MAP OF STRUCTURAL VEGETATION TYPES AND DRAINAGE ON SUBANTARCTIC MACQUARIE MAP OF STRUCTURAL VEGETATION TYPES AND DRAINAGE ON SUBANTARCTIC MACQUARIE ISLAND P.M. Selkirk and D.A. Adamson School of Biological Sciences, Macquarie University, N.S.W. 2109, Australia INTRODUCTION The vegetation of Macquarie Island reflects its isolation in the Southern Ocean, low average summer temperature, extremely strong winds, and the island's geology and hydrology. The island, 34km north-south by 2-5km east-west, comprises an undulating plateau bounded by steep coast-facing slopes and cliffs. Drainage on the island is complex and, to date, poorly mapped. The balance between surface and underground flow of water has profound effects on streams, lakes and on the distribution of structural vegetation types. Isolation and Flora. At 54° S 159° E Macquarie Island is approximately 1500km south east of Tasmania, 1000km south west of New Zealand. It is one of only seven subantarctic islands or island groups close to the Polar Front in the Southern Ocean. It has never been part of or close to a continental land mass. Its entire flora has arrived by long distance, over-ocean dispersal. The vascular plant flora is small, 46 species (Hnatiuk 1993), and shows strong similarities to the floras of other subantarctic islands, to New Zealand and , less strongly, to Australia (Selkirk et al 1990). Non-vascular plants constitute an important part of the Macquarie Island flora: 83 species of mosses, 51 species of liverworts (Selkirk et al 1990), 141 species of lichens (Kantvilas and Seppelt 1992), at least 400 species of non-marine algae (T.P. McBride pers. comm.). Low average summer temperature. Substantial woody growth is commonly found where the average temperature for three summer months exceeds 10° C. In common with other subantarctic islands, summer temperature on Macquarie Island (6.7° C mean 1948-86, Jacka, Christou and Cook 1984) is such that little woody growth is produced. The largest terrestrial plants, the non-woody tussock grass, Poa foliosa and the forb Stilbocarpa polaris, reach about 1.5m tall. Most plants are non woody and shorter than about 0.3m, but three species, Coprosma perpusilla, Acaena magellanica and A. minor, produce woody creeping stems up to about 0.5cm diameter. Strong winds. The severe climatic conditions result in a dynamic interaction between plants and landforms. Wind, snow, ice and water produce active erosion even on low slopes. Complex vegetation patterns occur over small distances, for example areas with high percentage of bare ground (feldmark) adjoin areas where the vegetation cover is complete. Geology. In the northern third of the island is a large area where the extremely sparse vegetation is explained by the presence of rock types which produce soil unfavourable for plant growth (Adamson et al 1993). Most of the southern two thirds of the island is a thick pile of pillow lavas and pyroclastic rocks whose intense fracturing promotes rapid infiltration of water. Bedding and faulting influence the flow of groundwater. As a result, areas of groundwater intake are generally well drained, while areas of outflow have abundant mires. Hydrology. Surface and underground flow of water is strongly influenced by geological features. Fault-bounded escarpments frequently control the direction of streams and location of lakes (Ledingham and Peterson 1984). Faults and fractures channel groundwater. Vegetation in lakes includes Myriophyllum triphyllum, various bryophytes, and abundant algae including at least 250 species of diatoms. Bare ground. On steep slopes between the plateau (altitude 100-400m a.s.l) and the coast, and on the plateau itself, bare ground is common. Strong wind, rain, sleet, and snow cause abrasion of plants and the bare ground. The progressive accumulation of peat, even on gentle slopes, results in mass movement of soil and its vegetation (Selkirk 1996). The resultant scars are subject to accelerated erosion and slow colonisation (Scott 1985). Nearshore marine vegetation. Offshore, the algal vegetation is dense. Brown algae grow to many metres in length. Southern kelps, particularly the bull kelp, Durvillaea antarctica, from the high tide limit to more than 15m depth, and Macrocystis pyrifera, form impressive masses of floating fronds, particularly prominent along the sheltered eastern coast. The great size of individual marine algal plants contrasts with the short stature of the terrestrial vegetation. THE MAP This map depicts, at a scale of 1:25 000, distribution of structural vegetation categories, with emphasis on height of the vegetation above the ground, and its density, defined as % of the ground surface covered by plants. The map does not show the distribution of species, nor does it attempt to establish and then map plant associations or comparable categories which reflect communities of plant species. Taylor (1955) mapped the vegetation of the whole island at small scale (approx. 1:63000, i.e. 1 inch to the mile), and the northern quarter at approx. 1:22000, using four essentially structural categories: wet tussock, subglacial herbfield, feldmark, bare ground. In the text accompanying the map he took a phytosociological approach, dividing the Macquarie Island vegetation into five main formations: grassland, herbfield, fen, bog and feldmark. Within each formation he described one or more alliances, associations and sub-associations. Since his pioneering work some aerial photography and satellite imagery have become available, much additional field work has been done, and a fuller assessment is now possible of environmental controls. Copson (1984) plotted the distribution of vascular plant species for the whole island on a 1 km x 1 km grid, showing the wide distribution of many species. In preparing this map, we have used as a basis the SPOT 3 merged pan/multispectral image of 22 December 1994 and the coastline prepared from this image by AUSLIG. We have consulted the black and white aerial photography of 6 and 7 February 1976, runs CAS 5465, CAS 8467 to 8473 and CAS 8480 to 8483. We have drawn extensively on our fieldwork and on our own and colleagues' field notes and photographs. Although the present map is more detailed and at a larger scale than previous studies, it is preliminary. Its most important role is as a basis for more detailed work in the future. Detailed work is probably best carried out on carefully selected small areas of the island where mapping and other data gathering can be done at high resolution. Knowledge of the present vegetation is needed to assess the consequences of environmental change including the effects that the planned eradication of cats will have on burrow-nesting bird and rabbit populations, and in turn on vegetation. The topographic map of Macquarie Island (Division of National Mapping 1971) is inaccurate with respect to the drainage system of lakes and streams (Selkirk and Adamson 1995). The map prepared by Blake in 1911-14 (Mawson 1943) is more accurate than the 1971 map. The geological maps of Macquarie Island (Tasmanian Geological Survey 1998) include more detailed representations of drainage than Blake's map or the topographic map. Although less detailed in places than the geological maps, the present map is in our view, more accurate in locating some stream and stream-lake connections. Definitive mapping of streams on Macquarie Island is yet to be done. CATEGORIES AND THEIR TERMINOLOGY Vegetation mapping categories were chosen to indicate foliage density and foliage height, similar to the scheme of Specht (Specht et al 1995), now widely adopted throughout Australia when mapping vegetation structure (AUSLIG 1990). It is appropriate to adopt Specht's boundary of 70% foliage projective cover of the ground in order to establish two categories, closed vegetation (foliage projective cover greater than 70%) and open vegetation (foliage projective cover less than 70%). Field checking showed that the SPOT imagery (22 December 1994) is a valuable aid in delineating boundaries between closed and open vegetation. Vegetation whose foliage stands higher than 0.4 to 0.5m above the ground is designated tall vegetation and it can be distinguished from short vegetation whose foliage is less than 0.4 to 0.5m in height. The term herb is applied to vegetation containing herbaceous , i.e. essentially non-woody, species which includes monocotyledons (such as grasses and graminoids), dicotyledons (such as forbs), and ferns. The terms grass and grassland are not used, first because forbs and graminoids (such as sedges that are not in the grass family) are abundant in almost all areas of short herb vegetation, and second because in places where the tall tussock grass Poa foliosa is dominant it often forms complex mosaics with the large forb Stilbocarpa polaris so that this vegetation is categorised as closed tall herb vegetation, rather than tussock grassland. macca_veg_map_doco.html[9/10/2015 12:02:29 PM] MAP OF STRUCTURAL VEGETATION TYPES AND DRAINAGE ON SUBANTARCTIC MACQUARIE The general term mire is applied to vegetation of waterlogged areas, in preference to the more restrictive terms bog or fen whose correct usage depends upon knowing attributes such as pH. Each mapping unit identifies areas which, when examined at greater resolution, prove to be complex mosaics, some features of which are described below. Vegetation cover 70% or greater (closed vegetation). Closed tall herb vegetation, northern and eastern coast. The tall herbs are mainly the tussock grass Poa foliosa and the forb Stilbocarpa polaris but in restricted areas in valleys draining into Sandy Bay and Green Gorge, thickets of the fern Polystichum vestitum also occur. Poa foliosa and S. polaris may occur in almost monospecific stands, but they commonly occur together. The tall vegetation is absent on gentler slopes near the coast where drainage is poor, and on unstable surfaces such as active scree, outwash fans, and crumbling rocky outcrops. Closed tall herb vegetation also occurs on the western and southern steep coast-facing slopes below the plateau, but there it is included in the category 'vegetation on western and southern coast-facing slopes'.