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- Materials and methods
Dispersal and establishment are of fundamental importance for the persistence of metapopulations, especially for species restricted to patchy substrates of limited duration (Herben et al. 1991; Hanski & Hammond 1995). Many bryophytes are examples of species restricted to patchy, transient habitats such as dead wood or deciduous tree stems. They form ‘patch-tracking’ metapopulations (Thomas 1994; Snäll et al. 2003): if new patches are not colonized before the occupied patches disappear, the species will be lost from the site. Bryophytes establish new colonies either from spores or from asexual propagules or fragments. Establishment is usually more effective from vegetative diaspores than from spores (Keever 1957; Mishler & Newton 1988; Kimmerer 1991). However, spores are small and can be dispersed over long distances by wind (van Zanten & Pócs 1981). Decaying wood and trunks of old deciduous trees host many threatened bryophytes (ECCB 1995; Berg et al. 2002) and their rarity may be caused by the lack of substrate patches or by dispersal limitation. Rare species could have a narrower fundamental niche than do common species (Gaston & Kunin 1997) and, for bryophytes, substrate quality might be most important during the establishment stage (Brown 1982; Bates & Bakken 1998). We need to increase our knowledge about establishment probability under different environmental conditions to improve understanding of the distribution and rarity of species.
Moisture is vital for spore germination; phosphorus availability and pH have also been shown to be important (Thomas, Proctor & Maltby 1994; Sundberg & Rydin 2002; Wiklund 2003). Establishment may also depend on temperature and light. There may also be interactions among those factors so that the response to pH may depend on the availability of moisture. The challenge of laboratory studies is to mimic the range of moisture conditions the species may encounter in the field. However, the use of natural substrates poses a problem because they are often inhabited by fungi or algae, but sterilization can alter substrate chemistry. Culture media offer a practical alternative. Polyethylene glycol (PEG) is often used to maintain culture media at fixed water potentials. PEG is a long-chain inert organic polymer that alters the matric potential of the solution (Steuter, Mozafar & Goodin 1981). PEG makes it easy to perform factorial growth experiments with moisture and pH.
We studied two bryophyte species from contrasting habitats, which are patchy in space and time. Buxbaumia viridis (DC) Moug. & Nestl. is an epixylic moss growing on dead wood in late stages of decay; Neckera pennata Hedw. is an epiphytic moss inhabiting stems of deciduous trees. Both species are red-listed in Sweden (Hallingbäck 1998) and in Europe (ECCB 1995). Wood in late stages of decay remains wet after rainfall because of its large water-holding capacity and because of the production of water as a by-product of cellulose metabolism during decomposition (Rayner & Boddy 1988). Bark, however, is prone to desiccation, and wet periods may offer brief windows of opportunity for germination. Phillips (1951) suggested that, in bark-inhabiting bryophyte communities, an abundance of moisture can compensate for low pH, for example. Another study (Wiklund 2003) indicated a possible interaction between moisture, pH and phosphorus. Our aim was to investigate the combined effect of pH and moisture on germination and protonemal growth of the two species. An important question is how long a period of wetness is required for germination success, and if the required length of the period depends on the levels of moisture and pH. We hypothesized that the effect of pH on germination depends on moisture, and that habitat differences are reflected in properties of spore germination. Spores of N. pennata are dispersed during dry weather conditions, while spores of B. viridis are dispersed during rain (personal observations; Patterson 1953; Schofield 1985). In nature, spores often land on a dry substrate surface and have to wait for rain before they can germinate. We made a study to test the survival time of such dry-stored spores
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- Materials and methods
Although bryophytes are generally desiccation-tolerant (Proctor & Tuba 2002), spore germination is a critical stage which requires water. Our study shows that moss spores have the capacity to germinate at water potential of −2 MPa, a value at which most seeds fail to germinate (Battaglia 1997; Roman et al. 1999; Sy, Grouzis & Danthu 2001), but only if pH >5. The interaction between pH and water potential effects on germination suggests that high moisture facilitates germination at suboptimal pH, or vice versa. Further, our study demonstrates the effect of pH and water potential on the length of the lag phase preceding germination, and on the germination rate. This time effect is ecologically important as slow spore germination increases the risk of desiccation or disappearance of spores through wind or predation.
Our results reflect the habitat of the species. Buxbaumia viridis appears to be adapted to germinate at low pH, and the faster onset of germination in N. pennata at reduced water availability would enable it to establish on bark. These results agree with those of other studies (Cameron & Wyatt 1989; Bosley, Petersen & Rebbeck 1998). Although the two species normally inhabit contrasting microhabitats and responded differently to the treatments, they showed a common pattern: the three-dimensional plots of final germination percentage vs pH and water availability (Fig. 1c,f) clearly show a split between a tenable and an untenable region of the response surface. We suggest a general trade-off between the ability of moss spores to colonize substrates with low moisture-holding capacity and low pH, with the effect that substrates prone to fast desiccation (such as the bark of living trees) can be colonized only if they have a fairly high pH. Substrates with a high water-holding capacity, such as wood in late stages of decay, or peat, can be colonized despite low pH. Interactions between environmental factors, preventing seemingly suitable substrates from being colonized, are probably common in nature. Our study emphasizes the interaction between moisture and pH, but other interactions are possible. For instance an interaction between light and CO2 was found to affect growth in an amphibious liverwort (Andersen & Pedersen 2002). Temperature is another factor to consider, influencing both the length of the lag phase and final cumulative germination (Newton 1972; Furness & Hall 1981).
Establishment constraints have implications for the understanding of metapopulation dynamics. Both B. viridis and N. pennata occupy a limited fraction of seemingly suitable substrate patches (Hagström 1998; Kuusinen & Penttinen 1999; Wiklund 2002). Possibly, many of those patches are of a lower quality, with lower probability of being colonized.
The better survival of dry-stored spores in N. pennata, compared with B. viridis, supports the hypothesis that xerophytic species have more drought-resistant spores (van Zanten & Pócs 1981). Consequently, spores of N. pennata have a higher chance of surviving a prolonged period of drought after dispersal, thereby increasing the probability of establishment. However, spores of both species retained vitality for a short period compared with several other bryophytes (Malta 1922; van Zanten & Pócs 1981; Dalen & Söderström 1999). The results are, however, in agreement with a study of fern spores, where survival of green spores was 48 days, whereas yellow-brown spores remained viable for 3 years as a mean (Lloyd & Klekowski 1970), and both B. viridis and N. pennata have green spore mass.
Buxbaumia viridis and N. pennata are classified as vulnerable in Europe (ECCB 1995) as a consequence of a reduction of decaying wood and host trees in forests. Spores of both species, in particular N. pennata, germinated at a lower pH in the experiments than those at which they normally occur in nature (Wiklund 2003; unpublished data). One explanation could be the interaction with moisture. In the lower pH range, germination was possible only if water availability was unlimited, which would apply only for short periods in the field. Another explanation could be the stronger effect of decreased pH on protonemal growth than on germination. In N. pennata protonemal growth was significantly (P < 0·05) reduced at pH 5 compared with pH 6 and 7. A stronger pH effect on protonemal growth than on germination has also been observed in three Splachnum species (Cameron & Wyatt 1989). Spore germination is of fundamental importance for B. viridis, which is a short-lived moss and entirely dependent on establishment from spores, but is probably also significant for N. pennata. In central Sweden spores of B. viridis are dispersed in June, and prolonged periods without precipitation could be a problem because of short-lived survival of spores. Although B. viridis can tolerate low pH, the probability of establishment increases with increasing pH. According to the models, B. viridis requires at least 25 days to germinate and develop a five-cell protonema at pH 3, but only 11 days at pH 5, provided water is unlimited. Throughfall and litter from deciduous trees increase pH and nutrient availability (Nihlgård 1970; Nordén 1991) for species growing on logs and stumps beneath the trees. Hence favourable germination conditions could be secured by increasing the density of deciduous trees in coniferous forests (see also Wiklund 2003). Neckera pennata, like most epiphytic mosses with frequent spore formation, depends on short, wet periods (temporary windows of opportunity) to establish new colonies. If a spore of N. pennata reaches a tree with a surface at pH 7, according to the models it requires 3 days to form the first germ tube and another 7 days to form a five-cell protonema – in all, a period of 10 days with unlimited moisture availability. At pH 5 or 4 the wet period required increases to 15 and 50 days, respectively. Probability of establishment of N. pennata benefits from a high density of host trees to increase spore load and overcome dispersal limitation (Snäll, Rydin & Ehrlén, in press) and from the high frequency of deciduous trees with a high bark pH, preferably in moist areas, to overcome establishment limitation.
In conclusion, many plant species are dependent on recurrent colonizations of newly formed habitat patches. Metapopulation models focus on interpatch distances to explain colonization events, and our study adds that interactions of habitat factors can strongly affect the probability of establishment for a diaspore once it has reached the patch. For the conservation of threatened species with such patch-tracking metapopulations, both the spatial arrangement and quality of the patches must be considered.