Donald C. Franklin

Savanna vegetation-fire-climate relationships differ among continents.

Surveying Savannas Savannas are structurally similar across the three major continents where they occur, leading to the assumption that the factors controlling vegetation structure and function are broadly similar, too. Lehmann et al. (p. 548) report the results of an extensive analysis of ground-based tree abundance in savannas, sampled at more than 2000 sites in Africa, Australia, and South America. All savannas, independent of region, shared a common functional property in the way that moisture and fire regulated tree abundance. However, despite qualitative similarity in the moisture–fire–tree-biomass relationships among continents, key quantitative differences exist among the three regions, presumably as a result of unique evolutionary histories and climatic domains. Evolution cannot be overlooked when aiming to predict the potential global impacts on savanna dynamics in a warming world. Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.

Seasonal use of savanna landscapes by the Gouldian finch, Erythrura gouldiae, in the Yinberrie Hills area, Northern Territory

The diet, attributes of feeding sites and patterns of seasonal movements of a population of the Gouldian finch, Erythrura gouldiae, were studied in the Yinberrie Hills area north of Katherine in the Northern Territory. In the dry season (April–November) Gouldian finches foraged mostly on burnt ground and fed on exposed seed of annual grasses, especially seed of spear-grass, Sorghum spp. In the wet season (December–March) Gouldian finches fed on seed of a sequence of perennial grass species, including Themeda triandra, Alloteropsis semialata, Chrysopogon fallax and Heteropogon triticeus. Gouldian finches undertake regular seasonal shifts in habitat, from breeding areas in hill woodland in the dry season to adjacent lowlands throughout much of the wet season, in response to seasonal changes in food availability. There is an annual pulse in abundance of fallen seed in the early dry season that is depleted to near zero levels by germination of annual grasses early in the wet season. Thereafter, finches depend on seed from other sources, principally ripe and ripening seed of perennial grasses. Observations over three successive wet seasons suggest that Gouldian finches track seed resources provided by seeding perennial grasses over an extensive area of lowland grassy woodland adjacent to the breeding area, favouring small patches of grassy woodland for brief periods until seed fall. There were subtle differences between years in the types of resources used. Management of Gouldian finch populations will entail protection and management of the full range of grassland habitats used throughout the annual cycle, and will require predictive knowledge of the causes of patterning of seed resources and probably an ability to exert control over the timing and extent of fires in fire-prone seasonal savanna landscapes.

Birds and Nectar in a Monsoonal Woodland: Correlations at Three Spatio-temporal Scales

The relationship between avian nectarivores and the availability of nectar was explored at three spatio-temporal scales over 23 weeks in tropical woodland near Darwin, Australia. Nectar was available in the study area throughout this period but its distribution varied spatially as the study progressed, and total availability was estimated to fluctuate 60-fold, being superabundant from mid-June to mid-August. The nectarivore community comprised 11 specialist nectarivores (two lorikeet and nine honeyeater species) and a variety of opportunists. The abundance and biomass of specialist nectarivores fluctuated five-fold and were positively correlated with nectar availability, but most of the fluctuation was due to just one species, the Little Friarbird, and at least six species were present in the area throughout the study period. Within the study area, the spatial distribution of all specialists combined was always positively correlated with nectar availability, but the correlation was weak and non-significant when nectar was most abundant. Correlations suggest that the Rainbow Lorikeet and Little Friarbird tracked nectar availability, but did so at different spatial scales. Evidence is presented that opportunists occupied niches left available by specialist nectarivores during the period of nectar abundance. The ‘failure’ of the specialist nectarivores to fully exploit the period of abundance may be related to the widespread availability of nectar in northern Australia during the middle of the dry season.

A comparison of the diet of three finch species in the Yinberrie Hills area, Northern Territory

Abstract The diet of three finch species, the Gouldian Finch (Erythrura gouldiae), the Long-tailed Finch (Poephila acuticauda) and the Masked Finch (Poephila personata) was studied at a site in the monsoonal tropics of northern Australia from June 1995 to May 1997. Seeds of native grasses dominated the food of all three species. There was seasonal variation in the diet of all three species, with seeds of annual grasses being important during the dry season, and seeds of perennial grasses being important during the wet season. Seed of annual spear-grass (Sorghum spp.) was important for all species during the dry season, but was especially prominent in the diet of the Gouldian Finch. Invertebrates, especially termites, were prevalent in the diet of the Long-tailed Finch in the late dry season but were not conspicuous in the diet of the other species. Relative to co-existing finches, the Gouldian Finch is a dietary specialist and thus potentially more exposed to changes in the distribution and abundance of key food resources. This may partially explain the current threatened status of this species.

Ecology of the Regent Honeyeater Xanthomyza phrygia

Data collected from archival material, literature surveys and responses to a questionnaire show that the Regent Honeyeater Xanthomyza phrygia is distributed mostly within 300 km of Australias south-east coast, from latitude 26°30′S (100 km north of Brisbane) to latitude 39°S (near Wilsons Promontory) and as far west as longitude 138°30′E (Adelaide). The species inhabits eucalypt woodlands and open-forest as well as treed farmland and urban areas. Regent Honeyeaters feed mainly on nectar and insects. Nectar is obtained chiefly from eucalypts and is clearly an important determinant of its lifestyle. The species aggressively defends nectar sources against other species of honeyeaters including larger species, although not always successfully. Other food is obtained mainly by foliage gleaning, although hawking and bark feeding are also important. Aggregations occur most often during autumn and winter and at nectar sources. Breeding occurs mostly from August to January. They may have adapted to the general unpredictability of eucalypt nectar flows by movements with two elements: semi-migratory longer-distance movement between regions where a supply of nectar is reliable and abundant; and local wandering in search of accessible nectar.

Resprouting and mortality of juvenile eucalypts in an Australian savanna: impacts of fire season and annual sorghum.

In northern Australian savannas, canopy tree species often have juvenile tree banks that are composed mainly of small individuals of indeterminate age that have resprouted repeatedly after fire. Little is known about their demography. We report the initial responses (mortality, topkill and resprouting type) of 3133 marked juvenile eucalypts to set fires of different seasons (early dry season, late dry season, wet season, unburnt) in a 32 400 m2 field experiment. Fire treatments were repeated in plots dominated by a native annual grass (sorghum) that becomes senescent before the early dry season and provides the main fuel of savanna fires, and in others with little or no sorghum, but instead other native grasses and forbs that remain green well into the dry season. Most juvenile eucalypts 22% in the low-intensity early dry season fire in plots with little or no annual sorghum, compared with <2% in all other fire/understorey combinations. We suggest results are related to fire behaviour, seasonal carbohydrate storage dynamics and competition with ground-layer plants.

Bamboo, fire and flood: regeneration of Bambusa arnhemica (Bambuseae: Poaceae) after mass-flowering and die-off at contrasting sites in monsoonal northern Australia

Bambusa arnhemica F.Muell., a long-lived, gregarious-flowering and semelparous bamboo endemic to north-western Australia, occurs in remarkably disparate but somewhat fire-sheltered flood-prone riparian forest and rocky hillside vine-thickets, but not in adjacent fire-prone savannas. We investigated the response of B. arnhemica seedlings to fire and flood at two contrasting sites over 2.5 years following a mass-flowering and die-off event. Seedlings grew vigorously notwithstanding either prolonged inundation or total loss of above-ground parts to fire within their first year. However, there was no evidence that such disturbance promoted regeneration, and several veins of evidence suggest that B. arnhemica is fire-retardant and refugial rather than fire-promoting. We suggest that creation of canopy gaps by parental death is a more parsimonious and generalisable hypothesis for the evolution of gregarious semelparity in bamboos than the recently advanced bamboo fire-cycle hypothesis. However, both hypotheses are potentially group selectionist, and resolution of dispersal distances and/or the spatial genetics of relatedness are required to resolve the problem.

Coexistence of shrubs and grass in a semi-arid landscape: a case study of mulga (Acacia aneura, Mimosaceae) shrublands embedded in fire-prone spinifex (Triodia pungens, Poaceae) hummock grasslands

The persistence of relatively fire-sensitive mulga (Acacia aneura F.Muell. ex Benth., Mimosaceae) shrublands within a landscape matrix of highly flammable spinifex (Triodia spp. R.Br., Poaceae) hummock grassland is a central question in the ecology of semiarid Australia. It is also a special case of questions about the coexistence of grasses and woody plants that have general application in semiarid rangelands and tropical savannas. With the use of field surveys and a 24-year fire history, we examined their coexistence on a sandplain in the Tanami Desert, Northern Territory, Australia. Mulga and spinifex each formed discrete monodominant stands with generally abrupt boundaries that did not correspond to obvious edaphic or topographic discontinuities. Spinifex hummock grasslands burnt almost three times as often as mulga shrublands and tended to occur on lighter soils with less biological crusting and more physical soil crusting. A combination of fire and soil variables described the environmental partitioning better than did either alone. Biological crusting increased with time since fire in both vegetation types. The demographic structure of mulga stands reflected their fire history, the more frequently burnt stands comprising almost entirely small plants. One fifth of mulga plants <0.5 m tall were resprouts. Our data provide support for the hypothesis that abrupt boundaries between mulga shrublands and spinifex hummock grasslands can be generated across diffuse environmental gradients by fire–soil–vegetation feedback loops. The oft-severe demographic impact of fire on mulga that is burnt raises questions about the appropriateness of frequent intense fires in this landscape.

Opportunistic Nectarivory: An Annual Dry Season Phenomenon Among Birds in Monsoonal Northern Australia

SummaryIn the Top End of the Northern Territory, 21 bird species other than honeyeaters, lorikeets and white-eyes were observed to probe flowers in apparent opportunistic nectarivory. Thirteen fami...

Bamboo, fire and flood: consequences of disturbance for the vegetative growth of a clumping, clonal plant

The clumping growth form of tropical bamboos suggests a consolidator strategy in the face of intense competition rather than an ability to exploit disturbance. We investigated the annual growth and culm demographic responses to disturbance by fire and flood of vegetatively mature clumps of a riparian stand of Bambusa arnhemica. Linear-mixed models were employed to control for the non-independence of culm fates within clumps and clump growth among years. As the stand was monodominant and the species is gregariously monocarpic, responses can be interpreted independently of interspecific competition and trade-offs with sexual reproduction. Disturbance depressed clump growth but the culm demographic responses to fire and flood were quite different. Few culms were destroyed during the fire but damage depressed their subsequent survival, leading to declines in clump basal area. Fire also triggered the release of ramet buds, but the additional recruits were small and short-lived. Prolonged and early flooding in one of the study years depressed culm recruitment in clumps low on the lagoon bank but there was possible compensatory recruitment in the following year. There was no convincing evidence that sparse clumps recruited culms better than dense clumps, though culm recruitment was negatively correlated with culm survival. Fire and prolonged flooding are inevitable elements of the environment occupied by B. arnhemica, but we interpret the species’ response as persisting in the face of disturbance rather than exploiting the opportunities created by it.