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.

In a commentary regarding Holocene pollen deposits from the Canary Islands, Carrión & Fernández (2009) argued that the discovery of Quercus and Carpinus pollen is noteworthy because ‘the prevailing concepts of natural potential vegetation in the study region imply that the pre-anthropic (mature phase or climax) vegetation’ would be an evergreen forest dominated by species of Lauraceae. Inferences of pre-anthropic vegetation made by palynologists were also compared with the potential natural vegetation (PNV; incorrectly quoted in the commentary as ‘natural potential vegetation’) proposed in the phytosociological literature at a number of sites in the Iberian Peninsula. The results of the two models indicated that, in many instances, the dominant species differed. Therefore they concluded that the PNV types determined in previous studies were the result of anthropogenic disturbance. Consequently, the authors polemically argued that there is a bias ‘in the conceptualization of the vegetational dynamics’ by ‘traditional vegetation science’, and resistance to abandon this bias ‘has little to do with scientific evidence’, in front of ‘a growing body of work questioning the floristic-phytosociological approach’. Unfortunately, this line of reasoning is based on two serious misunderstandings regarding the PNV concept.

First and foremost, PNV is not defined by vegetation scientists as ‘pre-anthropic (mature phase or climax) vegetation’. On the contrary, PNV is defined as the plant community that ‘would become established if all successional sequences were completed without interference by man under the present climatic and edaphic conditions (including those created by man)’ (Mueller-Dombois & Ellenberg, 1974, p. 422; our emphasis; see also: Westhoff & van der Maarel, 1973; Ellenberg, 1988; Ricotta et al., 2002). European landscapes exhibit present soil conditions that are often dramatically different from their original state, due to recent or ancient but irreversible human disturbance (cf. Dupouey et al., 2002). Consequently, it is an essential part of the PNV theory that the potential vegetation of a site can be very different from the pre-anthropic vegetation at the same site (e.g. Mueller-Dombois & Ellenberg, 1974; Chytrý, 1998; Moravec, 1998; Zerbe, 1998; Gamisans, 1999). The PNV concept was introduced (Tüxen, 1956) to express the present (‘heutige’) potential of a region or site as a useful reference to define a target for restoration ecology and ecological engineering projects, or for landscape management purposes (e.g. to forecast and manage landscape evolution on a time-scale of a few decades) (Rodwell & Patterson, 1994; Härdtle, 1995; Miyawaki, 1998; Zerbe, 1998; Verheyen et al., 2006; Dostalek et al., 2007).

It is quite surprising that Carrión & Fernández (2009) completely ignored the large body of works addressing and defining PNV theory. Furthermore, it is perplexing they assumed that ‘climax vegetation’ and PNV are considered synonyms in vegetation science. On the contrary, it is well known that the idea of PNV arose as an outcome of (and reaction to) the long-lasting debate on the ‘climax’ concept (Zerbe, 1998; Ricotta et al., 2002). European vegetation scientists have questioned the concept of climax for decades (Mueller-Dombois & Ellenberg, 1974; Chytrý, 1998; Schulze et al., 2005), and we now acknowledge that vegetation is not returning to an alleged, past equilibrium, but is adapting continuously to a changing abiotic environment and biotic interactions. In addition, the inferred climax phase requires a long period of succession, which introduces not only the effects of long-term climatic changes, but also those of vegetation-induced soil modifications. Finally, the climax concept was developed to study the phytogeography of North America, a continent featuring abiotic homogeneity over large areas, a condition rarely verified in Europe.

The second fundamental mistake in Carrión & Fernández (2009) was their assumption that PNV is a concept exclusive to the phytosociological school (i.e. of the Braun-Blanquet approach to plant community entitation, sampling and classification; see e.g. Westhoff & van der Maarel, 1973). Although originally developed by phytosociologists, the PNV model does not stem from the assumptions of phytosociology, nor do the ‘potential communities’ need to be defined and classified through the phytosociological system. PNV-based models can be usefully applied even if vegetation is classified on a purely physiognomic basis (e.g. Liu et al., 2009).

Furthermore, despite the fact that PNV arose in a historical context, when succession had a linear deterministic interpretation, it is interesting that even in modern ‘chaotic’ models of succession, the PNV hypothesis is supported, as it can be viewed as the strange attractor where succession trajectories converge (Anand & Orloci, 1997; Ricotta et al., 2002). Thus the comparison between the distribution of actual vegetation patterns and PNV types is presently integrated in the most current applied ecological research, for example: to assess the effects of disturbance on pattern and diversity in landscape ecology studies (Ricotta et al., 2002; Bajocco et al., 2010); to study the effectiveness of protected area networks (Rosati et al., 2008); and to contribute to habitat monitoring (Mücher et al., 2009).

As a consequence, the use of PNV concepts is definitely not restricted to a few ‘academic refuges [of phytosociology]’ in Spain or Italy (Carrión & Fernández, 2009). PNV-based methods are largely adopted today in countries where the phytosociological approach is not traditionally applied, such as the USA and China (e.g. Küchler, 1964; Brohman & Bryant, 2004; Liu et al., 2009; Zou et al., 2009), and/or within disciplines not related to phytosociology, including forestry and wildlife ecology (e.g. Lexer et al., 2002; Aubry et al., 2007; Kennedy & Wimberly, 2009; Strand et al., 2009). Recently, PNV concepts were integrated in major vegetation-mapping projects, based on extensive international collaboration (Bohn et al., 2004; Walker et al., 2005).

Therefore it is unclear why any shortcomings in the PNV model (see Zerbe, 1998 for limitations and caveats) should ‘question the floristic-phytosociological approach’ and why any flaws of the phytosociological approach should weaken the usefulness of the PNV concept. Incidentally, it should be noted that the conflicts between the assumptions of the phytosociological method and the latest views in ecology form a complex and long-lasting debate (see e.g. Dengler et al., 2008 and references therein), which cannot be summarised here, and have little to do with the issues raised by Carrión & Fernández (2009).

Furthermore, it is not clear why Carrión & Fernández (2009) quoted the Willis & Birks (2006) review to support the claim that ‘the palaeoecological literature is full of overwhelming evidence against [the phytosociological] notions of vegetational dynamics’. Instead, the need for a multi-method approach to a modern, integrated framework for the study of plant communities (as advocated by Willis & Birks, 2006) should be stressed. We believe in a multi-disciplinary approach to community ecology, where short-term secondary succession studied by vegetation scientists and long-term dynamics reconstructed by palynologists are two views of the same process studied at two different spatial-temporal scales, both of which should contribute to the development of a unified, multi-scale model of vegetation dynamics (Davis et al., 2005; Willis et al., 2007; Pickett et al., 2009; Zou et al., 2009).

Editor: John Lambshead