What is described by phytosociology?
Some weak points of Central European phytosociology
Radim H é d l
Botanický ústav AV ČR, oddělení ekologie, Poříčí 3b, 603 00 Brno, a Ústav geologie
a pedologie LDF MZLU, Zemědělská 3, 618 00 Brno. E-mail rhe@centrum.cz
Abstract
Main weak points in methods of Central European phytosociology are described and
commented. First, preferential sampling design implies a twofold subjective classification
already in the field. Subjectively distinguished and preferred vegetation types have been
sampled, which distorts representativeness of the data, regarding the real vegetation.
Second, vegetation has been changing continuously, while phytosociology gives rather a
static description, being gradually overcome by changes. Third, classification based strictly
on species composition makes the phytosociology applicable only in some vegetation types,
mostly the species-rich and ecologically specific ones. Finally, three controversial concepts
of potential natural vegetation are briefly discussed. Objectivized vegetation sampling
designs should be applied to assess the real abundance of vegetation types. A revision of
the phytosociological system of vegetation units should be done regarding the vegetation
changes. Methods enabling a unified classification of floristically and ecologically unequal
vegetation types should be developed. The present concept of potential natural vegetation
seems to be speculative, requiring a re-evaluation.
Introduction
Quite recently, two papers giving a qualified judgement of the present-day phytosociology
were published1. They have been written by two foremost European phytosociologists, M.
Chytrý and J. Ewald. The first one (Chytrý 2000) compares the classical and formalized
methods of sampling and classification, perceiving them as two complementary approaches.
The second one (Ewald 2003, in brief Ewald 2004) is basically the defense against the
„exact“ ecological sciences. Chytrý & Rafajová (2003) wrote a paper on the properties of the
1
The whole this paper concerns the prevailing Zurich-Montpellier school of phytosociology (Moravec
1994, chapter 6.2.5), or according to Ewalds (2003) the „Central European approach“ as a parallel to
Anglo-American Ecology. I omit here the other, in the Czech Republic especially Zlatnik-based,
systems.
1
data sources, namely of the Czech National Phytosociological Database; see also
http://www.sci.muni.cz/botany/dbase_cz.htm.
It is a delightful fact that even after a century of its existence the phytosociology still
undergoes processes of self-criticism and inovations. The main purpose is the methodology
objectivization, which has for long remained more or less similar as in the very beginnings at
the break of the 19th and 20th centuries. Consequently, the phytosociology still often has, in
the mouth of the other plant ecologists, a bitter aftertaste of a rotten pseudo-science rather
than an exact, modern ecological science. The breaking progress and approaching to the
„real science“ has been recently achieved in the field of formal classification (see e.g. Chytrý
et al. 2002). Due to this, the computer programmes providing controllable and repeatable
classification of large datasets are widely available.
The stressed advantage of phytosociology is its uniqueness as a system decribing
the vegetation variability at large spatial scales, providing the adequate amount of the
necessary information – the complete species composition. Phytosociology has in fact
created the so far only utilizable (or better: used) system of vegetation types 2, no matter how
geographically limited and burdened with subjectivity. The influence of observer’s subjectivity
or the seasonality concerning the data sampling had been examined for example by Smith
(1944), Gotfryd & Hansell (1985), Kirby et al. (1986) or Lepš & Hadincová (1992) the
heterogeneity or incompletedness of data (meaning the proportion of the described
associations) influencing the properties of the vegetation databases were pointed at by
Chytrý (2001) and Chytrý & Rafajová (2003).
Rather than the above-mentioned shortcomings of a „technical“ character, a much
more striking problem is, in my opinion, the fact that the existing system of syntaxa 3 cannot
be perfectly applied to the most of the existing vegetation – concerning its variability at one
hand, and the spatially prevailing vegetation types at the other. Doubtless, the universal
applicability should be an obvious requirement put on the phytosociology. Hence, let us
attempt a purely ideal definition of the phytosociology aims and scope (for the explanation of
terms see Moravec et al. 1994): „ The basic task of phytosociology is to create, for the entire
area described, a universally valid, whenever and by whoever repeatable and applicable,
hierarchically structured system of syntaxa. The clearly defined criterion for syntaxa
delimitation is specific combination of species, so called diagnostic plant species.“
That the system of phytosociological syntaxa is simply not easily applicable to any
vegetation even within some well-examined territory (such as of the Czech Republic), this
2
I use the term vegetation type in the broadest sense here, meaning that the vegetation can be
classified into defined units. The phytosociology-specific parallel is a syntaxon.
3 Holub et al. (1967), Moravec et al. (1995), Moravec (1998–cont.), Chytrý et al. (2001), Chytrý & Tichý
2003.
2
can be at least guessed by the most of the phytosociologists and practitioners. The reasons
for it partly lies in the conservative approach, which remained unchanged since the
pioneering beginnings, when the easily-recognizable vegetation types of Western Alps, the
adjacent Mediterranean and Central Europe had been described. The second serious
problem lies in the fact that the phytosociology has always worked with site ecology and
vegetation formations, while these criteria remain hidden in the system backgrounds (in
contrary to the related forestry classifications). Further, from the floristics-based classification
criterion it follows that the species richer the vegetation is, the larger number of associations
can be reached (M. Chytrý, personal communication). Can the objective of the purely
floristically based classification be kept even in case of the prevailing, i.e. species-poor and
badly-delimited vegetation? This concern would most probably be soon enlightened, as the
large amounts of data is re-classified using the modern methods. Apparently, the crucial
problem is not the unsufficient amount of data because only in the database comprising the
territory of the Czech Republic, around 70 thousand phytosociological relevés is included
(http://www.sci.muni.cz/botany/dbase_cz.htm#stav).
The main aim of this paper is to show that, the phytosociological relevés is a very
badly representative sample of vegetation variability. It is of course nothing new or
discovering, for a comprehensive reading see especially the above-cited papers by Chytrý
and co-workers. My arguments concern the following: (1) spatial structure of sampling; (2)
temporal changes of vegetation variability. Besides these, I would like to give some
comments on still discussed and unsufficiently solved question of (3) classification into
discontinuous units. Finally I would like to point at the heterogeneity and even the
oppositional character of the (4) different concepts of potential vegetation, which are besides
their attractivity from the theoretical point of view one of the few influential applications of
phytosociology.
1. Spatial structure of sampling
A phytosociological relevé is nearly a standardized unit of phytosociological data collection
(Mucina et al. 2000, Chytrý & Otýpková 2003). The relevés are however not naturally
delimited objects – in contrary to biological individuals or their parts, the objects of population
biology, for instance. They have to be delimited arbitrarily and placed somewhere, and their
number has to be decided, too (see e.g. Greig-Smith 1983, Podani 1984). The question of
the sample representativeness comes to play. There are exact and simple rules ensuring the
sample representativeness – the most important being the random selection (Cochran 1977,
Sokal & Rohlf 1994). This concerns, in the case of vegetation plots, the plots placement.
3
There are several alternatives of a plot-set design (Kershaw 1964, Greig-Smith 1983, Podani
1984), each suitable for different purposes.
Three types of randomized selection require the randomness at certain step of
placement (Kenkel et al. 1989). They are simple random selection, systematic selection and
stratified random selection. The latter includes, in fact, a sequence of transitions between the
systematic (on a regular, yet randomly initiated, lattice) and simple random placement
(entirely randomized positions), see Podani (1984). The fourth type of selection is
preferential positioning, which places the particular plots subjectively, depending purely on
the researcher’s decision. The vast majority of phytosociological data has been recorded
right this way, which influences substantially their representativeness.
In the reality, the individual phytosociologists concentrate on selected, subjectively
favoured vegetation types, whose delimitation is not an obvious task, anyway. The subjective
selections can be to a certain extent unique and do not need to reflect their perceivnig by the
other phytosociologists. The favoured vegetation types are henceforth studied by the
researcher for a long time, sometimes even for his/her whole life, and sampled. A limited
number of researchers is consequently responsible for the sample material describing the
particular broader vegetation types, while the vast majority of authors ever records only one
or very few relevés (Chytrý & Rafajová 2003). Within such subjectively perceived vegetation
types, a phytosociologist continues in preferential positioning of relevés, chiefly based on the
immediate recognition of the sampled spot directly in the field. The reasons for such relevé
positioning can be various; they can be generally characterized as a gradual supplying of a
subjectively built system of vegetation types.
The result is a double preferential selection – a set of phytosociological relevés
preferentially positioned within preferentially selected and apriori recognized vegetation
types. The fact that such not (-perfectly) formalized vegetation surveys are merely a
presentations of apriori field classification, was sufficiently commented by Chytrý (2000, see
also Chytrý et al. 2002). Merging of sets of authors’ contributions into one (for instance a
national) vegetation survey gives not quite an optimistically-looking result. It is all well
reflected in a structure of higher syntaxa because this „scaffolding“ had been created
subjectively, and not only concerning the species compositon but also combining with the
ecology and physiognomy, see below.
The authors subjectivity in sampling implies the addressed main drawback of
phytosociolgical data, thier low representativeness. While mainly the „favourite“ vegetation
types have been selectively sampled, the whole picture of the real vegetation variability is
much biased. Preferred vegetation can be strogly represented in datasets although it is in the
real nature rare; and in opposite, many potentially definable vegetation types are virtually
4
missing. We do not know quite reliably what we can claim about the real features of our
vegetation, based on the available preferentially collected data.
This statement can be documented using the available analysis of the Czech National
Phytosociological Database (Chytrý & Rafajová 2003), or by further Database analyses,
some of which being presented below. The most commonly represented classes are, with
the exceptions of temperate forests (Querco-Fagetea) and wet-mesic meadows (MolinioArrhenatheretea), rare in nature. Even the vegetation of the two mentioned classes does not
occupy a large space within the Czech Republic. The far most often sampled alliance
Calthion makes up only a small fragment of the vegetation cover, which holds also for the
most „common“ forest associations Melampyro nemorosi-Carpinetum and Dentario
enneaphylli-Fagetum. Except for the overestimation due to frequent sampling this can be
because of the fact that a various mixture of relevés has been attributed to them, which does
not correspond to their (however vague) delimitation in the literature. The existing system
often provides no other option, though.
2. Temporal shifts in vegetation variability
The vegetation is not constant, it makes up a mosaics of vegetation types changing in time.
Vegetation types spatially decrease, increase, vanish or establish, they transform4. It is a pity
that there is no stable set of syntaxa, because soon no other, inovative vegetation surveys
would be needed and the situation would be nice and smooth. And the scientists could go for
the deserved retirement, or devote themselves purely to computer modelling.
The vegetation dynamics is driven by the rate of changes of factors controlling the cooccurrence of certain species constituting the communities. The humans are in the European
conditions the so far most important factor, which can be testified by anybody when visiting
sparsely inhabited areas. The dynamics of direct and indirect human influences has
dramatically increased in the past decades. Many formerly occurring vegetation types can be
hardly found today while being dependent on the permanent human interventions, or the
management; let an example be a grassland vegetation (Blažková 2003) or „natural“
woodlands (Hofmeister 2002, Hofmeister et al. 2002). Their successional derivates are
prevalent in the present-day landscape. Quite new vegetation types originate, too: plant
invasions and expansions are the important forming factor of the new-age vegetation (Pyšek
& Tichý 2001), often due to human induced decay of ecosystems (see immision, blowdown
4
The term vegetation type has been used here as a pragmatic denotion, see Note 2, not as something
existent.
5
and bark-beetle forest clearings). The invasions do not even avoid from reserves (Pyšek et
al. 2003). The current changes are witfully commented by J. Sádlo and P. Pokorný (e.g.
Pokorný & Sádlo 2004).
What consequences does this has for phytosociology? Phytosociological relevés are
temporarily unequally spreaded (the production increases in time, Chytrý & Rafajová 2003),
though they capture several periods with partly different vegetation cover. A sum of all
relevés within syntaxa cannot be therefore used for vegetation surveys without concerning
the temporal aspect (or changing the title of a vegetation survey). Relevés old only 20 or 30
years often describe the history, i.e. the state of a no more existing vegetation. With some
exaggeration, the phytosociology can be denoted a kind of plant community archaeology.
This is further combined by the above disputed matters, meaning that each of the periods of
the recent vegetation development is a subjective picture by one or several researchers (for
particular vegetation types) 5.
The present system of syntaxa had been created mainly in the past, based on
sampling material capturing the vegetation sometimes no more existing in this shape, or is
very rare, restricted to refugia of either „natural“ or for some reason traditionally managed
sites. Or has this vegetation changed, moved in another direction. Most of the present-day
vegetation cover slowly start to build up the communities, which we are able to classify
merely to alliances – already wide units, but how well delimited? We surely do not want the
phytosociology become just a descriptive tool of „natural monument protection“ of our nature,
or even the mentioned vegetation archaeology.
On the other hand, a very attractive possibility of utilizing the phytosociological data
as an archive telling about the former shaping of vegetation, repeated sampling of the sites
comes to consideration. The main danger lies in an inexact localization of relevés (see Hédl
2001), however it is a parallel to permanent plots, which have to be permanently maintained
and with some exception, they usually do not go much back to the past. There is already a
number of studies repeating the old relevés, examples can be found in almost any periodical.
An important application can be in the field of nature protection (monitoring of changes).
An example
5
This shall be seen rather as a fantasy and simplification. Every vegetation cover comprises relics of
former prevailing vegetation covers, „embryonal“ stages of newly establishing species combinations,
and the currently prevailing recent vegetation. The former recent can be the today’s relict, the former
novum the teday’s recent. From this, discrepancies between older and newer sample sets and
vegetation surveys follow.
6
I would like to document the statements from above by an example. As a phytosociologist
subjectively preferring and sampling the beechwoods vegetation, I asked in the beginning of
2004 for all relevés from the Czech National Phytosociological Database with the beech tree
cover of more than 10% in the tree layer. The set of 2890 relevés is not burdened by apriori
classification within the used system of syntaxa. This dataset has the following structure.
Relevés were mostly recorded in two periods, roughly in the 1960 and around the end
of 1990s (Fig. 1). As the autorship concerns, the most relevés have been recorded by only
three authors (Fig. 2), 33%. Together with another twelve most productive authors (including
all anonyms), they have recorded 67% of the relevés. At least 20 relevés have been
recorded only by another 22 authors. The total of 37 authors is responsible for 91% of
relevés, 77 remaining authors for 9%; 25 authors recorded a single relevé.
The attractivity of vegetation has changed over time. Number of species statistically
significantly decreased (Fig. 3), the same for the herb layer coverage (Fig. 4). Species poor
and loosely undergrown beechwods become attractive only in the last decades; a notable
feature is maxima of species numbers in the 1960s and 1970s (five cases exceeding the
species number of 70 were excluded, and so were the relevés with the absenting value of
herb layer coverage). Number of species positively correlates with the coverage of tree layer:
loosely closed beechwoods are species poorer, the highest species numbers are by the
coverage of abour 80%. Besides this „attractivistic“ hypothesis, it is quite well possible that
the species richness of beechwoods is really changing in time, i.e. decreases (e.g. Hédl
2004).
3. Classification to strictly delimited units
A characteristic feature of phytosociology is the concept of communities delimited strictly by
occurrence of plant species, which is unique for each syntaxon (Moravec et al. 1994: 105;
Chytrý et al. 2002). As noteed in Introduction, such an approach had been induced in
conditions of well-delimited Alpine vegetation. If we want to classify also our (and even
worse, the nordic), hardly species-unique vegetation types, we have to choose carefully in
the field, a lot of vegetation simply ignore, or even exclude it from the analyses (which used
to be a common, sometimes recommended practice). When we want to label any piece of
vegetation in the field, we have no chance for success using this approach.
Hence, is it useful to classify the vegetation keeping purely on the species
composition? Clearly no. If so, the other criteria should be applied openly. Even expert use
frequently the site ecology when determining the syntaxa, quite sadly even the geographical
7
origin of relevés; this is somewhat a schizophrenic state of mind, which has been reflected in
sometimes vague formulations of texbook phrases (Moravec et al. 1994, chapter 6.2.5).
Would not it be better to modify the by time petrified approach? Three possibilities are readily
here: (a) leave the strictly species-based classification and introduce also other criteria
recordable in the field or derivable otherwise; (b) clearly species-defined units to be taken as
„cores“ and the transitional zones divide according some similarity criteria, as suggested by
Moravec (1989); (c) take the existing deductive method of K. Kopecký (Kopecký & Hejný
1974, 1978, 1980).
Ad a. Other information than strictly species composition already consciously interfere
the modern classification methods (Cocktail method: Bruelheide & Chytrý 2000, Kočí et al.
2003). They have been considered already when constituting the classification by the
researcher him/herself, which makes a modern continuity of classic expert-created systems.
The product is again strictly by diagnostic species combination defined syntaxa, which is the
differential of phytosociology especially against the ecologically-based forestry systems
(Plíva 1991, Buček & Lacina 2002). Diagnostic groups of species should however be better
constituted by independent data on ecological requirements of species.
Ad b. A good analogy is the taxonomical way of classification of genus Hieracium,
where main and subsidiary species have been recognized (Chrtek 2002). This concept is in
syntaxonomy feasible, concerning the data analysis possibilities (Kočí et al. 2003). Even
without a re-classification of the relevé material, it is possible to consider the recognized
syntaxa the „cores“ of the system, and define them by diagnostic, constant and dominant
species, as recently done by Chytrý & Tichý (2003). Transitional stages of vegetation
(relevés not assignable to any core-syntaxa) should have been classified otherwise, either
with an existing (ad c) or a newly developed method.
Ad c. As stated by Kopecký & Hejný (1992), the disadvantage of the deductive
method is the resulting not-easy system of communities. Such is the nature’s reality, though.
Important is, how detailed a system we want: if a large number of units recognizable even
subjectively in the nature, or rather fewer broader units. Except for synanthropic vegetation,
the dductive method has almost not been applied, although it would suit also to the other
vegetation types (like species-monotonous forests).
If we want to apply the phytosociological criteria really to the entire vegetation, a
feasible method, including the brand-new Cocktail method (which remains confined to
species well-defined vegetation), is the use of a system of central and transitional units.
Because Kopecký’s deductive classification produces too many units and names
conservatively tied to the present system, it would be better to introduce some objectivizing
criteria, namely mathematically defined similarity. The transitional cases between clearly
defined units do not even need to get their unique names.
8
4. Concepts of potential vegetation
The thinking of how a natural vegetation would look like is still attractive even today. One of
the most influential concepts is the idea of potential natural vegatation originated within the
phytosociology (Tüxen 1956, Neuhäusl 1994). It tightly concerns the ecological indication
ability of plant species and communities. Besides the phytosociological, there are other,
seemingly more speculative concepts (e.g. Vera 2000). The thinking circulates around two
basic questions: the relation between forest and non-forest, and the composition of natural
vegetation. Because indisputable facts are virtually missing, the resulting picture depends
much on an arbitrary decision for one from a trinity of factors (or their combinations): (1)
inter-species competition, (2) site ecology, (3) intervention of humans and large herbivores.
Simplified implications are as follows.
1. „Competition hypothesis.“ Forest-free sites almost do not exist, because the
succession leads unequivocally to a forest. There is an assumption supported by
observations that there are such competitively strong species (trees) that the other have a
chance to establish only at margins of ecological gradients. In Central Europe, European
beech (Fagus sylvatica) should be the species. It should entirely or partly dominate from the
lowest altitudes up to a narrow belt of moutain spruce forests, except for rocky outcrops,
sands and too wet places. That the beech is locally missing is probably due to its selective
cut-out by the humans, or that it did not spread by migration yet. This concerns especially the
lowlands and the present-day oak-hornbeam forests, acidic and xerothermic oakwoods,
which should rapidly transform via „ash-tree phase“ into beech dominated communities. The
current maps of potential vegetation are therefore not perfect. The „beech hypothesis“ is
influential probably thanks to works of German authors, namely H. Ellenberg (Ellenberg
1996, Tab. 71, Fig. 87). In our more continental conditions, this does not need to have a
universal validity.
2. „Ecologial hypothesis.“ Even though there are competitively differently strong tree
species, the decisive role play ecological conditions. They alter in a rather coarse mosaics,
which enables to create maps of potential vegetation (the phytosociological approach itself).
Non-forest is merely at ecologically extreme sites. Even if the ecological mosaics would be
shaped as microsites, it would enable a co-existence of more species (of trees) in one kind of
environment. A welcomed factor is doubtless small natural disturbances, which in forest
repeatedly suppress competitively strong tree species. This hypothesis is difficult to test in
the real world, because it requires human-untouched ecosystems.
9
3. „Historical hypothesis.“ In contrary to the former, basically „successional“
hypotheses, this one does not ask about the potential state, rather than attempts to
reconstruct the historical development and its consequences. Vera (2000) claims, in contrary
to successional hypotheses, the decisive natural influence of big herbivores, whose impact
keeping the landscape of lowlands covered with a forest-steppe had been quit not so long
ago. Further, the human cultural development lasts for the whole Holocene, hence purely
natural communities had no chace to establish, which holds especially for lowlands and hilllands utilized for agriculture from the Neolite. As described by Ložek (2004), the non-forest is
a main feature of the Quaternary landscape and its considerable remnants had played an
important role in interglaciales. Also archaeological findings and evidences of historical
management show the far-reaching role of the human influence.
Observations from our „primeval“ forests (Vrška & Hort 2004) indicate that the beech
really could be a natural dominant vigorously growing almost anywhere. A main
controversion is that lessons from several isolated places should not be generalized to the
whole larger territory, especially the drier conditions of lowlands. Except for the fact that the
recent observations do not need to be valid in a long-term perspective. There is no reliable
proof that the succession really leads to a closed climax forest. In the European conditions, it
is simply not possible to ignore the humans, the nature is a cultural product with natural
components. Does it make any sense to concern with something that possibly never existed
at all, or perhaps could exist one day? Such an idealistic vision are right the successionallybased (including phytosociological) concepts of potential natural vegetation. Their meaningful
use can be found somewhere else, because they are a complex representation of site
conditions (and this is how forestry classification systems work). The main utilization of the
products of the past efforts should be directed this way.
Possible solutions
The main drawback of phytosociology is that it describes above all subjective, temporarily
changeable selections, not a majority of the real vegetation. We should decide if we want the
phytosociology system representing only selected, in time still more and more minute
vegetation types (and ignore the „uninteresting“ rest), or to try include also so far not
sampled but increasing vegetation types.
At the beginning, it would be desirable to make a notion of how the particular
vegetation types are represented in the nature. This is rather easy to make for selected
model territories, where we should apply also other than preferential samplings. A suitable
option is stratified random arrangement (see Austin & Heyligers 1989, Grabherr et al. 2003).
Quite randomized sampling can provide a representative picture of vegetation variability,
10
capturing the rare vegetation types however requires too large sample size. How would a
comparison of synchronically made samplings – one randomized, another preferential, or
even a comparison with the existing relevés – would look like? Such important questions
have unfortunatelly never been asked by phytosociology.
A question of itself is the possibility of application of phytosociology to the entire
vegetation cover. Should we be concerned rather with (semi)natural vegetation types
reflecting mainly natural factors, or also with agri- and silvicultures? What is the formal
difference between a forest culture and a monospecies growth of Lemna (and the vegetation
of hygrophytes in general), or of a fern (e.g. Polypodium)? Phytosociology should declare
what kind of vegetation it is able to describe with its methods and what shall be left to more
general, let us say „formations“ approaches. Phytosociologically capable vegetation is only
that definable by combinations of ecologically quite narrowly defined species. The
corresponding sites are usually the species-richest places in the landscapes, hot-spots of the
so important biodiversity. This implies the importance of phytosociology for the nature
conservation. Vegetation composed of ecologically poorly delimited species can however be
only classified using some formation-based system, or one of the approaches suggested in
part 3. Looking at the landscape, especially the recent one, makes one to assume that the
phytosociology is not applicable to more than 10% of the ground, depending on the regional
differences.
It is obvious that the fresh-minded phytosociologists are well aware of the mentioned
shortcomings. Indicated solutions are attainable and there are doubtless many other
approaches even to newly-emerged problems. The purpose of this paper was nothing more
than contribute to a serious discussion about the Central European vegetation science.
Acknowledgements
I would like to thank Milan Chytrý for his comments which increased the quality of this paper.
I am indebted to Ilona Knollová for providing the relevés from the Czech National
Phytosociological Database, used in the illustration example. Inspiring thoughts followed
from numerous discussions with Karel Boublík and Péter Szabó. I thank to Jiří Danihelka for
his careful language correction.
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