release thresholds for moss spores: the importance of turbulence and sporophyte length

Release thresholds for moss spores: the importanceof turbulence and sporophyte lengthVictor Johansson1*, Niklas L€onnell1, Sebastian Sundberg2 and Kristoffer Hylander1

1Department of Ecology, Environment and Plant Sciences, Stockholm University, SE-106 91 Stockholm, Sweden; and2Swedish Species Information Centre, Swedish University of Agricultural Sciences, Box 7007, SE-750 07 Uppsala,Sweden

Summary

1. Adequately describing the dispersal mechanisms of a species is important for understanding andpredicting its distribution dynamics in space and time. For wind-dispersed species, the transportationof airborne propagules is comparatively well studied, while the mechanisms triggering propagulerelease are poorly understood, especially for cryptogams.2. We investigated the effect of wind speed and turbulence on spore release in the moss Atrichumundulatum in a wind tunnel. Specifically, we measured the amount of spores released from sporo-phytes when exposed to different wind speeds, in high and low turbulence, using a particle counter.We also related spore release to variation in vibrations of the sporophyte and investigated how thevibrations were affected by wind speed, turbulence and sporophyte length (here including capsule,seta and the top part of the shoot).3. We show that in high turbulence, the amount of spores released increased with increasing windspeed, while in low turbulence, it did not, within the wind speed range 0.8–4.3 m s�1. However,there was a threshold in wind speed (~2.5–3 m s�1) before large amounts of spores started to bereleased in turbulent flow, which coincided with incipient vibrations of the sporophyte. Thresholdsin wind variation, rather than average wind speed, seemed to initiate sporophyte vibrations. Thevibration threshold increased with decreasing sporophyte length.4. The deposition of spores near the source decreased with increasing wind variation during the timeof their release, based on simulated spore deposition from another study of moss dispersal.5. Synthesis. We suggest that vibration of moss sporophytes is an important mechanism to regulatespore release and that turbulence and sporophyte length regulate the onset of sporophyte vibration.Spore release thresholds affect dispersal distances and have implications for our understanding andpredictions of species distribution patterns, population dynamics and persistence. The mechanismsof this phase of the dispersal process are also important to explore for other species, as there maybe a substantial variation depending on the species’ different traits.

Key-words: abscission, bryophytes, diaspores, dispersal, propagules, vibration, wind speedvariation, wind tunnel

Introduction

Dispersal is a fundamental biological process. Information ondispersal is important for understanding the spatial patterns ofspecies and our ability to predict their future dynamics andpersistence (e.g. Bullock, Kenward & Hails 2002; Levin et al.2003; Bohrer, Nathan & Volis 2005). Therefore, it is crucialto adequately describe the various mechanisms regulating dis-persal processes, as well as understanding the establishmentphase leading to realized colonization.

The diaspores of most terrestrial cryptogams and many vas-cular plants are passively dispersed by wind. The dispersalprocess includes release, transportation and deposition of dis-persal propagules. Modelling dispersal is a large area ofresearch (e.g. Kuparinen 2006; Nathan et al. 2011), which toa large extent has focused on the transportation of airbornepropagules under different conditions, including seeds (e.g.Nathan et al. 2002; Tackenberg 2003), pollen (Aylor, Boehm& Shields 2006) and spores (Norros et al. 2012, in press;L€onnell 2014). Less attention has been paid to the initiationof the dispersal process (Greene 2005; Kuparinen 2006), andthe mechanisms triggering propagule release are, hence, still*Correspondence author: E-mail: [email protected]

© 2014 The Authors. Journal of Ecology © 2014 British Ecological Society

Journal of Ecology 2014, 102, 721–729 doi: 10.1111/1365-2745.12245

poorly understood for most species. Mechanistic dispersalmodels, therefore, most often assume that propagules arereleased randomly in relation to, for example, wind conditions(e.g. Nathan, Safriel & Noy-Meir 2001; Tackenberg 2003;Kuparinen et al. 2007). However, this is most likely not truefor many species (e.g. Greene 2005; Skarpaas, Auhl & Shea2006; Borger et al. 2012; Pazos et al. 2013). The timing ofpropagule release may affect the probability of long-distancedispersal (Soons & Bullock 2008; Savage et al. 2012; Pazoset al. 2013) and significantly alter the size of the area inwhich propagules are deposited (Savage et al. 2010), whichmay affect our ability to predict species distributions and per-sistence (e.g. Bohrer, Nathan & Volis 2005).Thresholds in wind speed, above which propagule release

is triggered (Schippers & Jongejans 2005), may filter dispersalevents towards certain wind conditions and increase the prob-ability of long-distance dispersal (Soons & Bullock 2008).For vascular plants, seed release has been shown to increasewith increasing wind speed (Jongejans et al. 2007; Soons &Bullock 2008; Borger et al. 2012), and the release can behigher in turbulent compared with laminar wind flows (Skar-paas, Auhl & Shea 2006). Also temperature, plant wetness,the condition and orientation of the flower heads (Jongejanset al. 2007; Borger et al. 2012; Marchetto et al. 2012) orcapsule design (Kadereit & Leins 1988) may affect seedrelease. For spore-dispersed species, however, our knowledgeis still very limited about the mechanisms triggering sporerelease and its relation to, for example, wind conditions.Mosses disperse their spores from a capsule that often is

elevated above the shoot by a seta (capsule and seta togethermake up the sporophyte) anchored to the top of the mossshoot in cushion growing species or along the shoot in mat-growing species. It has been suggested that the elevated cap-sule promotes dispersal by lifting it above the laminar bound-ary layer, to increase exposure to more windy conditions(Crum 1972). Moss spores are generally rather small (mostly<50 lm; Hill et al. 2007) and are thus likely to be easily car-ried long distances by wind once they have reached higher airmasses. However, our knowledge about the release mecha-nisms, which would increase our understanding of how andwhen spores can enter the transportation phase of the dis-persal, is still very limited. Some bryophytes have activerelease mechanisms in which the spores are violently dis-charged, for example, Sphagnum (Sundberg 2010; Whitaker& Edwards 2010), but most have passive spore release. Evenif spore release is passive in most species, it is likely that it isnon-random in relation to for example wind patterns (Delga-dillo & P�erez-Band�ın 1982), which we explore in this study.Quite contradictory results have emerged regarding dis-

persal distances for bryophytes. Several studies of occurrenceand colonization patterns suggest dispersal limitations withina few hundred metres (Sn€all, Ehrl�en & Rydin 2005; L€obel,Sn€all & Rydin 2009), while other studies indicate frequentdispersal (with subsequent establishment) over longerdistances, sometimes up to tens of kilometres (Hutsem�ekers,Dopagne & Vanderpoorten 2008; L€onnell et al. 2012;L€onnell, Jonsson & Hylander in press). Measurements of

spore rain suggest that moss spores can travel by wind forhundreds of kilometres (Sundberg 2013). The potential forlong-distance dispersal may be affected by spore size (Wilkin-son et al. 2012), but among other aspects, variation in sporerelease thresholds regarding wind conditions may explaininterspecific variation in dispersal capacity.The aim of this study was to increase the understanding of

spore release mechanisms. Specifically, we investigate howwind speed and turbulence affect the amount of sporesreleased from moss sporophytes. We hypothesize that sporerelease increases with increasing wind speed and that it ishigher in wind flows with high turbulence compared with lowturbulence. A second aim was to investigate how sporerelease relates to vibrations of the sporophyte, and how thischanges with wind speed, turbulence and sporophyte length.We hypothesize that vibrations in the sporophyte increasespore release and that the wind speed required for triggeringvibrations is lower in high turbulence compared with low tur-bulence, but that the required wind speed increases withdecreasing sporophyte length.

Materials and methods

STUDY SPECIES

All experiments were carried out using Atrichum undulatum (Hedw.)P. Beauv., an acrocarpous moss in the Polytrichaceae family with a1- to 3-cm-long and ~300-lm-thick seta growing from the top of a 2-to 5-cm-erect shoot (Flora of North America Editorial Committee2007; Fig. 1). The species grows in open to wooded habitats onmoist, clay-rich soil (Nyholm 1969). Fertilization occurs during thesummer, and the sporophyte matures during the autumn/winter andreleases its spores, from the laterally oriented capsule opening(Fig. 1), after the lid dehisces during winter/spring (Arnell 1875; Ny-holm 1969). The spherical spores are around 20 (10–28) lm in diam-eter (Nyholm 1969; Boros et al. 1993). The number of spores percapsule has been estimated to 300 000–400 000 (Miles & Longton1992). In the globally distributed genus Atrichum, like in all generaof the Polytrichaceae, the tips of the peristome teeth are attached to amembrane (the epiphragm), which stretches across the capsule open-ing and is likened to a salt-shaker (Crum 1972; Fig. 1). It has beensuggested that this type of capsule may need vibrations to release itsspores (Ingold 1965), but it has never been tested.

Capsule-bearing shoots of the study species were collected in Octo-ber 2012 from a natural population in a forest margin at Tumba, east-central Sweden (N59°11.80, E17°51.50), and were kept in a glasshousewhere we followed the maturation process over the winter. We con-sidered the capsules to have mature spores, ready to be released,when the capsule dropped its lid. This was confirmed, by observingspore release when shaking capsules (not used in the experiments)with recently dropped lids, under a dissecting microscope. All experi-ments were carried out during March–May 2013 in a wind tunnel atStockholm University (Appendix S1 in Supporting Information).

SPORE RELEASE IN RELAT ION TO WIND SPEED

AND TURBULENCE

In the first experiment, we investigated how spore release wasaffected by wind speed and turbulence. We exposed one sporophyte

© 2014 The Authors. Journal of Ecology © 2014 British Ecological Society, Journal of Ecology, 102, 721–729

722 V. Johansson et al.

at a time to either low or high turbulence wind flows over a range ofwind speeds (0.8–4.3 m s�1; Appendix S1). These wind speeds arerealistic in, at least, open habitats of the species (as judged by long-term field measurements at 10–20 cm above-ground in open and for-ested biotopes; S. Sundberg, unpubl. data). We tested 20 capsules: 10in low turbulence and 10 in high turbulence (see below). In each trial,we randomly collected a shoot with a mature sporophyte from theglasshouse. The sporophyte was fixed vertically in the wind tunnel byattaching its gametophyte shoot, 10 mm under the base of the seta, toa crocodile clamp in the centre of the tunnel (Fig. 1; Fig. S1). Thispoint of attachment was chosen to mimic the potential for wavingmovement of sporophytes in nature, which is likely to be affected notonly by the length of the seta but also by a flexible gametophyteshoot below the seta. Setae lengths showed little variation (16–22 mm). Turbulence was created by inserting five horizontal 1.5-cm-thick threaded rods across the wind tunnel (Skarpaas, Auhl & Shea2006; Jongejans et al. 2007). The rods were placed 1.5 cm apart atthe height of the capsule and 12 cm upwind. This created a mean tur-bulence intensity (I = r

U/�U, where �U is the mean horizontal windspeed and r

U is the standard deviation) across wind speeds of 0.053in the high turbulence flow compared with 0.030 in the low turbu-lence flow. Hence, we could not completely remove the variation inwind speed to create a laminar flow, but the variation in the low tur-bulence flow was considerably lower than in the high turbulence flow(Fig. 2). The reason that we could not create lower turbulence, as in,for example, the study by Jongejans et al. (2007), may be the windtunnel size (ours being much smaller), and, for example, a shorter dis-tance to the fan (as the flow may be more turbulent closer to the fan).In both treatments, the air flow started at 0.8 m s�1 and was turnedup in 10 steps until the maximum speed (~4.3 m s�1) was reached(see Appendix S1 for details). Each step was maintained for 2 min.As the steps may vary slightly between trials, we continuously loggedthe horizontal wind speed (ten measurements per second) usingSwema 3000 hotwire anemometer with the SWA 31 probe (Swema,Farsta, Sweden) located 3 cm sideways and 3 cm downwind of the

sporophyte. The amount of spore release was estimated with a CI-1054 particle counter (Climet Instruments Company, Redlands, CA,USA), with its suction opening located 5 cm downwind of the cap-sule, continuously logging (four measurements per second) the num-ber of particles >5 lm. This measure is, however, not a perfectestimation of the absolute number of spores released – the number ofspores may be overestimated due to a background level of particles inthe wind tunnel and underestimated because spores may clumptogether or because they are not all caught by the counter. The mea-sure is therefore only used for comparing relative levels among treat-ments.

For each trial and wind level, we judged the degree of sporophytevibrations visually (through a transparent window in the side of thetunnel, with the capsule being illuminated by a LED lamp fromabove; Appendix S1) on a three-point scale: no vibrations, weakvibrations (barely visible) and strong vibrations (clearly visible). Atthe end of each trial, we triggered release of spores mechanically byclicking the base of the crocodile clamp below the sporophyte. Thiswas carried out to confirm that there were loose spores available inthe capsule, for those capsules that did not release spores during thetrial, as it was difficult to be sure of when collecting the sporophyte.This made us exclude and replace five sporophytes with low numbersof loose spores (two in high turbulence and three in low turbulence)that already might have released their spores in the glasshouse.

VIBRATIONS OF THE SPOROPHYTE IN RELAT ION

TO SPOROPHYTE LENGTH

In the second experiment, we investigated how the vibrations of thesporophyte were affected by sporophyte length. We used 10 sporo-phytes. Each sporophyte was attached to the crocodile clamp at threesuccessive positions on the shoot: 10 mm down the shoot, 5 mmdown the shoot and 0 mm (i.e. at the seta base). For simplicity, weuse the term ‘sporophyte length’ to discriminate among these threetreatments, although more correctly, it is the combined length of thesporophyte and the part of the shoot between the seta base and thecrocodile clamp. For each position treatment, we exposed the sporo-phyte to the high turbulence wind flow and slowly increased the windspeed. We judged the level of vibrations of the sporophyte visuallyusing the same scale as in the first experiment (i.e. no, weak or strongvibrations). For the levels ‘weak’ and ‘strong’ vibrations, we mea-sured wind speed with the hotwire anemometer for 1 min when thelevel was reached. For comparison, we also exposed each sporophyteto the low turbulence wind flow when attached 10 mm down theshoot. As we did not record spore release in this experiment, weremoved the filters on the inflow of the fan and could thereforeincrease the maximum speed up to ~7 m s�1 (for details see Appen-dix S1). We also tested to attach the sporophyte 5 mm down theshoot and at the seta base for a few sporophytes in the low turbu-lence, but the maximum wind speed was too low to trigger substantialvibrations in these trials.

DATA ANALYSIS

We modelled the mean amount of particles detected per 0.25 s(experiment 1) based on wind speed and high turbulence (included asa factor: yes or no) using a Bayesian hierarchical model (Gelmanet al. 2004). For each of the 11 levels of wind speed, we calculatedthe mean wind speed and the mean number of particles detected (i.e.creating 11 data points for each sporophyte) and included sporo-phyte identity as random factor to account for difference between

(a) (b)

(c)

Fig. 1. A sporophyte of Atrichum undulatum (a) with a 21-mm-longseta growing from the top of the gametophyte shoot, and a 3-mm-long capsule (b) where the tips of the peristome teeth are attached toa membrane (the epiphragm), which stretches across the 0.9 mm cap-sule opening (c).


Release thresholds for moss spores 723

sporophytes and variation in background particle levels during the dif-ferent runs. The mean number of particles per wind level was log-transformed to improve normality. We chose the Bayesian frameworkas it is well suited for hierarchical models and because model predic-tions can include parameter uncertainty (Fig. 3; Gelman et al. 2004).We built the model by comparing models with different combinationsof the two explanatory variables (wind speed and turbulence) andtheir interaction term using the deviance information criterion (DIC;Spiegelhalter et al. 2002). DIC is an information-theoretic approachwith properties similar as Akaike’s Information Criterion (Burnham &Anderson 2002). The final model was the one with the lowest DIC(Table 1). However, this model should only be seen as a way of test-ing and illustrating differences among treatments, and should not beextrapolated outside the tested wind range. The assumed linear rela-tionship between wind speed and spore release is simplified, as thereseem to be thresholds for spore release (see Results). We thereforealso show a Lowess curve (smoother span = 0.5) for the data in thehigh turbulence wind flow (Fig. 3a). For one capsule, the mean num-ber of detected spores declined at the highest wind speed due to highrelease at lower wind speeds (just above the threshold), and hence, alimited number of spores were available at the highest speed. TheBayesian models were fitted using OpenBUGS 3.2.2 (Thomas et al.2006). We ran two MCMC chains for 110 000 iterations. 10 000 iter-ations were used for estimation, after removal of the first 10 000 iter-ations (burn-in) and thinning by 20. We used uninformative priordistributions (Table S1).

SIMULATIONS OF SPORE DISPERSAL

To illustrate the implications of our results for the dispersal distancesof moss spores, we used data from another study on moss dispersal(L€onnell 2014). In L€onnell (2014), spore deposition was modelledwithin 100 m from a source as a function of meteorological variables

in a real landscape for the species Discelium nudum with sphericalspores ~25 lm in diameter. This was done using a Lagrangian sto-chastic dispersion model (L€onnell 2014; cf. Norros et al. in press)during approximately 400 h of wind data divided into 10-min periods.Input variables were as follows: average wind speed, vertical momen-tum flux, variances of three wind speed components, dissipation rateof turbulent kinetic energy and vertical gradients of these statistics, aswell as spore release height (10 cm) and settling speed. For each per-iod, the trajectories of 10 000 spores were simulated. Here, weextracted the simulated deposition patterns for 30 of these periods thatfulfilled the criteria of wind variation of interest. We chose periodswith a standard deviation in wind speed of 0.05, 0.10 and 0.20 (10periods for each level of variation). We then plotted the mean propor-tion of spores dispersing over at least x metres (1–100 m) for eachlevel of wind variation. As D. nudum has somewhat larger sporesthan our study species, the simulations may underestimate the deposi-tion distances slightly (Norros et al. in press), but the relative differ-ence among them should be similar.

Results

At low wind speeds (<2 m s�1), the number of detected parti-cles was similarly low for both treatments (Fig. 3a). However,at wind speeds of approximately 2.5–3 m s�1, spore releasein high turbulence increased considerably, which is seen as athreshold in a nonlinear Lowess curve fitted to the high turbu-lence data (Fig. 3a). This threshold seemed to coincide withthe induction of strong vibrations of the sporophyte (Fig. 3b),which did not release much spores at no or weak vibrations.In contrast to high turbulence, the number of particlesdetected in low turbulence slightly decreased with increasingwind speed (Fig. 3a), and strong vibrations were only induced

Win

d sp

eed

(m s—1

)

0 500 1000 1500

12

34

5

(a) Low turbulence

0 500 1000 1500

12

34

5

(b) High turbulence

Time (s)

Num

ber

of p

artic

les

0 500 1000 1500

(c)

Time (s)

0 500 1000 1500

050

100

150

200

250

050

100

150

200

250

(d)

Fig. 2. Two examples of measuredhorizontal wind speed (a, b) in relation todetected number of particles (c, d) over time:one in low turbulence (a, c) and one in highturbulence (b, d) wind flow.



for one sporophyte at wind speeds >4 m s�1 (Fig. 3b). Thefinal model (i.e. the one with the lowest DIC, Table 1) forspore release included wind speed, wind speed squared, highturbulence (yes or no) and the interaction between wind speedand high turbulence (for parameter estimates see Table S1).

Vibrations of the sporophyte were triggered at lower windspeeds and lower wind variation for long compared with shortsporophytes (i.e. the total length of the capsule, seta and partof the shoot above the crocodile clamp; Figs 1 and 4). Thelevel of wind variation at which all sporophytes of the samelength were vibrating was similar in high and low turbulence(Fig. 4b), while the mean wind speed differed markedlybetween the two treatments (Fig. 4a).The simulated deposition of spores near the dispersal

source decreased with increasing wind variation during thetime of their release (Fig. 5). In periods with wind variationabove the threshold for sporophyte vibration (i.e. SD = 0.20,as shown in the wind tunnel experiment; Figs 3b and 5), 46%of the spores travelled further than 100 m from the source.The corresponding percentage for periods with lowerwind variation (SD = 0.05 and 0.10) were 6% and 28%,respectively.

Discussion

Based on wind tunnel experiments we show that (i) therelease of moss spores is much higher in high compared withlow turbulence wind flows given the same horizontal windspeed, at least up to 4.3 m s�1, (ii) the mechanism triggeringsubstantial spore release is vibrations of the sporophyte thatare induced at lower wind speeds in high compared to lowturbulence as a result of higher wind speed variation, and (iii)that the threshold of wind speed or its variation, triggeringvibrations, decreases with increasing sporophyte length.

THE IMPORTANCE OF TURBULENCE AND SPOROPHYTE

LENGTH FOR SPORE RELEASE

We show that the release of moss spores is higher in highcompared with low turbulence wind flows, in accordance withour first hypothesis. This also agrees with studies of seedrelease (Skarpaas, Auhl & Shea 2006; Jongejans et al. 2007).However, in studies of seed release, the actual mechanism bywhich the seeds are detached was not investigated (Skarpaas,Auhl & Shea 2006). Our results for the selected moss speciessuggest that the mechanism triggering substantial sporerelease was vibrations of the sporophyte, which were inducedat lower wind speeds in high compared with low turbulence.We suggest that it is the increasing variation in windspeed, and not increasing wind speed per se, which triggerspropagule release. There may, hence, be thresholds in windvariation, rather than mean wind speed, above which vibra-tions, and thus, substantial spore release is triggered. Weshow that spores released above this threshold travel muchfurther than spores released below the threshold (Fig. 5).Propagule release thresholds may thus increase dispersalunder specific wind conditions, which may increase the med-ian dispersal distance and the probability of long-distance dis-persal events (e.g. Schippers & Jongejans 2005; Soons &Bullock 2008; Savage et al. 2012). We propose that increasedwind variation under short time periods (i.e. wind gusts) maytrigger substantial propagule release, and we agree with Pazos

Table 1. DIC of models for the amount of spores released with dif-ferent combinations of the explanatory variables (wind speed and highturbulence)

Model description DIC

Null model (only including the random intercept) 512.1Wind speed 505.7High turbulence 504.0Wind speed + (wind speed)2 499.3Wind speed + high turbulence 479.9Wind speed + (wind speed)2 + high turbulence 491.6Wind speed + turbulence + wind speed 9 high turbulence 420.9Wind speed + (wind speed)2 + high turbulence + wind speed9 high turbulence

410.9

log(

part

icle

s)/0

.25

s

0 1 2 3 4 5

−2

−1

01

23

45

High turbulenceLow turbulenceLowess

(a)

Wind speed (m s–1)

Pro

port

ion

vibr

atin

g sp

orop

hyte

s

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

WeakStrong

WeakStrong

(b)

High turbulence

Low turbulence

Fig. 3. (a) The mean number of particles detected per 0.25 s in rela-tion to increasing wind speed. Observed data (points), mean (continu-ous line) and 95% credible interval (thin broken lines) of predictionsbased on the final Bayesian model, and a nonlinear lowess curve(thick blue broken line) for the data in the high turbulent wind flow.(b) The proportion vibrating sporophytes in relation to increasingwind speed, where dotted lines show weak vibrations (barely visible)and continuous lines strong vibrations (clearly visible).



et al. (2013) who emphasized the importance of measuringwind speeds at short time intervals. Further investigations onhow frequency, amplitude and direction in the wind variationrelate to vibration of sporophytes might be valuable, and howoften wind conditions that may trigger spore release occur inthe field for different species (where wind patterns are morecomplex than in a wind tunnel). There is a lack of realisticwind data at relevant points in the terrain from different habi-tats. In closed forests, the required wind conditions for ourstudy species seem rare, but may occur with a low frequency(S. Sundberg, unpubl. data). However, based on field datafrom a relatively open area with scattered trees (L€onnell2014), we calculated that during 2 weeks in May (when a dis-persal experiment of another moss species was conducted),the standard deviation in wind speed (at 20 cm above-ground)was above 0.20 approximately 30% of the time, which maytrigger spore release (Fig. 4b) in our study species. Those2 weeks were representative for normal weather conditions inthis part of Sweden (Uppland) during the spring when thespecies is dispersing (L€onnell 2014). In a national perspective,western Sweden is generally windier and central Sweden gen-erally calmer than the study site (SMHI 2014). This example

suggests that wind variation triggering spore release in thewind tunnel is realistic. However, this small data set cannotbe used to quantify how often such conditions occur. More-over, different habitats might have very different wind pro-files, which need to be monitored in situ and cannot beextrapolated from, for example, weather station data.We also show that the length of the sporophyte may affect

the spore release threshold in that vibrations were induced atlower wind speeds and at lower wind variation for long com-pared with short sporophytes. The likely reason may be thatvibrations of the capsule are more easily induced when hav-ing a long swinging arm, compared with a short. Long sporo-phytes may therefore have an advantage over shortsporophytes (given the same seta thickness), when it comes toinducing spore release at low wind speeds. Hence, there maybe a selection pressure for longer setae in, for example closedforests, where wind speeds are lower.The modelled relationship (Fig. 2a) between spore release

and wind speed might be somewhat simplified and we sug-gest that there is in fact a release threshold which is supportedby the vibration result (Fig. 2b) as well as the Lowess curve(Fig. 2a). It is also likely that the amount of spores releaseddo level out at higher wind speeds, and that if the first windgust is strong, a large proportion of the spores would bereleased more quickly. Moreover, the release of spores maystart to increase in low turbulence, at higher wind speeds,when the wind variation reaches the threshold for vibration(probably above the maximum wind speed we could have inexperiment 1 due to technical constrains). The slightly declin-ing amount of spores with increasing wind speed in low tur-bulence could be a result of decreasing background levels ofparticles over time. One source of background particles couldbe spores that were released when we attached the sporophyteto the crocodile clamp before each trial started.

GENERALITY OF OUR RESULTS FOR OTHER SPECIES

AND EVOLUTIONARY IMPLICAT IONS FOR MOSSES

Vibration is a likely mechanism for shaking out loose propa-gules from some sort of container in other organisms as well.Examples are seeds in species with capsules (e.g. Kadereit &Leins 1988), or spores in club mosses (Lycopodiaceae) and

Wind speed (m s–1)

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port

ion

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atin

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1 2 3 4 5 6

0.0

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(a)

Variation in wind speed (sd)

0.0 0.1 0.2 0.3 0.4 0.5

(b)

High turbulence 10 mmHigh turbulence 5 mmHigh turbulence 0 mmLow turbulence 10 mm

Fig. 4. The proportion of vibratingsporophytes in relation to (a) wind speed and(b) variation in wind speed, and sporophytelength (measured as distance from the setadown to the point of attachment on theshoot) in high turbulence wind flow. Thelongest sporophyte length in low turbulenceis shown as a contrast.

Distance (m)

Pro

port

ion

of s

pore

s di

sper

sing

ove

r

a

t lea

st x

met

ers

1 10 100

0.0

0.2

0.4

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1.0

0.050.100.20

Fig. 5. Proportion of moss spores (25 lm) dispersing at least xmetres within 100 m from a dispersal source, under different levels ofwind variation (SD = 0.05, 0.10 and 0.20), simulated with a Lagrang-ian stochastic dispersion model based on meteorological data (L€onnell2014). Note the logarithmic (log10) x-axis.



horsetails (Equisetaceae). This could in part be due to that thecontainers are tilted beyond some threshold angle whenvibrating (Thompson & Katul 2013). For species that do nothave loose propagules, vibration may instead be important fortearing the connection between the propagule and the plant(‘wear and tear’; Pazos et al. 2013; Thompson & Katul2013). For our study species, where most spores are loose,the vibration threshold could to some extent be explained bythe threshold angle hypothesis. However, as only a limitedamount of spores fall out when holding the capsule upsidedown, we believe that the most important function of thevibrations is rather to successively transport spores to theopening of the capsule, and separating clumped spores, inorder for them to pass through the peristome openings. With-out the shaking caused by the vibrations, spores becomearranged in such a way that they block the openings, and thespore release threshold may thus be due to the energyrequired to move spores enough. Perhaps vibrations not onlyfavour dispersal but are actually necessary in the family Poly-trichaceae, which has a ‘salt-shaker’ peristome (Fig. 1c;Ingold 1965; Crum 1972). The ‘wear and tear’ mechanism(Pazos et al. 2013; Thompson & Katul 2013), do not applyfor the loose spores but could be important for successivelydetaching maturing spores from the columella (where theydevelop) in the capsule. Among the adaptations that regulatewhen spore release from a capsule occurs are certain peri-stome traits: for example, if a peristome is present or not andif it opens or closes with increasing moisture (Ingold 1965).Another important difference among mosses is the orientationof the capsule, which can vary from erect to pendent (withthe capsule mouth directed towards the ground). However,when the capsule is open, vibrations of the sporophyte shouldhave the potential to cause substantial release of spores inmost species, irrespective of capsule orientation. Hence, webelieve that vibrations are a likely spore release mechanism inmost species that have a seta or in which the whole mossshoot can move.Mosses show a wide variation in sporophyte length

(Heden€as 2012). Our study species has a rather long seta,which may increase the potential for wind-induced vibra-tions. However, a shorter seta (especially on a short shoot)might ensure that it starts to vibrate in stronger wind gusts,which might increase the potential of long-distance dispersal.On the other hand, spores released from long sporophytesshould more easily be caught by the wind and are alsoexposed to higher wind speeds, since they are higher up inthe wind profile. Little has been done on variation of setalength and fitness trade-offs in relation to seta (and shoot)length. There is also a possibility of different selection pres-sures in different habitats that vary in moisture (Heden€as2001) and wind patterns (e.g. forests vs. open habitats), andbetween epiphytes and ground-living species (Heden€as2012). In species with very short setae or shoots, vibrationscaused by wind may be unlikely, and to what extent wind-induced vibrations occur in closed forests is a question openfor investigation. The potential for wind-induced sporophytevibrations may also depend on other sporophyte traits, such

as seta thickness, stiffness and capsule size, which differamong species. A better understanding of how wind inducesvibrations, and spore release, may require more detailedstudies of the wind flow around different types of sporo-phytes (Marchetto et al. 2010).Variation in vibration thresholds may explain differences in

dispersal distance among mosses (Fig. 5). However, dispersaldistances may also be affected by other factors, such as rela-tive humidity when the spores are released. Several epiphyteswith dispersal limitations (Sn€all, Ehrl�en & Rydin 2005; L€obel,Sn€all & Rydin 2009) have peristome movements that maypromote dispersal under humid conditions (Ingold 1965),which may not be ideal for long dispersal.

NON-RANDOM PROPAGULE RELEASE

We show, to our knowledge, the first example of non-randomrelease of passively released moss spores with respect toenvironmental conditions and appearance, while the literatureon non-random release of seeds has increased during recentyears (Skarpaas, Auhl & Shea 2006; Jongejans et al. 2007;Soons & Bullock 2008; Borger et al. 2012; Pazos et al.2013). This has implications for how to model dispersal; abetter understanding of propagule release mechanisms hasbeen suggested important for future progress in dispersalmodelling (Kuparinen 2006; Nathan et al. 2011). We suggestthat moss spores are almost only released over certain thresh-olds of wind variation, which leads to longer dispersal dis-tances. Hence, mechanistic models that assume thatpropagules are released randomly in relation to wind condi-tions (e.g. Nathan, Safriel & Noy-Meir 2001; Tackenberg2003; Kuparinen et al. 2007) may underestimate the proba-bility of long-distance dispersal (Greene 2005; Savage et al.2012; Pazos et al. 2013), and the area over which propagulesdeposit (Savage et al. 2010).

Conclusions

Studying how environmental conditions and species traitsaffect propagule release is important for understanding whenand where species disperse (Wright et al. 2008), which hasimplications for our understanding and predictions of speciesdistribution patterns, population dynamics and persistence(e.g. Bohrer, Nathan & Volis 2005). We demonstrate thatmoss spores are not released randomly and that vibration inthe sporophyte can be an important mechanism for triggeringconsiderable spore release for certain species. High turbulenceand a long seta (and a flexible shoot) facilitate these vibra-tions, and thus the release of spores at lower wind speeds,compared with in low turbulence wind flows and when thesporophyte is short.Our results highlight the importance of studying propagule

release mechanisms. The generality of these mechanisms forspore release in other species is, however, still unknown.Therefore, we suggest that future research should focus onstudying this part of the dispersal process also for otherspore-dispersed species with different traits (e.g. variation in



seta length and thickness, capsule orientation and peristomeproperties). This could, apart from wind speed and turbulence,also include effects of, for example temperature, moisture andmaturation (Jongejans et al. 2007; Borger et al. 2012), andtheir effects on vibrations and peristome movements and ulti-mately spore release.

Acknowledgements

We thank class Te08 at the gymnasium H€ogbergsskolan in Tierp led by teacherMattias Hjellstr€om, Jan Sundberg at Munters Europe AB in Tobo, and StefanBj€orklund for building the wind tunnel. We also thank €Ullar Rannik for lettingus use the simulated spore deposition data. The work was funded by the Swed-ish research council Formas, Grant 217-2008-1193 to K.H.

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Received 20 September 2013; accepted 6 March 2014Handling Editor: Eelke Jongejans

Supporting Information

Additional Supporting Information may be found in the online ver-sion of this article:

Appendix S1. Wind tunnel description and methodological issues.

Figure S1. Wind tunnel description.

Table S1. Prior distributions and final estimates of model parameters.



release thresholds for moss spores: the importance of turbulence and sporophyte length

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