Environmental Determinants of Allosyncarpia ternata Forests That Are Endemic to Western Arnhem Land, Northern Australia

Abstract

Allosyncarpia ternata S. T. Blake dominates closed forests on rugged sandstone escarpments on the western edge of the Arnhem Land Plateau, northern Australia. The forests occur in a floristic continuum between fire-protected wet monsoon forests in the base of canyons and frequently burnt, eucalyptdominated savannas that occur in all other topographic positions. An indirect gradient analysis of 69 quadrats from these three vegetation types at six different localities showed that no measured edaphic variable was correlated with this floristic transition. A detailed study of a single patch of Allosyncarpia on level terrain with rock-free, sandy soils showed that Allosyncarpia trees occur on sites with significantly deeper soils and higher concentrations of available K in the surface soil compared to surrounding eucalypt savannas. However, there was no significant difference in dry season surface soil moisture content between these communities. There is also evidence that there is no significant difference in subsoil moisture supply. Xylem pressure potential of Callitris intratropica (which is equally abundant in both Allosyncarpia and eucalypt communities) was found to be statistically similar at the beginning and end of the dry season. The Allosyncarpia forest was made up of a mosaic ranging from stands co-dominated by savanna species with grassy understoreys to stands co-dominated by monsoon forest species with dense understoreys. No measured environmental factors were significantly related to the patterning of these stands within the forest. Seed throw of Allosyncarpia is limited to several metres from the canopy edge and seedlings were only observed beneath the canopy. Field experiments demonstrated that seedling survival in the savanna can be enhanced by the provision of shade, and nursery experiments demonstrated that the growth of seedlings provided with ample water is suppressed by full sunlight. Dry season fires in grass fuels ranging from 2 to 8 t ha-1 were found to be lethal to seedlings <100 mm tall. Although most Allosyncarpia trees recover following fire damage, the present distribution of the species may be best explained as a consequence of wildfires. However, there is need for confirmation that the species range is contracting under current fire regimes. This is probably best derived by analysis of existing remote sensing data.