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Keywords:

  • Conifer stands;
  • Invasive plants;
  • Natural vegetation;
  • Nature conservation;
  • Restoration targets;
  • Succession

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References

This paper deals with the recent debate on the potential natural vegetation (PNV) concept. After reviewing its limitations with respect to understanding of the processes involved in secondary succession, spatial and temporal scaling and data processing, we still consider it a useful tool for summarizing knowledge about a territory in a way that can guide good practice in nature conservation, and for hypothesis generation. Mapping PNV has a descriptive aim and offers the possibility of depicting not only a ‘natural’ scenario according to the extant vegetation types and current environmental factors, but also an ecological description of the territory. It is not a commitment to build any ideal stage of nature but it can contribute to better management by providing targets for restoration and improving naturalness, ecosystem conservation and biodiversity preservation. Constant development of the concept, through discussion, improvement of methods and incorporation of new knowledge, is necessary, but we are reluctant to accept the idea of abandoning it because a part of what has been achieved to date in nature conservancy management using PNV could be lost, as arguments supporting naturalness would be weakened.

… through the graves the wind is blowing … (L. Cohen)

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References

This article is concerned with the recent debate about the validity and usefulness of the potential natural vegetation (PNV) concept, which, after starting as a street brawl, has evolved to a true scientific level (Chiarucci et al. 2010). The opinions and ideas put forward here will not repeat those presented by Loidi et al. (2010), Farris et al. (2010) and Mucina (2010), rather we will attempt to complement these publications and will address the main questions raised in the critique of the validity and usefulness of PNV. We are aware that this debate involves two levels that are all too frequently confused: one concerns the specific procedures adopted by practitioners of PNV mapping, and the other relates to the unpredictability of ecological succession and hence the validity of any prediction about its trajectories and outcomes. To use arguments from the second level against the first level and at the same time defend other procedures for identifying something like potential vegetation seems to lack scientific rigour. Therefore, we will set out to introduce new perspectives, with the aim not only of crediting or discrediting the approach but also of clarifying conflicts derived from the dogmatism under which it has been applied and interpreted, to evaluate its strengths and weaknesses and to compare it with alternative approaches. We strongly desire to enlighten the issue sine ira et studio instead of obscuring it, in order that the scientific community as a whole and nature conservancy services in particular can benefit from the values of this concept and the documents it produces.

General issues: on the validity of the construction

  1. Top of page
  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References

In the cases in which PNV has been applied and used in areas where Braun-Blanquetian phytosociology has a certain tradition, criticisms appear to be targeted more against the Braun-Blanquet approach than the PNV itself. The debate about the validity of European phytosociology is very old and we have no intention of entering into it here, its current status being established by the papers published in volume 42 of Folia Geobotanica (2007) and some others (Ewald 2003; Willner 2006). Returning to PNV, we agree with the concerns expressed by Härdtle (1995) and Zerbe (1998) about the difficulties of constructing PNV from remnants of supposedly natural or near-natural vegetation and extending their potential area to places where natural conditions are similar; it is also true that PNV has been built by means of chronosequences (space-for-time substitutions), which has the methodological flaws indicated by Chiarucci et al. (2010). Mapping PNV certainly cannot pretend to survey the processes and mechanisms of succession (Pickett 1989; Glenn-Lewin & van der Maarel 1992): its scale is coarser and its aim is descriptive; it provides a framework for the ecological description of a territory.

The idea that PNV is an exclusive product of Braun-Blanquetian phytosociology is false. Nevertheless, it is true that the description of floristically defined vegetation types, as in phytosociology, and the acceptance of succession as a phenomenon that involves different phases, leads to each community type being labelled with its position in the successional trend, and in former times it was assumed that such a trend was linear and deterministic. The result is that one of the recognized vegetation types is considered as the most ‘natural’ to be found in the area and attributed with being the living representation of the PNV. The more accurate the phytosociological surveys, the more precisely PNV units can be defined. So, naturally, in the phytosociological school PNV has been largely accepted and its cartography carried out.

An important point to consider is whether it is really feasible to identify a PNV. Research on succession has shown that processes in this field are much less deterministic than was previously thought, as stated in Chiarucci et al. (2010), but there are some examples that indicate that the idea of the existence of a mature and fairly stable vegetation in balance with environmental conditions (climate and soil) is not so far from reality. There are numerous examples of sacred sites in many areas of the world, particularly in Africa (Morocco, Burkina Faso, Togo, Tanzania, Ghana and Ethiopia) and Asia (India, Indonesia, China and Japan) around which human alterations have been severely restricted or are simply not allowed (Deil et al. 2009 and references therein). Outstanding, well studied examples are the North African Muslim shrines (marabouts) whose sacred groves (besides epitaphs) show excellent examples of natural or near-natural vegetation (Taïqui et al. 2005). Similarly, forests surrounding the Japanese Shinto shrines (Chinju-no-mori) have been taken as the main reference for establishing the PNV in that archipelago (Miyawaki 1979; Itow 1991). Another factor to consider is the growing interest in old-growth forests among conservation biologists, not only in North America (Spies 2004) but also in Europe (Nilsson et al. 2002; Burruscano et al. 2009). Such interest is based on the assumption that they represent the maximal naturalness in a given area. The idea of a sort of chaos reigning in succession everywhere implies perpetual uncertainty. Factors such as disturbances (human-induced or natural), biological invasions, changes in environmental conditions, etc., can co-exist perfectly with the idea of PNV.

Critical issues on methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References

Scale and resolution

Chiarucci et al. (2010) point to (at least) three main methodological problems in dealing with definition of PNV. The first, derived from the subjectivity of phytosociological sampling, is at most relative, because PNV identification can also be based on other sampling approaches or descriptions of vegetation units (Farris et al. 2010 and references therein). Spatial scale issues are interesting because they involve intrinsic assumptions of the PNV concept. Characterization of PNV types from their extant remnants is ineluctably fine-grained, and hence lacks large-scale properties. But it has been stated that PNV mapping is feasible at small scales (large extent) but faces increasing uncertainty at more detailed scales (Chytrý 1998; Zerbe 1998). This apparent contradiction stems from the process by which pieces of land are ascribed to a given PNV type among several competing or possible types. Ascription problems are expected to be minimal in the core area of a PNV type, but increase towards its boundaries with other types, as the information from indicators (remnants, discriminating species and communities, environmental factors) becomes more imprecise or scarce. Therefore, the ascription process implicitly excludes any large-scale property of the hypothesized PNV and must be interpreted in other terms. Passing from a map of current vegetation to a PNV map implies a strong smoothing of the vegetation mosaic, justified in theory by the homogenizing effects on environmental conditions attributed to PNV (Tüxen 1956; Zerbe 1998). Again, it is clear that some degree of spatial variation (and hence uncertainty) should be accepted within the territory ascribed to a PNV unit due to environmental variation or divergent successional trajectories induced by local conditions, uneven rhythms, random processes, etc., and that this variation is partially subsumed when mapping is carried out at a small scale (low spatial resolution). Hence, the pixel-by-pixel transfer of a complex landscape to PNV units can in theory be reformulated under a probabilistic (Bayesian) view as the expectation that the PNV unit would represent the dominant or the more probable outcome of successional trajectories in the territory ascribed to it. This view introduces corresponding measures of uncertainty and heterogeneity to PNV maps, which have too often been viewed as a prediction of continuous, monotone and even single-species forests covering immense landscapes. In practice, detailed PNV maps include information about such heterogeneity (permanent communities, metastable seral stages, invaders, stands that are difficult to replace, such as tree plantations) in the supplementary descriptions of the PNV units.

PNV mapping includes among its objectives a characterization of PNV units based on their remnants that must at least provide their differentiating elements with respect to neighbouring PNV types. Despite the aforementioned methodological constraints, extant remnants are the reference because they must provide the sources of propagules for the dominant and engineer species of the PNV unit, as well as for the other species associated with them. A parallel, loosely defined concept is that of the ‘reference’ or ‘target’ ecosystem used in restoration ecology (SER 2004; Clewell & Aronson 2007). The description of a reference ecosystem should include at least those structural, compositional and functional characteristics that allow a restoration project to be monitored to assess its degree of success (of course, a reference ecosystem does not need to belong to any PNV type). On the other hand, PNV mapping is conceived to provide complete territorial coverage, based on the analysis of spatial links between remnants, indicators and environmental conditions, besides additional rules to ensure the geographical coherence of results. The degree of consistency of such links, which is expected to be weaker in highly transformed landscapes, should provide additional uncertainty in the ascription of territories to PNV units.

Characterization of units and spatial resolution and coverage may be used to compare the performance with that of other approaches. The interpretation of natural vegetation based on paleobotanical records is obviously limited by sampling scarcity and the uneven compositional and spatial distribution of pollen records; macro-remains can give finer spatial resolution (e.g. Rubiales et al. 2007) but they are usually even scarcer. Interestingly, and for the same reasons, in restoration ecology it is advised that caution be exercised in the use of target ecosystems based on paleobotanical references (Millar & Brubaker 2006; Clewell & Aronson 2007).

Distribution modelling can potentially offer key contributions to explaining the relationships between environmental factors and PNV units (Tichý 1999; Zimmermann & Kienast 1999). Nevertheless, this type of modelling technique, after a burst of development, currently offers a huge variety of approaches (including differing model assumptions and initial parameters), and models often project considerably different distributions (Elith et al. 2006; Araújo & New 2007). Techniques for community modelling are less well developed (Ferrier & Guisan 2006; Baselga & Araújo 2010), although progress is expected in the short term and this kind of approach would have closer links with PNV interpretation. Species distribution modelling has been mainly applied at coarser scales than those used in PNV mapping, but finer analyses are also possible with data of higher resolution. In any case, distributional input data for modelling are essentially the same as those used for PNV mapping and hence they are subject to similar biases, in particular those derived from human modification of species distributions and assumptions about their degree of environmental adjustment.

Anatomy of a moment

The ‘instantaneous’ (schlagartig, Tüxen 1956) solution proposed for the determination of PNV is the most controversial and misinterpreted aspect of the approach. For some researchers (e.g. Andrič & Willis 2003) PNV should refer to the vegetation existing before human transformation of the landscape (up to several millennia ago in Europe or several centuries in North America), while others demand a time reference in the future (at least 50–150 yr according to Stumpel & Kalkhoven 1978). As noted before, each of these approaches adds different kinds of uncertainty that are difficult to resolve based on current knowledge. Tüxen's PNV concept simply eludes these uncertainties, focusing the interpretation on the real distribution of vegetation by assuming that remnants must act as sources for the coming successional trajectories and the links among remnants–seral vegetation–geophysical constraints must be based on their current distributions. The newly recognized uncertainties (identification and characterization of PNV units, heterogeneity within PNV, role of disturbances, invasions or pests, etc.) could be at least partially reduced by improving our knowledge of successional processes and making PNV schemes more flexible, but the starting point is focused on restricting the range of possible successional outcomes. The frame of co-occurrence relationships among plant communities and environmental factors is the basis of the mapping units, but one of the mature vegetation types is chosen as the reference to guarantee the relative homogeneity of the territory ascribed. Incidentally, we consider that the term ‘projection’ (Solomon et al. 2007) is more correct than ‘prediction’ for the PNV concept, precisely because of the forcing assumptions involved.

Data processing flaws

Surprisingly, the critique of Chiarucci et al. (2010) does not extend to the methods used to infer PNV units, which we consider a main drawback and an aspect that needs revision. Currently, the process of building a PNV map is too often a kind of black box from which only results emerge, in the form of PNV distribution maps and lists of their environmental conditions and indicator species or communities. Given this, the repeatability of procedures is not guaranteed and, in particular, heterogeneity and variation tend to be poorly reflected. Contributions to the literature reporting analysis of, for example, discordances among remnants and indicators are scarce (Šamonil et al. 2009; Muñoz & Raya 2010). Curiously, an inductive way to establish and characterize PNV units similar to the phytosociological tabulation procedure was proposed at the outset (Tüxen 1973; Rivas-Martínez 1978) but it has rarely been put into practice since then, and hence has not been optimized. The causes of this methodological precariousness are unclear, but typology instability, inefficient application of diagnostic species (for which objective methods have only been proposed in recent times; Dengler et al. 2008) and the limited use of GIS techniques may have contributed.

A consequence of this weakness seems to be an inflation of terms. When many terms are used to indicate the same thing, or nearly the same thing, confusion encroaches on the core issue and risks discrediting it. An excess of nomenclature (Terradas 2001) has certainly occurred in the case of PNV and related concepts over time and has had negative impact on the degree to which they have been generally accepted by ecology researchers. Far from being intentional, this is the result of a long series of reflections by many authors who have written about the issue and clear explanations, are available in the literature (Bolòs 1963; Tüxen 1977; Géhu & Rivas-Martínez 1981; Schwabe 1990; Theurillat 1992; Alcaraz 1996; Géhu 2006a; Rivas-Martínez 2007). Despite some of the terms proposed being synonymous or near synonymous to PNV, certain of them, such as vegetation series, sigmetum or synassociation, have been preferred by some authors because they highlight the focus on the whole pool of plant communities as the basis for their definition. Among other related terms, as discussed by Ricotta et al. (2002), are potential site-adapted vegetation (PSV; Leuschner 1997), reconstructed natural vegetation (Moravec 1998) and potential replacement vegetation (PRV; Chytrý 1998), all of them with similar premises in the construction of imaginary natural or near-natural vegetation. In our opinion, PNV is not at all the most accurate and eloquent term as it relates to the supposed final stage of succession, but a wide and flexible understanding of the concept is advisable for this discussion. A debate on these nomenclatural issues is needed, and in particular those terms not supported by explicit validation methods should be discarded.

It is beyond the scope of this paper to formulate improvements for PNV data processing, but some guidelines can be mentioned. An operative sampling unit (in the sense of Chiarucci 2007) must be defined for recording data. Unbiased sampling also requires the a priori establishment of possible indicators, either selected species or communities. GIS thematic layers, now widely available, should be used both to stratify sampling and to properly ascribe territories to PNV units (Capelo et al. 2007). In addition, characterization (including structure and regeneration estimates of the main species) and distribution patterns of remnants should be better documented. The hypothesis framework also ought to be enhanced with current knowledge of past vegetation, trajectory analyses of vegetation changes, distribution modelling and the outcome of mechanistic models of succession.

Old ghosts and future troubles

  1. Top of page
  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References

In pursuit of conifers

Conifers have been historically favoured for economic reasons in many countries, and there are large areas currently covered by conifer plantations and spontaneously grown forests whose degree of naturalness is difficult to state in some places. This issue has implications for PNV interpretation in some regions like the Iberian Peninsula, where it has become an important topic in the debate (Blanco et al. 1997; Gil 2008; Génova et al. 2009). From the evolutionary point of view, conifers are an ancient group emerging in the late Paleozoic and had their optimal development during the Mesozoic era. Thereafter they were widely replaced by angiosperms (Bond 1989; Coomes et al. 2005) in most of the resource-rich habitats, as angiosperm newcomers were more efficient in exploiting soil nutrients (Berendse & Scheffer 2009). The result is that in modern times conifers mainly occupy habitats or regions that are stressed in terms of climatic (low temperatures) or edaphic (shallow rocky, permanently flooded or nutrient-poor soils) conditions in various territories (Urbieta et al. 2011). This is only a general statement because there are many examples of conifers dominating natural forests in mesophytic environments in different parts of the world, particularly in those areas that have experienced fewer extinctions in recent history (Pleistocene), as is the case of some North American redwood forests (Sequoia, Sequoiadendron). Europe is a continent that experienced the stress of several successive ice ages that caused massive extinctions of tree species, particularly in those taxa adapted to mesic conditions. The contrast between European dendroflora with that of North America or East Asia is overwhelming (Tallis 1991). Specifically, Europe hosts fewer gymnosperm trees than its two homologous regions and so it has fewer conifer species to compete with angiosperms in mesic habitats. However, in comparison with the Eurosiberian region, the Mediterranean region (southern Europe, the Near East and North Africa) is relatively rich in tree species, including conifers, and they certainly have a role in the plant communities of the area. The more carefully we document their population dynamics in the past and the better data we gather about their current dynamics, the more accurately it will be possible to describe their communities, and changes in the consideration of some PNV units have and will be introduced. It is widely accepted that spatial heterogeneity and disturbance regimes favour the co-existence of pines and oaks in Iberian landscapes (Zavala & Zea 2004), and this indicates the need for a further review of the PNV assumptions in many areas. Some researchers are reluctant to accept certain of the proposed PNV models because they are difficult to tally with the suggestions derived from the results of surveys of paleoremains (Carrión 2000). Certainly, historical records have to be carefully taken into account, as we stated unambiguously (Loidi et al. 2010), because the past conditions affect the present status of ecosystems, but the past cannot condition the future in terms such that the current and coming environmental conditions tailor terrestrial ecosystems independently of what happened in the past. The idea that pines have been disregarded in the current documents describing Iberian PNV types has been supported by palynological surveys, but, in relation to this, there are cases in which pollen diagrams show abundant pine pollen even in areas where pines are scarce in the landscape; pine pollen is sometimes overrepresented in sediments and has to be interpreted with caution (Roc et al. 2002). Pollen rain as a representation of extant vegetation is not always very reliable (Vázquez & Peinado 1993). If we examine Tüxen's paper (1956), several pages are devoted to explaining the difference between primitive vegetation and current (heutige) PNV due to changes in environmental conditions caused by human influence or by natural processes since the original vegetation was altered or has disappeared. This means that the historical element is essential in the construction of the idea of PNV for two main reasons: first because of the human disturbances and second because paleoremains show us the biological material (species) existing in a particular area in the past. Such species may or may not survive local extinctions that are happening now or in the past. All these factors explain why existence in the past does not necessarily correspond to suitability in the present day. Although management and tree selection have played a role we should be careful not to exaggerate their contribution. In any case, modern global change means abandonment of large areas in many European countries, particularly in mountainous regions. This has triggered secondary succession in a substantial part of the territory, which has revealed that in a relatively short period of 40–50 yr (after the rural population fled), vegetation develops towards maturity at a faster pace than expected. Perhaps we will not need to wait too long to see good examples of ecosystems changing towards maturity in order to glimpse the final or mature stages of secondary succession in vegetation, and the real impact of natural or human-induced disturbance. The experiment, provided unintentionally by socio-economic changes and inscribed in what we could call ‘abandonment ecology’, is in motion.

Biological invasions

These have taken place in Europe since at least Neolithic times (Kornaś 1990), and a large number of plant species that migrated and established under human influence have, since then, come to be seen as native in their new territories after a certain amount of time has passed. They stabilize demographically and ecologically, becoming part of the native plant communities, i.e. they reach a sort of balance in the competitive dynamics with other native or alien plants involved in the same community. Eventually, they are effectively incorporated into the ecosystems and hence they have been duly incorporated in the current descriptions of flora and vegetation. The newcomers are those which have not yet reached such a degree of stabilization or steady state; many of them become extinct (ephemeral) while others are in an expansive phase (invasives). In any case, most of the alien flora of a territory is found in disturbed habitats (Kornaś 1990; Chytrý et al. 2005, 2008; Lambdon et al. 2008; Campos 2010; Pyšek et al. 2010) and they likely will withdraw as succession develops to more mature stages. Nevertheless, all new plants, in the past and nowadays, that come to seem native in any territory will participate in the development of its plant communities with all the associated consequences, such that they become part of various vegetation types (Hobbs et al. 2006; Vítková & Kolbek 2010).

Management and conservation of natural resources and PNV

  1. Top of page
  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References

The uses of PNV and related concepts have been demonstrated over many years, during which their cartography has been used in many countries for applied vegetation science. Basically, PNV provides guidelines for good practices for nature conservation in the frame of land-use planning (Pedrotti 2004; Miyawaki & Box 2006), taking into account that optimally it should be combined with real vegetation maps (Zerbe 1998): planning of infrastructure, land evaluation for agriculture, livestock and forestry, environmental impact assessment, ecological restoration and landscape architecture; together with reserve selection, design and management. It is not a commitment to restore, favour or build the PNV as the only possibility for conservation; rather it provides a framework of information on the territory (pool of communities, environmental factors) that can be applied for sustainable management. PNV is also a keystone of landscape analysis and fine delimitation of biogeographical units (Alcaraz 1996; Schwabe 1997).

One of the most typical and common products of PNV is maps, of which numerous have been produced, in a wide range of countries and at different scales. The map of natural vegetation of Europe at the scale of 1:2 500 000 (Bohn et al. 2003) is an example of how maps based on natural vegetation have been produced in territories in which Braun-Blanquetian phytosociology had not been established, including the former USSR, Fennoscandia and the British Isles, among other countries (i.e. most of the mapped territory). Handbooks on vegetation mapping recognize and explain the natural vegetation concept and the mapping of its potential territory (Küchler & Zonneveld 1988; Pedrotti 2004). Such maps provide the spatial pattern of all the environmental features that can be used as indicators for vegetation (bedrock, climatic conditions, biogeographical scenarios) from which all the aforementioned useful information is derived, making PNV documents much more meaningful than a static description of existing units (Terradas 2001).

A collateral point of criticism in the debate concerns the EC Habitats Directive. This document establishes a guide for focusing conservation efforts on a couple of ‘habitat types’ (certainly the term is used here with a different meaning to that defined in ecology) that are loosely related to phytosociological units. Phytosociological typology relies on over 4.36 million relevés in Europe alone (Schaminée et al. 2009), is widespread and based on similar sampling protocols, and provides a practical tool for vegetation classification, evaluation and conservation (Géhu 2006b). Its value and potential applications have been recognized even by authors critical of phytosociological sampling procedures (Lepš & Šmilauer 2007; Lájer 2007; see also Jennings et al. 2009). A large proportion of the units designated as European conservation targets in Annex I of the Directive correspond to seral stages (scrub, heathlands, grasslands, open grazed forests such as dehesas or montados, etc.), whose conservation unavoidably requires maintenance of the corresponding disturbance regimes or management practices: fire, grazing, pruning, etc. In our opinion, it is wrong to accuse the Directive of having a bias to conservation of pristine or natural vegetation units. On the contrary, it is particularly sensitive to historically altered (cultural) ecosystems.

PNV mapping requires a high level of mastery of the flora and vegetation of the surveyed area, and such experience is essential in the definition and delimitation of each PNV unit; decisions often made in the field in sight of the reality of the extant vegetation. This knowledge is to a great extent integrated in the legends of PNV units, with the result that such documents constitute a valuable advisory guide for land-use planners and conservation policy-makers. Management involves day-to-day decision-making, and usually managers cannot wait for long and complicated research to yield solutions or for advice on what to do or not to do in a particular area.

Clarifying the concept: some last words, but not yet epitaphs

  1. Top of page
  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References

We have tried to explain that an operational definition of PNV must take into account two conceptual aspects. First, the repertoire of PNV types that can be recognized in a geographical space must be established according to the analysis of remnants of mature or near mature vegetation, as well as to knowledge on vegetation dynamics and history of vegetation and land uses. These PNV types correspond to the most natural and successionally mature vegetation remnants. They do not represent the vegetation prior to the historical anthropogenic disturbance (primitive vegetation) or the unknown vegetation at the end of a sufficiently long and undisturbed succession. Such a succession is subsumed to an unavoidable uncertainty of spatial–temporal events and processes. PNV simply represents the more natural vegetation reflecting the current environmental conditions of a given territory. Second, the ascription of a location to one of the PNV types is evaluated through geographical, environmental (geophysical) and biotic (indicator species and communities) evidence of its closeness to the respective conditions associated with the accepted PNV types.

Most of the criticism against the PNV approach is addressed to the first aspect. We have stressed that characterization of PNV units is based on remnants. The minimum characterization required should consist in the differential elements with respect to other PNV units (structure, dominant species and differential species). There are many examples of PNV maps in which the characterization of PNV units is very general or merely physiognomic. PNV maps based on phytosociological units can furnish more detailed descriptions, and remnant samples can be compared among them according to criteria such as age structure, stand continuity, core area, indicators of land-use pressures, etc., in order to establish which of their structural or compositional traits are closer to the hypothesized maturity. But a detailed and reliable prediction about how the true mature state would appear is outside the capabilities of the PNV concept, particularly in territories heavily managed for centuries, where remnants may be a pale and biased image of the future mature vegetation. Even if the pool of vascular plant species proper of such remnants could be approached through a detailed analysis, the structural and compositional details of other species pools relevant for forest functioning (forest lichens, fungi, birds, mammals, insects …) would remain largely unknown. We argue that despite the relatively loose characterization of PNV units, the approach is still valid as an integrated analysis of the actual vegetation patterns. In fact, the validation procedures of the hypothesized PNV frame rely on this second aspect of the analysis.

In spite of all the difficulties and flaws, the PNV concept is a useful one in that it depicts not only a ‘natural’ scenario according to the extant vegetation types and current environmental factors, but also an ecological description of the territory in terms of extant plant communities. The challenge is to improve the maps through discussion and supported by new knowledge, not abandon the concept. It is easier to demolish than to build. If, however, we abandon what has been achieved to date using PNV, ceasing to work in that line of research and thereby provide government bodies with valuable documents for conservation management of the territory, we believe it would be a great victory for forces that are not particularly interested in nature and biodiversity conservation. Any changes can potentially be admitted if they are supported by evidence in the field and research findings; paleoremains, quantitative ecology, population ecology, alien plants, etc. will all influence the ever-shifting PNV constructions. It is an ethical duty to provide society with an accessible guide to what the science yields to serve as a source of advice for managers and decision-makers. Currently, PNV provides this.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References

We thank to W. Willner and two anonymous referees for their critical comments, which greatly improved the original manuscript. The authors were funded by the MICINN project CGL2009-13317-C03 (Sivim).

References

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  2. Abstract
  3. Introduction
  4. General issues: on the validity of the construction
  5. Critical issues on methods
  6. Old ghosts and future troubles
  7. Management and conservation of natural resources and PNV
  8. Clarifying the concept: some last words, but not yet epitaphs
  9. Acknowledgements
  10. References
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