Herb layer species as indicators of biodiversity of managed and unmanaged beech forests
Wolfgang Schmidt
Institute of Silviculture and Forest Ecology, Georg-August-University of Göttingen, Büsgenweg 1, D-37077 Göttingen, Germany.
wschmid1@gwdg.de
Abstract
Information on changes in the herb layer of unmanaged forest nature reserves may be useful to assess present-day issues of management relating to sustainability and biodiversity in forestry and nature conservation. Selected floristic and vegetation studies from forest nature reserves in the German states of Lower Saxony and Hesse showed that the number of vascular plant species was higher in forests that are managed than in completely unmanaged forests. Species richness declined significantly during a ten-year period after the active management of the forest reserves had stopped. In both cases, species richness was related to non-forest plant species that depend on high light intensity and soil disturbances. Beech (Fagus sylvatica)became dominant in unmanaged forests due to its high shade tolerance, providing it with a competitive advantage over most other species. However, after large-scale disturbances such as windthrow, species richness increased again even in unmanaged forests.
Keywords: reference conditions, species diversity, vegetation structure, Ellenberg’s indicator values, forest species, beech forest, disturbance, windthrow
1 Introduction
Since the United Nations Conference on Environment and Development Process (UNCED) of Rio de Janeiro (1992) and the Ministerial Conference on the Protection of Forests in Europe of Helsinki (1993), biodiversity has become one of the main topics of forest management. Biological diversity implies the variety of living organisms and includes diver- sity within species, between species and of ecosystems and the ecological processes of which they are a part (GASTONand SPICER2004). Species diversity is considered to be one of the key parameters characterizing ecosystems and a key component of ecosystem functioning (HUTCHINSON 1959; SCHULZE and MOONEY 1994; LARSSON 2001; LOREAU et al. 2002;
SCHERER-LORENZENet al.2005). Accordingly, the causes of different species numbers and the factors determining differences in diversity have been intensively investigated. Site disturbance in managed and unmanaged European forests provides the driving force for forest dynamics and regeneration through structural change, the initiation of succession and creation of habitat diversity (PICKETTand WHITE1985; QUINEet al.1999). According to the disturbance hypothesis of CONNELL (1978), the highest number of species is expected in areas with an intermediate frequency and amplitude of disturbance, a pattern first described
by EGGELING(1947). Randomly occurring periodic disturbances by management or natural catastrophic events such as wind storms are known to maintain high species richness and limit competitive exclusion (HUSTON1979; PETRAITISet al.1989; BRUNETet al.1996, 1997;
OHEIMB 2002). Diversity cannot be characterized adequately by the number of species alone; the quality or functional aspects of the species within the forest ecosystem have to be included (SCHULZEand MOONEY1994;VANANDEL1998; LOREAUet al.2002; SCHERER- LORENZEN et al. 2005). To evaluate diversity of forest ecosystems two methodological questions were posed at the beginning of the scientific discussion:
1 The first question relates to indicators: Which groups of species are suitable for efficiently measuring biodiversity in forests? Forest ecosystems differ in structure and species com- position. A forest ecologist distinguishes forest ecosystems by the vertical and horizontal structure of tree species, whereas a vegetation ecologist describes forest ecosystems using all visible plant species, i.e. higher plant (cormophyte) taxa, mosses and lichens. Due to the high number of species, their specific site requirements and ease of assessment in the field, higher plants of the understorey flora and vegetation are most suitable indicators for site conditions, environmental changes, human impact and forest dynamics (SCHMIDT
1999; SCHMIDTand WECKESSER2002).
2 The second question relates to reference conditions: What kind of baseline is adequate to assess biodiversity in forests? Reference plots or so-called standard values (boundary or threshold values) play an important role in monitoring research and in the certification of forests. Forest ecosystems that have not experienced any direct or indirect human impact are in a quasi-balanced stage in terms of time and space and are preeminently suited for establishing reference conditions. However, such natural forests are rare in central Europe. Even some of the most intact virgin forests, such as Fontainebleau, Rothwald or Hasbruch have long suffered from direct human impacts (BÜCKING1997; PARVIAINEN
et al. 1999) and have also been subjected to indirect impacts such as eutrophication caused by nitrogen input (BOBBINKet al.1998; SCHMIDT1999). Possible alternatives for use as baseline data sources include unmanaged forests such as forest nature reserves or core areas of national parks or biosphere reserves (THOMASet al.1995; SCHMIDT and WECKESSER2002). However, these reserves are not yet in a balanced stage as they have been left unmanaged only for the last few decades (in some cases for about a century).
The potential for and limits of using species lists of higher plants to answer both questions are presented from a number of beech forest sites in central Germany (Fig. 1). The sites included both unmanaged forest nature reserves and managed forests. Comparisons were carried out at two levels: 1) unmanaged forests were directly compared with paired managed forests sampled at the same time; and 2) before-and-after permanent plot data were com- pared both from intact forest stands in which management had been discontinued as well as in stands subjected to large-scale natural disturbance (windthrow). Data from forest nature reserves are increasingly important as a baseline against which data from managed forests can be compared, provided they are sampled using the same methods and parameters.
Baseline data from forest nature reserves thus provide essential arguments in favour of sustainable forestry, including the protection and maintenance of biodiversity (SCHUCKet al.
1994; BEESE1996). In the past the scientific and public discussion was often determined by the opinion that forest management reduces species diversity of forests while virgin forest per se has the highest species richness. Besides answering the above-mentioned method- ological questions, the results presented should also be used to test the following three hypotheses:
1 Species diversity in managed forests is higher than in the unmanaged forest nature reserves.
2 The proportion of plant species from open and/or disturbed habitats decreased and those of specialized forest plant species increased with time in unmanaged forests.
3 Plant species diversity increased only after large-scale disturbance (i.e. catastrophic events) in unmanaged forests.
Finally, the causes of differences in higher plant species diversity of forest ecosystems and the importance of biodiversity for forest nature conservation will be discussed.
Fig. 1. Location of the investigated unmanaged forest nature reserves and adjacent managed forests.
2 Material and methods
A permanent plot concept was developed for understorey vegetation studies that proved suitable both for unmanaged forest nature reserves and for managed forests (THOMASet al.
1995). Study sites are marked in a grid network system of 100 m ×100 m (Fig. 2). This system is the basis for assessing existing plant species per grid square (1 ha) and locating vegetation relevés on permanent sample plots around gridded dots or within core areas or transects (size of subsamples 10 m ×10 m square: 100 m2). In the case of gridded dots, different plot sizes were used in the different study areas (Hünstollen: quarter circle of r = 17.84 m: 250 m2; Hohestein, Wattenberg/Hundsberg: r = 20 m: 314 m2; Staufenberg: 20 m × 20 m square around the centre of the grid dot: 400 m2; see Fig. 2) In all cases, the size of the sample plots fulfils the minimal area of temperate forest undergrowth plant communities (DIERSCHKE
1994). Vegetation relevés include visual estimation of the cover of all terrestrial plant species using direct percentage values broken down into tree, shrub, herb, and cryptogam layers. The parameters obtained were vertical structure of the stands, the number of species (α-diversity), Ellenberg’s indicator values (ELLENBERG2001) and the relationship between the numbers of forest and non-forest species using the recent list of typical forest species of the German uplands by SCHMIDTet al.(2003).
To test the potential and limitations of the measuring strategy, a comparison was made between managed and unmanaged forests by taking the results of two nature reserves and adjacent managed forests in Hesse (Fig. 1): Wattenberg/Hundsberg (submontane mixed beech forest with basalt ridges) and Hohestein (submontane beech forest on limestone). The validity of this procedure is based on correctly identifying plot pairs which have similar site conditions and stand structure. Neither of the forest nature reserves have been managed commercially for at least ten years, while in the nearby paired, managed forests, a harvest volume of 33 to 50 m3wood is commonly removed every five years (SCHMIDT2003).
Permanent plot studies provide a useful technique to monitor long-term changes in forest diversity in relation to environmental impacts, forest management practices or successional processes. In order to test the development of species richness in undisturbed (after the cessation of forest management) beech forest, species lists and vegetation relevés were collected on permanent plots at ten-year intervals in two nature reserves, at Grosser Staufenberg (beech forest on nutrient-poor grauwacke and nutrient-rich diabas in the lower Harz Mountains, 1988 and 1998) and Hünstollen (beech forest on limestone near Göttingen, 1992 and 2002), respectively (Fig. 1). Following a local wind event in June 1997, plant suc- cession in windthrown beech forest stands on base-rich gypsum and dolomite soils was studied on permanent plots in the unmanaged nature reserve at Hainholz (foothills of the Harz Mountains, Fig. 1) from 1998 to 2001 (SCHMIDT2002, 2003; KOMPA2004; KOMPAand SCHMIDTin press).
Fig. 2. Flora and vegetation sample inventory design for forest nature reserves (THOMASet al.1995, SCHMIDT1999): Species lists from each of the 100 ×100 m squares of the grid network. Vegetation relevés from sample plots around gridded dots by sectors of a circle (a: r = 17.84 m = 250 m2 [Hünstollen] or r = 20 m = 314 m2[Wattenberg/Hundsberg, Hohestein]) or by squares (b: 20 × 20 m = 400 m2[Staufenberg]).
3 Results
3.1 Species diversity of managed and unmanaged beech forests
Species diversity (α-diversity) for the vascular plants is expressed not only by the total number per whole study areas with different sizes (ha) but also by the mean number of species per ha, which permits a comparison between the investigated sites. Species diversity showed clear trends when the areas of managed and unmanaged beech forests at Wattenberg/ Hundsberg and Hohestein (Fig. 3) sites were compared. The total number of species for the whole area and the mean number of species per ha were higher for the man- aged plots than for the unmanaged forest nature reserves. Owing to the larger area (42 ha for Wattenberg versus 34 ha for Hundsberg) and heterogeneous site conditions, the total number of species of the Wattenberg forest nature reserve is higher than at the corresponding managed forest area of Hundsberg. The high number of species at Wattenberg was mainly related to the large fringe zones bordering on the open landscape.
The vascular plant species recorded within the 1 ha grid plots indicated that a significantly higher proportion of plant species of dense (closed) forest occurred in the unmanaged forests. However, plant species that commonly occur in equal proportions in forest and open habitats and plant species which preferred primarily open habitats were more abundant in the managed forests (Fig. 4).
A number of factors affect the differences in species diversity and the proportion of forest- related species groups, including site history, management practices, time since cessation of management and the existing site conditions. The light factor as an indicator value of canopy cover was higher for the managed areas than for the unmanaged plots, and was related to the lower degree of cover of the tree layer and higher degree of cover of the shrub and herb layers (Fig. 3, 4). This resulted from the greater canopy opening through woodland clearings (vegetation type: Epilobietea angustifolii, Table 1, SCHMIDT 2003) in the commercially managed forests. Despite the similarity in vegetation structure and composition of the unmanaged forest nature reserves and reference areas, slight differences in site conditions were present that could not be attributed to former management. For example, indicator values for nitrogen and soil acidity (Fig. 4) differed from the results from vegetation and soil site mapping. 76 % of the area at the Wattenberg (forest nature reserve) site and 52 % of the area at the Hundsberg site was classified as eutrophic (vegetation type: Hordelymo- Fagetum, a species-rich beech forest community on basic or calcareous, often shallow soils), and the remaining area as mesotrophic (vegetation type: Galio-Fagetum, a beech dominated forest community on moderately acidic, deep soils with good water supply, Table 1, SCHMIDT
2003). This uneven distribution resulted in higher values of soil base richness and nitrogen indicators at Wattenberg than at Hundsberg (Fig. 4).
Soil acidity and nitrogen supply values also differed at Hohestein, as evidenced in the forest nature reserve by 1) a higher percentage of the species-poor Hordelymo-Fagetum typicum (49 % of forest nature reserve area compared to 3 % on the corresponding plots of the managed stands) and 2) the prevalence of strongly loamy soils with weakly to moderately acidic conditions (70 % of the forest nature reserve area compared to 50 % in the managed forests). The predominance of the Hordelymo-Fagetum lathyretosum on the rendzina soils (limestone with low loam content) in the managed forests indicated a beech community significantly richer in species and with higher cover of the herb layer (Table 1, Fig. 3, SCHMIDT2003).
Forest reserves can thus be used as reference plots for studying changes in managed forests provided the site conditions are similar. Stratification of vegetation relevés following vegetation and soil site mapping data showed that species diversity was generally higher in
managed stands (Table 1). Nearly identical numbers of species were recorded for the forest gaps or woodland clearings (Epilobietea angustifolii) in both the unmanaged forest nature reserve and managed forest of the Wattenberg/Hundsberg sites (Table 1). Species of open and disturbed habitats persisted for several years in successional plant communities. Their presence may be related to the short period that has elapsed since management of the sites was discontinued. The comparatively high species numbers in openings and clearings may be taken as an indication that similarly high numbers could be expected in disturbed beech forests, regardless of whether the disturbance is natural or anthropogenic.
Fig. 3. Species diversity and vertical structure of unmanaged and managed forests within the forest nature reserves at Wattenberg/Hundsberg and Hohestein (Hesse, Fig. 1). Database: Species lists of the total area (complete column) or of the complete grid areas only (hatched column), vegetation relevés at all grid points (vertical structure). Significant higher values (± standard error) are indicated by * (p < 0.05 differences between management type based on Wilcoxon test).
Table 1. Number of higher plant species of the vegetation relevés around grid dots (sample size: 314 m2) of unmanaged and managed forests within the forest nature reserves at Wattenberg/Hundsberg and Hohestein following stratification based on the main vegetation types. Significant higher values (± stan- dard error) are indicated by * (p≤0.05 differences between management type based on Mann-Whitney test).
Study site – Vegetation type Unmanaged forest Managed forest Mann-
n x ± se n x ± se Whitney p
Wattenberg/Hundsberg
– Hordelymo-Fagetum 17 23.3 ± 2.5 5 31.2 ± 1.3 0.071
– Galio-Fagetum 9 13.0 ± 3.1 8 31.3 ± 1.7 0.002 *
– Epilobietea angustifolii 10 31.2 ± 1.5 17 31.3 ± 1.2 0.880 Hohestein
– Hordelymo-Fagetum lathyretosum 14 26.0 ± 1.0 29 29.6 ± 1.2 0.038*
– Hordelymo-Fagetum typicum 12 22.9 ± 2.0 2 23.5 ± 5.5 1.000 Fig. 4. Typical forest species relationship and Ellenberg’s mean indicator values of unmanaged and man- aged forests within the forest nature reserves at Wattenberg/Hundsberg and Hohestein (Hesse, Fig. 1).
Database: Species lists of the complete grid areas only. Significant higher values (± standard error) are indicated by * (p < 0.05 differences between management type based on Wilcoxon test).
3.2 Development of species richness in undisturbed beech forests
The floristic analysis of the total area and of the complete grid squares of the unmanaged beech forest reserves at Staufenberg and Hünstollen showed that the total number of species and the mean number of species per ha declined in both beech-dominated forest nature reserves after the course of ten years (Fig. 5). The decrease of species numbers was mainly caused by the decrease of semi-shade-tolerant species of clearings (1.2 in Fig. 6 which includes Sambucus racemosa, Rubus idaeusand Epilobium angustifolium) and open, often disturbed habitats (2.2 in Fig. 6, which includes non-forest species). However, shade-tolerant understorey plant species from closed, dense forests (1.1 in Fig. 6, which includes Carex sylvatica, Festuca altissima, Melica uniflora and Calamagrostis arundinacea) were more abundant in the beech forest nature reserves in 1998 or 2002 than ten years earlier.
Fig. 5. Dynamics of species diversity and vertical structure of the forest nature reserves at Staufenberg and Hünstollen (Lower Saxony, Fig. 1) during a ten-year period since cessation of management. Data include: species lists of the total area (complete column) or of the complete grid areas only (hatched column), vegetation relevés at all grid points (vertical structure). Significant values (± standard error) are indicated by * (p < 0.05 differences between years based on Wilcoxon test).
The decrease of radiation for the growth of herb layer vegetation is related to the higher cover of the tree and shrub layers, as indicated by the significantly lower light indicator values (Fig. 5, 6). This is mainly caused by changes of the dominant, shade-tolerant beech (Fagus sylvatica),which forms a densely closed canopy when the sites are not managed.
Fagus sylvatica in general and other tree species at the Hünstollen site such as Acer platanoides, A. pseudoplatanusand Fraxinus excelsiorbecame more abundant within the shrub and herb layers due to natural regeneration, resulting in reduced light for understorey vegetation. Other site factors, e.g. water supply, nitrogen supply, and soil acidity (given as F, N, and R values in Fig. 6) did not differ during the ten-year period in which no management was carried out.
Fig. 6. Dynamics of typical forest species relationship and Ellenberg’s mean indicator values of the forest nature reserves at Staufenberg and Hünstollen (Lower Saxony, Fig. 1) during a ten-year period since cessation of management. Data include species lists of the complete grid areas only. Significant values (± standard error) are indicated by * (p < 0.05 differences between years based on Wilcoxon test).
3.3 Development of species richness in unmanaged beech forests after windthrow In the unmanaged and undisturbed beech forests of Staufenberg and Hünstollen, the number of higher plant species decreased within one decade after management was stopped. How species diversity behaves in beech forests with large-scale disturbance is however still not fully known. After a wind event in June 1997 in the unmanaged forest nature reserve at Hainholz (65 ha) that resulted in extensive windthrow, a total of 341 plant species (Spermatophyta and Pteridophyta) were recorded. With a mean of 92 species per complete grid area (1 ha) this forest nature reserve has the highest species diversity of all beech forests so far investigated.
From two areas where windthrow was severe, the number of species recorded on perma- nent plots after four years is shown in Figure 7. Core area I occurred on a dolomite soil and showed a rapid natural regeneration by fast growth of existing saplings of Fraxinus excelsior, Acer pseudoplatanusand Fagus sylvatica.Core area II occurred on a gypsum soil and exhi- bited slow tree regeneration. In both cases numbers of species in the shrub and herb layers increased in the first three years after windthrow. During the same period, natural regen- eration formed a dense shrub layer in core area I, resulting in a decrease in the total number of species, mainly of the herb layer, due to lack of light. In core area II, the number of species is still increasing, reflecting the openness and an increase in species of open habitats. Large- scale disturbance by wind events does not eliminate typical plant species from dense forests;
species such as Mercurialis perennisor Allium ursinumare still present following the wind event. However, plant species of clear-cuts, such as Atropa belladona, or open sites ruderals such as Chelidonium majusor Verbascum thapsusoccurred on newly disturbed sites.
Fig. 7. Dynamics of species diversity after wind throw in the shrub (a) and herb layer (b) of two core areas of the unmanaged forest nature reserve Hainholz (Lower Saxony, Fig. 1).
4 Discussion
The results presented here demonstrate that understorey flora and vegetation are suitable indicators for site conditions, human impact and forest dynamics, and therefore a useful and easy tool to measure and evaluate biodiversity in order to characterize sustainable or ecosystem-based forest management. Results from the Wattenberg/Hundsberg and Hohe- stein research areas show that a comparison of species diversity is useful only if like veg- etation or habitat units are compared with each other (Table 1). As long as site conditions and history are strictly the same, forest nature reserves provide a convenient baseline for the evaluation of impacts in managed forests (GRAAEand HESKJAER1997).
Both abiotic factors and niche diversification play a role in mediating species diversity.
From vegetation relevés of small sub-sample plots (100–500 m2), we know that species richness of beech forests increases with increasing base-richness (BRUNET et al. 1996;
LEUSCHNER1999; SCHMIDTin press). As predicted by the species pool hypothesis (ZOBEL
1997), this relationship has been found again by the floristic inventory of large sample plots (1 ha). Following the environmental heterogeneity hypothesis (HUSTON1994), small-scale differences in topography with a wide range of soil nutrients and water supply over small distances should increase species diversity. Finally, species diversity will be higher in inter- mediate disturbance ecosystems than in undisturbed forests (CONNELL1978, DECOCQet al.
2004). These mechanisms working in combination (GRACE 1999) could explain why the Hainholz forest nature reserve has the highest species diversity of all forest reserves so far studied in Germany (SCHMIDT2002, 2003). Small-scale site conditions (from shallow, dry and nutrient-poor gypsum soils to deep dolomitic sites with good water and nutrient avail- ability) as well as different types of disturbances (from closed stands without windthrows to small gaps and extensive areas of windthrow) are the reason for the high species numbers.
After cessation of forest management, species diversity of beech forests in the optimal successional stage decreases (SCHMIDT1999, in press). In unmanaged stands,Fagus sylvatica becomes dominant over other tree species. Beech is extremely competitive, mainly owing to its shading behaviour (LEUSCHNER1998; LEUSCHNERand HAGEMEIER2003). Transmission of photosynthetically active radiation in beech canopies is significantly lower than in canopies of other deciduous tree species. This inhibits both the establishment and develop- ment of shade-intolerant herb-layer and tree species. Beech seedlings and saplings survive in much deeper shade than does the regeneration of nearly all other European forest tree species. Therefore,Fagus sylvaticain general and other tree species at the Hünstollen site such as Acer platanoides, A. pseudoplatanusand Fraxinus excelsiorbecame more abundant within the shrub and herb layers due to natural regeneration, resulting in further reduced light for herbaceous understorey vegetation (LEUSCHNER1998; LAMBERTZand SCHMIDT
1999; SCHMIDTin press).
Only after large-scale disturbances such as windthrows can early successional shrub species such as elder (Sambucus sp.) and raspberry (Rubus idaeus) form dense patches (KOMPAand SCHMIDT2003; KOMPA2004). Depending on site conditions in the Hainholz forest nature reserve, shade-tolerant tree species (mainly Acer platanoides, Acer pseudo- platanus, Fagus sylvatica, Fraxinus excelsior and Ulmus glabra) of the pre-storm woody regeneration grew up rather quickly so that the proportion of light-demanding herbs and grasses from open landscapes was rather low during the first years after windthrow (KOMPA
2004).
The decrease of large herbivores (ungulates) with a low impact of browsing on tree regeneration may also directly or indirectly be an important factor for decreasing species richness in beech forests (SCHMIDT 1978; OHEIMB et al. 1999; SCHÜTZ et al. 2000). The extent to which herbivores can cause diversity changes in forest ecosystems may depend on
the scale of disturbance. WOODWARDet al.(1994) and KRÜSIand MOSER(2000) concluded from their studies in Olympic National Park and the Swiss National Park, respectively that
“while ungulate herbivory may indeed affect forest regeneration at the scale of individual tree gaps, their influence following large catastrophic disturbances seems inconsequential by comparison” (WOODWARDet al.1994). Similarly, the impact of ungulates on species diversity may be much more important in forest nature reserves such as Hünstollen and Staufenberg with slow tree regeneration in small gaps than in the Hainholz forest nature reserve with the large areas disturbed by windthrow and rapid regeneration of young trees.
Besides the changed light conditions (greater closure of the tree and shrub canopy), the lack of soil disturbance (abandonment of forest road network, lack of forest harvest activi- ties, BRUNET et al. 1996, 1997; EBRECHT and SCHMIDT 2005) may have decreased plant species diversity in the undisturbed beech forests. The floristic analysis of the grid squares showed a significant decline of species richness contributed mainly by a decrease of non- forest species. The species in decline frequently include short-lived species with long-term persistent seeds from ruderal and clear-cut sites (THOMPSONet al.1997; KOMPA2004). These plants require bare mineral soil to establish (BONNand POSCHLOD1998) during an early successional period with low interspecific competition (FISCHER 1998; MÄRKL and EGELSEER2001; KOMPAand SCHMIDT2003; SCHMIDT2002, 2003). At the same time plant species typical of closed forests are not or only little hampered by forest management as they are to a certain extent adapted to soil disturbances such as pits and mounds after wind- throws or wild boar digging activities (BRUNETet al.1996, 1997; ABSet al.1999; SCHMIDT
and WECKESSER2002; OHEIMB2002). Individual tree gaps without soil disturbances are not enough to stop the decline in species diversity. Only in extensive windthrown stands with much bare mineral soil do species numbers again increase. These differences in species diversity of undisturbed (Hünstollen, Staufenberg) and disturbed forest nature reserves (Hainholz) agree with results from beech forests in southern Sweden, where alpha diversity increased with increasing forest management intensity (from unmanaged forests to near clear-cutting of the stands) (BRUNETet al.1996). Finally, the pattern and process of increasing species diversity in unmanaged beech forests after large-scale disturbance by windthrows may to a certain extent mimic the development of a ‘close-to-natural’ managed beech forest (QUINEet al.1999).
The Staufenberg and Hünstollen sites are nutrient-rich. High nitrogen inputs have had no measureable effect on species diversity of these nature reserves during the last ten years.
This is in contrast to beech forests on acidic, nutrient-poor soils (Luzulo-Fagetum) in which a significant increase in species richness – not only in managed but also in unmanaged forests – has been documented during the last decades (BOBBINKet al.1998; SCHMIDT1999, in press; OHEIMB2003; SCHMIDTand WECKESSER2004).
Acknowledgements
Field data of Luise Ebrecht, Dirk Garbitz, Rainer Ermert, Thomas Kompa, Katja Megner, Sven Melcher, and Dirk Schreiber were evaluated for this publication. Andreas Parth supported data analysis and preparation of the figures. Claus Holzapfel, Partap K. Khanna, Toby Spribille and two anonymous reviewers helped to improve the paper. All this assistance is gratefully acknowl- edged.
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Accepted June 8, 2005
