Allosyncarpia‐dominated rain forest in monsoonal northern Australia

Abstract

AbstractFollowing recent classifications of rain forest vegetation in northern Australia this paper examines the biogeo‐graphical status and condition of a rain forest type endemic to that region, dominated by the sclerophyllAllosyncarpia ternata (Myrtaceae).These forests are restricted to the Arnhem Land region of the Northern Territory, which includes Kakadu National Park. They cover an area of 1138 km2, or 41 % of all rain forest in northern and northwestern Australia. DCA of floristic data from 140 sites illustrates thatAllosyncarpiaforests occupy a range of sandstone‐derived substrates, from moist valley sediments to steep, freely draining, rocky sites. DCA of floristic transect data illustrates thatAllosyncarpiais by far the dominant canopy species over this topographic‐moisture sequence, but especially on seasonally dry substrates where it provides over 80 % basal area and effectively the entire canopy. DCA of floristic quadrat data from a floristi‐cally singular site illustrates major death of the fire‐sensitive gymnospermCallitris intratropica, and, to a lesser extent,Allosyncarpiaitself, on the forest‐savanna boundary.Biogeographical implications arising from the Gondwanic distributions ofAllosyncarpiaand its close relatives,Arillas‐trum(New Caledonia),Eucalyptopsis(eastern Malesia), and an as yet undescribed taxon (eastern Australia), suggest that taxa ancestral to this group were extensive in the late Cretaceous. Their current restriction is in marked contrast to the success of their near relatives, the eucalypts. Given the tolerance ofAllosyncarpiato a wide range of substrate moisture conditions in the present day, it is argued that fire regulates patch margins of this forest type. Although tolerant of light fires, canopy trees at patch margins are susceptible under a regime of frequent, intense late dry‐season fires, such as are prevalent in Arnhem Land today. For effective conservation of fire‐sensitive communities in this floristically significant region, greater attention must be given to management and monitoring of the fire regime.