Porsild's bryum (Haplodontium macrocarpum) COSEWIC assessment and status report: chapter 6

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Biology

Reproduction

Despite its dioicy, Mielichhoferia macrocarpa produces sporophytes in most populations (Cleavitt 2002a, Brassard & Hedderson 1983). Sporophytes occurred on 10.7% of the mapped colonies with high variation between sites (Cleavitt 2002a). Spore germination for M. macrocarpa on agar was 55.7±4.1% (mean±stdev), and no spores germinated on natural substrate. Gametophore growth from protonema was very low. Further experimentation on reproduction from spores is needed for this species.

Cleavitt (this report) also observed asexual reproduction by this species. The leaves of the original gametophyte fragment always go chlorotic and then secondary protonema grow out from the red stem. The protonema produce rhizoids and gametophore buds. Direct sprouting from the stem has also been observed, but it is less common than regeneration via secondary protonema (Cleavitt 2002a). Evidence for occurrence of asexual reproduction was provided by one population of genetically homogeneous male plants found in the study area (Cleavitt unpbl. isozyme data). Mielichhoferia macrocarpa had significantly lower regeneration than five other moss species with only 25% (±30) fragments established in the field and 8% (±7) established under growth chamber conditions (Cleavitt 2002a).

Survival

Short-term (three-year) monitoring of three Alberta populations in the Cadomin vicinity has revealed that 52.9%(±6.15) (mean±stdev) of the colonies grew from 1997 to 2000. In the same period, 13.6%(±3.82) remained the same size, 18.8%(±8.01) became smaller and/or died, and 14.7%(±4.41) disappeared from the cliff. There was also evidence of replacement for lost or dead colonies as several new colonies were also found in these populations in 2000.

Mielichhoferia macrocarpawas compared to a common congener, Bryum pseudotriquetrum, in terms of colony growth and survival of reciprocal colony transplants. Percentage of expanded colonies for M. macrocarpa was higher than for B. pseudotriquetrum (Cleavitt 2002a). However, Bryum pseudotriquetrum transplants had higher survival than Mielichhoferia macrocarpa at both M. macrocarpa and B. pseudotriquetrum sites. Mielichhoferia macrocarpa does not survive transplantation well (Cleavitt 2002a). Reconnaissance work on populations in 2002 revealed the susceptibility of M. macrocarpa populations to disturbance by drought and ice scouring. These natural and unpredictable disturbances strongly decrease survival of individual colonies, but long-term herbarium records suggest that the populations on the whole are resilient.

Physiology

The physiology of this species is quite complex and cannot be inferred from habitat data (Cleavitt 2002b). As noted previously, the species is physiologically restricted to basic substrates (Cleavitt 2001). Mielichhoferia macrocarpa had significantly higher photosynthetic yield (a proxy for the efficiency of photosynthetic machinery) than the common B. pseudotriquetrum (Cleavitt 2002b). In an experimental comparison between six moss species, M. macrocarpa had the slowest rate of photosystem recovery (50 minutes to reach ½ pre-treatment levels) after rehydration of plants that had been subjected to three days in a dry state. However, within 24 hours the colonies did recover to levels not significantly different from pre-drying levels and constantly hydrated control samples (Cleavitt 2002b). The ability to recover from desiccation was greater when plants were desiccated as colonies rather than as fragments (Cleavitt 2002b).

The temptation to infer physiological tolerance from habitat characteristics should be avoided. Although Mielichhoferia macrocarpa occurs in wet, dark sites, the species was not physiologically limited by either desiccation tolerance or light levels (Cleavitt 2002b). This finding is somewhat counter-intuitive given that there is ample evidence for a correlation between habitat moisture regime and bryophyte desiccation tolerance (Brown & Buck 1979, Seel et al. 1992, Oliver et al. 1993, Deltoro et al. 1998, Csintalan et al. 1999, Robinson et al. 2000). This departure from the usual relationship between habitat and desiccation tolerance may be explained if habitat moisture regimes are more rigorously classified. For instance, although M. macrocarpa occurs at sites that are hydric throughout the growing season, these sites dry out in autumn when the seep water freezes and remain dry without protection from snow cover until late spring/early summer (pers. obs.). Therefore, this species would be expected to possess some type of desiccation tolerance.

There are two types of desiccation tolerant plants, poikilochlorophyllous and homoiochlorophyllous, and both types occur in bryophytes (Tuba et al. 1998). Poikilochlorophyllous bryophytes experience breakdown of their chlorophyll in response to drying-wetting cycles and survive in habitats that are generally mesic and slow drying such that drying-wetting cycles tend to be both longer in duration and less frequent (Oliver et al. 1998, Tuba et al. 1998). Homoiochlorophyllous bryophytes retain their chlorophyll through drying-wetting cycles and occupy more xeric, exposed habitats that experience more frequent, brief, rapid drying events; however, even for these mosses fast desiccation leads to a prolonged recovery period (Oliver et al. 1998). Therefore, the frequency, rate and duration of habitat drying throughout the year are important in accurately describing the relationship between desiccation tolerance and moss habitats (Oliver et al. 1993, Oliver et al. 1998, Tuba et al. 1998). Based on the facts that Mielichhoferia macrocarpa is desiccation tolerant and that it occurs in habitats which dry out infrequently for long periods of time, this moss is most likely poikilochlorophyllous. This hypothesis is also supported by M. macrocarpa's relatively slow rate of recovery in photosynthetic yield after rehydration (Cleavitt 2002b).

Movements/dispersal

Cleavitt (2002b) investigated the dispersal ability for gametophyte fragments of this species via air and water. Mielichhoferia macrocarpa had higher fragment viability after storage in air rather than water after four months, but there was no difference between fragment viability in air versus water after only one month (Cleavitt 2002b). The likelihood of water transport for this species depends on the number of vertical rock seeps encountered by a waterway along which it occurs. The unexpected high viability of fragments stored dry makes transport by wind, especially during the winter, another plausible mode of asexual dispersal for M. macrocarpa. Unpublished evidence for M. macrocarpa indicates that this species can establish at suitable, but unoccupied field sites. However, the potential for M. macrocarpa to increase its area of occupancy is hampered by the apparent inability for successful dispersal. Population genetic studies would greatly increase our understanding of dispersal in this species.

Interspecific interactions

There have been no experimental tests of the importance of interspecific interactions for this species. Because vascular plant cover in Mielichhoferia macrocarpa habitats is negligible, the most likely competitors are other bryophytes. In scoring neighbor contact for several moss species, M. macrocarpa had relatively lower frequency of neighbor contact than the common B. pseudotriquetrum (Cleavitt 2002a). By the same method, M. macrocarpa had a relatively low number of encounter losses (times when it was overgrown by another species) suggesting that competition may play a relatively small role in the persistence of this species at a site. However, experimental evidence is needed to verify this hypothesis.

From additional plot data comparing attributes of suitable occupied and unoccupied sites for Mielichhoferia macrocarpa, we know that the species was absent from sites with higher percent cover of other moss species. Sites with M. macrocarpa had a higher percentage of bare rock (73%±28) than sites without M. macrocarpa (17%±22) (Cleavitt, unpub. data). Together these results point to the importance of competition in determining where M. macrocarpa will establish rather than affecting its continued persistence at a site where it currently exists.

Continued exploration into ecological limitations of Mielichhoferia macrocarpa should include the relative effects of environmental parameters and neighbors on the rates of establishment from gametophyte fragments and spores and subsequent colony expansion. The most important point is that this species has great difficulty establishing new populations, but seems capable of long-term local persistence once it has established. Therefore, habitat preservation is crucial and transplantation of populations is not recommended.