P.G. Ladd

Biodiversity and management of riparian vegetation in Western Australia

This paper considers the nature of riparian vegetation in south west Western Australia and issues relevant to its management. Riparian forests and vegetation in Western Australia are generally less species rich than adjacent upland communities. This correlates with the few studies in other parts of Australia and South Africa, but is in contrast with the situation in Europe. Structurally, the vegetation is usually more complex than that of upland areas. Riparian vegetation has been severely degraded in many parts of Western Australia and plant species biodiversity is threatened by a number of environmental problems. Foremost among these are clearing, grazing, salinization, weed invasion, urbanisation and frequent burning. The relative importance of these problems is related to the geographical area in question. Management practices to maintain biodiversity need to be tailored to the conditions of each particular area. In relatively undegraded areas an integrated management approach, especially in relation to weeds, is likely to provide the best outcome. In more degraded areas, biodiversity is greatly decreased from that of natural systems. The priority in these areas should be to enhance the value of the system through revegetation and moderation of degrading influences. A primary need in this area is a data-bank of recruitment requirements of suitable riparian plant species to enable their re-establishment on riverbanks.

Comparative population structure and reproductive biology of the critically endangered shrub Grevillea althoferorum and two closely related more common congeners

Grevillea althoferorum is a critically endangered, sprouting shrub known from two disjunct populations within the South-West Botanical Province of Western Australia. This study compares the conservation biology of G. althoferorum and two closely related but more common congeners, the non-sprouter G. rudis and the sprouter G. synapheae subsp. pachyphylla in order to determine whether there are differences in reproductive and ecological attributes that might explain why G. althoferorum is rare. In contrast to the more common species, neither population of G. althoferorum exhibited evidence of seedling recruitment. However, the northern population was confirmed to be clonal and was actively recruiting from root suckers. Both populations of G. althoferorum were found to have reduced amounts of viable pollen on stigmas in comparison with the other species. The fruit set at the southern population of G. althoferorum was considerably lower than that found for the common species, with only 0.15% of flowers setting fruit and no fruit was produced in the northern population. In addition no evidence of a soil seed bank was found for either population of G. althoferorum, but G. rudis and G. synapheae subsp. pachyphylla both had soil stored seed which germinated following treatment with aqueous smoke solution. Sexual recruitment at both populations of G. althoferorum was absent, and reproduction appears to be predominantly clonal in the northern population. Management strategies for G. althoferorum should therefore focus on the protection of adult plants from accidental destruction.

The use of quartz grain microtextures in the study of the origin of sand terrains in Western Australia

The origin of extensive sand terrains which lie inland from the coastal margin of Western Australia is contentious, with the debate centering around an in situ vs. an aeolian origin. To resolve this debate the shape and surface features of sand grains are reported for sandplains, sand dunes and bedrock for the Victoria Plateau sandplain, which lies in the central west coastal region of Western Australia. All three components of the Victoria Plateau are similar in grain shape and microtexture. Quartz grains show a combination of chemical dissolution and precipitation micromorphology. Characteristics of dissolution include etch patterning, triangular shaped etch pits and solution features. Precipitation forms include edge rounding, silica veneers and plate- and sheet-like structures. Features such as peeling plates, complex precipitation forms projecting from grain surfaces, adhering particles and preserved grain contact faces indicate stability rather than a transport dominated environmental history. The combination of preserved dissolution and precipitation forms, a dearth of mechanically derived features and a correspondence between grains in rock and overlying sands points to local siliceous sedimentary rocks as the source of the sands.

Early inflorescence development in the grasses (Poaceae)

The shoot apical meristem of grasses produces the primary branches of the inflorescence, controlling inflorescence architecture and hence seed production. Whereas leaves are produced in a distichous pattern, with the primordia separated from each other by an angle of 180°, inflorescence branches are produced in a spiral in most species. The morphology and developmental genetics of the shift in phyllotaxis have been studied extensively in maize and rice. However, in wheat, Brachypodium, and oats, all in the grass subfamily Pooideae, the change in phyllotaxis does not occur; primary inflorescence branches are produced distichously. It is unknown whether the distichous inflorescence originated at the base of Pooideae, or whether it appeared several times independently. In this study, we show that Brachyelytrum, the genus sister to all other Pooideae has spiral phyllotaxis in the inflorescence, but that in the remaining 3000+ species of Pooideae, the phyllotaxis is two-ranked. These two-ranked inflorescences are not perfectly symmetrical, and have a clear “front” and “back;” this developmental axis has never been described in the literature and it is unclear what establishes its polarity. Strictly distichous inflorescences appear somewhat later in the evolution of the subfamily. Two-ranked inflorescences also appear in a few grass outgroups and sporadically elsewhere in the family, but unlike in Pooideae do not generally correlate with a major radiation of species. After production of branches, the inflorescence meristem may be converted to a spikelet meristem or may simply abort; this developmental decision appears to be independent of the branching pattern.

Comparison of ecosystems in the Mediterranean Basin and Western Australia

A comparison between fire-prone ecosystems in the Mediterranean basin and Western Australia shows convergent adaptations in structure, life forms and flora evolution. Both areas have in common summer drought and winter rainfall. Reproductive biology is mainly correlated with fire ecology; the different length of human impact and history seem to be responsible for floristic divergency in both areas. The ecological significance of pollination by insects, birds and mammals and problems of the seed banks are discussed.

Habitat preference and reproductive traits of a major Australian riparian tree species (Casuarina cunninghamiana)

The population structure and reproductive biology of the river oak (Casuarina cunninghamiana) were studied along the Murrumbidgee River on the Southern Tablelands of eastern Australia. The species has cone-like infructescences but is not serotinous, with samaras released when they are mature. Samaras were mostly distributed by wind close to female trees but were also carried by water. The first year of study (1985) appeared to be a mast year for seed production with much lower seed fall in the following year. Seedling establishment was spatially very variable, mostly under female canopies and appeared earlier on soil within the river channel than on the bank. Turnover was high and seedlings in the river channel generally died after being inundated. Most trees were within 3 m of the mean river level. However, the total distribution of adults was within the envelope of maximum floods in the area but establishment was not dependent on floods. The population structure was the result of yearly recruitment, although episodic events (floods, drought) may enhance or decrease establishment. Pot-trial results paralleled the field situation with substrate and water-table level not affecting germination of seed but strongly influencing seedling growth. Plants grew best on cobble substrates under a low water-table regime and poorly on cobbles with high water and sandy substrate under all water-table levels. Cobble banks seemed the best substrate for growth within the river channel and establishment may be prolific. Less-abundant seedling establishment occurred upslope but controls over this were not investigated.

Pollen presenters in the South African flora

In contrast with the majority of flowering plants, where pollen is released directly from the anthers to travel to the female organ to effect fertilization, the pollen in certain species belonging to fifteen families worldwide is initially deposited on the female part of the flower before transport to another flower occurs. The structure on which the pollen is deposited is (in almost all cases) a modification of the style called the pollen presenter. In South Africa, pollen presenters are ubiquitous in the Asteraceae, Campanulaceae, Lobeliaceae, Goodeniaceae and Proteaceae; they also occur in almost half of the genera in the Rubiaceae, and in Polygala and some Muraltia (Polygalaceae), in Turraea, Trichilia and Ekebergia (Meliaceae) and a small proportion of taxa in the Fabaceae. The modifications of the style take various forms and can be summarized into actively and passively operating types. The active forms act like a piston to push the pollen away from the anthers, while the passive forms are static, receiving the pollen from the anthers before the anthers fall away to leave the pollen ready to be removed from the presenter by animals or the wind. In the past, pollen presenters have either not been recognized or have been described as styles or stigmas. This leads to confusion about processes involved in pollination and wastes useful information which is of value in taxonomic studies.

Buzz pollination in the Epacridaceae

Conostephium (Epacridaceae) has flowers that conform with a buzz-pollination syndrome but, unlike most plants with this form of pollination, the anthers are hidden within the corolla tube. Vibrations generated by bees grasping the apices of the corolla tubes must be transferred via short broad filaments to the anthers. The anthers do not have pores but each dehisces from the apex by a slit that elongates over the time the flowers take to senesce (up to 10 days). This may limit self-fertilisation as the stigma is receptive as soon as it appears from between the very short corolla lobes, so little pollen is released at first but later this would increase as the slit elongates. Visitation by pollinators has rarely been seen but several observations of native bees (Leioproctus and Lasioglossum) working the flowers are presented. The bees visit the nectarless flowers of Conostephium only for pollen and must forage at other kinds of flowers to obtain nectar. Pollen tubes occurred in the stigmas of most older flowers of C. pendulum, so pollen delivery does not seem to limit seed set. Despite this, the species sets few fruit. From examination of the taxonomic positions of likely buzz-pollinated taxa in the family, it appears that pollination by sonication has arisen independently several times in the Epacridaceae, with primarily two different floral configurations.

Serotiny in southern hemisphere conifers

Serotiny is a widespread trait in angiosperms in the southern hemisphere; however, it is less common in conifers and has been little examined in the only two genera of southern conifers (Callitris and Widdringtonia) that have serotinous cones. There is variation across the family in the size of cones, the amount of seed contained and the time over which the cones stay closed on the plant. Cones from most of the species were collected in the field and various morphometric measurements made including cone wet and dry weight, the number of seeds contained and their likely viability. Cones from a selection of species with different cone sizes were heated to increasing temperatures, to investigate the ability of cones to protect the contained seeds from heat. In comparison to the flowering plants, serotiny has developed comparatively recently in southern conifers (in the last 10-20 million years). In Widdringtonia, serotiny is relatively weak, whereas in Callitris, it varies from strong to non-existent. Cone size and fertile-seed production across the two genera varies and the number of fertile seeds produced is positively related to the size of the cone. In some species, there are sterile seed-like bodies. These may have developed to confuse seed predators, so fertile seeds have a better chance of survival. Larger (heavier) cones are more effective in protecting the contained seeds from the heat of fires than are smaller ones. There is no simple relationship between the cone size and type of environment occupied by the species. In regions where fire is unlikely, predictable but mild or completely unpredictable, the species tend to be non-serotinous. In temperate regions where hot fires are likely to have been a selective agent, the species tend to be more strongly serotinous, although fire is not essential to open the cones. The community and environment in which a species has evolved is likely to have influenced the development of the degree of serotiny for each species and this may still be a variable property among populations of some species, depending on the fire regime of the area in which they grow.

Reproductive biology, post-fire succession dynamics and population viability analysis of the critically endangered Western Australian shrub Calytrix breviseta subsp. breviseta (Myrtaceae)

Calytrix breviseta Lindl. subsp. breviseta is a critically endangered, obligate-seeder shrub within fire-prone kwongan of south-west Western Australia. Little is known about the species’ reproductive biology and how threatening processes, particularly altered fire regimes and exotic species invasion, will impact the long-term viability of the species. This study aims to elucidate the species’ reproductive biology and patterns of seedling recruitment during succession after fire. The effects of changes to the fire return interval and exotic species invasion on the long-term viability of the species is also described. The species exhibits abundant recruitment following fire and the application of a smoke treatment significantly improves germination, similar to many other Western Australian shrubs. However, significant inter-fire recruitment was observed up to 10 years following fire, leading to the presence of multi-aged subpopulations, although seedling recruitment was negligible >20 years after fire. The juvenile period is short at 3–4 years to first flowering. Population viability analysis (PVA) predicted that the optimal fire return interval to maintain C. breviseta subsp. breviseta was dependent on the carrying capacity (K) of the community and the number of individuals present. Carrying capacity will be related to site quality and competition from invasive species. PVA showed that if K remains high, then the optimal fire return interval is ~15–20 years, but under lower carrying capacity, (i.e. weed competition) fires decrease the likelihood of population survival.