B. A. Wilson

Response of Eucalyptus forest and woodland to four fire regimes at Munmarlary, Northern Territory, Australia

Abstract: (1) A Eucalyptus tetrodonta forest and a Eucalyptus confertiflora woodland were found on different soils in the coastal plains of the Northern Territory. Within the forest and woodland the understorey communities were correlated with slight changes in soil characteristics. (2) Experimental annual early and late dry-season, and biennial early-season fires resulted in little floristic or structural differences after thirteen years of treatment in the forest and twelve years in the woodland. (3) Protection against fire resulted in the development of a dense understorey in both the forest and woodland. The understorey in the forest was dominated by two species of Acacia, and Erythrophleum chlorostachys. In the woodland the understorey was dominated by Acacia holosericea and two broadleaf species. (4) Grass cover was lowest in the forest and highest in the woodland sites burnt late in the dry-season. Grass persisted on the unburnt treatments. An inverse relationship between shrub cover and grass cover was apparent in the woodland but not in the forest, possibly because of the greater abundance of broadleaf species in the woodland. (5) There was no systematic response of individual species to burning. Of the species which showed a significant change in cover, 36% in the forest and 47% in the woodland also showed a significant interaction with experimental block. This interaction between fire-treatment and site was also apparent in the different responses to treatments by species which occurred in both the forest and woodland. (6) The results of this experiment differ from those of similar studies in African savannas where rainforest invades protected plots and grass cover is dependent upon frequent burning. (7) It is concluded that the Eucalyptus communities have had a sufficiently long history of dry-season burning for vegetation patterns to be primarily determined by edaphic factors.

Fuel characteristics of coastal monsoon forests, Northern Territory, Australia

Fuel characteristics of four forest types (Eucalyptus forest, Callitris forest, Mixed Eucalyptus/monsoon thicket, Pure monsoon thicket) with distinct fire regimes, were compared. The Eucalyptus forest had the highest frequency of fires (2-3 years interval). It had a sparse litter layer with a low moisture content but the greatest biomass of grass fuels. These grasses had rapid rates of flame spread, variable sustainabilities and low energy contents. Callitris forest has a small grass component and the lowest mass of fuel of the four sampled communities. The Callitris forest litter was more moist than the Eucalyptus forest, possibly because of its greater canopy cover. Of the infrequently burnt communities the mixed Eucalyptus/Monsoon thicket had more evidence of previous fires than the adjacent Pure monsoon thicket. Both communities has similar loads of energy rich, flammable litter, but differed in the positively related measures of little moisture content and canopy cover. Moisture content of the litter before the wet season was one half that after the summer rains. With the exception of the grasses there was no corresponding decrease in the flammability of individual fuel components. The interrelated measures of litter moisture, fuel density and abundance of grass appear to best explain the differences in the fire regimes of the four communities studied. The lack of corresponce between community fire-regime and measures of fuel flammability suggest that there has been no natural selection for fire-promoting characteristics in northern Australian vegetation, perhaps reflecting the extremely high frequency of natural and anthropogenic fires. Circularity of the hypothesis that vegetation evolves characteristics to promote fire makes validation or rejection extremely difficult.

Conservation of coastal wetlands of the Northern territory of Australia: The Mary River floodplain

Abstract The seasonally inundated coastal floodplains of the Northern Territory contain Australias largest relatively unmodified wetlands. Past exploitation has been confined chiefly to extensive pastoralism, but that situation is changing rapidly. We report a floristic survey of the Mary River floodplain, and compare among river systems the vegetation pattern and factors influencing it. Management and conservation implications of changing land use on the Mary River and elsewhere are examined. Flora is cosmopolitan and plant communities relatively simple. Water depth and persistence of floodwaters determine vegetation patterning. Distribution and relative abundance of vegetation types within and between plains is influenced by an interaction between erratic rainfalls and inter-plain variation in topography and hydrology. Catchment differences in soils and hydrology add to inter-plain variation. But the conservation significance of the floodplains is founded neither in floristic uniqueness nor unusual plant communities. In combination the plains reliably provide large areas of favourable wildlife habitat. Diversity is maintained despite erratically variable rainfalls by temporal and spatial shifts of favoured habitats among and within river systems. Sustained diversity maintains larger populations than could persist under a uniform vegetation response to below average falls. Increasingly manipulative land management practices, introduced plants and saltwater intrusion threaten to reduce the total area of unmodified freshwater habitat, weakening the resilience of the ecosystem to natural environmental perturbation such as delayed or reduced rainfalls. The nature of the changes demand that ‘passive’ conservation measures such as creation of reserves be supplemented by active management practices. An ecosystem-wide approach to management is required to achieve conservation goals, especially maintenance of habitat diversity.

Monsoon forests in Northwestern Australia I. Vegetation classification and the environmental control of tree species

An indirect gradient analysis was performed on woody species basal area data collected from 144 monsoon forests situated throughout northewestern Australia. In order to eliminate edge effects each monsoon forest was differentiated from surrounding vegetation by inspection of sorted lists of tree species recorded from quadrats

Factors influencing tree growth in tropical savanna: Studies of an abrupt Eucalyptus boundary at Yapilika, Melville Island, northern Australia

(10 \times 20 m)

Preliminary Biogeographic Analysis of the Northern Territory Vascular Flora

placed on a transect that traversed each forest boundary. On each transect the monsoon forest quadrats were amalgamated to yield a single averaged sample of tree species basal area and environmental characteristics to represent each fo the 144 sampled forests. Surface soil physical and chemical characteristics were related to compositional patterns of tree species abundance data in one-, two-and three-dimensional Hybrid Multi-dimensional Scaling ordination solutions. The unidimensional ordination recovered a major environmental gradient where dry fertile soils, often on rocky slopes occur at one extreme and infertile waterlogged soils charcterize the other. Mean basal area of the fifty species with the highest importance values showed a continuously varying sequence of unimodal curves over the unidimensional ordination axis, although individual species distribution and abundance showed considerable variation on the gradient. Forest biomass, as measured by mean canopy height and mean total basal area, increased linearly over the gradient. With the exception of the dry extreme, stem density was relatively uniform across the gradient. Both the wet and dry extremes of the gradient have lower species richness per

Sandstone vegetation pattern in the Jim Jim Falls region, Northern Territory, Australia

200 m^2

Fire, storm, flood and drought: The vegetation ecology of Howards Peninsula, Northern Territory, Australia

and fewer species with peaks of abundance. There appears to be a relatively sharp environmental and physiognomic discontinuity along the gradient as indicated by surface soil moisture content, and the cover of ferns and palm regeneration. This beak is consistent with a primary dichotomy of wet and dry forest formations as derived by other floristic and structural classifications of this vegetation. Of the common tree species (149), 53% of the dry forest species (125) only occurred in that community, while 29% of the wet forest species (83) were restricted to the wet forest.

A transect study of the Eucalyptus forests and woodlands of a dissected sandstone and laterite plateau near Darwin, Northern Territory

Most of the land surface of Melville Island, Australias second largest island, is covered in Eucalyptus savanna. One exception is an area at Yapilika where a large tract of savanna is dominated by Acacia shrubs. An ordination analysis of 122 quadrats revealed that the boundary of Eucalyptus dominance did not correspond to a major change in floristic composition. Detailed transect studies at one site on the boundary showed that Eucalyptus trees were abruptly replaced by a band of Grevillea trees which gradually gave way to Acacia shrub dominance. There was a gradual change in the floristic composition of the savanna across the boundary. The distributional limit of Eucalyptus was found to be independent of any hydrological discontinuity. There was a slight decrease ( -0.9 MPa) during the dry season. Over a 12 month period Eucalyptus tetrodonta and E. miniata seedling growth was not significantly different on the Acacia or Eucalyptus savanna, although this result may be due to the counteracting effects of greater soil fertility and tree competition in the Eucalyptus savanna and lower soil fertility in the treeless, and hence competition-free, Acacia savanna. This hypothesis is supported by the significantly greater growth of Eucalyptus seedlings on fertilized Acacia savanna soils. The limited production, dispersal and establishment of Eucalyptus seeds and the greater frequency of fires in the Acacia savanna probably explains the abrupt limit to Eucalyptus dominance along the edaphic gradient.