ZOBODAT - www.zobodat.at Zoologisch-Botanische Datenbank/Zoological-Botanical Database Digitale Literatur/Digital Literature Zeitschrift/Journal: Tuexenia - Mitteilungen der Floristisch-soziologischen Arbeitsgemeinschaft Jahr/Year: 2013 Band/Volume: NS_33 Autor(en)/Author(s): Kasari Liis, Gazol Antonio, Kalwij Jesse M., Helm Aveliina Artikel/Article: Low shrub cover in alvar grasslands increases small-scale diversity by promoting the occurrence of generalist species 293-308 Tuexenia 33: 293–308. Göttingen 2013. available online at www.tuexenia.de Low shrub cover in alvar grasslands increases small-scale diversity by promoting the occurrence of generalist species Leichte Verbuschung von Alvar-Kalkmagerrasen hat einen positiven Effekt auf die Artenvielfalt durch die Förderung generalistischer Arten Liis Kasari1,*, Antonio Gazol1, Jesse M. Kalwij1, 2, Aveliina Helm1 1Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, EE-51005 Tartu, Estonia 2Department of Zoology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa *Corresponding author, e-mail: [email protected] Abstract Dry calcareous grasslands in Europe are renowned for their high plant diversity. However, declining habitat areas and highly fragmented distribution threaten the long-term persistence of this valuable habitat type. In Estonia the decline of traditional grassland management and subsequent encroachment of shrubs has resulted in a substantial loss of alvar grasslands – a particularly rare and species-rich type of calcareous grassland. It is known that a shrub cover of more than 70% decreases the alvar grassland species richness. At the same time, a shrub cover of 30% is considered optimal for alvar grasslands and thus a target state for habitat restoration. However, very little is known about the effect of low shrub cover on environmental conditions and species composition of alvar grasslands. Our aim was to detect to what extent the small-scale plant diversity and species composition is influenced by low shrub cover (less than 30%). We hypothesized that even a low shrub cover can have an effect on the environmental conditions of alvar grasslands. We sampled small-scale plant species richness, shrub cover and abiotic environmental conditions such as light, soil moisture, soil pH and soil depth in 10 metre long transects (n = 33) subdivided into 10 cm × 10 cm plots in Estonian alvar grasslands. Structural equation modelling was used to quantify the direct and indirect effects of shrub cover on the richness of characteristic alvar species and on the richness of generalist species. We found that low shrub cover of up to 30% increased total and generalist species richness directly and indirectly by increasing the light heterogeneity. Alvar characteristic species richness was not relat- ed to low shrub cover values. This suggests that when estimating the effects of shrub cover on species richness and on conservation needs of grasslands, habitat specificity of species needs to be taken into account. Keywords: alvar grassland, grassland conservation, species richness, structural equation modelling, woody plants Erweiterte deutsche Zusammenfassung am Ende des Textes Manuscript received 31 October 2012, accepted 19 March 2013 Co-ordinating Editor: Eszter Ruprecht 293 1. Introduction Temperate, oligo- to mesotrophic, semi-natural grasslands have the highest small-scale plant species richness in the world (WILSON et al. 2012), yet these habitat types are globally endangered (CEBALLOS et al. 2010). In Europe most of the historically developed semi- natural grasslands have either been abandoned or the traditional extensive grassland man- agement has been replaced by intensive agricultural practices during the past century (ROSENTHAL et al. 2012, STRIJKER 2005). Both types of land use change resulted in shrub encroachment in grasslands and in changed environmental parameters and species composi- tion (AAVIK et al. 2008, PETŘÍK et al. 2011, PIHLGREN & LENNARTSSON 2008, PÄRTEL & HELM 2007, PÄRTEL et al. 1999a), ultimately leading to large decreases in grassland habitat area (POSCHLOD et al. 2005). Shrub encroachment in grassland is often followed by a decline in characteristic grassland species richness (GRIME 2002, LIMB et al. 2010, PÄRTEL & HELM 2007, ÖCKINGER et al. 2006). At the same time, shrub encroachment can facilitate the estab- lishment of species that are better adapted to these new conditions, e.g. generalist species or species from other habitat types (KIVINIEMI & ERIKSSON 2002). The mechanisms by which shrub cover influences grassland species composition includes a reduction in light availabil- ity (LIMB et al. 2010), a shift in soil nutrient and water availability (HUXMAN et al. 2005, MIWA & REUTER 2010), and a decrease in soil pH (JOBBAGY & JACKSON 2003). Environmental changes related with shrub encroachment not only modify the quantity and quality of different parameters, but also their spatial heterogeneity (PÄRTEL & HELM 2007). Spatial environmental heterogeneity is caused by a patchy distribution of resources in space (WIENS 2000). PÄRTEL & HELM (2007) found that small-scale environmental factors such as soil moisture and nutrient content varied notably more in forests than in grasslands. This is because the extensive root systems of woody plants use resources patchily, and rela- tively homogeneous grassland environments will become more heterogeneous after shrub encroachment (BEKELE & HUDNALL 2006, LIU et al. 2011, PÄRTEL & HELM 2007). Habitat heterogeneity can also have influence on the species diversity (CLARK et al. 1998). At larger scales the influence of environmental heterogeneity on plant diversity is usually presumed to be positive according to niche theory (CLARK et al. 1998, TILMAN 1982); at smaller scales, however, the influence can also be negative (LAANISTO et al. 2012, PAUSAS & AUSTIN 2001, TAMME et al. 2010). In this study we focus on the effect of low-cover shrub encroachment on alvar grassland plant species richness and composition. Alvars are calcareous species-rich semi-natural grasslands, mostly occurring on the outcrops of Ordovician or Silurian limestone (LAASIMER 1965). Alvar grasslands are characterised by very shallow (< 20 cm) soils and a distinctive species composition consisting of calciphilous, light-demanding species (LAASIMER 1965, PAAL 1997). Distribution of alvar grasslands is very limited: they mostly occur in Sweden and Estonia, and to lesser extent a similar habitat type is also present in Canada and Russia (SCHAEFER & LARSON 1997, ZNAMENSKIY et al. 2006). For centuries alvar grasslands have been more or less continuously mown and grazed, which has kept the shrub cover low or even entirely absent (LAASIMER 1965, POSKA & SAARSE 2002; Fig. 1). In Estonia alvar grasslands are currently mostly abandoned and rapidly overgrown by pine (Pinus sylvestris) and juniper (Juniperus communis) (Fig. 2). Since the 1930s the surface area of alvars has decreased from 43,500 hectares to approx. 5000 hectares (HELM et al. 2006). Without large- scale restoration measures, their further persistence in Estonia is doubtful (HELM et al. 2006, PÄRTEL & HELM 2007). 294 Fig. 1. Rajametsa alvar grassland in western Estonia. One of the few well-preserved and currently continuously grazed alvar grassland (photo: A. Helm). Abb. 1. Magerrasen des Rajametsa-Alvars in West-Estland. Es handelt sich dabei um einen der weni- gen noch gut erhaltenen und derzeit noch kontinuierlich beweideten Alvare in Estland (Foto: A. Helm). Fig. 2. Abandoned and highly overgrown (mostly by pines and junipers) Pivarootsi alvar grassland in western Estonia (photo: L. Saar). Abb. 2. Aufgegebener und stark mit v. a. Kiefer und Wacholder verbuschter Magerrasen des Pivaroot- si-Alvars in West-Estland (Foto: L. Saar). 295 Alvar grasslands with a shrub cover of 30% are considered to be in the best condition, and this cover is also recommended as a desirable state following habitat restoration (HELM 2011). However, although several studies have shown that a shrub cover of more than 70% rapidly decreases alvar grassland species richness (PÄRTEL et al. 1999b, REJMÁNEK & RO- SÉN 1988), little is known of how a low shrub cover (up to 30%) influences the environmen- tal conditions and species richness of alvar grasslands. The influence of woody species cover on herbaceous vegetation is most pronounced at small scales (REJMÁNEK & ROSÉN 1992). It is also important to take into account that habitat characteristic species and more widespread generalist species may be differently related to low shrub cover due to the possibly differing requirements for habitat conditions (PYKÄLÄ et al. 2005, REJMÁNEK & ROSÉN 1992). The aim of this study is to detect the extent to which shrub encroachment affects plant species richness and composition in currently open and visually well-preserved habitat patches. We hypothesize that even a low cover of shrubs (up to 30%) influences the small- scale richness of grassland plants via altering the environmental conditions or increasing environmental heterogeneity. More specifically we test whether the richness of alvar charac- teristic species and generalist species are influenced differently by environmental changes triggered by low shrub cover. 2. Material and methods A total of 33 alvar grasslands (EC Habitats Directive habitat type 6280* “Nordic alvar and precam- brian calcareous flatrocks”) on the Estonian islands of Saaremaa and Muhu (Fig. 3; ca. 58º N, 22–23º E) were visited during August 2010. Each grassland belonged