Long‐term vegetation stability and the concept of potential natural vegetation in the Neotropics

When vegetation trends over time are analysed from an appropriate long‐term perspective using palaeoecological records, the concept of potential natural vegetation (PNV) is unsupported because of continual vegetation changes driven by natural or anthropic forcings. However, some palaeoecological records show long‐lasting (i.e. millennial) vegetation stability at multidecadal to centennial time scales in the absence of natural and human drivers of change, which fits within the definition of PNV. A more detailed palaeoecological analysis of these records shows that they are an exception rather than a rule, and that they cannot be differentiated from other transient ecological states. Therefore, long records of vegetation stability cannot be considered to be valid evidence for PNV. From a palaeoecological perspective, the PNV concept is concluded to be unnecessary, even in cases of multidecadal to centennial vegetation stability in the absence of environmental disturbance.     

Potential natural vegetation and pre‐anthropic pollen records on the Azores Islands in a Macaronesian context

This paper discusses the concept of potential natural vegetation (PNV) in the light of the pollen records available to date for the Macaronesian biogeographical region, with emphasis on the Azores Islands. The classical debate on the convenience or not of the PNV concept has been recently revived in the Canary Islands, where pollen records of pre‐anthropic vegetation seemed to strongly disagree with the existing PNV reconstructions. Contrastingly, more recent PNV model outputs from the Azores Islands show outstanding parallelisms with pre‐anthropic pollen records, at least in qualitative terms. We suggest the development of more detailed quantitative studies to compare these methodologies as an opportunity for improving the performance of both. PNV modelling may benefit by incorporating empirical data on past vegetation useful for calibration and validation purposes, whereas palynology may improve past reconstructions by minimizing interpretative biases linked to differential pollen production, dispersal and preservation.     

Towards a more transparent use of the potential natural vegetation concept – an answer to Chiarucci et al.

In this paper, the concerns of Chiarucci et al. ( 2010 ) regarding use of the potential natural vegetation (PNV) concept are addressed, as voiced in the forum section of the Journal of Vegetation Science. First, we rectify some unfounded expectations concerning PNV, including a relationship with prehuman vegetation and phytosociology. Second, we point out issues that pose considerable challenges in PNV and require common agreement. Here, we address the issue of time and disturbance. We propose to use the static PNV concept as a baseline, a null model for landscape assessment and in comparisons. Instead of changing the PNV concept itself, we introduce a new term, potential future natural vegetation (PFV) to cover estimations of potential successional outcomes. Finally, we offer a new view of PNV with which we intend to make the use of PNV estimates more transparent. We formalize the PNV theory into a partial cause‐effect model of vegetation that clearly states which effects on vegetation are factored out during its estimation. Further, we also propose to assess PNV in a probabilistic setting, rather than providing a single estimate for one location. This multiple PNV would reflect our uncertainty about the vegetation entity that could persist at the locality concerned. Such uncertainty arises from the overlap of environmental preferences of different mature vegetation types. Thus reformulated, we argue that the PNV concept has much to offer as a null model, especially in landscape ecology and in site comparisons in space and time.     

Natural,potential and actual vegetation in North America

The potential natural vegetation (PNV) concept has parallel applications in Europe and North America. Paleoecological studies in parts of North America provide records of vegetation patterns and dynamics under little or no human disturbance. Something resembling PNV emerges at millennial temporal scales and at regional to subcontinental spatial scales. However, at finer spatial and temporal scales, actual vegetation often displays properties of inertia, contingency and hysteresis, most frequently because of climatic variability across multiple timescales and the episodic nature of disturbance and establishment. Thus, in the absence of human disturbance, the actual vegetation that develops at a site may not resemble a particular PNV ideal, but could instead represent one of any number of potential outcomes constrained by historically contingent processes. PNV may best be viewed as an artificial construct, with utility in some settings. Its utility may diminish and even be detrimental in a rapidly changing environment.     

Precise vegetation restoration in arid regions based on potential natural vegetation and potential normalized difference vegetation index

Precise vegetation restoration is critical in drylands, as some inappropriate restoration attempts have even increased water scarcity and degradation in afforestation areas. Potential natural vegetation (PNV) is widely used to provide a reference for the appropriate location and vegetation type of restoration programs while the appropriate restored areas remain unknown. Therefore, we proposed a PNV–potential normalized difference vegetation index (PNDVI) coupling framework based on multiple machine learning (ML) algorithms for precise dryland vegetation restoration. Taking the lower Tarim River Basin (LTRB) with a total area of 1,182 km² as a case study, its present suitable restoration locations, area, and appropriate planting species were quantitatively estimated. The results showed that the model developed by incorporating PNDVI into PNV with easily measurable and available data such as temperature and soil properties can accurately identify dryland restoration patterns. In LTRB, the potentially suitable habitats of trees and grass are closer to the riverbank, while shrubby habitats are further away from the course, covering 1.88, 2.96, and 25.12 km², respectively. There is still enormous land potential for further expansion of the current trees and grass in the LTRB, with 2.56 and 1.54% of existing land supposed to be trees and grass, respectively. This study's novel aspect is combining PNV and PNDVI to quantify and estimate precise restoration patterns through multiple ML algorithms. The model developed here can be used to evaluate the suitable reforestation locations, area, and vegetation types in drylands and to provide a basis for precise vegetation restoration.     

Application of the Ancient Forest Concept to Potential Natural Vegetation Mapping in Flanders, A Strongly Altered Landscape in Northern Belgium

Construction of potential natural vegetation (PNV) poses particular challenges in landscapes heavily altered by human activity and must be based on transparent, repeatable methods. We integrated the concept of ancient forest (AF) and ancient forest species (AFS) into a four-step procedure of PNV mapping: 1) classification of forest vegetation relevés; 2) selection of those vegetation types that can serve as PNV units, based on AF and AFS; 3) merging of selected vegetation types into five PNV units that can be predicted from a digital morphogenetic soil map; 4) mapping of three additional PNV units based on additional environmental data. The second step, concerning the selection of reference forest vegetation, is of particular interest for PNV construction in Flanders (northern Belgium), where forest cover has been subject to temporal disruption and spatial fragmentation. Among the variety of extant forest recovery states, we chose as PNV units those vegetation types for which a high proportion of relevés had been located in AF and that contained many AFS. As the frequency of AFS depends on site conditions, we only compared and selected vegetation types that are found on similar sites according to average Ellenberg indicator values. While succession is irrelevant for the definition of PNV, colonization rates of AFS can be used to estimate the time required for PNV to be restored in a site.     

The power of potential natural vegetation (and of spatial‐temporal scale): a response to Carrión & Fernández (2009)

A commentary by Carrión & Fernández (Journal of Biogeography, 2009, 36 , 2202–2203) compared Holocene pollen records with models of potential natural vegetation (PNV) proposed in the phytosociological literature and concluded that the predicted PNV resulted from anthropogenic disturbance. However, the authors misinterpreted PNV, leading to two serious flaws in their assumptions: (1) PNV is not defined as a pre‐anthropic or climax plant community; and (2) PNV is not a concept restricted to the phytosociological method. Therefore we criticize the conclusions expressed in the commentary, and we stress the need for an interdisciplinary approach based on multi‐temporal and multi‐spatial scales to achieve a modern framework for the study of plant communities.     

Potential natural vegetation: validity and applicability in landscape planning and nature conservation

Abstract. Since the introduction of ‘potential natural vegetation’ (PNV) as a concept in vegetation science by Tüxen (1956), many PNV-maps with different scales have been made. Tüxen emphasized the great value of PNV-maps for different purposes in land use, landscape planning and nature conservation, in particular with regard to forestry, agriculture and landscape management. Different aspects are discussed in order to examine the validity and applicability of PNV-maps in landscape planning and nature conservation. PNV-maps are useful for the differentiation of natural and landscape units on a small scale (< 1 : 100 000). However, maps of the potential natural vegetation are less useful for purposes of detailed planning on larger scales (> 1 : 100 000). Problems arise, for example, from the often highly hypothetical character of the construction and the practice of taking remnants of ‘natural’ vegetation as a reference object for the PNV. With regard to the goals of modern landscape planning and nature conservation purposes (e.g. conserving biodiversity in the cultural landscape of Central Europe) the exact documentation of the actual real vegetation (ARV) on intermediate and large scales gives much more detailed information than a hypothetical PNV.     

Potential replacement vegetation: an approach to vegetation mapping of cultural landscapes

Abstract. The concept of mapping potential replacement vegetation (PRV) is proposed as a parallel to potential natural vegetation (PNV). Potential replacement vegetation (PRV) is an abstract and hypothetical vegetation which is in balance with climatic and soil factors currently affecting a given habitat, with environmental factors influencing the habitat from outside such as air pollution, and with an abstract anthropogenic influence (management) of given type, frequency and intensity. For every habitat, there is a series of possible PRV-types corresponding to the different anthropogenic influences, e.g. grazing, mowing, trampling or growing cereals. The PRV-concept is especially useful in large-scale mapping (scales > 1 : 25 000) of small areas where replacement vegetation is the focus of attention for managers and land-use planners, for example in nature reserves where the aim is conservation of replacement vegetation managed in a traditional way, or in restoration ecology where the concept may be used for defining restoration goals and evaluating the success of restoration efforts. At smaller scales, PRV-mapping may be useful for revealing the biogeographical patterns of larger areas which may be different from the corresponding PNV patterns, because replacement vegetation and natural vegetation may respond to environmental gradients at different scales. An example of medium-scale PRV-mapping through the coincidence of diagnostic species of vegetation types, based on species distribution grid data, is presented. In cultural landscapes, the advantage of using the PRV-concept instead of PNV is its direct relationship to the replacement vegetation. In the habitat mapping with respect to the replacement vegetation, the PRV concept yields more valuable results than the mapping of actual vegetation, as the latter is strongly affected by spatially variable anthropogenic influences which may be largely independent from climatic and soil factors.     

Traditional land uses enhanced plant biodiversity in a Mediterranean agro-silvo-pastoral system

Mediterranean agro-silvo-pastoral systems play a key role in view of the positive contribution that they could offer to a sustainable development of European agriculture. The knowledge of the vegetation dynamics and of the processes and land uses favoring different vegetation types related to the same actual potential natural vegetation (PNV) could represent a sound reference framework for monitoring and managing plant biodiversity in these systems. The aim of the research was to evaluate plant diversity along a gradient of use intensity comparing the actual vegetation versus the PNV. The results of our research showed that in the studied Mediterranean agro-silvo-pastoral system, included in the same environmental unit, human activities enhanced plant biodiversity. Moreover, the case study presented here confirmed the effectiveness of those landscape approaches comparing actual vegetation versus the PNV for plant biodiversity monitoring and reinforced previous studies showing the effect of human activities on plant community diversity at the environmental unit scale in different biogeographical contexts.     

On the theoretical concept of the potential natural vegetation and proposals for an up-to-date modification

In the extent to which it is used, the concept of the potential natural vegetation (PNV) is one of the most successful novelties in vegetation science over the last decades. However, previous applications of the concept have shown that the theoretical principles were used inconsistently or interpreted in an incorrect sense. The present problems in application (which become evident when visualizing historical aspects of the concept) mainly result from (a) inconsistent treatment of the construction criteria; (b) failure to distinguish between the “potential natural vegetation”, the “reconstructed natural vegetation” and the vegetation developing during succession, (c) the lack of a precise definition for reference terms to construct potential natural vegetation (e.g. treating reversible vs. irreversible changes of vegetation). For a sensible application of the concept it is suggested (a) to construct the potential natural vegetation on the basis of natural site conditions as well as permanently effective site changes as a consequence of human impact, (b) to consider the PNV to be in balance with all site conditions taken as basis for its construction. In practice, however, the construction basis may also derive from a particular question underlying the making of a PNV-map. A suggestion for a re-definition of the term “potential natural vegetation” as well as a key for PNV-mapping (valid for landscapes of Northern Germany) are given.     

Model for the potential natural vegetation mapping of Friuli Venezia-Giulia (NE Italy) and its application for a biogeographic classification of the region

ABSTRACT

The aim of this paper is to present a joint vegetation data base and GIS application to produce a model to map the potential natural vegetation (PNV) of the Friuli-Venezia Giulia region (NE Italy) and to show how the map can be used to draw a biogeographic classification of the region. All the natural arboreal coenoses growing below the timber line, as well as the dwarf shrubs and prairies developing above this limit, were considered. Some cross sections, extracted from the potential vegetation map, were tested against transects of real vegetation distribution.     

The concept of potential natural vegetation: an epitaph?

We discuss the usefulness of the concept of Potential Natural Vegetation (PNV), which describes the expected state of mature vegetation in the absence of human intervention. We argue that it is impossible to model PNV because of (i) the methodological problems associated to its definition and (ii) the issues related to the ecosystems dynamics.We conclude that the approach to characterizing PNV is unrealistic and provides scenarios with limited predictive power. In places with a long‐term human history, interpretations of PNV need to be very cautious, and explicit acknowledgement made of the limitations inherent in available data.     

Improving Transboundary Maps of Potential Natural Vegetation Using Statistical Modeling Based on Environmental Predictors

The potential natural vegetation (PNV) is a tool for landscape planning, nature preservation and the assessment of naturalness. It is mostly constructed by the application of expert knowledge. This paper shows the advantages of using a more sophisticated and formalized PNV construction that overlays vegetation types and site factor maps by applying a Bayes model and herewith improving existing PNV maps solely based on expert knowledge. The investigation was conducted in the forest complex of the Bavarian Forest National Park (Germany) and the adjacent ?umava National Park (Czech Republic). The project reached two major results: (1) The existing heterogeneous country-specific databases of natural site conditions and of vegetation types could be adapted to each other to construct a solid scientific basis to deduce a PNV map. The habitat requirements of the occurring harmonized vegetation types can now be quantitatively described in a formalized way. (2) The combination of terrestrial PNV mapping and numerical modeling allows the synthesis of the views of the different experts that generated the maps used for model calibration. However, the modeled map loses the details of the expert-based map that cannot be derived from the underlying site maps. A common modeled PNV map of both national parks covering an area of about 92,000 ha was created. While the former expert-based PNV maps display breaks along the country border, the modeled PNV presents a harmonized view based on the common database of both national parks.     

Gap analysis comparing protected areas with potential natural vegetation in Tuscany (Italy) and a GIS procedure to bridge the gaps

The aim was to compare the protected area (PA) network in Tuscany, Italy, with the areas referenced to different types of potential natural vegetation (PNV), to determine whether they are adequately represented for plant and habitat conservation purposes. For PNV, an existing but slightly updated and modified Italian Vegetation Series (VS) map was used. Each VS type corresponds to vegetation complexes that live under homogeneous environmental conditions and can each be considered an ecological land unit at the working scale employed here. Using GIS techniques, the geographic layers of PAs and VS were processed with spatial intersection to extract and quantify the VS contained within the boundaries of PAs. As a minimum conservation goal, we used the widely accepted 10% target threshold. It was found that, even though Tuscan PAs covered almost 20% of the total land surface, 94% of VS types resulted to be included in PAs with a percentage of at least 10% of their total area. The survey shows that the VS with the higher degree of inclusion in PAs are distributed in the Mediterranean Tuscany (coast and Tuscan Archipelago) and in some inner areas such as Apuan region, northern Apennines, Amiata Mt. and Farma-Merse Valley. Two VS types must be considered under-protected (i.e. contained in an existing PA network with percentages < 10%). We propose a simple GIS procedure based on certain priority assumptions: (a) existing PAs should be enlarged rather than new ones created and (b) their naturalness taken into account. This procedure produces a suitability map useful for identifying the best areas in which a local administration might look for solutions to bridge the gaps.     

Bioclimatic classification of US vegetation along a coast-to-coast macrotransect crossing central United States. I: Mediterranean vegetation

This report presents the first of two parts of a bioclimatic classification of the vegetation of the United States. Using a geographical information system, 987 weather stations were located along a longitudinal macrotransect from the shores of the Atlantic to Pacific on four maps: Map of the Physiographic Divisions of the Conterminous US, US Potential Natural Vegetation Map, US Ecoregion Map, and Terrestrial Ecosystems-Isobioclimates Map of the Conterminous United States. Based on these maps, bibliographic resources and field data, we deduced the potential natural vegetation (PNV) of each weather station; then, we assigned the different PNV types to alliance or association levels using the US National Vegetation Classification (USNVC). In a next step, USNVC groups were related with similar level phytosociological syntaxa described in the study area. The bioclimatic distribution of the USNVC units defined was then interpreted using the bioclimatic classification proposed in successive approximations by S. Rivas-Martínez. The distribution of USNVC alliances was mainly linked to the macrobioclimates (Mediterranean, Temperate, and Tropical) of the longitudinal gradient examined, though some edaphic factors induced the appearance of specialized plant groups. Herein, we present our data for the Mediterranean macrobioclimate, in which 53 alliances and 28 isobioclimates were identified.     

Unified classification of European natural forests: the approach of the vegetation map of Europe

With the aim of evaluating the accessible data on vegetation structure and composition for the Vegetation Map of Europe as far as possible, a reasonable classification of natural vegetation has been proposed which facilitates the identification of vegetation units used in different Schools of vegetation science. The proposed universal classification makes full use of several different principles. The highest units are based on physiognomic-ecological features and correspond to formation units of different rank. The European natural forest vegetation (including shrubs) has been divided into 10 formation units. Each formation is further divided into subunits according to the most important features. Ecological and functional interpretation has priority in the hierarchic structure of the system which was used in the legend to the Vegetation Map of Europe. The resulting system shows the most important features of latitudinal (vegetation zones and subzones), longitudinal (oceanic to continental analogues) and altitudinal (vegetation belts) regularities, further azonal vegetation types and their differentiation as well as the edaphic, geographic and florogenetic varieties of the natural plant cover. This arrangement constitutes a framework in which the vegetation units of different Schools can be classed.     

Quantitative comparison of the diversity of landscapes with actual vs. potential natural vegetation

Abstract. In the past 20 years, several metrics have been developed to quantify various aspects of landscape structure and diversity in space and time, and most have been tested on grid‐based thematic maps. Once landscape patterns have been quantified, their effects on ecological functions can be explained if the expected pattern in the absence of specific processes is known. This type of expected pattern has been termed a neutral landscape model. In the landscape‐ecological literature, researchers traditionally adopt random and fractal computer‐generated neutral landscape models to verify the expected relationship between a given ecological process and landscape spatial heterogeneity. Conversely, little attention has been devoted to distribution patterns of potential natural vegetation (PNV) as an ecological baseline for the evaluation of pattern‐process interactions at the landscape scale. As an application for demonstration, we propose a neutral model based on PNV as a possible reference for a quantitative comparison with actual vegetation (AC V) distribution. Within this context, we introduce an evenness‐like index termed ‘actual‐to‐potential entropy ratio’ (H_A/P = H_ACV/H_PNV, where H is Shannon's entropy). Results show that, despite the hypothetical character of most PNV maps, the use of PNV distribution as a baseline for a quantitative comparison with ACV distribution may represent a first step towards a general model for the evaluation of the effects of disturbance on vegetation patterns and diversity.     

Recent fire regime characteristics and potential natural vegetation relationships in Spain

Abstract. In heavily altered landscapes, where vegetation is not natural and where people are the main source of ignitions, relationships between fire occurrence and climate conditions may be unclear. The objective of this study was to evaluate to what extent territories with similar Potential Natural Vegetation (PNV) in peninsular Spain differ in their forest fire characteristics. From 1974 to 1994, more than 174 000 fires occurred. We used (1) the Spanish data base of forest fires, (2) a PNV map and (3) a land use map. Separate fire characteristics, based either on the number of fires occurred or the area burned, were obtained for each of the ca. 5000 grid‐cells (10 km × 10 km) into which peninsular Spain is divided in the UTM projection. Also, meteorological conditions at the time of fire ignition, cause of ignition and present forest cover were referred to the same grid‐cells as external factors potentially determinant of fire occurrence. The relationships between fire regime characteristics and PNV units were explored with Principal Components Analysis (PCA). The role of the three sets of external factors in the fire characteristics was evaluated with Redundancy Analysis (RDA). Groups of similar PNV types were clearly segregated, suggesting a gradient of fire characteristics. Higher fire incidence (higher frequencies and spatial incidence of fires, but lower proportions of grid‐cells affected by large fires) was associated with Atlantic, warm territories with deciduous forests as PNV. Intermediate fire frequency and rotation period, but with a higher relative incidence of medium and large fires occurred in Mediterranean PNV units, dominated by sclerophyllous oak forests. Low fire frequency and long rotation periods, with strong seasonal and yearly variability occurred for PNV units in the cold uplands (Fagus, Pinus, Abies, Juniperus) or in the semi‐arid, shrubby PNV units. The cause of ignition best explained the patterns of forest fire characteristics, followed by weather conditions. Our results indicate that, even in human influenced regions, climate and soil conditions exert control on the resulting forest fire characteristics, as indicated by the high segregation of the PNV types. However, the role of man was crucial in shifting the patterns of fire incidence. This was so that highest fire incidence occurred in regions that, otherwise, would be expected to have a much lower one, thus posing a serious threat for such areas. PNV maps, by providing a phytogeographical framework for characterizing forest fires, could be valuable tools for applying research results to forest fire management policies, taking properly into account the underlying determinant factors.