Dispersal of bryophytes across landscapes. Niklas Lönnell

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1 Dispersal of bryophytes across landscapes Niklas Lönnell

2 Niklas Lönnell, Stockholm University 2014 Front cover illustration: Niklas Lönnell Back cover photograph: Carita Lönnell Paper cover: 270g Scandia 2000 Paper insert: 100g Color copy naturalwhite ISBN Printed in Sweden by US-AB, Stockholm 2014 Distributor: Department of Ecology, Environment and Plant Sciences, Stockholm University

3 Some things in life are bad They can really make you mad Other things just make you swear and curse. When you're chewing on life's gristle Don't grumble, give a whistle And this'll help things turn out for the best... (Eric Idle)

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5 Contents Glossary... 6 Abstract... 7 List of Papers... 9 Introduction Dispersal Bryophyte dispersal Objectives of the thesis Methods Study areas Study system 1 Dicelium nudum Study species Study system 2 limed mires Results and discussion References Svensk sammanfattning Tack... 36

6 Glossary Anemochory Wind dispersal Antheridium The male elliptical reproductive organ from where the spermatozoids have to swim to the archegonium. Archegonium The female bottle-shaped reproductive organ with a long neck with a canal which lead down to the venter where the eggcell is situated. Bryophytes A generic term for the polyphyletic group consisting of mosses, liverworts and hornworts Deposition (syn. landing) The final stage of dispersal where the diaspores come to rest Diaspore (syn. propagule) A dispersal unit that could be a seed, spore, gemmae or even a fragment (as bryophytes can regenerate from any part of the gametophyte). Dispersal Transportation of diaspores away from the plant of origin (See introduction) Impaction A deposition process when a diaspore continues its path towards an object instead of following the air stream around it Interception A deposition process when a diaspore travels so near an object that it touches it Lagrangian stochastic dispersion model A model that simulates a large number of trajectories of a single diaspore taking into account the variation in wind components. Liming A practice to spread calcium compounds (to increase the ph) Release (syn. abscission, take-off) The process when the diaspores leave the place of origin, in the case of bryophyte spores the capsule. Sedimentation A deposition process when a diaspore is deposited due to gravity Settling velocity/speed (syn. falling/fall velocity/speed, terminal velocity/speed) A measure of how fast a diaspore falls. It is defined as the diaspore speed when the drag force (upwards) and the force by gravity (downwards) are equal and the particle will not increase its speed. Vector (syn. agent) A carrier of diaspores, e.g. wind, water and animals -chory a suffix used to describe different types of dispersal, for example wind dispersal would be anemochory -phily a suffix used to describe different types of pollen dispersal, but could also be used to describe spore dispersal, wind dispersal would be anemophily 0:6

7 Abstract Doctoral dissertation Niklas Lönnell Department of Ecology, Environment and Plant Sciences Stockholm University Lilla Frescati SE Stockholm Sweden Dispersal, especially long-distance dispersal, is an important component in many disciplines within biology. Many species are passively dispersed by wind, not least spore-dispersed organisms. In this thesis I investigated the dispersal capacity of bryophytes by studying the colonization patterns from local scales (100 m) to landscape scales (20 km). The dispersal distances were measured from a known source (up to 600 m away) or inferred from a connectivity measure (1 20 km). I introduced acidic clay to measure the colonization rates over one season of a pioneer moss, Discelium nudum (I III). I also investigated which vascular plants and bryophytes that had colonized limed mires approximately years after the first disturbance (IV). Discelium effectively colonized new disturbed substrates over one season. Most spores were deposited up to 50 meters from a source but the relationship between local colonization rates and connectivity increased with distance up to 20 km (I III). Also calcicolous wetland bryophyte species were good colonizers over similar distances, while vascular plants in the same environment colonized less frequently. Common bryophytes that produce spores frequently were more effective colonizers, while no effect of spore size was detected (IV). A mechanistic model that take into account meteorological parameters to simulate the trajectories for spores of Discelium nudum fitted rather well to the observed colonization pattern, especially if spore release thresholds in wind variation and humidity were accounted for (III). This thesis conclude that bryophytes in open habitats can disperse effectively across landscapes given that the regional spore source is large enough (i.e. are common in the region and produce spores abundantly). For spore-dispersed organisms in open landscapes I suggest that it is often the colonization phase and not the transport that is the main bottle-neck for maintaining populations across landscapes. Keywords: anemochory, bryophytes, colonization, connectivity, diaspores, dispersal kernel, establishment, spore dispersal, long-distance dispersal, mechanistic model, mosses, realized dispersal, spore release, Lagrangian stochastic model, wind dispersal 0:7

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9 List of Papers This thesis is based on the work contained in the following papers, referred to by Roman numerals in the text I. Lönnell, N., Hylander, K., Jonsson, B.G. & Sundberg, S. (2012) The fate of the missing spores patterns of realized dispersal beyond the closest vicinity of a sporulating moss. PLoS ONE, 7, e II. Lönnell, N., Jonsson, B.G. & Hylander, K. Production of diaspores at the landscape level regulates local colonization: an experiment with a spore-dispersed moss. Accepted for publication in Ecography. DOI: /j x III. Lönnell, N., Sundberg, S., Norros, V., Rannik, Ü., Johansson, V., Ovaskainen, O. & Hylander, K. Colonization patterns of a wind-dispersed moss in relation to modelled dispersal based on meteorological data. Manuscript IV. Lönnell, N. & Hylander, K. Calcicolous plants colonize limed mires after long-distance dispersal. Manuscript 0:9

10 Introduction Dispersal Dispersal in a biological context can be defined as "intergenerational movement" or "movement leading to gene flow" or "movement of individuals to new locations away from their parents" and as it is important component in many different both theoretical and applied disciplines it has rendered an increasing attention during the last decades (Bullock, Kenward & Hails 2002; Cousens, Dytham & Law 2008; Clobert et al. 2012). Dispersal can be active, where the individual animal itself can decide on the direction, length and final goal of the dispersal event, or passive,where the individual or dispersal unit (diaspore) is transported by some other vector e.g. animals (zoochory), water (hydrochory) or wind (anemochory) (Pijl 1982). Many fields of ecology and conservation biology are dominated by research concerning animals, which is reflected in the high number of studies that have been devoted to active dispersal and animal-mediated pollination and dispersal. A tendancy for such a bias towards viewing dispersal from an animal perspective could be detected in the conceptual framework of movement of an individual, where four mechanistic components that influences a movement path are distinguished: internal state (why move?), motion capacity (how to move?), navigation capacity (when and where to move?) and external factors (Nathan et al. 2008a). Consequently, an attempt to apply it on a wind-dispersed fungi encountered some challenges: for example to distinguish between navigation and motion capacity is not unambiguous and the internal state of both the mother plant and the diaspores could be affected independently by external factors (Norros 2013). Wind disperses a plethora of organisms: for example vascular plants (pollen and seeds), bryophytes, lichens, fungi, bacteria, and protozoan. Also some larger animals rely on passive wind dispersal, for example spiders that fly with help of strands of silk (Bonte 2012). That wind could be an efficient vector is indicated by the fact that the strategy to have very small seeds (so called dust seeds) have evolved several 0:10

11 times among flowering plants (Eriksson & Kainulainen 2011). Wind is also an important vector for passive long-distance dispersal of species which otherwise has active short-distance dispersal (e.g. insects) (Compton 2002; Sturtevant et al. 2013). A majority of wind-dispersed diaspores will end up in an environment where they cannot survive and establish. One extreme example is all pine pollen that miss the very small surface of a stigma and can be seen colouring the ground and water surfaces yellow in May-June. For all wind-dispersed species is the number of produced spores (the source strength) accordingly an important factor. The source strength is influenced by the number of individuals, how often they reproduce and how many diaspores that are produced per individual (Fig. 1). Figure 1. The different phases related to dispersal. Release, transport and deposition are the phases traditionally included in dispersal. If establishment is included the term realized dispersal could be used. The arrows represent the filters that could influence which species from the regional species pool that you find in a local community according to Morin (1999). (Morin 1999).. 0:11

12 Release conditions and diaspore characteristics are two ways that plants can influence the dispersal distance of a single diaspore and could be classified into the component navigation capacity in the movement ecology paradigm (Nathan et al. 2008a; Norros 2013). Besides seasonal variation in the production of diaspores there seem to be diurnal variations in production and/or release of diaspores judged from measurements of spore concentration in the air, for many fungi at least (Pady, Kramer & Clary 1967; Gregory 1973). One mechanism that could facilitate long-distance dispersal is release triggered by some environmental conditions such as high wind speeds. Such nonrandom release has been suggested for an increasing number of species (vascular plants, fungi and bryophytes) (Johansson et al.; Aylor 1990; Greene 2005; Skarpaas, Auhl & Shea 2006; Jongejans et al. 2007; Borger et al. 2012). Some spore-dispersed organisms, e.g. peat mosses Sphagnum spp. and some fungi, have a violent discharge of their spores (Sundberg 2002; Roper et al. 2010; Whitaker & Edwards 2010), which is still another mechanism that could enhance the probability of being transported further away. The settling velocity, a measure how fast a diaspore falls and will reach the ground, is one important property to know in order to predict how long horizontal distance a diaspore will be transported under a given wind speed. The heavier a diaspore is the faster it will reach the ground, given that the species not has any adaptations to increase the buoancy such as plumes on its seeds or air sacs on its pollen. For a spherical diaspore with the same density the diameter could be used as a good proxy of settling velocity (and thus transport capacity). Many fungal spores are ellipsoid and ca 10 μm long (Weber & Hess 1976), the spherical spores of mosses and liverworts have a diameter of (-200) μm (Hill et al. 2007), the majority of ferns have a diameter of μm (Tryon 1970) and orchids have elongated seeds from 0.1 mm to 4 mm long, but more commonly around 1 mm (Bojnansky & Fargasova 2007), while most wind-dispersed pollen have a diameter of μm (Faegri & Pijl 1979). Differences in diaspore survival after desiccation, UV-radiation and freezing have been reported and could influence a species ability for a successful long-distance dispersal (Zanten 1978; Zanten & Gradstein 1988; Wiklund & Rydin 2004; Löbel & Rydin 2010; Norros 2013). One critical stage in the transport of diaspores is for them to reach above the canopy where the wind speeds are higher and it is easier to be swept away to higher altitudes and longer horizontal distances. Another mechanism is for diaspores to be lifted with warm air, so called 0:12

13 thermal upheaval, that could be effective in many cases (Tackenberg, Poschlod & Kahmen 2003). Deposition occurs through several mechanisms, whose relative importance varies with the size of the diaspore, properties of the collecting elements (e.g. a pine needle or a leaf) and the flow conditions. Interception (when a diaspore travels so near an object that it touches it), impaction (when a diaspore continues its path towards an object instead of following the air stream around it) and sedimentation (when a diaspore is deposited due to gravity) are the mechanisms that mainly are affecting particles above 2 μm (note that turbulent impaction or turbophoresis is also a mechanism of importance, but in general there is not very good consensus about the importance of this mechanism (Üllar Rannik pers. comm.). Interception is important for diaspores 2 10 μm but influences also larger diaspores, while impaction is the dominant deposition mechanism for diaspores μm in coniferous forest (Petroff et al. 2008). Impaction increases with increasing wind speed, increasing diaspore size (mass) and decreasing diameter of the object and the stickiness of the surface of the object (Gregory 1973). Sedimentation is especially important under still conditions with wind speeds below 2 m s -1 and for heavy diaspores. Finally if a viable spore arrives at a habitat the establishment phase could be limited by abiotic factors such as ph, phosphorous, moisture (Sundberg & Rydin 2002; Wiklund & Rydin 2004; Löbel & Rydin 2010) or by biotic factors such as competition or absence of a symbiont. Bryophyte dispersal Besides by spores, bryophytes disperse with gametophyte fragments as well as a wide array of specialized asexual diaspores: gemmae, tubers, bulbils, fragile braches and leaves (Laaka-Lindberg, Korpelainen & Pohjamo 2003). These are often larger than the spores but have a higher germination rate and their production are less costly and could begin in earlier life stages, at least in epiphytes (Löbel & Rydin 2009, 2010). However, as spores are smaller than most asexual diaspores they are generally thought to have a higher probability of longdistance dispersal. The only well documented specialized dispersal with animals as vectors that can be found among the mosses is in the family Splachnaceae 0:13

14 where both visual (broad hypophysis) and olfactory (odours that imitate the smell of dung) ques are used to lure the animal vector to land on the sporophyte (Koponen 1990; Marino, Raguso & Goffinet 2009). Flies then transport the spores to new suitable substrates e.g. in the form of fresh dung. Unspecialized bryophyte dispersal with other animals has been suggested (Glime 2007) and from a long-distance dispersal perspective the speculations about possibilities of bird mediated dispersal are the most interesting. Otherwise the main vector for longdistance dispersal of most bryophyte spores is wind. Bryophyte spore size Frequency Spore size (micrometre) Figure 2. A histogram over the average spore size (µm) for 1035 mosses and liverworts in Great Britain (Hill et al. 2007). The vertical line marks the spore size of Discelium nudum (25 µm). Most species with spores above 50 μm are liverworts while the outlier with spores around 200 μm is the acrocarpous moss Archidium alternifolium. 0:14

15 Most bryophyte spores are spherical and μm in diameter even if there are some exceptions with spores up to μm in the British bryophyte flora (Hill et al. 2007) (Fig. 2). Sexual reproduction (i.e. spore production) occur in most bryophyte species. However, there are species that not has been found with capsules. The sporophyte production not only vary among species, but also abiotic factors, such as frost and precipitation during the development of the sporophyte, could influence the final spore output (Sundberg 2002; Ruete, Wiklund & Snäll 2012). The number of spores produced per capsule varies with capsule size and size of the diaspores and can vary even within species (Sundberg & Rydin 1998). The interspecific differences are large and range from Million spores in Buxbaumia viridis (Wiklund 2002) to as few as 16 in Archidium alternifolium (Miles & Longton 1992a) (Fig. 3). Figure 3. The spore output per capsule versus spore size plotted in a log-log-space for 92 species for which data were available (Ingold 1959; Schuster 1966; Kreulen 1972; Ingold 1974; Longton 1976; Mogensen 1978; Söderström & Jonsson 1989; Miles & Longton 1992; Boros et al. 1993; Sundberg & Rydin 1998; Wiklund 2002; He & Zhu 2010; Cuming 2011). Some genera are overrepresented e.g. Riccia and Sphagnum. The figure has been modified from Lönnell (2011). Discelium nudum is marked by a cross, even if it belongs to the group acrocarpous mosses. (Ingold 1959, 1974; Schuster 1966; Kreulen 1972; Longton 1976; Mogensen 1978; Söderström & Jonsson 1989; Miles & Longton 1992a; Boros et al. 1993; Sundberg & Rydin 1998; Wiklund 2002; He & Zhu 2010; Cuming 2011; Lönnell et al. 2012) 0:15

16 Objectives of the thesis The main objective of this thesis was to study spore dispersal of bryophytes by an experimental approach to help to remedy the lack of empirical evidence within this field. Moreover, we wanted to quantify the dispersal over scales (temporal and spatial) that could be relevant for a species to persist in a landscape. The specific objectives for the single studies were To experimentally quantify dispersal beyond those few meters that most studies have used and describe a dispersal kernel up to 600 m from a spore source. (I) To assess the influence of the landscape connectivity on the colonization ability over one season for our study species (Discelium nudum) in a region where it is naturally occurring. (II) To investigate to what extent a mechanistic model based on a number of measured meteorological parameters could predict the colonization pattern of the study species up to 100 m from a spore source and assess if incorporating realistic release threshold could improve the fit. (III) To quantify on what spatial scale connectivity influence the colonization of calcicolous vascular plants and bryophytes in limed mires and to test if some traits (regional frequency, how often they reproduce and diaspore size) have an effect on which species and how often such species colonize the mires. (IV) 0:16

17 Methods The two study system used in this thesis were the Discelium nudumsystem where we studied colonizations on translocated clay over one season (I III) and the limed mire system were we studied colonization Figure 4. A).The 52 analysed limed mires in study IV B) The location for the study sites in study I (filled circle), study II (triangles) and study III (square). 0:17

18 of calcicolous species years after the first liming event (IV). Study areas The studies were all performed on mires in the boreal zone in Sweden: (I) A raised bog in the county of Gästrikland (N E ), (II) Sphagnum-dominated mires around Umeå (N63 64 E19 20 ), (III) a raised bog in Uppland (N E17.328) and (IV) limed mires in the middle part of Sweden (N E N E ) (Fig. 4). Study system 1 Dicelium nudum Acidic clay was retrieved with an excavator from some meters depth. We then translocated it onto mires in pots (I, III) or heaps (II) as spore traps for the moss Discelium nudum. The reason for performing the studies on mires was primarily that the clay should be moist to secure good colonization conditions for our study species. Moreover, a mire is not a suitable habitat for the species (besides the introduced clay patches), which facilitates to control the spore sources. In two studies a spore source were translocated to a central point and pots where placed in four directions at distances up to 600 m (I) or in eight direction at distances up to 100 (III). The colonizations were then recorded during the autumn either in the field (I, II) or in the greenhouse (III). Study species Flag moss Discelium nudum (Dicks.) Brid is an acrocarpous moss, i.e. the spore capsule grows from the top of the shoot. When a moss spore germinates filamentous threads that look like a green alga develop; called protonema. Unlike many other moss species this stage is very conspicuous and long-lived in Discelium and can cover several square centimetres. However, adjacent protonemata grow into each other and the delimitation of one individual is not feasible. From one protonema several moss shoots are eventually formed. These are on the other hand very tiny (ca. 2 mm high) and only consist of a few leaves. The male shoots can readily be distinguished like orange dots on the green protonema mat, since the orange antheridia are not entirely covered by the surrounding leaves, while the female shoots are more inconspicuous. Discelium is said to be pseudo-dioecious, i.e. there are female and male shoots, but they come from the same protonema and hence spore 0:18

19 (Nyholm 1989). From the female shoot and a fertilized archegonium the sporophyte develops. The capsule is usually horizontally orientated mm situated on a 1 3 cm seta (I) and the mouth of the capsule have a double peristome (Shaw & Allen 1985). The spores are spheroid with a diameter of 25 ( ) mm (Boros et al. 1993) and in one capsule there are ±2500 [SE] spores (I). Its life cycle is completed within one year. The spore release occurs during a few weeks in April May. The exact timing is dependent on the weather conditions during the spring, such as the duration of the snow cover and temperature and hence varies between years and latitudes. The fertilization occurs in summer and is vividly described by Hampus Wilhelm Arnell (Arnell 1875). The sporophytes develop dur- Figure 5. The life cycle of flag moss Discelium nudum. The colonizations emanating from spore dispersal in April May were recorded during the autumn when gametophyte shoots (male shoot the lower picture) and/or young sporophytes (the left picture) could be found. The growth of the protonema (right picture) could be detected already after a few weeks. The upper picture depicts a capsule from above with the reddish peristome at the capsule mouth. 0:19

20 ing autumn and mature during spring after the snow has melted away. På ex, från Ångml. Hernosand 11 Juli 72 iakttogs en befruktningsakt. Några han- och honplantor voro preparerade i vatten för mikroskopet. Preparatet hvimlade av svärmande antherozoider. Ett arkegon, som, då det först kom inom synfältet, var slutet, men visade en tydlig kanal, öppnade sig, under det förf. betraktade detsamma. Alla antherozoider inom synhåll syntes i detsamma dragas till arkegoniets mynning; derefter drogo de sig så småningom ned genom kanalen, tills de kommo till centralblåsan. Denna försattes af antherozoiderna i en starkt vaggande rörelse, tills de slutligen upphörde att svärma och började att absorberas af centralblåsan, hvilken derigenom fick ett papillöst utseende ifrån att förut hafva haft en jemn yta. Arnell, H.W. (1875) The quotation translated and interpreted into a modernized version: On a specimen from Härnösand in the county of Ångermanland 11 July 1872 was a fertilization observed. Some male and female plants were prepared in water for the light microscope. The preparation was filled with swarming spermatozoids. An archegonium, which was closed but showed an obvious canal as it came into view, opened in front of the eyes of the author. All the spermatozoids within view were attracted to the mouth and dragged themselves down through the canal until they reached the eggcell. It was by the spermatozoids put into a swinging motion until they stopped to swarm and started to be absorbed by the eggcell, which then got a papillose appearance; before this it had a smooth surface. The species is found in Europe, North America and Asia in the northern hemisphere (Nyholm 1989). In Sweden the main distribution is on clay in the lower parts of the river valleys in the northern part of the country, but have scattered occurrences also in the southern part of the country. On a national scale it is rare, but can locally be rather abundant in its core area. The substrate consists of clay or silt. It seems to be more abundant on glacial clay (which in the area often have a greyish colour) than on silt. The clay is often rather acidic: ph 4 5 (measured in CaCl 2, which is thought to better reflect the ph for organisms under field conditions) or ph 5 6 (measured in deionized water) based on clay samples from a number of sites with Discelium (unpublished data). Preliminary results from a pilot study of germination gave also indications of a preference for acidic clay. I observed almost no establishment on clay with ph 8 (measured in deionized water) and intermediate establishment on clay with ph 6 (measured in deionized water) compared to a fast and prolific establishment on clay with ph 4.5 (measured in de- 0:20

21 ionized water) (unpublished data). The species is dependent on disturbance as it will be overgrown by other bryophytes or vascular plants within a few years without a renewed disturbance. Oddly enough it is very rare in arable fields where I only have spotted it a few times and then often in the margin of the field. More frequently it is found in ditches and road verges, in tracks in clear cuts and along watercourses. Discelium was chosen as model organism for studying dispersal for the following reasons: 1) It is substrate specific, which is an advantage when it comes to spotting potential habitats in the field and using translocated substrate as spore traps. 2) It is an effective colonizer of suitable substrate and grows rather fast so the colonizations could be detected within some months. 3) It is rather rare. Study system 2 limed mires Liming was used as early as 1920 for favouring fish production. In the late 1960's fish kills and low ph in the lakes in southwest Sweden were found to be related to the deposition of airborne anthropogenic sulphur and nitrogen compounds. In 1977 a liming project of lakes started. This activity became permanent with governmental financial support in 1982 and has increased and is now also occurring in the northern parts of Sweden (Bernes 1991). This works well in large lakes with slow residence time. However, in small shallow lakes and in running water the effect of the liming event fast disappears. To remedy this and especially meliorate the acid surges in the spring the authorities have started to deposit the lime in streams in small but frequent doses by a dosing device or in wetlands to let it slowly leak out in the surrounding limnic environments. This activity is in fact a massive disturbance that highly affects the vegetation, often consisting of acidophilous mire species. Peat mosses Sphagnum spp. which often are the dominant part of the vegetation are killed off and the vascular plant vegetation become denser, with several graminoids especially favoured (Rafstedt 2008). In this study we used this large scale nationwide transformation of habitats not to study the extinctions of acidophilous species, but instead the colonizations and hence dispersal capacity of species that can withstand high ph-environment. In a 50 m 50 m plot in each of the 52 sampled limed mires the abundance (in a scale 1 5) of all bryophytes and vascular plants were recorded. The mire area outside the plot was surveyed for presence of additionally calcicolous species. In each mire was also ph measured 0:21

22 and five peat samples were taken (to get an indication of the vegetation before the liming event). We then analysed the occurrences of calicolous wetland species in the surveyed mires. Connectivity measures for several distances (5, 10, 20 km) were calculated from the number of 1 km 2 squares with occurrences of at least one strictly calcicolous wetland species (based on records from the Swedish species gateway ( A region or study area was defined as 100 km around all the surveyed mires and the number of 1 km 2 squares within this area with occurrence of species was used as measure of their respective regional frequency. The relationships between the number of colonized mires and connectivity measures, ph and Sphagnum-content were investigated. Moreover were regional frequency, how often the species produce capsules (for the bryophytes) and spore (or seed) size analysed against which species that colonized and the number of mires that they had colonized. 0:22

23 Results and discussion The overall result was that the realized dispersal capacity of the studied bryophyte species was good (both for a pioneer species and species colonizing limed mires). The simulations from the used mechanistic model (Lagrangian stochastic dispersion model) fitted rather well to the observed colonization pattern especially if observed spore release thresholds in wind variation and humidity were accounted for (III). Colonizations around a spore source of Discelium showed that the density of spores decreased up to m and after that it leveled out. However there were still colonization at the furthest distance both up to 100 m (III) and 600 m (I). The proportion that traveled further than 50 m was approximated to somewhere between 0 10 spores per square metre from this rather small patch source. This implies that dispersal over 1 km is probably not such a rare occasion for this species (cf. Nathan et al. 2008b). This finding initiated the idea of investigating dispersal over longer distances in a core area for the species around Umeå. In this region where the species is widespread the effect of connectivity on colonization rates of Discelium over one season increased with distance up to the largest scale analysed, 20 km (II). The approximation of the spore source, based on a survey of the focal species in two landscapes, showed that the colonization rates not were unrealistic given a high germination rate. This comply with observations that Discelium seems to be very effective in tracking newly created suitable substrates along forest roads, ditches and in disturbed areas in clear cuts in this area Extending the temporal scale to years in a study of colonization of limed mires gave a similar result of an effective colonization of bryophytes over long distances. Over this time period a high percentage (61 %, 54 species) of the calcicolous wetland bryophytes in the study area had colonized one or more of the 52 surveyed mires (IV). No effect of the connectivity up to 20 km could be detected in this 0:23

24 study. However, the average number of bryophytes per mire was significantly lower in the southern part of the region compared with the middle and the northern part, which might indicate that the relative proximity to areas with higher densities of rich fens in the province of Jämtland influenced the pattern. The species regional frequency (a gradient from common to rare species) and capsule frequency (how often a species produce capsules) showed a positive relationship with the proportion of mires (out of the 52) that a species had colonized, whereas spore size did not. Compared to the bryophytes a much lower number (15 species) and proportion (29 %) of the calcicolous wetland vascular plants in the study area had colonized one or more of the 52 surveyed mires. The species regional frequency showed a positive relationship and seed size showed a negative relationship with the proportion of mires (out of the 52) that a vascular plant had colonized. As indicated by Fig. 6 the studies in this thesis bridge a gap between previously performed studies, analyzing dispersal over a short time scale (1 30 years) and relatively large spatial scales ( km). The figure only incorporates some examples of different kind of studies to show the temporal and spatial context that this thesis addresses. Another example of a study that combine dispersal over large spatial scales and short temporal scales concluded that connectivity to mires in a circle of 200 km best explained Sphagnum spore deposition over one year (Sundberg 2013). In the lower left hand corner we find studies of spore deposition over hours and days and a few meters (2 15 m) where 2 70 % of the spores were accounted for within the measured distances (Söderström & Jonsson 1989; Miles & Longton 1992b; Stoneburner, Lane & Anderson 1992; Roads & Longton 2003; Sundberg 2005; Pohjamo et al. 2006). This large range shows that there can be a substantial difference in the number of spores available for long-distance dispersal. Two examples from the other end of the gradient with dispersal over large both spatial and temporal scales are a genetic study of Cinclidium species which found an extensive haplotype sharing between sites on different continents (Piñeiro et al. 2012) and an analysis that showed that floristic similarity between continents in the southern hemisphere could be explained by wind connectivity during the vegetation season for bryophytes, lichens and pteridophytes (Muñoz et al. 2004). The majority of studies have been performed on intermediate temporal and spatial scales. Studies of colonization of newly created habitats indicate a good colonization capacity over a few to tens of kilometers over half a century (Bremer & Ott 1990; Miller & McDaniel 2004; Hutsemékers, Dopagne & Vanderpoorten 2008). On the other hand studies of the spatial distribution and genetic structure in epiphytes indicate a more restricted dispersal for many 0:24

25 epiphytes from local to landscape scale over the life time of a tree (Hedenås, Bolyukh & Jonsson 2003; Snäll et al. 2004a; b; Löbel, Snäll & Rydin 2006). Also investigation of genetic structure in other species e.g. limnic species over landscape scales show a restricted gene flow (Hutsemékers et al. 2010; Korpelainen et al. 2013), while e.g. a study on a forest floor species on islands could detect no isolation by distance (Cronberg 2002). A study of a small scale (20 80 m from a forest) could not show any effect of connectivity on colonization of forest species on a clearcut over a time of 50 years (Hylander 2009) Time (years) Space (km) Other studies This thesis Figure 6. Some examples of studies of bryophyte dispersal plotted after which temporal and spatial scale that they have analysed or tested. One explanation for these different results could be for how long time the substrate has been available for colonization. However, the ephemeral patches of Discelium that are overgrown in a few years time seem often to be colonized (II), while species on trees that last for tens to hundred years seem to strongly positively relate to connectivity (Snäll et al. 2004b; Löbel et al. 2006). The same could be said for bryophytes on calcareous boulders that have been available for colonization for thousands years, and still are structured by connectivity (Virtanen & Oksanen 2007). So even if time per se increase the 0:25

26 probability for successful colonization, there must also be other factors that could explain the differences in dispersal capacity among bryophyte species. Another suggestion is that other substrate characteristics such as how easily it is colonized, could increase the probability of colonization after longer distances. It seems more likely that a diaspore comes to rest on a horizontal surface such as a mire or a clay surface compared to a vertical surface of a tree trunk or a boulder. Moreover, could the time the substrate is moist enough for colonization differ between an exposed elevated surface and a clay surface at ground level (cf. Hylander et al. 2005). Also the water-holding capacity of clay or peat compared to bark or stone differ. Contrary to this, one could argue that species usually are adapted to the substrate they inhabit. Discelium seems certainly adapted to fast occupy a disturbed clay surface and reproduce and disperse to new patches. However, other species can be highly adapted to more harsh substrates but the environment may still limit how fast they can colonize it and grow. Also in what kind of habitat the species grows could have an effect on dispersal capacity, and the degree of tree cover in the habitat is an important gradient for wind-dispersed organisms (Nathan et al. 2008b). All the studies in this thesis has been performed in open mires (I-IV), while some that have found clear dispersal limitation have been performed in more sheltered habitats (Snäll et al. 2004b; Löbel et al. 2006; Norros et al. 2012). Higher wind speed and wind variation could also trigger spore release under conditions that could favour long-distance dispersal (cf. III) and these are conditions that are more common in open habitats. However, this could to some extent be compensated for by the higher release height for epiphytes. Not only abiotic factors, but also biotic factors, such as competition, could influence which species that you find in a community (Zobel 1997). Competition between shoots of similar-sized species may rarely lead to competitive exclusion (Mälson & Rydin 2009), and can even lead to facilitation under some conditions (Bu et al. 2013). However, in the case of colonization, it might be different. A founder individual could monopolize the habitat and block further colonization (Waters, Fraser & Hewitt 2013). Both of the study systems in this thesis are disturbed habitats that should facilitate colonization (I IV) and the results have demonstrated a good dispersal and colonization capacity across landscapes. This is in concordance with other studies of 0:26

27 colonization of bryophytes in disturbed or newly created habitats and substrates such as planted forests, peat pits, mortar and slag heaps (Bremer & Ott 1990; Soro, Sundberg & Rydin 1999; Miller & McDaniel 2004; Hutsemékers et al. 2008). Transport of diaspores is easily underestimated when studying colonization patterns of a species which is heavily establishment limited. The fact that our study systems were disturbed may be one important explanation to that we managed to show such high colonization across landscapes (II, IV). Many species in other habitats might be confined to small windows of opportunities in space or time to be able to establish (Økland, Rydgren & Økland 2003; Löbel & Rydin 2010) even if they perhaps have dispersed there. If that is the case the spore source strength (how many spore that are available for dispersal) and not small differences in transport capacity of the spores becomes crucial (IV). For wood-inhabiting fungi this seems to be the case, where very large deposition densities could be needed for a successful colonization (Edman, Kruys & Jonsson 2004; Norros et al. 2012). This would imply that generalists, that not only manage to built up much larger source strength but also have a much larger target area of suitable substrates, would be less dispersal limited than specialists (Nordén et al. 2013). Even if we found a positive relationship between colonization frequency and both capsule frequency and regional frequency (which both influences the source strength) we also found some rare species with small populations in the lowland, like Meesia uliginosa and Catoscopium nigritum, rather frequent in the limed mires (IV). This raises questions on what spatial and temporal scales and for what species connectivity matter. For Discelium, a rather rare species, habitat quality and availability seem to matter much more than connectivity on landscape scales (II). The results from this thesis can certainly not be generalized to all species since dispersal and colonization can be highly species specific. In many cases it is probably not the transport but the colonization that is be the bottle-neck. However, this thesis highlights the good dispersal capacity in open habitats for many prolific species under relatively short to moderate time scales. 0:27

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