Survey of threatened plant species in South East Queensland biogeographical region queensland cra/rfa steering committee survey of threatened plant species in
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3.6 Allocasuarina rigida subsp. exsul L.A.S. Johnson3.6.1 SummaryAllocasuarina rigida subsp. exsul is known only from Mt Cooroora in SF 963, 2 km west south west of Pomona. Approximately 4000 plants are present in an area of approximately 1875 m2. They grow in a shrubland community on very steep hillslopes with a southerly to easterly aspect. The soils are derived from trachyte and are shallow, dark sandy loams with a acidic reaction. Potential threats to the survival of Allocasuarina rigida subsp. exsul arise from human interference in the environment. These threats are inappropriate fire regime and physical disturbance. Monitoring the population to obtain information on the ecology of the species and to gauge the effect of pedestrian traffic on the population is recommended. 3.6.2 Species description and identificationThe genus Allocasuarina is placed in the family Casuarinaceae. Allocasuarina is endemic to Australia where there are approximately 59 species distributed throughout the southern part of Australia with four species extending to north-east Queensland. Allocasuarina rigida subsp. exsul L.A.S. Johnson was formally described 8 years ago (Johnson 1989) from material collected by Mr P. Sharpe from Mt Cooroora near Pomona. The species belongs to the section Cylindropitys (Wilson and Johnson 1989). The botanical description of Allocasuarina rigida subsp. exsul is as follows: Dioecious shrub up to 1.5 m high with smooth, grey bark. Branchlets ascending, to 30 cm long, articles 10-15 mm long, 0.7-1.5 mm diameter, smooth, usually glabrous; phyllichnia angular to sometimes rounded, with median ridge; teeth 7-10, erect to slightly spreading, overlapping slightly only when young, 0.4-0.9 mm long, somewhat marcescent. Male spikes strongly moniliform, 1-7 cm long, rarely to 9 cm, 4-6.5 whorls per cm; bracteoles persistent; anther 0.7-1.2 mm long. Cones cylindrical to ovoid, pubescent; peduncle 2-9 mm long. cone body 9-19 mm long, 6-11 mm diameter. Samara 3.5-4.5 mm long (Wilson & Johnson 1989) (see Plate 1). The characters used by Wilson & Johnson (1989) to distinguish A. rigida subsp. exsul from A. rigida subsp. rigida are presented in Table 3.6.1. There are three other rare or threatened species of Allocasuarina in the Sunshine coast area. These are A. emuina from Mt Peregian and surrounding coastal heathland, A. thalassoscopica from Mt Coolum and A. filidens from Mt Coonowrin and Mt Beerwah. A. rigida subsp. exsul is distinguished from these other species by having 7 to 9 teeth and usually smaller cones. Table 3.6.1. Characters distinguishing Allocasuarina rigida subsp. exsul and Allocasuarina rigida subsp. rigida
3.6.3 Current conservation statusAllocasuarina rigida subsp. exsul is presently not listed on the schedule of threatened wildlife of the Queensland Nature Conservation Act 1992 or the Commonwealth Endangered Species Act 1992. However it is currently pending inclusion on the Queensland Schedule of threatened wildlife as ‘Vulnerable’. It is proposed to be added to the schedule because it has a restricted distribution and is not in a formal conservation reserve. 3.6.4 Distribution and abundanceDetails from Queensland Herbarium specimens of A. rigida subsp. exsul are listed in Appendix 1.5. The species is only known from a single location near the summit of Mt Cooroora, approximately 2 km west south west of Pomona. Detailed locality data for this site is presented in Appendix 2.1. The site is within State Forest SF 963 (Fty 517). The area is presently managed jointly by Department of Natural Resources and the Department of Primary Industries, Forestry. The base of the mountain supports forests of commercial timber species, but the summit of the mountain has no commercial timber values. Previous reports on population size indicated that it was of only a few hundred individuals and that below average rainfall in the early 1990’s had killed the majority of the population (P. Sharpe pers. comm.). The present study examined the upper slopes (ie. above the cliff lines) of Mt Cooroora and observed that A. rigida subsp. exsul is restricted to the south-eastern end of the mountain summit. The species is the dominant plant species in a shrubland above a cliffline over an area of approximately 1875 m2. A population of approximately 4050 individuals was estimated. However because of the large confidence interval (± 2271) this estimate can only be used as a guide. The population consists of a relatively mature even-aged stand of individuals all 1-1.5 m high. The population appear generally healthy. No seedlings of A. rigida subsp. exsul were observed at the site. Low levels of seedling recruitment are typical of serotinous species in habitats where fire has been absent for sometime (Specht 1981). Twenty-six (56 %) of the forty-six plants examined had large crops of mature cones along the stem. No other likely habitats were examined during this present study. Other peaks in the area which have been investigated for this species in the past include Mt Ninderry, Mt Cooroy and the Glasshouse Mt Peaks (A.R. Bean & P.R. Sharpe pers. comm.). 3.6.5 HabitatMt Cooroora is an isolated trachyte plug. A. rigida subsp. exsul is confined to the steep rocky upper slopes of the mountain. The single site is situated 390 m above sea level with a easterly to southerly aspect and is exposed to the prevailing winds. The shallow soils are well drained brownish black sandy loams with a pH 4.6-4.8. The vegetation type is a mid-tall to tall shrubland with emergent low to mid-tall trees. A. rigida subsp. exsul is a dominant species in the shrub layer with Xanthorrhoea latifolia, Monotoca scoparia and Leptospermum polygalifolium. Scattered emergent trees of Eucalyptus racemosa and E. exserta are also present. The date of the last prescribed burn for the state forest was approximately 1992, but it is unclear whether the A. rigida subsp. exsul population was burnt by this fire. The area is scheduled for a hazard reduction burn during winter of 1998. On the eastern slope of Mt Cooroora there is a walking track which leads to its summit. There is substantial erosion and vegetation destruction along the walking track. Track maintenance has been undertaken in the last couple of years to reduce erosion and stabilise the track surface. Within the vicinity of the population of A. rigida subsp. exsul, the track in the past divided in two with one track traversing the northern end of the population while the other track passed along the northern edge of the population. The use of the track through the population has been discouraged and the area appears to be slowly regenerating. On the mountain summit the main walking track leads northwest away from the population of A. rigida subsp. exsul. However, there are minor tracks down through the A. rigida subsp. exsul population to a cliff face. At present the damage to the habitat is minimal. 3.6.6 Life history and ecologyVery little is known about the life history and ecology of A. rigida subsp. exsul. The majority of the following information is extrapolated from our understanding of other species of Allocasuarina and Casuarina. A wide range of chromosome numbers have been observed along with differing degrees of polyploidy in a number of species of Casuarina species s.l.. Barlow (1959a & b) has reported a diploid number of 22 for A. rigida subsp. rigida. A. rigida subsp. exsul is a dioecious shrub with an unknown life span. The main method of reproduction is by sexually produced seed. An isolated female plant of A. rigida subsp. exsul in cultivation has been observed to set a small quantity of viable seed (P.R. Sharpe pers. comm.). This suggests that A. rigida subsp. exsul is capable to some degree of apomixis. Apomixis has been reported to occur in other species of Allocasuarina and Casuarina (Barlow 1959a). Allocasuarina spp. are reported to be wind-pollinated (Torrey 1983). The flowering period has not been recorded. Observations of A. rigida subsp. exsul during the survey in October 1997 found numerous female inflorescences present but an absence of male flowers. In south east Queensland A. rigida subsp. rigida has been recorded flowering in August and September. Other mountain species of Allocasuarina in the region have been observed to have their main flowering period from May to July. As with other species of Allocasuarina, A. rigida subsp. exsul has serotinous fruit. The seeds are released only on the death of the parent plant or shoot supporting the cones. Fire or extreme desiccation are usually the major causes of such events. Detailed information on the period of retention of the seed in the cones is lacking for this species. However from field observations it would appear that the cones remain unopened for several years. The longevity of the seed while retained in the fruit is unknown. Pannell and Myerscough (1993) have reported a gradual decline in viability of seed retained in fruit of A. distyla and A. nana over a period of 12 years. It is reported by Turnbull and Martensz (1982) that there is considerable variation in the viability of seed collected from Australian stands of Casuarina s.l. Seeds of serotinous species generally show no form of germination dormancy once released from the protective fruits. In other species of Allocasuarina after the seed is released from the cone the seed coat readily absorbs water to produce mucilaginous gel on the seed surface. Turnbull and Martensz (1982) suggested this trait assists rapid germination in conditions where the water supply may be transient. There are numerous reports on the germination and cultivation of Allocasuarina and Casuarina (Elliot and Jones 1982, Midgley et al. 1983). The seeds readily germinate given reasonable moisture and temperature conditions. A fresh sample of 346 seeds was exacted from 40 cones from the herbarium voucher taken from the population in October 1997. Seventy six seeds (22%) were found to lack a viable embryo, contain only a mass of white woody tissue. These seeds appeared well formed and were indistinguishable on their outward appearances from seeds with embryos. It is unknown what causes these woody seeds to form. Similar woody seeds have been observed in A. emuina, A. thalassoscopica, A. littoralis and Casuarina equisetifolia. The level of viability of seeds with embryos was high (225 seeds (83 % of seeds with embryos and 65 % of seeds sampled)). The response of A. rigida subsp. exsul to fire is unknown, however it is believed that above-ground parts are fire-sensitive and are killed by fire. It is unknown whether it can regenerate from the root stock. 3.6.7 ThreatsThere is no quantitative data to indicate that the populations of A. rigida subsp. exsul have declined in the past or are presently declining. However, potential threats to the survival of A. rigida subsp. exsul arise from human interference in the environment. These threats are an inappropriate fire regime and physical disturbance by human visitation. Fires are either unplanned (wildfires) or planned (hazard reduction burns). The occurrence of fire in the habitat of A. rigida subsp. exsul should not be viewed as being incompatible with the long term survival of the species. However the lack of ecological information about this species’ response to fire makes it impossible to assess what would be an appropriate level of frequency and intensity for the long term benefit of the species. If fires are too frequent, the plants will have insufficient time to build-up a canopy seedbank to replace plants that are killed in the fire, and this will lead to population declines. The shallow soils and low vegetation are sensitive to physical disturbance. Physical disturbance can lead to the trampling of seedlings, branches being broken off, general compaction and disturbance to the soil, and the introduction of exotic weed species. The present level of physical disturbance due to human visitation appears to be minor and unlikely to critically threaten the species in the short term. However the long term effect of such continued disturbance may lead to the modification of the habitat to the detriment of A. rigida subsp. exsul. The degree on physical disturbance is currently being informally monitored by Department of Natural Resources staff during routine track maintenance (P. Leeson pers. comm.). 3.6.8 Management, Research and Conservation MeasuresIt seems unlikely A. rigida subsp. exsul will be found at any other locations. In view of this, it is important that the single location be managed to afford long-term protection to the species. Management presupposes knowledge about a species’ response to particular management practices. There is little information available on the ecology and reproductive biology of A. rigida subsp. exsul . The forthcoming hazard reduction burn for the state forest is an ideal opportunity for the commencement of a monitoring program to obtain information on the species’ response to fire. Physical disturbance by pedestrian traffic at the site should also be monitored. The present track passes along the northern edge of the population. If there are planned changes to the walking track then it would be appropriate to relocate it away from the habitat of A. rigida subsp. exsul. When assessed against the IUCN (1994) criteria for threatened wildlife A. rigida subsp. exsul falls into the category of Vulnerable, ie. it is facing a high risk of extinction in the wild in the medium-term future, as defined by criteria D.2. (ie. Population is characterised by an acute restriction in its area of occupancy (typically less than 100 km2) or in the number of locations (typically less than 5)). A. rigida subsp. exsul is presently being considered for inclusion on the schedule of threatened wildlife of the Queensland Nature Conservation Act 1992 as Vulnerable. This status is considered to be appropriate. There appear to be few opportunities to improve the conservation status of this species in the long term. |