Studies on spatial and temporal distributions of epiphytic lichens

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Linköping Studies in Science and Technology Dissertation thesis No. 1310

Studies on spatial and temporal

distributions of epiphytic lichens

Håkan Lättman

School of Life Science Södertörn University SE – 141 89 Huddinge, Sweden

Department of Physics, Chemistry and Biology Division of Ecology

Linköping University SE – 581 83 Linköping, Sweden

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Front cover: The cartoon is produced by Jan Berglin Paper divider: Photo by the author

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Lyrics

Pay close attention Prodigy God damn right it’s a beautiful day Eels I got run over by truth one day D.A.D.

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List of papers

The following papers are included in the thesis and are referred in the text by their Roman numerals:

Paper I Lättman H, Lönn M, Milberg P & Mattsson J-E. The succession of epiphytic lichens quantified on Quercus robur L. Manuscript. Paper II Lättman H, Brand A, Hedlund J, Krikorev M, Olsson N, Robeck A,

Rönnmark F & Mattsson J-E. (2009) Generation time estimated to be 25–30 years in Cliostomum corrugatum (Ach.) Fr. The Lichenologist 41: 557–559.

Paper III Lättman H, Lindblom L, Mattsson J-E, Milberg P, Skage M & Ekman S (2009) Estimating the dispersal capacity of the rare lichen Cliostomum

corrugatum. Biological Conservation 142: 1870–1878.

Paper IV Lättman H, Milberg P, Palmer MW & Mattsson J-E (2009) Changes in the distribution of epiphytic lichens in southern Sweden using a new statistical method. Nordic Journal of Botany 27: 413–418.

Paper V Lättman H, Lönn M, Mattsson J-E & Milberg P. Regional gradients in occurrence and size of the epiphytic lichen: Hypogymnia physodes (L.) Nyl. in southern Sweden. Manuscript.

Published papers are reproduced with kind permission from the publisher.

My contributions to the papers

I have, together with co-authors, set up and designed the field work on Paper I–III and V. All field work was done by the undersigned on Paper I–III and V and as co-worker on Paper IV. Laboratory work, i.e., DNA extraction, PCR amplification and sequencing were carried out by me in Paper III as well as editing and alignment. The statistical analyses were done by the undersigned in Paper I–II and V as well as some minor analyses in Paper III. I have done most of the writing on Paper I and V, contributed substantially to Paper III and a bit less to Paper II and IV.

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Abstract

Lättman, H. 2010. Studies on spatial and temporal distributions of epiphytic lichens Doctoral dissertation

ISSN 1652–7399; ISBN 978–91–7393–405–3; ISSN 0345–7524

Lichens are an important group of organisms in terms of environmental issues, conservation biology and biodiversity, since lichens are sensitive to changes in their environment. Therefore it is important that we develop our understanding of the factors that affect lichen distribution. In this thesis both spatial and temporal distributions of epiphytic lichens at different scales have been studied in southern Sweden.

Brokind was chosen as the study site to investigate the succession of epiphytic lichens on Quercus robur using a chronosequential approach. Fourteen of the investigated taxa out of 50 proved to be significant. The taxa were divided into three groups according to whether they occurred on young, middle-aged or old trees.

Generation length of the red-listed lichen Cliostomum corrugatum was examined using Bjärka-Säby as the study site. The results showed that the average age of an individual of C. corrugatum is 25–30 years at the onset of spore production.

The rarity of C. corrugatum was also examined. DNA of an intron from 85 samples, collected at five sites in Östergötland, yielded 11 haplotypes. Results from the

coalescent analysis, mantel test and AMOVA indicated that C. corrugatum have a high ability to disperse. The study concluded that its rarity is most likely connected with the low amount of available habitat, old Q. robur.

The changes in the distribution of epiphytic lichens in southern Sweden between 1986 and 2003 were compared. For each year a centroid was calculated on all combinations of tree and lichen species. The three significant cases showed that the centroid movement pointed toward a north-east or north-north-east direction.

Regional gradients of abundance and size of Hypogymnia physodes at 66 sites in southern Sweden were examined. The coordinate system rotating the reference system

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Authors address: Håkan Lättman, School of Life Sciences, Södertörn University,

SE-141 89 HUDDINGE, Sweden; IFM Division of Ecology, Linköping University, SE-581 83 LINKÖPING, Sweden.

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Preface

It has been a long and winding journey from where I started my professional career until where I am today. I started in the building construction industry in mid 1970s as a carpenter. About ten years later, in middle of the 80s, Sweden went into a small economic crisis and I had difficulties in maintaining a livelihood in this business. But in a neighbouring profession, as a carpet layer, there were still some jobs available; consequently I started to lay rugs for about eight years. The last two years in this occupation I had my own company “Lättmans Golv”.

In the beginning of the 90s Sweden went into a second economic crisis. I guess the whole country was close to going bankrupt. The situation was severe and my company “Lättmans Golv” was ruined. The chieftain by this time, Ingvar “Shoe” Karlsson encouraged the unemployed to start studying but there was also an option at “The Employment Service” to practice a profession that was new to the job applicant. This was a great opportunity and I worked at a small and a large bakery and in a restaurant. After a few months I decided that I wanted to educate myself and from that day onwards I have been studying, which is a decision that I have no regrets about whatsoever.

Since I only had two years of high school I had to supplement this with a 3rd _year. One year later I applied to the biology program at Linköping University. After three and a half years of study my journey continued at Lund University, when after one and a half years I finished my thesis where I investigated an avenue on their epiphytic lichens, supervised by Ingvar “Mäster” Kärnefelt. After I finished my Masters degree I had a brief inventory job at “The National Road Administration” to search for red-listed and rare epiphytic lichen species on avenue trees in Östergötland. By this time I had become interested in lichens and their world and I were really glad when I got a position as a PhD-student at Södertörn University.

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Background

The environment has been changing throughout the Earth’s history for different reasons. Undoubtedly today humans have the greatest impact on the environment and on its species. The development of our culture and civilisation has resulted in the widespread exploitation of nature with significant degrading effects, including recent global climatic changes. Because of our large and unprecedented impact on our surroundings, there have been suggestions that the current geological epoch Holocene has come to an end and that we are beginning a new epoch called Antropocene (Zalasiewicz et al., 2008). Humans have a great impact on the landscape in the form of agriculture and forestry where, armed with arguments of efficiency, they tend to grow only one crop species over large expanses. This has had, and is continuing to have, a large negative impact on biodiversity. Due to our actions, more and more of the Earth’s surface are exploited resulting in increasing habitat loss and fragmentation of the landscape. This has led to the decline in abundance and distribution of many species and even to extinction for others. In order to understand and predict how species will respond to human activities in the environment basic knowledge about species behaviour is vital. It is important to study the succession of species, their dispersal ability, distribution, establishment, and population structure in order to conserve biodiversity. In Europe especially, the broad-leaved forests have suffered. Many species are solely dependent on these forests and are unable to extend their range to other habitats e.g. many lichens, insects, and fungi. Globally, lichens are a group of organisms that have been less studied than other comparable multi-cellular organisms. Thus, there is a gap in the scientific knowledge concerning lichen species' dispersal capacity and establishment on different substrates, their habitat requirements, and population structure. Our lack of knowledge of lichens is explained by their inconspicuousness and their small thalli, which may make them difficult to indentify. This might also explain why Carl von Linnae (1707–1778) did not pay any attention to lichens. Fortunately his student, Erik Acharius (1757–1819) did, and made great progress by identifying many lichen species, estimated at more than 300 taxa (Krempelhuber, 1867). Hale (1974) reported the number of lichen species to be 17.000 and then ten years later Hawksworth and Hill (1984) reported the number to be 13.500. At present 18.803 lichen species have been described (Feuerer, 2009). Red-listed species are threatened in any way for their existence. There are species that have decline by their abundance because of the some threat from the environment and must be protected. Due to human activities, severally lichens have been affected and decreased in their abundance and, unfortunately, become red-listed. In which way will human activities such as agriculture, forestry and global warming affect lichens in the future?

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Aims of the thesis

The overall goal of this thesis is to present methods to reveal the factors behind the change in time and space in the occurrence of epiphytic lichen species and their communities. The complexity of these factors and their synergistic effects make it necessary to undertake studies on different spatial and organisational levels. This ranges from genetic, individual and population levels to community and biotope levels. Thus, a number of different studies with different objectives were designed in order to target the overall aims of the thesis.

The objective of Paper I was to describe the succession of epiphytic lichen

communities or single species on the trunks of the tree species Quercus robur L. as the tree grows. A chronosequence approach was used to investigate tree trunks of different ages to predict how succession will develop.

Estimating the generation time of the red-listed crustose epiphytic lichen

Cliostomum corrugatum (Ach.) Fr. was the objective of Paper II. A method for

assessing generation times, from meiospore to meiospore, is often necessary in order to describe evolutionary history both in the short as well as in a long run perspective. Here the results achieved were used in the following study (Paper III).

In Paper III the question was if C. corrugatum is restricted to sites with long temporal continuity or to sites where the substrate and microhabitat meet the species ' particular ecological requirements. The more general question here was how to distinguish between limiting habitat and dispersal factors.

Paper IV deals with the problems of describing changes in the lichen flora on a regional scale. From field surveys conducted in 1986 and 2003 of common epiphytic lichens in southern Sweden, the change in position of the centroids over this time was monitored. A centroid of a species is the mean position of its sites in an area.

Finally, in Paper V the demographic structure on populations of the common lichen,

Hypogymnia physodes (L.) Nyl. in southern Sweden was studied in order to find a

method to detect whether populations are newly established, healthy or vanishing based mainly on thallus size. Based on these demographic data regional gradients in the health of populations may be revealed.

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Human-caused threats to lichens

One reason explaining the sensitivity of lichens to human impact is their complex structure. However, somewhat paradoxically, this is also the key to their survival in very harsh environments. Lichen symbiosis always consists of a fungus together with an alga or a cyanobacterium. The association between the fungus and the photobiont in lichens has been a successful relationship lasting at least 400 million years (Taylor, 1995). In partnerships the two species are able to withstand harsh environments that they could not withstand by themselves. For example, the lichens Rhizocarpon

geographicum (L.) DC. and Xanthoria elegans (Link) Th.Fr. have even survived in

space (Sancho, 2007). The environment in space is harsh and unfriendly, i.e. vacuum, wide fluctuations of temperature, extra terrestrial solar electromagnetic radiation and cosmic radiation.

On the other hand, lichens are sensitive to changes in abiotic and biotic factors. Since lichens do not have a cuticle like, e.g., spermatophytes airborne particles may easily enter the cells and affect the lichen negatively. Some of the most significant threats to lichens are air pollution, global warming, forestry, habitat loss and fragmentation of habitats.

Air pollution

In the 19th_{century independent observers in England (Grindon, 1859), Munich (c.f.} Gries, 1997) and Paris (Nylander, 1866) documented that lichens were disappearing from the cities. Hundred years later, in the beginning of the 20th_{century, this city-based} phenomenon was recognised across the whole of Europe. The toxic agent was first ascribed as the dust from coal but it was later realised that sulphur dioxide (SO2) was the main toxic agent. Hawksworth and Rose (1970) showed that lichens could be used to monitor the SO2 content in the air. They used a ten degree scale using lichens with different sensitivity to SO2.

Soon after the industrial revolution of Europe in the late 18th_{century, air quality had} started to change. Sulphur dioxide emission and deposition increased rapidly all over the world from the early 20th century (Erisman and Draaijers, 1995; Mylona, 1996). Sulphur dioxide is one of the most lethal substances for lichens and is largely suspected to be responsible for most of the decrease and death of lichens (Gilbert, 1968).

International co-operation with the goal to reduce the effects of air pollutants on the environment has been fruitful, and a dramatic decrease in SO2 has been observed in recent decades (UNECE, 1999). Schopp et al. (2003) predicted a continued decrease of SO2 in Europe up until the end of 2030. Recolonization of lichens in London has been shown by Hawksworth and McManus (1998) and has been attributed to the decrease in sulphur dioxide.

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Global warming

During the last decades, many studies on global warming have revealed its enormous impact on numerous different organisms (Vitousek, 1994; Hughes, 2000; Saxe et al., 2001; Parmesan and Yohe, 2003; Root et al., 2003; Sanz-Elorza et al., 2003; Thuiller et al., 2005; Parmesan, 2006). A study from the Netherlands, (van Herk et al., 2002) revealed that lichens have responded to a warmer climate. Their results suggest that epiphytic lichens on trees, and other transient substrates, are likely to respond relatively quickly to large-scale climate change. van Herk et al. (2002) used checklists, field data, herbarium material and long-term monitoring data to study large-scale changes of lichens. The study was based on the documentation of 329 lichen species collected in 1979, 1984, 1989, 1995 and 2001. Their conclusion was that lichens with a southern distribution limit in the Netherlands have declined. On the other hand, species with a general northern distribution limit are at present expanding their range into the country. van Herk et al. (2002) have shown that lichens are an important tool for monitoring the effects of climate change.

Forestry

Forestry practices in Sweden have changed during recent centuries. Different kinds of forestry operations have different impacts on forest habitat for non-target organisms. Out of all land areas in Sweden (41.3 million ha), 23 million ha consists of production forest (NBF, 2007). The human impact on the production forest is significant and they are managed, primarily to preserve economic values. Prevailing forest management is unfortunately often inconsistent with the necessary life conditions for lichens dependent on old tree trunks, standing and prostate dead wood - such habitats have become scarce. Thus, the change in the demography of forests towards a greater proportion of younger trees is a negative factor for many lichen species.

Habitat loss and fragmentation

As our population increases we use more and more land area: we claim more space. As a consequence, the amount of available habitat reduces for most other organisms. This unceasing demand to exploit new areas and there resources for our benefit has create severe situations for many other non-human organisms, particularly those that requires large areas for their survival. Furthermore, remaining habitats become fragmented and thus divided from each other into patches often bordered by different kinds of intense human activity, such as agriculture, urban areas, roads and railway tracks. Organisms with a limited ability to disperse face a severe situation in fragmented areas.

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and rubber (personal field observations). On the other hand some lichens, often red-listed species, are restricted to a specific habitat or substrate.

Several papers have been published arguing that some lichen have limited dispersal abilities. Knowledge is often lacking about whether dispersal or substrate is the limiting factor for a specific lichen population to survive in the long run, both locally and regionally. To fully understand the importance of these different factors in isolation or in combination it is necessary to study both dispersal efficiency and the impact of substrate abundance and microhabitat characteristics. Dispersal is never successful unless diaspores spread and establish functional thalli on new substrates in new environments. Thus, it is important to know the range of the dispersal area, the abundance of suitable habitats within this area and the abundance of suitable substrates within the habitats. Also the stability of the sites over a long period of time has to be assessed.

Population stability is another important factor which may be assessed by demographic methods. Stable populations have a comparatively high proportion of small newly established thalli and lower proportions of larger ones.

The factors discussed here may not be the only ones affecting lichen occurrences but all of them are of great importance individually or in combinations. In order to

understand the reason for observed changes in distribution areas, substrate preferences, community composition, or population structures, etc., it is necessary to design methods for assessments of these factors’ importance in different situations. In order to be reliable these methods have to focus on understanding processes based on descriptions of snapshots analysed to depict the ongoing changes.

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Successional changes and chronosequence

The term succession, when used by ecologists, describes the development of

communities over a period of approximately 1000 years or less. The general pattern of a succession on, e.g., abandoned arable land, meadow or a cut forests are the changes in occurrence of vegetation, animals and microorganisms over time. On a particular site, the vegetation, animals and microorganisms that are part of successive communities will be different. There are two different types of succession, i.e., primary and secondary succession. Primary succession is when a newly exposed area that has not been previously influenced by organisms, e.g., lava flows, craters caused by meteors, substrate exposed by the retreat of a glacier or sand dunes, is colonized by organisms for the first time. If there has been a community on a site that has been partially or completely removed, but still has well developed soil with remaining in situ seeds and spores, the subsequent sequence of species is called a secondary succession. The species that enter the succession in an early stage are referred to as pioneer species or early successional species. These species are characterized by a high growth rate, smaller size and a high degree of dispersal. Late successional species generally have a lower rate of dispersal and colonisation and tend to be larger and live longer.

For all epiphytic organisms, including the epiphytic lichens, it is a matter of perspective to regard succession as primary and secondary. In general terms, epiphytic lichen populations and communities established in a forested area may change in size and composition as the forest ages representing different secondary succession stages. On every tree the trunk and branches will be colonized as they develop and this may be regarded by some as primary succession. On a stand level, the succession is secondary but on a tree level it may be regarded as primary.

There are several papers written about lichen succession on trees. Hedenås and Ericson (2000) studied succession on whole trees while Yarranton (1972), Olsson (1995), Bates and Brown (1981) and Ellis and Coppins (2007) studied tree trunks and Hilmo (1994) studied succession only on branches.

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listed, Caloplaca lucifuga G. Thor; Near Threatened (NT), Cliostomum corrugatum (Ach.) Fr. NT, Schismatomma decolorans (Turner & Borrer ex Sm.) Clauzade & Vezda NT, Sclerophora coniophaea (Norman) J. Mattsson & Middelb. NT and Lecanographa

amylacea (Ehrh. ex Pers.) Egea & Torrente Vulnerable (Gärdenfors, 2005).

Of all the lichens identified taxa 14 were significantly related to trunk circumferenc. They were divided into three groups according to the model used: Early in the

Succession (ES), Intermediate in the Succession (IS) and Late in the Succession (LS). The results showed that on trees with similar diameter of the trunk, similar lichen species was found but also the variation in species composition was greater between small sized trees than between larger trees. The general pattern of lichens and the different communities in the succession is that they were most common during a certain period of the life-cycle of Q. robur (Fig. 1). It is worth noting that the different groups have low abundance outside of the stages where they were most frequent. It was only 14 taxa that were significant due to the small amount of data derived from only 13 tree trunks. It would be interesting to extend the study by investigating where more species fit in the succession stages.

Figure 1. A general pattern of the succession of the four groups, Early in the Succession (ES), Intermediate in the Succession (IS) and Late in the Succession (LS).

Dispersal capacity of lichens

Dispersal of a lichen is the activity when the diaspores of an individual move from one place to another. To establish offspring in new habitats fast dispersal over long

distances may be promoted by easily spread diaspores. This dispersal may be passive or active. The passive dispersal refers to a situation when a vector, e.g., wind, water or animals carries the diaspore, whereas active dispersal is when the individual itself actively moves to another site. Conditions of crowding and high competition are known to influence whether individual lichens engage in acts of active dispersal. The success of the dispersal is related to the abundance of acceptable habitats and substrates. Often a specific combination of aforementioned factors is necessary for stability in a population or successful colonization.

There are different views though, about the ability of lichens to disperse and establish. Armstrong (1987, 1990), Tapper (1976), Heinken (1999) and Lorentsson and

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although the studies did not include the success rate of establishment. Dispersal limitation has also been reported for lichens on trees, e.g., within tree stands, between compact tree stands, or between trees up to a few kilometers apart (Dettki et al., 2000; Sillett et al., 2000; Hilmo and Såstad, 2001; Johansson and Ehrlén, 2003; Walser, 2004; Öckinger et al., 2005). However, genetic studies of Xanthoria parietina (L.) Th. Fr. and

Lobaria pulmonaria L (Hoffm.) suggest that effective dispersal at a few kilometres did

not show any sign of being restricted, although ascospores have been found to disperse, on average, at a longer distance than heavier vegetative diaspores (Lindblom and Ekman, 2006, 2007; Wagner et al., 2006; Werth et al., 2006a, 2006b). At a large spatial scale, for populations separated by hundreds of kilometers or more, genetic studies of lichen populations revealed considerable gene flow (Printzen et al., 2003; Palice and Printzen, 2004; Walser et al., 2005), whereas studies relying on biogeographic patterns (Munoz et al., 2004), trapping of lichen fragments in the atmosphere (Harmata and Olech, 1991) and observations of lichen fragments on bird feet (Coppins and James, 1979) concluded that effective dispersal was frequent. Finally, the small size and weight of the ascospores has been taken as indirect evidence that lichens are able to disperse “widely” (Nordén and Appelqvist, 2001).

Knowledge about generation length is often important, especially when evolutionary processes are assessed. The time span between meioses events is important to estimate as these events have a potential for genetic recombination and mutation, while the vegetative phase of organisms is more inert at the genetic level. Thus, knowledge of species generation time is essential for calculations of the speed of evolutionary changes and also for the dispersal assessments. In Paper II the generation length of Cliostomum

corrugatum was studied in Bjärka-Säby, Östergötland, Sweden. The trees CBH and the

area of the largest thallus were measured. Since Johansson et al. (2009) and Ranius et al. (2008) had shown that the occurrence of C. corrugatum on trees with a small CBH is low only larger trees were included. The results showed that C. corrugatum has to reach an average age of 25–30 years before it is able to produce and disperse its spores (Fig. 2). I find it interesting and a bit surprising that the generation time, from spore to spore, takes so many years. Since this is the first time that generation time has been

determined in a lichen one could ask the interesting follow up question: What differences are there between the generation times of different lichen species?

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In Paper III we investigated whether the rare lichen C. corrugatum was restricted to sites with long temporal continuity or to sites where the substrate and microhabitat meet the species’ particular ecological requirements. The investigation was conducted in central Östergötland, south-eastern Sweden at five sites. The laboratory methods involved DNA extraction, PCR amplification and sequencing of a group 1 intron at the end of the small subunit (SSU). Attempts were made on other parts of the DNA (ITS and IGS) but the variability of these regions was too low for our purposes. Out of the 96 collected samples of C. corrugatum, 85 were successfully extracted and shown to represent 11 haplotypes (Fig. 3).

Several statistical methods were used to analyse the genetic variation and the gene flow between the populations. First, a coalescent simulation showed that the gene flow was considerable between the five investigated sites. Second, a mantel test showed that there were no significant correlation between the genetic and the geographic distances matrices. Third, an AMOVA test showed that 0.4% of the variation was between the populations and 99.6% of the variation was within the populations. All three tests indicate that C. corrugatum does not seem to have any difficulties dispersing from one place to another. In addition our results indicate that the five sites behave more or less as a single, sexual interbreeding population, that is: a panmictic population.

Consequently, C. corrugatum rarity is likely to be connected with the limited amounts of the suitable habitat, old oaks. During several hundreds of years an oak trunk serves as a suitable substrate for this lichen, a time during which it disperses spores almost every year with exception of the first 25. As C. corrugatum is also found in isolated oaks in different habitats it unlikely that it is dependent on specific habitats but relies mainly on old oaks. In the light of our results, it is worth noting that the distribution of Quercus

robur encompasses more or less the whole of Europe, but its abundance has decreased

during recent centuries. Furthermore the age distribution is uneven because old Q. robur have become rare.

Figure 3. The unrooted network of a group 1 intron on the epiphytic lichen Cliostomum

corrugatum are represented by 11 haplotypes. The most common haplotypes 1 (N = 30)

and 2 (N = 46) is in the centre of the network, are most likely the oldest. The terminal haplotypes are rare (N = 1) and have evolved through haplotypes 1 and 2.

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Distribution and climate change

Large-scale environmental changes have similar affect on different organisms, e.g., promoting range shifts in similar directions. Current global warming is likely to be a prominent factor behind rapid changes in distribution areas, community composition and population structure. Most studies of distributional changes attributed to global warming in the northern hemisphere have shown a northward expansion of the organisms studied (e.g. Thomas and Lennon, 1999; Warren et al., 2001; Parmesan and Yohe, 2003; Root et al., 2003; Hickling et al., 2005). Studies concerned with changes in population densities (Thomas and Lennon, 1999; Warren et al., 2001; Hickling et al., 2005), or focussed on the likelihood of encountering a species (Bridle and Vines, 2007) are fewer. Few data sets exist at the appropriate spatial and temporal scales. Further, available data are often not straightforward and rarely detailed; e.g., scarce field notes from sites with long intervals, data collected without a uniform protocol, or data with low information content. Regardless, data of this type may be useful if large, even-quality data sets are established thereby providing possibilities for interesting analyses.

Also simple methods for analyses of uneven-quality data would be valuable and these have sometimes been used. As epiphytic lichens are likely to respond relatively quickly to broad-scale changes of climate (strong signal) also comparatively poor quality data (large error) may be used for analyses. One example of this is the study by van Herk et al. (2002) who used checklists, data from field meetings, herbarium material and long-term monitoring data to study large-scale changes of lichens in the Netherlands. van Herk et al. (2002) found that lichens has respond to a warmer climate within the last couple of decades. Their results showed that lichens with a southern distribution limit in the Netherlands have declined; while southern species with a northern distribution limit are at present invading the country.

Similar results were achieved in Paper IV where the movement of the “centre of distribution” within southern Sweden, the centroid movements, of some common epiphytic lichens were studied. The study was conducted at 64 sites and the inventory in field was carried out both in 1986 and 2003 and 56 epiphytic lichen species and 22 tree species were included. Thirty cases were analyzed, out of which three were significant. The centroid movements of the lichens Hypogymnia physodes (L.) Nyl. and Vulpicida

pinastri (Scop.) J.-E. Mattsson and M. J. Lai on the tree species Juniperus communis L.

were 50 km and 151 km (p-value 0.0258, 0.0002) with the direction 27° and 48°, respectively. The movement of the centroids of Hypogymnia physodes on Pinus

sylvestris L. was 41 km (p-value 0.0066) with the direction 30° (Fig. 4). All three

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Figure 4. The three arrows indicate direction and distance the centroid had moved between the years 1986 and 2003 of Hypogymnia physodes and Vulpicida pinastri on the tree species Juniperus communis but also Hypogymnia physodes on the tree species

Pinus sylvestris.

The data set was fairly small with low informative content; only the occurrence of epiphytic macrolichens on different substrates was recorded. The sites were roughly described; however, information on tree size and tree species abundance and information about trees without lichens was not recorded. Nevertheless, some significant changes were recorded for a couple of common species in the area. Furthermore, these species possess characteristics that provide it with strong resilience against small changes in environmental conditions. This indicates a larger impact of global warming on the epiphytic lichen flora than previously presumed.

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use size structure as a proxy for the demographic structure of populations. Hypogymnia

physodes is by far the most common epiphytic macrolichen in the area (data on some

other common lichens were also collected, but were too rare to be included in the analyses). All occurring tree species present on a site were investigated. Our results showed that there was a gradient of increasing probability of occurrence in a north-north-east direction (15°) and that the average size of thallus diameter increased in a west-north-west direction (304°) (Fig. 5). The increase of probability of occurrence going in a north north-east direction in Paper V corroborates the direction of centred shift identified in Paper IV.

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Both Paper IV and Paper V showed similar results although they differ in

methodology. They cover the same investigation area and the studied sites are similar in number, 64 and 66 respectively. There are differences in site selection. In Paper IV the sites were selected out of knowledge of earlier observations of Vulpicida juniperinus (L.) J.-E: Mattsson & M.J. Lai or V. pinastri in order to study the present (1986) situation of these lichens. During the field work other lichens were recorded. These sites were studied later in 2003 in a similar way and were analyzed for changes in lichen occurrences. The studied sites from Paper V were selected in order to get, in combination with the sites of Paper IV, an even distribution of sites within the area. Here, sites only covered by a forest were included while in Paper IV all types of sites including, pastures, farm yards, rural areas etc., are represented. In the demographic study (Paper V) the analyses was based on population structure at one occasion while the study in Paper IV was based on comparisons of occurrences on two occasions. A study based on population structures needs a larger data set which is discernable in the results concerning only one species.

The used and modified methods

All the methods used were originally designed or are modifications of methods used by others. In most cases this work was undertaken in co-operation with the different project groups but the main ideas were already sketched during the early planning stages of this thesis. The over-riding idea was to use simple, easily obtained, preferably large, data sets to analyze them in order to find patterns. Although these studies are comparatively small, resulting in a fairly modest number of significant results within each data set, the methods developed are potentially powerful.

One necessary condition for obtaining large sets during a limited amount of time is a relatively high abundance of the objects of the investigation. Thus, common species, e.g., in a geographic area (Paper IV and V) or on a specific substrate (Paper I) are easy to study. Although Cliostomum corrugatum is fairly rare it is possible to study it on its most common substrate, Quercus robur using these methods.

The method for determining the shortest generation length of lichens (Paper II), using a small number of observations, is easy to use and also extremely efficient.

Although we used some loci well known for their genetic variability this variation was absent in C. corrugatum (Paper III). The used gene showed small a variation, which indicates problems when using only a limited number of genetic markers during the development of standardized methods.

In summary, the methods described above constitute a methodological tool box useful for different types of assessments, not only of lichen, but also of other organisms. At the same time, the results concerning common species make it also possible to draw conclusions about less abundant species. If common well adapted species show differences in their distribution, it is reasonable to assume similar patterns for less abundant species and to design studies in order to achieve data sets large enough for reliable results.

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Populärvetenskaplig sammanfattning

En lav består av en svamp och en alg som lever i symbios vilket betyder ungefär; att leva tillsammans. Ibland ansluter sig även en cyanobakterie till symbiosen. Svampen kallas för mykobiont och är för det mesta en sporsäckssvamp, vid några tillfällen så är det en basidsvamp. Algen, för det mesta en grönalg, eller cyanobakterien kallas för fotobiont. Dessutom finns ett fall beskrivet där algen en brunalg. I Sverige finns det drygt 2100 lavarter som är vetenskapligt beskrivna och i hela världen nästan 19000. Lavar är en viktig organismgrupp att studera eftersom de kan ge oss svara på många frågor när det gäller förändringar i vår miljö. De är känsliga för yttre förändringar samtidigt som de är tuffa och kan leva i ogästvänliga miljöer där många andra organismer har svårt att överleva. Ett exempel på deras tålighet är kartlav som har tillbringat 14 dagar ute i rymden och överlevt både vakuum, solens elektromagnetiska strålning och kosmisk strålning. Å andra sidan så har de svårt att klara t.ex. luftens föroreningar i urbana miljöer. En lav som förekommer på växter kallas för en epifyt. I den här doktorsavhandlingen har jag studerat utbredning och spridning av epifytiska lavar i tid och rum i södra Sverige.

I Brokinds skolhage, Östergötland, undersöktes ekar av olika åldrar för att se i vilken ordningsföljd epifytiska lavarter och lavsamhällen avlöser varandra (Papper I). Den här processen kallas för succession och är egentligen svår att studera eftersom det tar lång tid för en ek att bli gammal. Metoden jag använde för att undersöka

successionen kallas kronosekvens. I stället för att studera unga träd i flera hundra år och se vilka lavarter och lavsamhällen som avlöser varandra i successionen så studeras både unga, medelålders och gamla träd, vid ett tillfälle. Våra resultat visar att somliga lavarter förekommer mest på unga ekar, dvs. tidigt i successionen medan andra arter förekommer mest på äldre ekar, dvs. sent i successionen.

I Bjärka-Säby, Östergötland, inventerades ekar med avseende på en sällsynt lav som kallas för gul dropplav (Papper II). Undersökningens syfte var att ta reda på hur många år det tar för en spor av gul dropplav att växa upp och själv bli ”förälder” och producera

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lavarter. För vart och ett av åren beräknades en medelpunkt, en så kallad centroid, för varje kombination av lokal, träd- och lavart. Utav alla tänkbara kombinationer var det endast 30 fall som motiverades en analys, varav 3 uppvisade en signifikant förflyttning av centroiden. Förflyttningen av centroiden för blås- och granlav på trädslaget en var 50 och 151 km i riktningen 27 och 48 grader för respektive lavart. Förflyttningen av centroiden för blåslav på gran var 41 km i riktningen 30 grader. Samtliga tre signifikanta fall visade att förflyttningen var i en nordöstlig eller nord-nordöstlig riktning. Vår undersökning kan tyvärr inte svara på varför lavarna har förflyttat sig den här sträckan inte heller varför de har den här riktningen, men våra tolkningar av lavarnas förflyttning kan ha att göra med en global uppvärmning. De tre lavarna har en nordlig utbredning så riktningen av förflyttningen mot norr på grund av ett varmare klimat är inte osannolikt.

Sökandet av den bästa riktningen för blåslavens största förekomst och största medeldiametern på bålen undersöktes på trädstammat från 66 lokaler i södra Sverige (Papper V). Hela koordinatsystemets 66 lokaler vreds runt sitt egen medelpunkt för att ta reda på riktningarna. Resultaten visar att sannolikheten för förekomst av blåslav ökar i en nord-nordöstlig riktning och medeldiametern på bålen ökar i väst-nordvästlig riktning.

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Acknowledgement

I am indebted to so many people and I am sure that I am neither the first nor the last with this debt. There are so many people I would like to thank who meant a lot to me during my time as a PhD-student. In order not to exclude anyone by mistake I

collectively want to thank all colleagues and others that have contributed, in any way, to my dissertation thesis. However, I cannot help but mention a few who has been

particularly important to me.

My first thanks goes to my supervisors Jan-Eric Mattsson and Per Milberg. Jan-Eric, thanks for all the enriched journeys we have undertaken together to participate in congresses, conferences and field excursions - it was great fun. On my own and together with you, I have during my past years as a PhD-student visited; Paris in France, Vienna in Austria, San Francisco in the United States of America, Cairns in Australia,

Düsseldorf in Germany, Ceské Budějovice in Czech Republic, Tartu in Estonia, Bergen in Norway, Lammi in Finland, Skibotn in Norway and a lot of different places here in Sweden. During the field work we undertook together in 2003 we really got to know each other. Also, thanks for your understanding in letting me grow at my own pace and your encouragement and sometimes confusing, but relevant ideas. To me it seems that I could always rely on you to have a solution to any problem we encountered along the way.

Per, thanks for your almost extra terrestrial help with comments and proposals for improvements of different texts, e.g., manuscripts, PowerPoint presentations and posters. Your feedback on my oral PhD-presentations, your fast reply to my e-mails and being patient and giving me answers on the same question several times, always with a smile on your face. Your help is greatly appreciated.

Thanks to Yann Bertrand and Ivailo Simoff for fruitful discussions and Yann for your hospitality. Thanks Stefan Ekman for your tremendous work on one of my papers and thanks Louise Lindblom for your help and encouragement. Thanks Mikael Lönn for quick answers to e-mails and support on R. Thanks Mats Gran for your help on

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Rundgren, Kerstin Johansson, Eva Mattsson, Anna Sundin and Agneta Johansson for your help with the administration but also Anita Gustavsson, Gunilla Hertzberg and Monika Karlsson for your help with room service. Thanks to the group members of Quercus-gruppen and Växt Ekologiskt Forum, for your input on my fieldwork and manuscripts. I would also like to say a big thanks to all PhD-students at the Linköping University for the Friday beers and the interesting discussions with the members of morning coffee breaks in Zenit, Fysikhuset.

Thanks to my loving mother Ingegerd and my sister Eva for all your support and encouragement. Eva I am glad that you know everything there is to know about computers but most importantly I am glad that you are my sister.

Last but not least, I want to thank my family Åsa, Elin, Gustav, Linn, Kim and David; I love to spend my time with you.

…mission impossible, accomplished…

Håkan Lättman January 2010

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References

Armstrong RA (1987) Dispersal in a population of the lichen Hypogymnia physodes. Environmental and Experimental Botany 27: 357–363.

Armstrong RA (1990) Dispersal, establishment and survival of soredia and fragments of the lichen, Hypogymnia physodes (L) Nyl. New Phytologist 114: 239–245.

Bates JW & Brown DH (1981) Epiphyte differentiation between Quercus petraea and

Fraxinus excelsior trees in a maritime area of South West England. Vegetatio 48:

61–70.

Bridle JR & Vines TH (2007) Limits to evolution at range margins: When and why does adaptation fail? Trends in Ecology and Evolution 22: 140–147.

Coppins BJ & James PW (1979) Birds and the dispersal of lichen propagules. The Lichenologist 11: 105–106.

Dettki H, Klintberg P & Esseen PA (2000) Are epiphytic lichens in young forests limited by local dispersal? Ecoscience 7: 317–325.

Ellis C & Coppins BJ (2007) Reproductive strategy and the compositional dynamics of crustose lichencommunities on aspen (Populus tremula L.) in Scotland. The Lichenologist 39: 377–391.

Erisman JW & Draaijers GPJ (1995) Atmospheric deposition in relation to acidification and eutrophication, Amsterdam, Oxford, Elsevier.

Esseen P-A & Renhorn K-E (1998) Edge effects on an epiphytic lichen in fragmented forests. Conservation Biology 12: 1307–1317.

Feuerer T (ed.) (2009) Checklist of lichens and lichenicolous fungi, Version 1 September 2009. http:www.checklist.de

Gilbert OL (1968) Bryophytes as indicators of air pollution in the Tyne valley. New Phytologist 67: 15–30.

Gries C (1997) Lichens as indicators of air pollution. In: Nash TH (ed.) Lichen biology 240–254 University Press, Cambridge.

(27)

Hawksworth DL & McManus PM (1989) Lichen recolonization in London under conditions of rapidly falling sulfur-dioxide levels, and the concept of zone skipping. Botanical Journal of the Linnean Society 100: 99–109.

Hedenås H & Ericson L (2000) Epiphytic macrolichens as conservation indicators successional sequence in Populus tremula stands. Biological Conservation 93: 43– 53.

Heinken T (1999) Dispersal patterns of terricolous lichens by thallus fragments. The Lichenologist 31: 603–612.

Hickling R, Roy DB, Hill JK & Thomas CD (2005) A northward shift of range margins in British Odonata. Global Change Biology 11: 502–506.

Hilmo O (1994) Distribution and succession of epiphytic lichens on Picea abies branches in a boreal forest, central Norway. The Lichenologist 26: 149–169. Hilmo O & Såstad SM (2001) Colonization of old-forest lichens in a young and an old

boreal Picea abies forest: An experimental approach. Biological Conservation 102: 251–259.

Hughes L (2000) Biological consequences of global warming: Is the signal already apparent? Trends in Ecology and Evolution 15: 56–61.

Johansson P & Ehrlen J (2003) Influence of habitat quantity, quality and isolation on the distribution and abundance of two epiphytic lichens. Journal of Ecology 91: 213– 221.

Johansson V, Bergman K-O, Lättman H & Milberg P (2009) Tree and site quality preferences of six epiphytic lichens growing on oaks in southeastern Sweden. Annales Botanici Fennici 46: 496–506.

Krempelhuber A (1867) Geschichte und litteratur der lichenologie von den ältesten zeit bis zum schlusse des jahres 1865. München, Selbstverlag.

Lindblom L & Ekman S (2006) Genetic variation and population differentiation in the lichen-forming ascomycete Xanthoria parietina on the island Storfosna, central Norway. Molecular Ecology 15: 1545–1559.

Lindblom L & Ekman S (2007) New evidence corroborates population differentiation in

Xanthoria parietina. The Lichenologist 39: 259–271.

Lorentsson S & Mattsson J-E (1999) New reports of soredia dispersed by ants, Formica

cunicularia. The Lichenologist 31: 204–207.

Munoz J, Felicisimo AM, Cabezas F, Burgaz AR & Martinez I (2004) Wind as a long-distance dispersal vehicle in the southern hemisphere. Science 304: 1144–1147. Mylona S (1996) Sulphur dioxide emissions in Europe 1880–1991 and their effect on

sulphur concentrations and depositions. Tellus Series B-Chemical and Physical Meteorology 48: 662–689.

NBF (2007) Statistical Yearbook of Forestry 2007. NBF, Jönköping, Sweden. Nordén B & Appelqvist T (2001) Conceptual problems of ecological continuity and its

bioindicators. Biodiversity and Conservation 10: 779–791.

Nylander W (1866) Les lichens du Jardin du Luxembourg. Bulletin de la Société Botanique de France 13: 364–372.

Olsson K (1995) Changes in epiphytic lichen and moss flora in some beech forests in southern Sweden during 15 years. Ecological Bulletins 44: 238–247.

Palice Z & Printzen C (2004) Genetic variability in tropical and temperate populations of Trapeliopsis glaucolepidea: Evidence against long-range dispersal in a lichen with disjunct distribution. Mycotaxon 90: 43–54.

(28)

Parmesan C (2006) Ecological and evolutionary responses to recent climate change. Annual Review of Ecology Evolution and Systematics 37: 637–669.

Parmesan C & Yohe G (2003) A globally coherent fingerprint of climate change impacts across natural systems. Nature 421: 37–42.

Printzen C, Ekman S & Tønsberg T (2003) Phylogeography of Cavernularia hultenii: Evidence of slow genetic drift in a widely disjunct lichen. Molecular Ecology 12: 1473–1486.

Ranius T, Johansson P, Berg N & Niklasson M (2008) The influence of tree age and microhabitat quality on the occurrence of crustose lichens associated with old oaks. Journal of Vegetation Science 19: 653–662.

Root TL, Price JT, Hall KR, Schneider SH, Rosenzweig C & Pounds JA (2003) Fingerprints of global warming on wild animals and plants. Nature 421: 57–60. Sancho LG, de la Torre R, Horneck G, Ascaso C, de Los Rios A, Pintado A, Wierzchos

J & Schuster M (2007) Lichens survive in space: Results from the 2005 LICHENS experiment. Astrobiology 3: 443–454.

Sanz-Elorza M, Dana ED, Gonzalez A & Sobrino E (2003) Changes in the high-mountain vegetation of the central Iberian peninsula as a probable sign of global warming. Annals of Botany 92: 273–280.

Saxe H, Cannell MGR, Johnsen B, Ryan MG & Vourlitis G (2001) Tree and forest functioning in response to global warming. New Phytologist 149: 369–399. Schopp W, Posch M, Mylona S & Johansson M (2003) Long-term development of acid

deposition (1880–2030) in sensitive freshwater regions in Europe. Hydrology and Earth System Sciences 7: 436–446.

Sillett SC, McCune B, Peck JE, Rambo TR & Ruchty A (2000) Dispersal limitations of epiphytic lichens result in species dependent on old-growth forests. Ecological Applications 10: 789–799.

Tapper R (1976) Dispersal and changes in the local distributions of Evernia prunastri and Ramalina farinacea. The New Phytologist 77: 725–734.

Taylor TN (1995) The oldest fossil lichen. Nature 378: 244.

Thomas CD & Lennon JJ (1999) Birds extend their ranges northwards. Nature 399: 213–213.

Thuiller W, Lavorel S, Araujo MB, Sykes MT & Prentice IC (2005) Climate change threats to plant diversity in Europe. Proceedings of the National Academy of Sciences of the United States of America 102: 8245–8250.

Turner IM (1996) Species loss in fragments of tropical rain forest: A review of the evidence. The Journal of Applied Ecology 2: 200–209.

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Walser JC (2004) Molecular evidence for limited dispersal of vegetative propagules in the epiphytic lichen Lobaria pulmonaria. American Journal of Botany 91: 1273– 1276.

Walser JC, Holderegger R, Gugerli F, Hoebee SE & Scheidegger C (2005)

Microsatellites reveal regional population differentiation and isolation in Lobaria

pulmonaria, an epiphytic lichen. Molecular Ecology 14: 457–467.

Warren MS, Hill JK, Thomas JA, Asher J, Fox R, Huntley B, Roy DB, Telfer MG, Jeffcoate S, Harding P, Jeffcoate G, Willis SG, Greatorex-Davies JN, Moss D & Thomas CD (2001) Rapid responses of British butterflies to opposing forces of climate and habitat change. Nature 414: 65–69.

Werth S, Wagner HH, Gugerli F, Holderegger R, Csencsics D, Kalwij JM & Scheidegger C (2006a) Quantifying dispersal and establishment limitation in a population of an epiphytic lichen. Ecology 87: 2037–2046.

Werth S, Wagner HH, Holderegger R, Kalwij JM & Scheidegger C (2006b) Effect of disturbances on the genetic diversity of an old-forest associated lichen. Molecular Ecology 15: 911–921.

Yarranton GA (1972) Distribution and succession of epiphytic lichen on black spruce near Cochrane, Ontario. The Bryologist 75: 462–480.

Zalasiewicz J, Williams M, Smith A, Barry TL, Coe AL, Bown PR, Brenchley P, Cantrill D, Gale A, Gibbard P, Gregory FJ, Hounslow MW, Kerr AC, Pearson P, Knox R, Powell J, Waters C, Marshall J, Oates M, Rawson P & Stone P (2008) Are we now living in the Anthropocene? Geological Society of America 2: 4–7. Öckinger E, Niklasson M & Nilsson SG (2005) Is local distribution of the epiphytic

lichen Lobaria pulmonaria limited by dispersal capacity or habitat quality? Biodiversity and Conservation 14: 759–773.

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Södertörn Doctoral Dissertations

Jolanta Aidukaite, The Emergence of the Post-Socialist Welfare State: The case of the Baltic States: Estonia, Latvia and Lithuania, 2004

Xavier Fraudet, Politique étrangère française en mer Baltique (1871-1914): de l'exclusion à l'affirmation, 2005

Piotr Wawrzeniuk, Confessional Civilising in Ukraine: The Bishop Iosyf Shumliansky and the Introduction of Reforms in the Diocese of Lviv 1668-1708, 2005

Andrej Kotljarchuk, In the Shadows of Poland and Russia: The Grand Duchy of Lithuania and Sweden in the European Crisis of the mid-17th Century, 2006 Håkan Blomqvist, Nation, ras och civilisation i svensk arbetarrörelse före nazismen,

2006

Karin S Lindelöf, Om vi nu ska bli som Europa: Könsskapande och normalitet bland unga kvinnor i transitionens Polen, 2006

Andrew Stickley. On Interpersonal Violence in Russia in the Present and the Past: A Sociological Study, 2006

Arne Ek, Att konstruera en uppslutning kring den enda vägen: Om folkrörelsers modernisering i skuggan av det Östeuropeiska systemskiftet, 2006

Agnes Ers, I mänsklighetens namn: En etnologisk studie av ett svenskt biståndsprojekt i Rumänien, 2006

Johnny Rodin, Rethinking Russian Federalism: The Politics of Intergovernmental Relations and Federal Reforms at the Turn of the Millennium, 2006

Kristian Petrov, Tillbaka till framtiden: Modernitet, postmodernitet och generationsidentitet i Gorbačevs glasnost´ och perestrojka, 2006

Sophie Söderholm Werkö, Patient patients?: Achieving Patient Empowerment through Active Participation, Increased Knowledge and Organisation, 2007

Peter Bötker, Leviatan i arkipelagen: Staten, förvaltningen och samhället. Fallet Estland, 2007

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Aleksei Semenenko, Hamlet the Sign: Russian Translations of Hamlet and Literary Canon Formation, 2007

Vytautas Petronis, Constructing Lithuania: Ethnic Mapping in the Tsarist Russia, ca. 1800-1914, 2007

Akvile Motiejunaite, Female employment, gender roles, and attitudes: the Baltic countries in a broader context, 2008

Tove Lindén, Explaining Civil Society Core Activism in Post-Soviet Latvia, 2008 Pelle Åberg, Translating Popular Education: Civil Society Cooperation between

Sweden and Estonia, 2008

Anders Nordström, The Interactive Dynamics of Regulation: Exploring the Council of Europe’s monitoring of Ukraine, 2008

Fredrik Doeser, In Search of Security After the Collapse of the Soviet Union: Foreign Policy Change in Denmark, Finland and Sweden, 1988-1993, 2008

Zhanna Kravchenko. Family (versus) Policy: Combining Work and Care in Russia and Sweden, 2008

Rein Jüriado, Learning within and between public-private partnerships, 2008 Elin Boalt, Ecology and evolution of tolerance in two cruciferous species, 2008 Lars Forsberg, Genetic Aspects of Sexual Selection and Mate Choice in Salmonids,

2008

Eglė Rindzevičiūtė, Constructing Soviet Cultural Policy: Cybernetics and Governance in Lithuania after World War II, 2008

Joakim Philipson, The Purpose of Evolution: ’struggle for existence’ in the Russian-Jewish press 1860-1900, 2008

Sofie Bedford, Islamic activism in Azerbaijan: Repression and mobilization in a post-Soviet context, 2009

Tommy Larsson Segerlind, Team Entrepreneurship: A process analysis of the venture team and the venture team roles in relation to the innovation process, 2009 Jenny Svensson, The Regulation of Rule-Following: Imitation and Soft Regulation in

the European Union, 2009

Stefan Hallgren, Brain Aromatase in the guppy, Poecilia reticulate: Distribution, control and role in behavior, 2009

Karin Ellencrona, Functional characterization of interactions between the flavivirus NS5 protein and PDZ proteins of the mammalian host, 2009

Makiko Kanematsu, Saga och verklighet: Barnboksproduktion i det postsovjetiska Lettland, 2009

Daniel Lindvall, The Limits of the European Vision in Bosnia and Herzegovina: An Analysis of the Police Reform Negotiations, 2009

Charlotta Hillerdal, People in Between — Ethnicity and Material Identity: A New Approach to Deconstructed Concepts, 2009

Jonna Bornemark, Kunskapens gräns — gränsens vetande, 2009

Adolphine G. Kateka, Co-Management Challenges in the Lake Victoria Fisheries: A Context Approach, 2010

René León Rosales, Vid framtidens hitersta gräns: Om pojkar och elevpositioner i en multietnisk skola, 2010

Simon Larsson, Intelligensaristokrater och arkivmartyrer: Normerna för vetenskaplig skicklighet i svensk historieforskning 1900-1945, 2010

Studies on spatial and temporal distributions of epiphytic lichens