Distribution of Bryophytes on Subantarctic Heard Island

Abstract

Heard Island is a small, isolated subantarctic island dominated by an ice covered volcano. Glaciers descend the volcano to the sea. Coastal ice free areas of tundra are present separated from each other by glaciers. Each of these areas can be considered as individual islands and Heard Island an archipelago. The distribution of 49 bryophyte taxa across five major and four minor ice-free areas was examined. Fifty percent of the bryophyte taxa exhibited widespread distribution, occurring on all ice-free islands examined. Absence of some conspicuous species from particular areas appeared to reflect situations where limited habitat variation has restricted the establishment of species. With present global warming, glacial retreat on Heard Island is rapid. Consequently, areas suitable for colonization by plants are increasing and previously separated ice free areas are now merging. This study provides baseline data against which future studies will be able to investigate possible changes in bryophyte species diversity and distribution on Heard Island. Subantarctic Heard Island (53?05'S, 73030'E) is approximately 4,850 km SE of southern Africa, 4,350 km SW of Western Australia and 1,650 km N of the Antarctic continent. The island is an emergent portion of a mid-ocean ridge with a mixed geology of pelagic limestones and volcanics (Clarke et al. 1983). A large active volcano 2,475 m in elevation and 20 km in diameter dominates the landscape (Figs. 1, 2a). Heard Island is 42 km long and has a surface area of 385 km2. Glaciation on Heard Island may have occurred as early as the Miocene (Quilty et al. 1983) and the island's position south of the oceanic Antarctic Convergence would have ensured major glaciation events during cool episodes of the Pleistocene. Today, Heard Island is severely glaciated and has been continuously since the Last Glacial Maximum (approximately 18,000 years ago, Climap 1976). Allison and Keage (1986) estimated that in 1980 approximately 80% of Heard Island was ice covered, with glaciers descending from 2,400 m to sea level. Ice free areas are coastal and rarely exceed 400 m in elevation. As tidewater glaciers separate these ice-free areas from one another, the ice free areas can be considered analogous to individual islands, and Heard Island analogous to an archipelago. With current climate warming, glacial retreat is occurring rapidly (Scott 1990), with some ice-free areas coalescing. Heard Island has a remarkably constant temperature regime both diurnally and yearly, moderated by the surrounding ocean. Mean annual temperature is 1.2 oC, with a seasonal range of 4.5 oC. Rain or snow falls on most days of the year and the island is severely battered by wind storms, mostly from NW to SW directions (Allison & Keage 1986). Approximately 5% or 20 km2 of the total surface area of Heard Island is covered by substantial vegetation. Herbaceous flowering plants and bryophytes are the major components of the vegetation. The flora is small, with only 11 flowering plant species and 58 bryophyte species recorded from the island (Bergstrom 1993; Bergstrom & Seppelt 1988; Scott 1989). Much of the coastal ice-free areas are covered by some of the most extensive angiosperm cushion vegetation on Earth. Bergstrom (1993), Hughes (1987), Jenkin (1980), Whinam (1989) have examined the vegetation on the island. Bergstrom (1993) identified five major vegetation types which are summarized and illustrated in Table 2 and Fig. 2. Continua and variations within the vegetation types can be found. Bryophytes are a major component of many of the vegetation types and in many cases, such as in closed bryophyte vegetation on shallow stable ground (Fig. 2E), are the dominant component. The geological history of the island necessitates that all members of the biota reached the island via some form of long distance, trans-oceanic dispersal. Bergstrom and Seppelt (1988) identified five phytogeographical elements amongst moss species from the island. These were species restricted to the subantarctic, species found in southern South America and subantarctic islands, southern hemi0007-2745/97/349-355$0.85/0 This content downloaded from 157.55.39.6 on Mon, 19 Sep 2016 05:53:17 UTC All use subject to http://about.jstor.org/terms 350 THE BRYOLOGIST [VOL. 100 I I I 731 5'E 73.30'E 73A45'E