or traced from large-scale overview maps (e.g. Germany or a federal state). Firstly, small-scale location variations are neglected and, secondly, small-scale, irreversible location changes by humans are ignored. For example, a potential natural vegetation in the form of a forest community of the natural landscape is usually indicated even for built-up areas in inner cities.
Criticism and extensions
The existing dynamics of ecosystems and the methodological problems in the construction of the potential natural vegetation have led in vegetation science to the fact that the original concept comes more and more into criticism. Modifications such as the potential site-appropriate vegetation (Leuschner) or the potential replacement vegetation (Chytry) are intended as variants of the original approach to help overcome these weaknesses. Numerous vegetation experts, however, argue in favor of abandoning the concept entirely, whereas other vegetation experts put this criticism into perspective as a result of the misinterpretations mentioned and recommend an orientation towards the original concept.
A fundamental problem in determining the potential natural vegetation is the unavoidable subjective aspect in determining typical or natural plant communities - what constitutes a natural plant community in an area can hardly be assessed objectively. In spite of improved analysis methods, it is already difficult to objectively define existing vegetation units. A precise definition of potential units is therefore only possible to a limited extent. Another problem in determining the potential natural vegetation is the determination of the sensible spatial extent of individual vegetation units. With the use of larger vegetation units one naturally achieves much higher numbers of species than with small areas. Species that occur side by side on a large scale can depend on different habitats and therefore be restricted to different areas of the area. The time course of succession processes is, however, the greater methodological problem. Little research has been done into how plant communities change over long periods of time. Only in the last few decades have data been recorded directly on this. Details of the original vegetation forms are scarce, especially for Europe. There is disagreement as to whether the forests of Europe would be a dense woodland with a closed canopy or a more savannah-like landscape without human influence.
General dynamics of ecosystems
Ultimately, ecosystems are only in the rarest of cases static structures and rather characterized by a dynamic that results from the interplay of different types and processes. In particular, the factors fire, soil development and the influence of herbivores ( herbivores ) are responsible for the dynamics of ecosystems. Predictions of potential natural vegetation that are based on original forms of vegetation can therefore often be challenged. During the last warm period ( Eem warm period ), the hornbeam, which played a marginal role in the current warm period, was one of the dominant tree species in Central Europe.
According to the mega-herbivore hypothesis developed by Frans Vera , large grazing animals (mega-herbivores) have a decisive influence on their habitats. The influence of large herbivores on vegetation today depends heavily on humans and makes it difficult to determine a potential natural vegetation. Some species such as aurochs and bison have been pushed back or even exterminated in the last few centuries, others such as fallow deer and mouflon were introduced over large areas. It must therefore be taken into account, for example, that the original forests, which today in Central Europe are mostly cited as a reference for potential natural vegetation, grew up under the influence of an animal world that differs from today's. In addition, trunk animals and giant deer disappeared from Europe . Other species such as bison, wild horse and aurochs were gradually pushed back. Their disappearance or their decline since the beginning of the Holocene and their regulation by humans in many regions of the world probably led to a down-regulation of this natural factor. According to the mega-herbivore hypothesis, the natural vegetation of large parts of Central Europe would not be a beech forest, but a varied forest and grassland mosaic with a distant resemblance to today's African savannas. The same is postulated for other vegetation zones, in particular temperate deciduous forests such as those found in North America.
Fire and floors
Large fires such as forest fires are kept under control by humans in many regions, but they are also caused by humans. The influence of earlier fire management on today's vegetation forms is difficult to determine. This makes it difficult to predict future developments. Long-term change in the soil is a factor that is difficult to incorporate into potential natural vegetation calculations. For example, the soils in the Mediterranean region are often barren and poor in nutrients due to centuries of overexploitation . It is difficult to estimate how the soil conditions would develop in the long term if there were woodland there for centuries.
Reconstructed natural vegetation as an alternative concept
Because the potential natural vegetation involves man-made changes, it is difficult to determine the potential vegetation of severely human-disturbed habitats. To avoid the problem, Neuhäusl introduced the concept of reconstructed natural vegetation (RNV). It is based on the extrapolation of units of the original vegetation to the habitat conditions. Reconstructed natural and potential natural vegetation are largely the same - with the exception of areas that have been heavily influenced by humans.
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- Map of the potential natural vegetation in Germany at Floraweb. Published by the Federal Agency for Nature Conservation .
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