Spatial pattern of occurrence of epiphytic lichens on oaks in a heterogeneous landscape
Introduction
Old growth trees are of particular interest for conservation, since they are an important substrate for a number of organisms from several taxonomic groups, e.g., epiphytic lichens and bryophytes and many insects (Ek and Johannesson, 2005, Jansson et al., 2009, Lindenmayer et al., 2012). Trunks of old growth oaks (Quercus robur) host a diverse lichen flora and with increasing age the bark of oaks becomes suitable for rare and threatened species (Johansson et al., 2009, Paltto et al., 2010, Thor et al., 2010, Jönsson et al., 2011). The decline of old oaks in the landscape may be the main reason why many epiphytic lichens only seem to exist in remnant populations in patches with high density of old oaks (Johansson et al., 2009, Johansson et al., 2012, Scheidegger and Werth, 2009), and why many oak lichen species are threatened and red-listed (Paltto et al., 2010, Ranius et al., 2008). Most often a reduction of habitat involves increasing isolation of habitat patches, and that may add to the expected decline in species richness (Ranius et al., 2008, Steffan-Dewenter et al., 2002, Löbel et al., 2006).
Conservation of rare and threatened epiphytic lichens need to estimate the densities of old oaks that are needed for continued survival. It is further important to know at what spatial scales to consider such densities. Furthermore, other landscape variables – in addition to substrate availability – may affect lichen species distribution, e.g. forest openness and landscape composition (Jüriado et al., 2003, Bolliger et al., 2007). One way to approach these problems is to explore current species occupancy patterns in relation to potential substrates in small to large scales (Paltto et al., 2006, Paltto et al., 2010, Snäll et al., 2003, Holland et al., 2004, Bergman et al., 2012, Musa et al., 2013), which could add to our understanding of the dynamics of species and build better strategies for preservation (Löbel et al., 2006, Sillett et al., 2000, Lättman et al., 2009, Lättman et al., 2014).
In a recent study, Paltto et al. (2010). showed that species richness and the occurrence of three of five red-listed epiphytic lichen species on oaks were best explained by increasing density of oaks within 0.5 km in a large region (10,000 km2). However, this study tested scales in fixed steps from 0.5 km and upwards (1, 2, 3, 4 km etc.). Considering these results it would therefore be valuable to explore the spatial scales around 500 m in more detail. For conservation, it is also important to know how to manage the immediate surroundings of valuable oak trees as, for instance, sun exposure of the oak and density of surrounding shrubs and trees can affect the probability of occurrence of lichens (Johansson et al., 2009, Johansson et al., 2012, Jüriado et al., 2003, Paltto et al., 2011).
The aim of this study was to investigate the spatial distribution of eleven epiphytic lichens species preferring large and old oaks in a heterogeneous landscape, and to identify the spatial scales, in the range of 28–1225 m, at which species occurrence is best explained. Landscape variables included density of other large oaks in the surrounding of a target tree, as well as potentially important land use types (the amount of houses, and water, agricultural and forested land (Styers et al., 2010, Svoboda et al., 2010)). Furthermore, two tree-specific factors that could be affected by management (sun exposure, and density of nearby trees and shrubs) were also considered. The study was conducted in one of the few remaining landscapes in Northern Europe with a high density of old oaks: the province of Östergötland, south-eastern Sweden (Antonsson and Wadstein, 1991).
Section snippets
Study species
The eleven lichen species targeted are known to be strongly associated with large oaks, but differing in frequency and abundance (Table 1). For most of these lichens, it has been confirmed that they are very rare outside old oaks (Ranius et al., 2008). All species except Chrysothrix candelaris are listed on the Swedish Red List or used as signal species in the woodland key habitat survey in Sweden indicating forests of high conservation value (Table 1 (Gärdenfors, 2010, Johansson et al., 2010
Results
Lichen species richness varied between 0 and 7 species per tree (mean 2.14; median 2). Nine of the eleven target species were found and their occupancy varied between 99% and 0.5% (Table 1).
Discussion
There are several factors affecting the occurrence of lichens (Pinho et al., 2008), but their relative contribution have been difficult to assess. We compared the relative effect of land use (forest, arable, houses and water), tree variables (circumference, sun exposure, trees and shrubs near the oak) and oak density at different spatial scales and found that they often showed a clear peak of response between 200 and 400 m. Moreover, oak density had the largest overall effect while tree
Conclusion
The occurrence and richness of red-listed lichens on oaks was mainly affected by oak density. The second most important factor was circumference while land use was of less importance. Furthermore, this study highlighted the importance of spatial scale for understanding the occurrence of epiphytic lichens, where the species responded to landscape factors at the scale of 20–50 ha. These finding have implication for conservation efforts – management, restoration, reserve allocation – in oak
Author contributions
LMW, KOB and PM conceived the study; all authors contributed to the design; UHM collected the data; UHM and LMW analysed the data; all authors contributed to evaluation and interpretation of the results; UHM drafted the text (as part of a MSc thesis) with all authors contributing substantially to the current version.
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