Dust Seed Production and Dispersal in Swedish Pyroleae Species

Dust seed production and dispersal in Swedish Pyroleae species

Veronika A. Johansson , Gregor M ü ller and Ove Eriksson

V. A. Johansson ([email protected]) and O. Eriksson, Dept of Ecology, Environment and Plant Sciences, Stockholm Univ., SE-106 91 Stockholm, Sweden. – G. M ü ller, Dept of Biology, Univ. of Konstanz, DE-78457 Konstanz, Germany.

Dust seeds are the smallest seeds in angiosperms weighing just about a few micrograms. Th ese seeds are characteristic of most orchids, and several studies have been performed on seed features, fecundity and dispersal of orchid dust seeds. In this study we examine seed features, seed production and seed dispersal in another plant group with dust seeds, the Pyroleae (Monotropoideae, Ericaceae), focusing on six species: Pyrola chlorantha , P. minor , P. rotundifolia , Chimaphila umbellata , Moneses unifl ora and Orthilia secunda . Seed production per capsule among these species was in the range between ca 1000 and 7800, and seed production per capsule bearing shoot was in the range between ca 7000 and 60 000. Combining our results with published information on pollen-ovule ratios suggests that these Pyroleae species have a generally effi cient pollination system. Th e most fecund species was P. minor, the only species among the investigated that is probably largely self-pollinating. Th e investigated Pyroleae species have a seed production comparable to the less fecund orchid species. We studied seed dispersal in the fi eld in one of the species, P. chlorantha . Despite the extremely small and potentially buoyant seeds, the vast majority of seeds are deposited close to the seed source, within a few meters. Further studies on the recruitment ecology of the investigated Pyroleae species are currently under way.

‘ Dust seeds’ are extremely small and are often produced lack chlorophyll and hence the capability to photosynthesize. in vast numbers. Although most common among orchids Instead these plants use fungi as a source of organic car- (Arditti and Ghani 2000), dust seeds occur in at least 12 bon. Fully mycoheterotrophic plants have received much diff erent families, most likely representing at least 15 inde- attention with particular focus on their physiology and pendent evolutionary origins (Eriksson and Kainulainen dependence on host fungi (Bidartondo and Bruns 2001, 2011). In many of these families, dust seeds have conver- Bidartondo 2005, Leake and Cameron 2010). For exam- gent features such as an elongated shape and large internal ple, it has been found that species in the genus Monotropa , air spaces, possibly benefi cial to enhance buoyancy in air or closely related to the Pyroleae, are extremely specialized in water. Furthermore, in most dust seeds the embryo is undif- the use of fungal hosts (Leake et al. 2004, Bidartondo and ferentiated. Arditti and Ghani (2000) compiled extensive Bruns 2005). In contrast to full mycoheterotrophs, Pyroleae information on seed size, seed shape and seed production in species are photosynthetic as adults, except for P. aphylla orchids. Th is information is not only important as descrip- (a species not included in our study) which remains fully tors of orchids, but serves also as an essential background to mycoheterotrophic as adult (Hynson and Bruns 2009). understand their dispersal features, in turn highly relevant However, Pyroleae species are considered to be not fully for studies of population and conservation biology (Diez autotrophic, but instead ‘ mixotrophic ’ (Selosse and Roy 2007, Jacquemyn et al. 2007, Swarts et al. 2010). In this 2009), i.e. partially mycoheterotrophic, implying that they paper we present data on seed features, seed production and partly use fungi as a source of organic carbon (Tedersoo dispersal from another group of plants with dust seeds, the et al. 2007, Matsuda et al. 2012). A possible exception is tribe Pyroleae (Monotropoideae, Ericaceae) (Pyykkö 1968, C. umbellata which in one study was found to be fully auto- Takahashi 1993, Kron et al. 2002). Our study concerns trophic (Hynson et al. 2012). Despite being partially myco- six of the species found in Sweden: Pyrola chlorantha Sw., heterotrophic as adults, or even autotrophic, all Pyroleae species P. minor L., P. rotundifolia L., Chimaphila umbellata (L.) are fully mycoheterotrophic as ‘ seedlings ’ , a below-ground W. P. C. Barton, Moneses unifl ora L. A. Gray and Orthilia stage that may last for several years (Whigham et al. 2008, secunda (L.) House (Fig. 1, Table 1). One of these species, Eriksson and Kainulainen 2011). Moreover, in contrast to C. umbellata , is presently red-listed as endangered in Monotropa spp., it seems as the partially mycoheterotrophic Sweden, and two of the other, P. chlorantha and M. unifl ora species in the Pyroleae, as adults, are generalistic concern- are currently declining (Jonsell 2010). ing their fungal partners (Tedersoo et al. 2007, Hashimoto A group of plants characterized by having dust seeds et al. 2012). Some evidence suggests that the host use is more is mycoheterotrophic plants (Leake 1994), i.e. plants that restricted during juvenile stages (Hashimoto et al. 2012).

209 Figure 1. SEM-photos of seeds of six Pyroleae species. (A) Chimaphila umbellata, (B) Moneses uniﬂ ora, (C) Orthilia secunda, (D) Pyrola chlorantha , (E) Pyrola minor and (F) Pyrola rotundifolia .

Th e present paper is a part of an ongoing study on limitation, seed production and seed dispersal ability. Seed recruitment biology of Pyroleae species. In Johansson and production is an important component of the realized seed Eriksson (2013) we showed that the study species diﬀ er dispersal potential, since increasing number of seeds from with regard to what limits their recruitment. Microsite a seed source in itself improves dispersal (Eriksson and availability was the main factor limiting recruitment in Jakobsson 1999). In addition, the capacity of single seeds P. chlorantha , C. umbellata and O. secunda , whereas seed to be carried by wind determines the seed shadow around availability was the main limiting factor in P. minor . For a seed source (Tackenberg 2003). Several studies have been P. rotundifolia and M. uniﬂ ora, seed and microsite limita- performed on seed dispersal in orchids with dust seeds tion contributed about equally. Apart from factors that con- (Murren and Ellison 1998, Jacquemyn et al. 2007, Jers á kov á cern the interactions between germinating seeds and fungi, and Malinová 2007), but as far as we are aware of, none have two other factors contribute to patterns of recruitment been conducted on Pyroleae species.

Table 1. Information on seed size, ﬂ ower/capsule number, plant height and habitat preferences of the six studied Pyroleae species.

Seed length Seed breadth Mumber of Flower Species (mm) a (mm) a ﬂ owers/capsules height (cm)b Habitatb Chimaphila umbellata 0.55 0.10 3– 5c 10– 25 Rare in old growth boreal forests Moneses uniﬂ ora 0.81 0.11 1b 5– 15 Fairly common in mesic-moist habitats Orthilia secunda 0.53 0.11 5– 10 c 10 – 20 Common in mesic-moist habitats Pyrola chlorantha 0.70 0.17 2– 6b 10– 25 Fairly common in dry-mesic boreal forests Pyrola minor 0.54 0.17 5 –10 c 5– 20 Common in mesic-moist habitats Pyrola rotundifolia 0.67 0.15 6 – 15b 15 – 30 Fairly common in mesic-moist habitats a Mean seed size values from Johansson and Eriksson (2013). b Occurence in Sweden, Mossberg and Stenberg (2010). c Pers. obs.

210 Th is study has the following objectives: 1) to examine diff erent directions, at an angle of 60° from each other. Th e seed production in the six Pyroleae species, and 2) to exam- seed traps were placed along each transect at the following ine the realized seed dispersal shadow around a seed source distances 0 cm, i.e. adjacent to the seed source (n ϭ 1); 10 cm in the fi eld. Th e second objective was focused on one of the (n ϭ 1), 31 cm (n ϭ 1), 56 cm (n ϭ 1), 100 cm (n ϭ 1), species, P. chlorantha , but considering the similarities in seed 177 cm (n ϭ 2), 316 cm (n ϭ 4) and 562 cm (n ϭ 8). Th e characteristics, we assumed that the results could be genera- n-values are the number of traps at each distance in each lized to all the six study species. transect, thus with an increasing number of traps at the outermost positions. Th ere were a total of 114 seed traps per seed source. In all, fi ve replications of seed sources were Material and methods used (one at Rä lla and four at Dö rby; the replications at Dö rby were located at a distance of ca 300 m from each Whole infructescences with mature seed capsules, hereafter other), thus making a total of 570 seed traps. Th e traps were referred to as shoots, were collected randomly from all study collected after ten days in the fi eld, which was the approxi- species from subjectively selected fi eld sites located on Ö land mate time the traps remained sticky. Th e tape was then cut (56 ° 41 ’ N, 16 ° 36 ’ E) and from the counties of Uppland off and viable seeds were counted under a dissecting micro- (60 ° 16N ’ , 18 ° 5 ’ E) and S ö dermanland (58° 53 ’ N, 17° 1 ’ E) scope. Th e actual number of available seeds from the seed in Sweden. Five sites were used for each study species from source varied, but as the analysis focused on the proportion which fi ve random shoots were collected resulting in a total of seeds deposited at each distance, we regard this as a minor of 25 shoots per species, except for P. minor where four sites source of error. In the fi nal analysis, the fraction of seeds were used, resulting in 20 shoots. We removed all seeds from deposited in relation to distance from seed source was exam- each collected shoot and they were dissolved in a solution ined by fi tting both an inverse power law function, and a of water and detergent to remove the surface tension. Seed negative exponential function. Th e results were similar, but numbers per shoot were counted by taking ten subsamples of the inverse power law function gave slightly better fi t, and is the seed solution for each shoot. Depending on the concen- thus shown in the following results. Statistical analyses were tration of seeds in the seed solution, between 25 and 100 μ l carried out in the program R. were sampled for easier handling. Th e total amount of seeds per shoot was then calculated by dividing the total volume of the seed solution with the volume of the samples taken Results and multiplied by the mean number of seeds counted for the ten samples. Seed number per capsule was counted by Th e six study species have dust seeds of similar size, but dividing the total number of seeds found with the number somewhat varying in shape, M. unifl ora seeds being the most of capsules per shoot. Furthermore, aborted seeds containing elongated (Fig. 1, Table 1). Th e species diff ered with regard no embryo were counted in the same way but these are not to the potential number of capsules per shoot (Table 1). As included in the total number of seeds produced. M. unifl ora only produces one fl ower per shoot it is con- Comparisons between species with regard to seed num- strained to produce only one capsule per shoot. Th e other ber/capsule and seed number/shoot were done by analysis fi ve species have multi-fl owered infl orescences and are of variance (ANOVA) followed by Tukey ’ s post hoc test to therefore able to produce several capsules per shoot. Th ere determine which species diff er from each other. was a signifi cant diff erence between species in seed produc- Th e fi eld study on seed dispersal was conducted on tion both per capsule (Fig. 2) and per shoot (Fig. 3). Th e two sites on Ö land, R ä lla (56° 46 ’ N, 16 ° 32 ’ E) and D ö rby average number of seed per capsule ranged between 1024 (56 ° 37 ’ N, 16 °39 ’ E) and included one of the species, P. chlo- ( O. secunda ) and 7882 (C. umbellata ). Th e actual seed pro- rantha. Both sites were on sandy podsol soils dominated duction potential of the Pyroleae species is however higher by old growth pine (Pinus sylvestris L.) with undergrowth for those species that have several capsules per shoot (all spe- mainly consisting of Vaccinium spp., Calluna vulgaris (L.) cies except M. unifl ora) (Fig. 3). Th e average seed produc- Hull, Linnaea borealis L., various grasses and mosses and tion per shoot ranged from 7324 (M. unifl ora) to 60 487 some deciduous trees like Quercus robur (L.) and Betula ( P. minor). Aborted seeds, containing no embryo, were pendula Roth. Th e method used was to place seed traps not included in the analysis but ranged between 16% for ( ‘ sticky tape seed traps ’ ) at various distances from a seed C. umbellata to 30% for P. chlorantha . source. Th e seed source consisted of mature capsule bear- Th e results concerning dispersal ability of Pyroleae dust ing shoots of P. chlorantha , with a mean of 41 shoots per seeds showed that the vast majority of seeds (82.5% of site (ranging from 33 to 56). Seed traps were placed around the total number of seeds deposited) were deposited close the center of each patch of P. chlorantha . Th e patch size was to the seed source, i.e. with a dispersal distance less than ca 0.5 m2 and the mean shoot height was 20.8 cm (rang- 1 m, and 95.7% were deposited within ca 5 m from the seed ing from 20 to 22 cm). Th e surroundings were checked for source (Fig. 4). additional fl ower stalks and if some were found these were collected and stuck down into the soft forest fl oor together with the main patch of shoots. Each sticky tape seed trap Discussion was constructed by wrapping masking tape around a petri dish (diameter 90 mm). Th e traps were placed along six Comparing the seed production of Pyroleae species with transects from each seed source. Transects were laid in the compilation from orchids made by Arditti and Ghani

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of potentially long-dispersing seeds may nevertheless be (Ericaceae): a fi eld manipulation and stable isotope approach. relatively high. – Oecologia 169: 307 – 317. In this study we have contributed information on seed Jacquemyn, H. et al. 2007. A spatially explicit analysis of seedling production and dispersal in a group of species for which recruitment in the terrestrial orchid Orchis purpurea . – New Phytol. 176: 448 – 459. these features are relatively poorly known. In order to gain a Jers á kov á , J. and Malinov á , T. 2007. Spatial aspects of seed disper- deeper insight into the population biology of Pyroleae spe- sal and sedling recruitment in orchids. – New Phytol. 176: cies one approach is to integrate studies on seed dispersal and 237 – 241. recruitment with identifi cation and mapping of host fungi, Johansson, V. A. and Eriksson, O. 2013. 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