Species In South-western Australia Research Articles

Biogeography of Fire-Killed and Resprouting Banksia Species in South-Western Australia

Banksia includes 38 fire-killed (seeders) and 20 resprouting species, and two species with contrasting ecotypes, in south-western Australia. There may be up to 12 seeders per 50 × 50 km grid cell in the southern sandplains and 12 resprouters in the northern sandplains. The patterns of distribution of species across soil type and eight climatic attributes is similar for both life forms, except that greater numbers of resprouting species occur at higher rainfalls and where there is greater seasonal spread of rainfall. Most seeders occur on white sands and rocky substrates, and resprouters occur on yellow sands and the more fertile lateritic soils. Nutrient requirements for both life forms appear to be similar. Resprouters are more widespread than seeders which suggests that resprouters show greater environmental tolerances. The distribution of grid cells containing each life form across soil types and eight climatic attributes is similar and any differences in climatic profile for all species in each category are considered biologically insignificant. Both life forms in section Abietinae are well represented in the climatically distinct southern and northern sandplains indicating no climatic preferences within the lineage. There are no consistent trends in environmental attributes from fire-killed to resprouting ecotypes of B. ashbyi E.G.Baker and B. violacea C.A. Gardner. Multiple-partitioning classification of the floristic data produced 10 groups varying greatly in geography, species richness, and proportion and endemism of each life form. The Lesueur (northern) district has four endemic seeders, six endemic resprouters and a mean of 10 resprouters per cell. The East Eyre (southern) district has five endemic seeders, no endemic resprouters and one resprouter per cell. Both groups have a mean growing season of 5 months. The relative aridities and fluctuations of present and past (Quaternary and late Tertiary) climates are invoked to explain the much higher proportion of resprouters in the northern than southern sandplains and the absence of seeders in the most marginal cells. The absence of endemic species yet high proportion of resprouters (73%) in the extreme south-western corner of the region might be explained by elimination of seeders through frequent burning by Aborigines in the late Quaternary. The increase in the proportion of fire-killed species along the south coast from 23% to 100% at the edge of the Nullarbor Plain also requires an explanation.

Dawn Water Potential and Root Depth of Trees and Understorey Species in Southwestern Australia

Water relations of selected tree and understorey species in the jarrah forest of south-western Australia were studied during summer drought and the results related to root morphology. Seasonal patterns of predawn water potential (Ψp) differed between species according to root depth and between sites according to average annual rainfall. Dawn water potentials fell most rapidly and by the greatest amount in plants with the shallowest roots. Dawn water potentials of medium and deep rooted species were not consistently different. Separation of Ψp between sites of different annual rainfall was less marked than was separation by root depth. Changes in Ψp, were consistent with a top-to-bottom drying of the soil profiles. We suggest that measurements of Ψp of plants of appropriate root depth can be used to monitor the drying of soils as an alternative to more expensive mechanical and electrical methods.

SPECIATION IN THE GENUS CRINIA (ANURA: MYOBATRACHIDAE) IN SOUTHERN AUSTRALIA: A PHYLOGENETIC ANALYSIS OF ALLOZYME DATA SUPPORTING ENDEMIC SPECIATION IN SOUTHWESTERN AUSTRALIA.

Where there are several closely related species with sympatric distributions occurring in an isolated habitat, the cause is generally ascribed to multiple invasion (Mayr, 1963 p. 504). White (1978p. 115) has criticized the indiscriminate application of this principle in the Australian biota in the following way: such hypotheses are produced solely to satisfy the a priori postulate that speciation is only possible where populations are completely separated by major topographic barriers. Interpretations on the origin of the frog fauna of southwestern Australia have suffered in this regard, in that the region is topographically featureless (Prider, 1966) and isolated by desert from the rest of Australia while the frog species there have largely sympatric distributions. Thus it was postulated that frogs from southeastern Australia crossed the arid Nullarbor plain during several cycles of increased rainfall caused by Pleistocene glaciation. Speciation occurred after populations were separated each time the Nullarbor plain again became arid. The direction of invasion was determined on the basis of the number of species found in each area so that invasion proceeded from centres of low species number to areas which now have many species. Among the birds of southern Australia, for example, invasion is postulated to have gone from west to east (Keast, 1961). In the case of frogs, the proposed invasions were from east to west, and most authors have assumed that each southwestern species is most closely related to a southeastern species (e.g., Main et al., 1958; Martin, 1972; Littlejohn, 1981) and that several southwestern species may be related to one southeastern species. White (1978 p. 113) has challenged this interpretation, suggesting that the number of species in southwestern Australia is the result of endemic speciation, as such an hypothesis makes fewer assumptions. According to this interpretation, each southwestern species is most closely related to some other southwestern species. These alternative biogeographic hypotheses imply different phylogenies and these can be tested by an independent phylogenetic analysis. In the present study the alternative evolutionary scenarios were tested using seven species from the genus Crinia (six of these species were formerly in Ranidella [Blake, 1973] but were returned to Crinia by Heyer et al., 1982). The characters used were allozymes as most of these species are morphologically indistinguishable. Crinia is of especial interest as the first hypothesis invoking multiple invasion for the origin of frog species in southern Australia was formulated for this genus, before being extended to include not only Heleioporus and Neobatrachus (Main et al., 1958) but three other genera as well (Main, 1968).

Mechanisms for enhancing nutrient uptake in plants, with particular reference to mediterranean South Africa and Western Australia

The major constraints to nutrient uptake by vascular plants in mediterranean South Africa and Western Australia are: very infertile soils, relatively low temperatures when water availability is high, and hot, dry summers. These constraints are partly overcome through increased efficiency of uptake, tapping novel sources of nutrients, and prolonging water uptake. Absorptive area per unit “cost” may be enlarged directly through increased fineness of the root system and proliferation of long root hairs. This reaches its greatest development in the root clusters of the Proteaceae (proteoid roots), Restionaceae (“capillaroid” roots) and Cyperaceae (dauciform roots). Absorptive area is increased indirectly through fungal hyphae which extend from hairless rootlets into the soil. Two major groups can be recognised: general (VA mycorrhizas) and host-specific (ericoid, orchid and sheathing mycorrhizas). Mycorrhizas are the most widespread specialised modes of nutrition and are probably universal in such major taxa here asPodocarpus, Acacia, Fabaceae, Poaceae, Asteraceae, Rutaceae, terrestrial orchids, Ericales and Myrtaceae. General mycorrhizas are the least drought-adapted of mechanisms for maximising absorptive area. All have been implicated in enhancing P uptake through increasing access to inorganic P, solubilisation and shortening the diffusion path. However, selective uptake of other nutrients, especially N, by host-specific mycorrhizas may be equally important. Included under novel sources of nutrients are free N2 (utilised by N2-fixing nodules), small-animal prey (carnivorous leaves) and persistent leaf bases (aerial roots ofKingia australis). Both legume and non-legume N2-fixing species are well-represented in these two regions, with stands of individual species in southwestern Australia estimated to contribute 2–19 kg N/ha/yr to the ecosystem. Free nitrogen fixation requires additional nutrients, especially Mo and Co, but is enhanced following fires and by supplementary uptake mechanisms, especially VA mycorrhizas. Southwestern Australia is particularly rich in carnivorous species. Nitrogen, P, K and S are important nutrients absorbed, with digestion aided by enzymes provided by bacteria and the glands. Parasitic plants both tap novel sources of nutrients and capitalise on any efficient water and nutrient uptake mechanisms of the hosts. Root parasites are better represented than stem parasites in mediterranean South Africa and Western Australia. Phosphorus and K in particular are absorbed preferentially by the haustoria, but much remains to be known about their modes of operation. Maximum activity of all uptake mechanisms, except those attached to some deep-rooted plants, is restricted to winter-spring. Most new seasons’s rootlets and specialised roots are confined to the uppermost 15 cm of soil, especially in or near the decomposing litter zone. Nutrient uptake is further enhanced by the tendency for the rootlets to cluster, trapping water by capillary action and prolonging nutrient release. As an early product of decomposition, N tends to be available as NH4 (rather than NO3) and it is absorbed preferentially by almost all specialised modes of nutrition. Microorganisms are required in the formation and/or functioning of all these structures, except haustoria. Uptake mechanisms which are optional to the plant reach their peak contribution to the root system at soil nutrient levels well below those required for greatest plant growth, when they may be absent altogether. It is only over the narrow range of nutrient availability, where shoot content of a nutrient is greater in the presence of the mechanism than in its absence (other factors remaining constant), that specialised modes can be termed nutrient-uptake “strategies.” For all specialised modes of nutrition, the component genera are better represented in these two regions than in the surrounding more fertile, arid to subtropical regions of much greater area. Endemism of species with each mode exceeds that for the two floras overall (75%). This is taken as preliminary evidence that specialised modes of nutrition are best represented in nutrient-poor soils. While they serve to limit nutrient loss from the ecosystem, their proliferation is therefore not necessarily a response to increasing “leaks” in the system. A hierarchical scheme of the functional/structural relationships between the various mechanisms is presented, starting with the rootless, VA-mycorrhizal plant as the most primitive condition. Taxa with many of the specialised modes of nutrition at present in southwestern South Africa and Western Australia have been evident in the pollen record since the early Tertiary Period. The absence of ectomycorrhizal forests in mediterranean South Africa, in marked contrast to Western Australia, can be traced to differences in their paleohistory. In both regions, the combination of fluctuating, but essentially diminishing, nutrient and water availability that began with the first mediterranean climate < 5 million years ago resulted in decimation of the less-tolerant rainforest ancestors on the one hand, and remarkable rates of speciation of the pre-adapted sclerophyll nucleus on the other.