The effects of light and soil conditions on the species richness of the ground vegetation of deciduous forests in northern Germany (Schleswig-Holstein)

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Abstract

This paper reports a study carried out in 91 deciduous forests to investigate the influence of light and soil conditions on the species richness of the ground vegetation (vascular plants, bryophytes, lichens). To examine the relationships between the species richness and environmental factors all data were evaluated using univariate and multivariate (principle component analysis) methods. At all the stands investigated, the conditions of soil and light have a vegetation type specific effect on the species richness of the ground vegetation. In moist forests (alder-ash forests of the alliance Alno-Ulmion), species richness has a close positive correlation with soil moisture, whilst light conditions and nutrient supply have in the main no effect on species richness. On the other hand, in meso- to eutrophic beech forests (beech forests of the alliance Fagion sylvaticae), species richness is closely correlated with soil activity and with the base and nitrogen supply. Improving the light conditions for the ground vegetation does not increase the number of typical woodland species as most of these are shade tolerant and have very limited light requirements. Species richness in acidophytic beech and mixed beech–oak forests (beech and mixed beech–oak forests on acid soils of the order Quercetalia roboris) is affected mainly by the canopy closure and the interior light conditions. Soil moisture, nutrient supply and light availability must therefore be evaluated differently for their effect on the number of species of the ground vegetation and with regard to individual forest communities as they are distinguishable according to their site conditions. If the relation between species richness and environmental factors like soil conditions is analysed not with reference to individual forest communities but rather with reference to the totality of central European forests, relationships may be found which are not helpful for an explanation of the species richness of a certain forest community and thus they are only of restricted usefulness for appropriate recommendations for forest management. The consequences of the results for the effects of silvicultural treatment on deciduous forest ecosystems of the central European lowlands are discussed.

Introduction

Although deciduous forests of the north German lowlands are still oligo- to mesohemerobic ecosystems, their species composition has changed more or less obviously over the past decades. Ecological forest management attempts a form of silvicultural treatment that balances the need to produce timber with a due regard for the requirements of nature conservation within forest ecosystems. In order to formulate recommendations for conservation-oriented management in forestry practice, it is helpful to have investigations which analyse the effects of silvicultural measures on the structure and species composition of forests and which describe the development of natural forests that have not been commercially used. Recent years have seen numerous studies devoted to the description and analysis of the effects of forest management on the structure and ground vegetation of the forests, as in this context the ground vegetation in particular is of great value as an indicator (Barkham, 1992, Grabherr et al., 1995, Grabherr et al., 1998, Halpern and Spies, 1995, Brunet et al., 1996, Christensen and Emborg, 1996, Graae and Heskjar, 1997, Nagaike et al., 1999). However, these investigations can only be meaningfully interpreted and proper conclusions drawn for silvicultural practice if at the same time the effect and the variability of natural conditions for growth, such as nutrient and water supply and light conditions, on the species composition and the species richness of the ground vegetation are adequately known. The variability of the forest ground vegetation within this gradient of natural site conditions, as regards soil and structure parameters, is particularly important. Roberts and Gilliam (1995) and Ehrlich (1996) have pointed out that additional research is needed to identify diversity patterns and mechanisms in respect of natural site conditions and quality (cf. also Hüttle et al., 2000).

The objective of this paper is to examine how environmental factors (light and soil conditions) affect the species richness and the species composition of the ground vegetation (vascular plants, bryophytes, lichens) of deciduous forests in northern Germany, using representative stands in Schleswig-Holstein as examples. In the foreground is the question of how the species number in the ground vegetation changes within a natural site gradient, particularly in respect of the nutrient, moisture and light gradients. Additionally we sought to compare the effect of these site gradients on diversity patterns of the ground vegetation with regard to different forest communities (moist forests with alder and ash, eu- and mesotrophic beech forests, acidophytic beech and mixed beech–oak forests). Based on this, prognoses can be formulated to describe how the species composition of the ground vegetation will change when the relevant environmental conditions are altered by silvicultural treatment.

Section snippets

Study area and forest communities

Between the years 1990 and 1998, a total of 91 sample sites in different stands of deciduous forests on ancient and recent moraines in northern Germany (Schleswig-Holstein; cf. Fig. 1) was investigated (ancient moraines: moraines of the penultimate (Saale) glacial period; recent moraines: moraines of the last (Weichsel) glacial period). The study area comprises a geographic area of approximately 15700 km2. Each sample site was square with a size of 100 m2 and represented a stand of a forest

Results

As a basis for the evaluation of the correlation matrices (Table 2, Table 3, Table 4) in Table 1 an overview is given to the mean values and the range found for each variable investigated for the three forest communities. As regards the moist forests (alder-ash forests of the Alno-Ulmion), the correlation matrix (Table 2) shows that a high species richness in the ground vegetation is correlated mainly with the mean Ellenberg indicator value for soil moisture. This means that the total number of

Discussion

The analysis of alder-ash stands (alliance Alno-Ulmion) shows that the species richness of the ground vegetation increases with increasing soil moisture. This can be explained by the fact that the species richness of moist forests is to a considerable extent due to the high proportion of moisture-loving or moisture-tolerant perennials which are also characteristic for moist meadows, but have their natural habitats in forests (e.g. Crepis paludosa, Filipendula ulmaria, Ranunculus repens,

Conclusions for silvicultural management

Various conclusions can be drawn from the results described here concerning the influence of silvicultural measures on deciduous forest ecosystems that have been investigated.

In moist forests, drainage leads in the first place to a clear loss of the species characteristic of those habitats, whereas the removal of individual tree trunks and the resulting increase of light through the canopy has hardly any effect on the number of species in the ground vegetation. In this context it has to be

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