Headwater springs in the German Low Mountain Ranges are local ecotone habitats and biogeographical islands embedded in and interlinked with their adjacent landscape. The structure of forests reflects the eco-hydrological conditions in substrate type occurrence, microhabitat richness and biodiversity in forest springs. This study considers effects from different forest land cover by comparing spring habitats in deciduous beech forests and coniferous spruce forests on eco-hydrological structures and biodiversity. Study areas include six different forest landscapes in the Low Mountain Ranges in Central Germany in Hesse and Thuringia. Hydro-morphological structure mapping and invertebrate sampling was executed within a multi-habitat sampling regime, which involves sampling plots being allocated according to the cover ratio of the occurring substrata. Aquatic and terrestrial spring zones are considered with respect to an ecotone approach. Some in situ measurements were implemented, such as pH values, to assess the acidity of the spring water. Results show obvious differences in acidity, substrate type cover ratios and biodiversity in deciduous and coniferous forest springs. Conifer forest springs were found tending to acidification while deciduous forest springs were slightly alkaline. Deciduous forest springs had higher cover ratios of organic microhabitats as well as a higher biodiversity in species richness and total number of individuals. Although it was not possible to clearly distinguish one direct key factor of fauna assemblages, negative effects from forest management practices (e.g. monoculture plantations of conifer forest) on spring habitats can be concluded.

springhead, headwater, microhabitats, substrate, habitat assessment, crenobionts

Mountainous headwater springs are mostly small water bodies where dominant groundwater occurs at the surface to form an intermittent or permanent discharge influenced by subsurface interflow and overland flow (Frisbee et al. 2013). Spring habitats are ecotones in two spatial dimensions. Firstly, springs are vertical aquatic below-above-ground-interfaces between groundwater and surface water. Secondly, springs are above-ground-horizontal terrestrial-aquatic interfaces between different adjacent areas within a continuum of aquatic, semi-aquatic or amphibian to hygrophilous terrestrial spring areas similar to the riparian zone ecotone (Gibert et al. 1990). Therefore, springs are normally nearly constantly cold and wet island habitats without clarifying benchmarks on the delimitation of hot springs (e.g. Pentecost et al. 2003). Glazier (2009) introduces the term “ambient springs” to characterise the water temperature regime with low minimum and maximum amplitudes and an annual mean related to the local yearly mean air temperature (Thienemann 1924). The relative constantly cold water temperature conditions or isothermy (Odum 1971, Teal 1957) lead to specific fauna adaptation of cold-stenothermic specialists and glacial relicts (Nielsen 1950, Ward and Stanford 1982, Fischer 1996). However, fauna composition is much more heterogeneous in springheads and is often explained as a consequence of a high degree of substratum heterogeneity, which can result in high species richness (Bonettini and Cantonati 1996, Lindegaard et al. 1998, Williams and Williams 1999, Staudacher and Füreder 2007, Koperski et al. 2011). The springhead can be built of different substrate types that build heterogeneous mosaic-like structures called patches, hence, springs which can also be termed patchy ecosystems on the micro scale (Pickett and White 1985). The first quantitative approach for analysing the microhabitat-fauna-relationship in springheads and for calculating ratios of invertebrate substrate preferences regarding the ecotone characteristics was completed by Reiss (2011) and considered mountainous headwater springs in Central Germany.

The objective of this study is to determine effects from different forest cover by comparing spring habitats in deciduous and coniferous forests on eco-hydrological structures and biodiversity. This research focuses on impacts from forest types as a determinant of the occurrence of corresponding microhabitat types, its substrate type composition and diversity, as well as its specific colonisation by invertebrates. Here, acidity is also considered, because conifer forests could be contributing to acidification of surface waters (Nisbet 1990, Cannell 1999).

Results show three main forest land cover impacts on eco-hydrological structures and biodiversity in mountainous headwater springs concerning deciduous and coniferous forest: 1) Acidification in conifer forest springs; 2) Higher cover ratios of organic substrate type based microhabitats in deciduous forest springs; 3) Higher biodiversity in species richness and total number of individuals as well as abundances in deciduous forest springs.

Acidification of springs in coniferous forests of the German Low Mountain Ranges is a well-known problem, especially in siliceous springs with poor acid buffer capacities and can causes a decrease in the total number of species and individuals (Klös 1984, Lükewille et al. 1984, Meijering 1984, Mauden 1994, Orendt and Reinhart 1997, Audorff and Beierkuhnlein 1999, Hahn 2000). Therefore, it can be expected that the lower values of the total number of species and individuals in conifer forest springs is mainly a consequence of acidification. This interpretation is congruent with findings from a multivariate statistical analysis to characterise the main components of environmental factors influencing the absent and present data on spring fauna using a principal component analysis in the entire data set (Reiss 2011). It is important to note that only in-situ measurements (on-site pH values) were taken into account and no further investigations could be executed to analyse the dynamics or additional reasons for low pH values in order to deduce more accurate and detailed interpretations about acidification.

Higher cover ratios of organic substrate type based microhabitats in deciduous forest springs compared with springs in coniferous forest can be seen as a higher hydro-morphological representation of specific microhabitats (e.g. emergent macrophytes or CPOM). However, conifer forest springs have the same quantitative share of occurring substrate types. CPOM (Coarse particular organic material), mainly composed by leaf litter from deciduous trees, is more important as allochthonous organic material as the most important food source for spring fauna communities with a high proportion of primary consumers in particular like herbivorous shredders (e.g. Gammarus fossarum, Gammarus pulex) or grazer and scrapers (e.g. Bythinella dunkeri, Bythinella compressa) (Teal 1957, Mninshall 1967, Whiles and Wallace 1997, Kemp and Boynton 2004). There is no microhabitat preference in coniferous litter, because it is commonly known that the chemical composition of needles tend to retard biological activity in decomposition (Olson 1963). Here, detritus processing takes a much longer time (Sedell el al. 1974, Whiles and Wallace 1997) and can be characterised as a low quality detritivore food resource (Taylor et al. 1989, Klemmedson 1992, Friberg and Jacobsen 1994). Furthermore, comparing deciduous and coniferous spring microhabitats, one main conclusion is that substrate types are more important than substrate heterogeneity in contrast to a more general finding by Kubíková et al. (2012). However, in this study, it is not possible to distinguish between acidification and substrate types for describing a prevailing key factor causing taxa and individuals’ richness in forest spring habitats.

Higher biodiversity in species richness, total number of individuals as well as abundances in deciduous forest springs in contrast to conifer forest springs, are the most obvious impacts caused by different forest stands. The result of a higher amount of spring dwelling taxa in conifer forest springs than in deciduous forest springs, in proportion to the total number of all found taxa, is consistent with findings by Hahn (2000). Hahn (2000) concluded that spring dwelling species are more tolerant to low pH-values than rhithrobiontic ones due to their adaptation to special crenal living conditions. However, conifer forest springs in our investigation, with their tendency to acidification, are characterised by considerably less individuals per spring ratios and a total number of individuals considering only spring dwelling taxa. Higher cover ratios of organic substrate type based microhabitats in deciduous forest springs compared with springs in coniferous forest seem to be less crucial for biodiversity. Nevertheless, although organic substrate type based microhabitats are of importance, they are more decisive on taxa composition. Some taxa which are under-represented in deciduous forest springs become more quantitatively relevant in conifer forest springs. The abundance of the shredding plecopteran Leuctra spp. is much higher in conifer forest springs, which is in line with the findings in a study about leaf litter decomposition in macroinvertebrate communities in pine and hardwood headwater catchments of the southern Appalachian Mountains in North Carolina, USA (Whiles and Wallace 1997). Leuctra spp. is known as an acid-tolerant aquatic invertebrate (Dangles and Guérold 2000), which is an indicator for acidification as an important factor on taxa composition. Otherwise, acid-sensitive amphipod Gammarus fossarum is similarly abundant, which means no acidification shift in taxa composition is noticeable. It can be inferred that both factors – acidification and microhabitat – govern fauna assemblages or biodiversity and no strong key variable can be characterised. However, this study indicates impacts from different forest cover when comparing spring habitats in deciduous and coniferous forest on eco-hydrological structures and biodiversity.

Different forest land cover causes considerable contrasts in microhabitat structures; obvious organic substrate type composition and cover ratios; as well as differences in species richness and invertebrate abundance of spring habitats in deciduous and coniferous forest. This means, land cover as an ecological mesoscale, properly determined by different forest types, has an impact on eco-hydrological structures and biodiversity on the micro scale. It implies an essential consideration for adjacent biotope type mapping and is an important integrative parameter for spring habitat assessment approaches. Furthermore, the recognition of substrate preferences of invertebrates, within an ecotone based assessment approach, characterises microhabitats explicitly for all parts of a springhead, regarding aquatic and terrestrial spring habitat zones. Here, the importance of forest land cover and the substrate type diversity relationship is taken into account within an ecological spring habitat assessment methodology and characterises its consequences on invertebrate biodiversity. Therefore, negative effects from forest management practices (e.g. forest conversion) within a nature conservation perspective can be included in decision-making and action plans to realise national or regional strategies on biodiversity.

Taxa determination for specific taxonomic groups were done by some colleagues with special thanks to Dr. Peter Martin (Kiel, Germany) for Halacaridae and Hydrachnellae of the order Acari (water mites); Dr. Axel Schönhofer (Mainz, Germany) for Opiliones (harvestmen); Christoph Bückle (Tübingen, Germany) for Auchenorrhyncha (cicadas) and Andreas Allspach (Frankfurt / Main, Germany) for Trichoniscidae (woodlice, isopods). The Hesse Federation for Cave and Karst Research supported the study with material resources. Thank you Stefan Zaenker, Chairman of the Hesse Federation for Cave and Karst Research in Fulda, Germany.