Plant Ecology 共2005兲 177:113–124 DOI 10.1007/s11258-005-2187-x
© Springer 2005
Relationships between the vegetation and soil conditions in beech and beech-oak forests of northern Germany Werner Härdtle*, Goddert von Oheimb and Christina Westphal Institute of Ecology and Environmental Chemistry, University of Lüneburg, 21332 Lüneburg, Germany; *Author for correspondence (phone: +49-4131-782842; fax: +49-4131-782807; e-mail:
[email protected]) Received; accepted in revised form
Key words: Fagion, Forest ecology, Forest vegetation classification, Northern Germany, Quercetalia roboris, Site gradients
Abstract In this study we examine the relationships between the vegetation of beech and beech-oak forest communities 共Hordelymo-Fagetum, Galio-Fagetum, Deschampsio-Fagetum, Betulo-Quercetum兲 and their soil conditions in the lowlands of northern Germany, based on 84 sample plots. In all plots the vegetation was recorded and soil parameters were analysed 共thickness of the O- and the A-horizons, pH, S-value, base saturation, C/N, mean Ellenberg moisture indicator value兲. The vegetation classification according to the traditional Braun-Blanquet approach was compared with the result of a multivariate cluster analysis. Vegetation-site relationships were analysed by means of an indirect gradient analysis 共DCA兲. Both traditional classification methods and the cluster analysis have produced comparable classification results. So far as the species composition is concerned, a similar grouping of sample plots was found in both approaches. Multivariate cluster analysis thus supports the classification found by the Braun-Blanquet method. The result of the DCA shows that the four forest communities mentioned above represent clearly definable ecological units. The main site factor influencing changes in the species composition is a base gradient, which is best expressed by the S-value. In addition, within the series Hordelymo-Fagetum – Galio-Fagetum – Deschampsio-Fagetum the C/N-ratios and the thickness of the organic layers 共O-horizon兲 increase continuously. By contrast, the floristic differences between oligotrophic forest communities 共i.e., Deschampsio-Fagetum and Betulo-Quercetum兲 cannot be explained by a base gradient and increasing C/N-ratios. It is suggested that a different forest management history in some cases 共e.g., promotion of Quercus robur by silvicultural treatments兲 is responsible for differences in the species composition, but on the other hand the result of the DCA indicates that Fagus sylvatica is replaced by Quercus robur with increasing soil moisture 共i.e., with the increasing influence of a high groundwater table兲. Summarizing these results, it can be concluded that the ecological importance of single site factors affecting the species composition changes within the entire site spectrum covered by the beech and beech-oak forests of northern Germany.
Introduction Meso- and eutrophic beech and acidophytic mixed beech-oak forests 共of the alliance Fagion and the order Quercetalia roboris兲 are the most important forest communities in natural forest landscapes in
Central Europe 共Bohn et al. 2000兲. Although the natural areal of beech and beech-oak forests has decreased considerably due to forest clearances since the Middle Ages and to the introduction of conifer plantations with Norway spruce 共Picea abies兲 and Scots pine 共Pinus sylvestris兲 during the 20th century,
114 beech and beech-oak forests still dominate large regions in Central Europe particularly in the submontane and montane altitudinal zones 共Diekmann et al. 1999兲. Recent years have seen numerous studies devoted to the description and analysis of the vegetation and the site conditions of beech and beech-oak forests, mainly in the submontane and montane zone 共Dierschke 1989; Müller 1989; Brunet et al. 1996; Ellenberg 1996; Ewald 1997; Graae and Heskjaer 1997; Grabherr et al. 1998兲. However, there are few if any studies on the vegetation of beech and beechoak forests combined with an analysis of their site conditions in the lowlands of northern Central Europe although the beech tree is the most competitive tree species in this region. In this context it is important to note that knowledge about vegetation-site relationships in the low mountain range is not automatically transferable to conditions in the lowlands, due to differences in the climate, pedogenetic processes and the different ecological behaviour of herbaceous woodland species in these landscapes 共Diekmann and Lawesson 1999兲. The main objective of this study is to examine the relationships between beech and beech-oak forest communities and their soil conditions in the lowlands of northern Germany, using stands in Schleswig-Holstein as examples. Two main questions have been addressed in this investigation: – To what extent do forest communities defined according to the traditional Braun-Blanquet approach 共Braun-Blanquet 1964; Westhoff and van der Maarel 1978; Dierssen 1990; Dierschke 1994兲 coincide with units obtained by a multivariate cluster analysis? Our comparison demonstrates whether ‘manual’ classification results are supported by a multivariate cluster analysis. – What are the main ecological differences and site gradients between beech and beech-oak forest communities in northern Germany, and which site parameters 共soil properties兲 are responsible for the differences in their species composition? The following four beech and beech-oak forest communities 共associations and subassociations兲 were considered in this comparison: Hordelymo-Fagetum, Galio-Fagetum, Deschampsio-Fagetum and Betulo-Quercetum 共in the following text we use the abbreviations HF, GF, DF and BQ兲.
Material and methods Study area The study area is located in the north-western part of the lowlands of Central Europe 共northern Germany, Schleswig-Holstein; 8° 30⬘-11°E, 53° 30⬘-55° N兲 and comprises a geographic area of approximately 15700 km2. It is characterized by sediments of the penultimate 共Saale兲 and last 共Weichsel兲 glacial periods and is situated in the temperate zone. The climate is subhumid/temperate with an annual precipitation of 550 mm 共eastern parts兲 to 900 mm 共north-western parts兲 and with mean monthly temperatures of approximately 0 °C in January and 17 °C in July. Vegetation analysis Between 1995 and 1998 in this area, a total of 84 sample plots was chosen to represent the range of edaphic conditions found in beech and beech-oak forests of the lowlands of northern Germany. Each plot was square 共100 m2 in size兲 and was established in different stands of beech and beech-oak forests within the study area. In each plot, the vegetation was recorded and the relevés finally processed into a synthesising synopsis following the Braun-Blanquet approach 共Dierssen 1990; Dierschke 1994兲. The vegetation units obtained were distinguished at the rank of subassociations, which could be assigned to the four beech and beech-oak forest communities 共HF, GF, DF and BQ兲. In order to compare different classification methods, all relevés were subjected to a cluster analysis 共agglomeration method: average-linkage, similarity index: Sørensen coefficient, Jongman et al. 1987; program: Sort 4.0, Ackermann and Durka 1998兲. As vegetation-site relationships are affected not only by recent soil conditions but also by historical factors 共e.g., historical management practices兲 and structural parameters 共Aude and Lawesson 1998兲, the following criteria underlay the selection of the sample plots in addition to the criteria of the Braun-Blanquet method: – the age of the trees ranged from 80 to 120 years; – all the stands investigated in this study were ancient woodlands 共in the sense of Rackham 1980 and Peterken 1996兲 and have existed for at least 250 years; – the species composition of the trees at all the stands was autochthonous;
115 – all the stands have been managed in a semi-natural manner, and no trees have been felled in the plots for at least 10 years. Nomenclature follows Oberdorfer 共2001兲 for vascular plants, Smith 共1980兲 for bryophytes, Dierschke 共1989, 1990兲 and Ellenberg 共1996兲 for syntaxa. Soil sampling and laboratory methods In previous studies dealing with vegetation-soil relationships, the soil chemical properties of the upper mineral horizon have often been considered 共i.e., the A-horizon兲. This procedure is consistent, provided that the upper mineral soil horizon contains the major part of the fine and coarse roots and thus reflects the ecological soil conditions under which the vegetation develops. So far as the forest communities considered in this study are concerned, this procedure would be problematic from two points-of-view: Firstly, the main root horizons 共of fine and coarse roots兲 of the forest vegetation on acid soils 共e.g., podzolic soil or Podzols兲 are not the upper mineral layers but the organic layers 共i.e., the Of- and the Oh-horizons兲, where more than 90% of the fine and coarse root biomass of the trees and the herbaceous plants is located 共Göttsche 1972兲. In strongly acid soils, therefore, only the organic layers are responsible for the nutrient supply of the forest vegetation 共Leuschner and Rode 1999, Leuschner et al. 2001兲, and the soil chemical properties of the upper mineral horizon do not reflect the growth conditions of forest plants on acid soils. Secondly, soil chemical properties in the upper mineral horizon differ more or less markedly from those in the organic layers 共Härdtle et al. 2004兲. Consequently, soil samples from the main root horizons 共for fine and coarse roots兲 were analysed in this study. In eutrophic forest communities, the main root horizon for the forest vegetation is the Ah-horizon 共forest communities nos 1–4 according to Table 1兲. In mesoto oligotrophic 共acidophytic兲 forest communities, the main root horizons are the organic layers 共Of- and Oh-horizon; forest communities nos 5–9 according to Table 1兲. In the centre of all 84 plots, we sampled the upper mineral horizon 共A-horizon兲 and, when developed, the organic layers 共Of- and Oh-horizon兲. Each soil sample consisted of three subsamples which were taken from the entire depth of a single horizon. Subsamples were thoroughly mixed to obtain one sample per soil horizon and plot. The main root horizon 共A-
or O-horizon兲 was detected according to Schlichting et al. 共1995兲, by estimating the density of fine and coarse roots on a 100 cm2 area in the O- and A-horizon of a profile. Each soil sample was dried for 24 h at 105 °C, crushed and passed through a 2 mm sieve. In each horizon we analysed parameters which are considered to be meaningful chemical features for the nutritional state of forest soils 共Gönnert 1989; Heinken 1995; Brunet et al. 1997; Matschonat and Falkengren-Grerup 2000兲. These parameters are: – pH共H2O兲-value, determination according to Steubing and Fangmeier 共1992; soil:distilled water ratio 1:2.5, measurement with a WTW glass electrode in the soil solution兲; – S-value 共mval/100 cm3兲 and base saturation 共%兲, determination according to the titration method of Brown 共1943; extraction of base cations and acid cations from a 20 g soil sample with 100 ml 1n acetic acid and 1n ammonium acetate, respectively; titration with 0.1n HCl 共titrisol兲 and 0.1n NaOH 共titrisol兲, respectively兲; – C/N-ratio; determination with a C/N analyser 共Heraeus Rapid兲. In addition to the soil chemical analyses, we investigated soil morphological properties 共thickness of the O-, Ah- and Ae-horizons in cm兲 and the lime content in the subsoil 共in 80 cm soil depth; determination according to Steubing and Fangmeier 共1992兲: 10 ml 5n HCl was added to 2 g of a soil sample, and the volume of the released CO2 was determined兲. These soil parameters give additional information on the trophic level and the pedogenetic state of a forest soil 共Rehfuess 1981, Ewald 2000兲. Analysis of vegetation-soil relationships To give an overview of the variability of single soil parameters within the forest subtypes investigated, mean values and standard deviation were calculated 共Table 2兲. Table 3 shows a correlation matrix for the soil parameters investigated. In order to analyse vegetation-site relationships, floristic data were subjected to indirect gradient analysis 共DCA, using CANOCO version 4.02, ter Braak 1991兲. DCA was used instead of CA to avoid the arch effect 共Jongman et al. 1987兲. For an ecological interpretation of the ordination result, scores of plots of the first two DCA axes 共with the highest eigenvalues兲 were correlated with corresponding measurements of environmental variables 共Spearman rank correlation, SPSS, version 11.5;
Stellaria holostea Anemone nemorosa Oxalis acetosella Milium effusum Polygonatum multiflorum Mnium hornum Athyrium filix-femina Hedera helix Convallaria majalis Poa nemoralis Mycelis muralis
Lamiastrum galeobdolon Galium odoratum Melica uniflora Sambucus nigra Atrichum undulatum Carex sylvatica Festuca altissima Dryopteris filix-mas Carex remota Impatiens noli-tangere Viburnum opulus Scrophularia nodosa Pulmonaria obscura Phyteuma spicata
CC
OC
Quercus petraea
Quercus robur
Fagus sylvatica
Trees Fraxinus excelsior
Vegetation type no. Number of sample plots Mean number of species
IV IV
V V IV
III IV
V
IV II
III V
IV IV IV III IV III
IV III
S-G IV
S-G
T1 G T1 T2 S G T1 G T1 G
1 4 36
II I I I I I I
IV V IV III II III
III V IV IV V IV III I I I
IV IV IV IV II III II I
2 15 34
V V V III IV IV I III III III I II I
V V V V III IV IV I II I I
II V V IV I V III II
3 20 34
III
II III III
V V V IV IV I
I I
V V V V IV I V I
I V V IV II V I II
4 5 28
GF circaeetosum
HF typicum
HF lathyretosum
HF corydaletosum
Galio-Fagetum
Hordelymo-Fagetum
I
III III II II I I
V IV IV III III
II I
V V V V III III V III
IV III II
I V V V
5 9 23
GF typicum
I
I
V IV V II II I III
I
V V V V IV IV II IV III
IV V IV II V II I
6 13 24
GF polytrichetosum
I
II I
V III IV IV IV V I II
V V II I V II III
7 6 27
DeschampsioFagetum
V V V V V V I II V
II III I III V II III II
8 7 28
BQ milietosum
I
I V
III
V
III II IV IV
I II II
9 5 29
BQ typicum
Betulo-Quercetum
Table 1. Species composition and species frequency of the forest communities investigated. Only species with a frequency of more than 40 % in at least one forest subtype are considered. Explanation of the abbreviations: T1/T2 ⫽ first and second tree layer, S ⫽ shrub layer, G ⫽ ground vegetation; CC ⫽ character species of the class Querco-Fagetea; OC ⫽ character species of the orders Fagetalia and Quercetalia roboris, respectively; AC ⫽ character species of the associations Hordelymo-Fagetum, Deschampsio-Fagetum and Betulo-Quercetum, respectively; d ⫽ differential species for subassociations. Definition of constancy classes: I ⫽ ⬎ 10 ⫺ 20%, II ⫽ ⬎ 20 ⫺ 40%, III ⫽ ⬎ 40 ⫺ 60%, IV ⫽ ⬎ 60 ⫺ 80%, V ⫽ ⬎ 80 ⫺ 100%.
116
Stachys sylvatica Mercurialis perennis Brachypodium sylvaticum Vicia sepium Fissidens taxifolius Ranunculus auricomus Hordelymus europaeus Aegopodium podagraria Equisetum telmateia Euonymus europaea Sanicula europaea
Actaea spicata Carex digitata Homalia trichomanoides Hepatica nobilis Campanula trachelium Lathyrus vernus
Anemone ranunculoides Allium ursinum Gagea lutea Corydalis cava
Ranunculus ficaria Viola reichenbachiana Circaea lutetiana Urtica dioica Gagea spathacea Adoxa moschatelina
Luzula pilosa Maianthemum bifolium Avenella flexuosa Polytrichum formosum Carex pilulifera Holcus mollis Agrostis tenuis Plagiothecium curvifolium Anthoxanthum odoratum Leucobryum glaucum
Hieracium laevigatum Hieracium murorum Luzula campestris agg.
Trientalis europaea
AC
d
d
d
OC
AC
AC
Table 1. Continued.
IV V
IV IV III III III II
III III IV IV IV III II III IV III III
V II II III I II
III II II II
III IV II I I II I II I I
V III V IV II I
I I
V IV III III I I III I
IV IV III IV IV III
GF circaeetosum
HF typicum
HF lathyretosum HF corydaletosum
Galio-Fagetum
Hordelymo-Fagetum
III
I I
GF typicum
IV V II III
I
I
GF polytrichetosum
IV III III
V IV V V V III IV II II II
DeschampsioFagetum
V
IV V V III III IV II II II I
BQ milietosum
V
IV IV V III IV III I II II I
BQ typicum
Betulo-Quercetum
117
Others T2-G
Sorbus aucuparia Deschampsia cespitosa Lonicera periclymenum Dicranella heteromalla Rubus fruticosus agg. Rubus idaeus Dactylis glomerata agg. Galeopsis bifida agg. Dryopteris dilatata Eurhynchium praelongum Hypnum cupressiforme Frangula alnus Brachythecium rutabulum Ulmus glabra Carpinus betulus Crataegus laevigata agg. Lophocolea heterophylla Dryopteris carthusiana Isopterygium elegans Taraxacum offıcinale agg. Isothecium myurum III
V
IV
1
IV III I
III I I I
III
I III I II II I III I
IV
2
I
II I I I I
I IV II I III IV IV II I III I
IV
3
I
I IV
III I I
II III I I I I
V
4
GF circaeetosum
HF typicum
HF lathyretosum HF corydaletosum
Galio-Fagetum
Hordelymo-Fagetum
G III G S-G V
T2-G
S-G S-G
S-G
G
Acer pseudoplatanus
Vegetation type no.
Molinia caerulea Pteridium aquilinum Dicranum scoparium Plagiothecium undulatum Vaccinium myrtillus Carex nigra
Table 1. Continued.
I I
III III
III V II IV IV IV
V
5
GF typicum
I
III I I
II I
V V V IV IV IV II I II
III
6
GF polytrichetosum
III
II
II
II
V III V V II I IV
V
7
I
DeschampsioFagetum
I III
I
III III I V IV
V II V IV V IV
III
8
I
III III II
BQ milietosum
III II
I
III III II III IV III
V II V IV IV III
9
V IV IV IV III III
BQ typicum
Betulo-Quercetum
118
10.4 0.0 0.5 8.3 1.3 0.5 0.5 18.0 0.0 1.5 16.7 21.4 3.3 5.9 1.5 0.8 0.3 5.5 2.2 0.4 0.3 0.6 3.7 0.7 9.8 2.3 0.4 0.2 0.9 1.8 1.3 9.5 4.5 0.3 0.2 O Ah S BS C/N pH M
1.0 25.0 130.2 100 12.6 7.5 5.3
0.0 8.7 31.9 0.0 1.3 0.3 0.2
1.1 34.2 34.1 75.2 14.3 5.5 5.5
0.4 1.5 10.4 20.9 43.7 7.2 19.0 52.0 2.3 14.5 1.1 4.3 0.2 5.7
0.6 2.7 6.1 15.0 1.8 5.3 22.2 32.8 1.6 19.4 0.6 3.7 0.1 5.6
0.4 4.0 2.6 11.3 1.9 3.2 10.5 17.6 2.7 21.4 0.2 3.4 0.2 5.2
4.4 9.9 2.2 25.5 23.0 3.3 5.2
sd mean sd mean
sd
mean
sd
mean
sd
mean
sd
mean
mean
4.5 5.3 1.9 22.6 26.3 3.3 5.2
0.9 7.1 4.8 0.7 0.8 1.1 11.1 14.4 1.2 21.1 0.5 3.1 0.1 5.2
sd mean sd
Classification of the forest communities according to the Braun-Blanquet approach
sd
6 14 8 5 20 15 4 number of site samples 共n兲
Table 4兲. The matrix of soil morphological and chemical variables was standardized to mean 0 and variance 1 prior to ordination.
Results
mean
5 7
9 8 7 6 5 4 3 2 1 forest community
Table 2. Mean values 共mean兲 and standard deviation 共sd兲 for all soil parameters investigated. Forest communities: 1: Hordelymo-Fagetum lathyretosum, 2: HF corydaletosum, 3: HF typicum, 4: Galio-Fagetum circaeetosum, 5: GF typicum, 6: GF polytrichetosum, 7: Deschampsio-Fagetum, 8: Betulo-Quercetum milietosum, 9: BQ typicum. Abbreviations for soil parameters: O ⫽ thickness of the organic layers 共cm兲, Ah ⫽ thickness of the Ah-horizon 共cm兲, C/N ⫽ C/N-ratio, BS ⫽ base saturation 共%兲, pH ⫽ pH共H2O兲-value, S ⫽ S-value 共mval/ 100cm3兲, M ⫽ mean Ellenberg indicator value for the soil moisture.
119
Table 1 gives an overview of the forest communities investigated in this study by means of a synthesising synopsis. Four forest communities 共associations兲 are distinguished, with a total of 9 forest subtypes 共subassociations兲. According to Table 1, each subtype is characterized by one or more groups of at least three differential species. In the Table, the four forest communities are assigned to the following column numbers: nos 1–3 HF, nos 4–6 GF, no 7 DF, nos 8–9 BQ. Compared with the descriptions given by Dierschke 共1989, 1990兲 and Ellenberg 共1996兲, the species composition of subtypes distinguished differs little or not at all from that of other regions of Central Europe. Comparison of the Braun-Blanquet classification and the cluster analysis Figure 1 shows the dendrogram obtained by the cluster analysis for the 84 plots. With regard to the syntaxonomic level of the order 共Fagetalia sylvaticae and Quercetalia roboris兲 and the alliance 共Quercion roboris and Luzulo-Fagion兲, the dendrogram coincides with the Braun-Blanquet classification. The orders Fagetalia and Quercetalia are distinguished on a similarity level of about 30%. The same applies for the alliances Quercion and Luzulo-Fagion on a similarity level of about 50%. Even on the levels of the association and the subassociation 共this applies particularly for plots of the BQ, the GF and the HF lathyretosum兲, the dendrogram corresponds well with the classification result given in Table 1. However, the most conspicuous disparity is with regard to plots of the GF circaeetosum, which are more closely linked to the HF typicum in the dendrogam. Summarizing the results of this comparison, it can be concluded that in principle the cluster analysis supports the classification result obtained by the traditional BraunBlanquet approach.
120 Table 3. Spearman rank correlation matrix for the soil parameters investigated. Abbreviations for soil parameters: O ⫽ thickness of the organic layers 共cm兲, Ah ⫽ thickness of the Ah-horizon 共cm兲, Ae ⫽ thickness of the Ae-horizon 共cm兲, C/N ⫽ C/N-ratio, BS ⫽ base saturation 共%兲, pH ⫽ pH共H2O兲-value, S ⫽ S-value 共mval/100 cm3兲, M ⫽ mean Ellenberg indicator value for the soil moisture, LC ⫽ lime content in the subsoil 共%兲. O Ah Ae C/N BS pH S M
Ah ⫺ 0.88**
Ae 0.61** ⫺ 0.64**
C/N 0.69** ⫺ 0.68** 0.32*
BS ⫺ 0.74** 0.73** ⫺ 0.52** ⫺ 0.75**
pH ⫺ 0.77** 0.74** ⫺ 0.45** ⫺ 0.76** 0.86**
S ⫺ 0.83** 0.82** ⫺ 0.60** ⫺ 0.81** 0.90** 0.88**
M ⫺ 0.36** 0.41** ⫺ 0.03 ⫺ 0.45** 0.31* 0.33* 0.38**
LC ⫺ 0.31* 0.40** ⫺ 0.16 ⫺ 0.46** 0.53** 0.55** 0.53** 0.07
** ⫽ p ⬍ 0.001, * ⫽ p ⬍ 0.01
Table 4. Spearman rank correlation between soil parameters 共n ⫽ 84兲 and ordination scores of plots for the first two DCA axes. Abbreviations for soil parameters: S ⫽ S-value 共mval/100 cm3兲, Ohorizon ⫽ thickness of the organic layers 共cm兲, Ah-horizon ⫽ thickness of the Ah-horizon 共cm兲, C/N ⫽ C/N-ratio, Ae-horizon⫽ thickness of the Ae-horizon 共cm兲, LC ⫽ lime content in the subsoil 共%兲, M ⫽ mean Ellenberg indicator value for the soil moisture.
S O-horizon Ah-horizon C/N Ae-horizon LC M eigenvalue % of variance
DCA axis 1
DCA axis 2
⫺ 0.88** 0.87** ⫺ 0.85** 0.75** 0.65** ⫺ 0.44** ⫺ 0.35* 0.55 10.6
⫺ 0.32* 0.30* ⫺ 0.37** 0.41** 0.01 0.04 ⫺ 0.62** 0.28 5.3
* ⫽ correlation significant on the level of p ⬍ 0.01; total inertia 5.20
Correlation of soil parameters The correlation matrix in Table 3 indicates a close correlation for some of the site parameters investigated. This particularly applies for parameters describing the base supply 共pH, base saturation and S-value兲. In addition, the C/N-ratio is related to some of the base parameters, but shows a much weaker correlation with the soil morphological properties 共e.g., thickness of the Ae-horizon兲. The weakest correlation with other soil properties is found for the soil moisture. These results suggest omitting two of the base variables when calculating the correlations between DCA ordination scores of plots and corresponding environmental variables. Among the base
variables, we selected the S-value because of its closest correlation with the first DCA axis 共Table 4兲. Comparison of vegetation and soil parameters In Figure 2 the species composition of the plots has been evaluated by means of a DCA. Correlations between the ordination scores of the plots and corresponding measurements of environmental variables are shown in Table 4 and by means of vectors in the ordination diagram. The first two DCA axes account for a total of 15.9% of the variance in species data. According to Table 4, the first axis in the DCA represents a ‘trophic gradient’, in which with increasing scores the base supply decreases 共expressed by decreasing S-values兲. In addition, C/N-ratios increase and the humus form changes from Mull to Mor 共raw humus兲 as expressed by the increasing thickness of the O- and Ae-horizons. In the ordination diagram, plots are almost clearly separated on the level of the association and partly on the level of the subassociation. Plots of the HF lathyretosum and the HF corydaletosum show the lowest scores on the first DCA axis. By contrast, plots of the DF and the BQ are characterized by high ordination scores. However, whilst plots of the HF, GF and DF form a trophic gradient represented by the first DCA axis, differences in ordination scores on this axis are low for the DF and the BQ. This particularly applies for plots of the DF and the BQ milietoseum. Obviously stands of the DF and the BQ differ with regard to both the degree of podzolisation of soils and the soil moisture, which is in the first place expressed by the second DCA axis 共Table 4兲. Furthermore, soil moisture continuously increases within the series GF polytrichetosum-GF typicum-GF circaeetosum-HF typicum-HF
121 Discussion Comparison of the Braun-Blanquet classification and the cluster analysis
Figure 1. Dendrogram obtained by the cluster analysis 共agglomeration method: average-linkage, similarity index: Sørensen coefficient兲 for the 84 plots; vegetation types according to Table 1: 1: HF lathyretosum, 2: HF corydaletosum, 3: HF typicum, 4: GF circaeetosum, 5: GF typicum, 6: GF polytrichetosum, 7: DF, 8: BQ milietosum, 9: BQ typicum.
corydaletosum. This is indicated by the vector for soil moisture in the ordination diagram. Coinciding with the result of the cluster analysis, plots of the GF circaeetosum overlap with those of the HF typicum.
Results of a cluster analysis mainly depend on the underlying algorithm. So if the classification results of a multivariate cluster analysis are compared with those of the Braun-Blanquet method, a similarity index must be defined that corresponds to a great extent to the criteria of the ‘manual’ method. In the cluster analysis we used the Sørensen coefficient as the similarity index, because in the Braun-Blanquet approach the quality more than the quantity of a given species composition is assessed 共Dierschke 1994兲. In addition, soil conditions in woodlands are reflected to a lesser extent by the degree of cover of a single species than by presence-absence data. This becomes particularly obvious when species of different growth forms and size are considered 共e.g., many species of the ground vegetation: bryophytes or Gagea spathacea on the one hand and Mercurialis perennis or Stachys sylvatica on the other兲. On the basis of a comparable similarity index, the result of the cluster analysis supports the classification obtained by the Braun-Blanquet method. This finding is in agreement with other attempts to detect parallelisms between the Braun-Blanquet method and numerical approaches 共Wildi 1989; Bruelheide 1995兲. Nevertheless, an important disparity in the cluster analysis is the assignment of plots of the GF circaeetosum to the HF. This indicates a heterogeneity of the GF from a floristic point-of-view, which may be interpreted as a result of the application of the ‘character species theory’. According to this theory, beech forests on mesophytic sites must be classified as a separate association as they lack their own character species 共Dierschke 1989, 1994兲. Consequently the Braun-Blanquet approach may lead to a somewhat artificial grouping, in which single units may be well defined floristically but do not reflect floristic discontinuities within a broader floristic gradient. Comparison of vegetation and soil parameters The result of the ordination demonstrates that floristic gradients within beech forests 共HF, GF, DF兲 coincide significantly with properties of the uppermost soil horizons. The main phytosociological gradients in our data set appear to be closely related to conservative features of the abiotic environment, notably the
122
Figure 2. DCA ordination of all sample sites 共n ⫽ 84兲, which are assigned to forest communities 共subassociations兲 defined according to Table 1 共abbreviations for forest communities: HF ⫽ Hordelymo-Fagetum, GF ⫽ Galio-Fagetum, DF ⫽ Deschampsio-Fagetum, BQ ⫽ BetuloQuercetum; two plots assigned to the HF lathyretosum show extrem values on axis 2兲. Vectors indicate correlations between ordinations scores of plots and corresponding environmental variables.
Ah-depth, the presence of base cations and the C/Nratio. This result coincides with other findings on the impact of these soil properties on the understorey vegetation of woodlands in Central Europe 共Gönnert 1989; Heinken 1995; Brunet et al. 1996, 1997a; Aude and Lawesson 1998; Ewald 2000兲. In agreement with the cluster analysis, the ordination diagram underpins the result of the heterogeneity of the GF from an ecological point-of-view. While the GF circaeetosum shows great affinity with the HF typicum, the GF polytrichetosum may form a transitional unit to the Quercetalia. Beech forest communities defined according to the Braun-Blanquet method may thus represent clearly definable ecological units, but are less appropriate for expressing ecological discontinuities within a broader ecological gradient 共i.e., within beech forest communities兲.
With regard to eutrophic forest communities 共HF, GF兲, the ordination indicates an increase in soil moisture with an increase in base supply. This phenomenon applies particularly to soil conditions in the lowlands, where the majority of bases such as Ca2⫹ or Mg2⫹ are released from the marly till with the groundwater and are transported to the plant roots 共Gönnert 1989兲. In addition, the mineralisation processes require a certain level of soil moisture and are assisted by high pH-values 共Pausas and Austin 2001兲. By contrast to the series HF-GF-DF, floristic differences between the DF and the BQ cannot only be explained by a base gradient or increasing C/N-ratios. Whilst plots of the BQ milietosum and the BQ typicum are clearly separated on the first axis due to increasing podzolisation features, plots of the DF and the BQ milietosum only differ with regard to ordina-
123 tion scores on the second axis. This confirms the findings of other authors according to whom acidophytic beech forests differ little or not at all in their base and total nitrogen supplies compared to acidophytic mixed beech-oak forests, provided that the humus layers are not disturbed or removed 共Heinken 1995; Leuschner and Rode 1999兲. In our opinion, one of the main reasons for the differences in the tree species composition of the DF and the BQ is in some cases primarily a different forest management history rather than differences in the soil parameters. The dominance of Quercus robur or Quercus petraea in stands of the BQ can be explained mostly by the promotion of oaks by forestry, as can be verified from forestry records from the history of silvicultural treatments in different lowland forest districts 共Wulf 1992兲. Consequently, as many characteristic species of acidophytic mixed beech-oak forests are photophilous, they become more abundant in the herb layer with an increasing proportion of oaks in the tree layer 共Heinken 1995兲. As a reverse trend, these species die out when the crown density increases, that is to say beneath the strongly shading canopy of the beech tree 共Brunet et al. 1997b兲. Under a natural succession, Quercus robur and Q. petraea would be replaced by Fagus sylvatica so that stands of the BQ would develop into acidophytic beech forest communities. This succession can be observed in forest reserves where no silvicultural measures are taking place due to the protective envelope of a natural forest development 共Koop and Hilgen 1987兲. The absence or low frequency of regenerating oaks due to the increasing competition from beech in the plots investigated support this hypothesis 共Table 1, vegetation types 8 and 9兲. Stands of the BQ where Fagus sylvatica is rare or even absent appear under natural conditions in the lowlands on Regosols, when the humus horizons are not or weakly developed. From a dynamic point-ofview, the BQ can be considered as a young stage of the DF, as the beech becomes more abundant according to the development of the humus horizons. In addition, beech may be absent on Podzols with a more or less high groundwater table. The higher the mean level of the groundwater table in Podzols 共e.g., Podzol-Gleysols兲, the lower is the competitive capacity of Fagus sylvatica 共Diekmann et al. 1999; Härdtle et al. 2003兲. The natural proportion of Quercus robur in the canopy therefore increases under these site conditions. However, these explanations can only be verified by incorporating historical parameters into the context of a forest community-site type comparison.
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