Geographic reconnaissance of Western Australian Seashore vegetation

Abstract

Field study of 32 sites along the Western Australian coast showed that pioneer vegetation includes 110 species in 40 angiosperm families. Less than 25 % are exotic, only 10 % are endemic, and the bulk are shared with the interior or with other coasts of Australia. Regional ranges and microdistributions, described from herbarium records and 50 sample transects, are diverse. Although unique in detail, species distributions show broadly coherent relationships between geographic origins and present habitat patterns.

Sheltered intertidal shores have two discrete types of land vegetation. Mangroves, sea-dispersed outliers of a richer Indo-Malayan assemblage, form extensive estuarine forests on the tropical coast, where tidal ranges are large, and a few stands of one species occur along the temperate coast. Low-growing chenopods and similar halophytes, with major populations in saline interior basins, are scattered discontinuously along the coast in shallowly flooded tidal marshes. They are subject to no apparent climatic control within the region.

Rocky coasts are even more disjunct and also lack any unique, autochthonous vegetation. Wet pockets in the spray zone bear ordinary salt marsh plants while dry rock shores and outcrops in the dunes have vegetation differing mainly quantitatively from neighboring sand areas.

Open beaches and dunes offer the most extensive coastal habitats and bear the most complex vegetation. Species groups derived from different geographic sources are loosely segregated by zones parallel to the shore. Toward the rear of the active dunes, some plants are simply outrunners from the stabilized inland sand vegetation : these are important dune pioneers in the drier sectors. Generally, the active dune zone is dominated by species peculiar to it, some endemic but mostly shared with temperate coasts east of the study region. All are adapted to tolerate salt spray, sand blast, and other special habitat conditions. Sclerophylions shrubs predominate, grading into small trees on parts of the moister, tem~erate coasts. Prostrate succulents, rhizomatous grasses and sedges, and other types of evergreen perennials are minor components. The active dune vegetation is floristically richer and more regionally diversified towards its inner limits, the few species native to the outer dunes having comparatively wide coastwise ranges.

On the outer faces of the foredunes and on the storm beach, the otherwise almost purely Australian sand vegetation is joined by exotics. Sea-dispersed paleotropical and pantropical beach vines, grasses, and other low-growing plants range along the north coast and a few penetrate beyond the tropic. A different, primarily inland group of old immigrants may have arrived indirectly via the arid interior to provide the extreme outpost vegetation of the drier coasts. Along the moister, temperate coasts, a belt seaward of the native dune vegetation is occupied by recently naturalized annuals and other low-growing herbs from temperate African and European beaches.

Except close to roads and settlements, little recent modification of the coastal vegetation is evident. Some blowouts, particularly on the drier margin of the tem- perate region, may stem from dune burning, but generally the plant cover regenerates rapidly after fire.