National recovery plan for Zieria formosa, Zieria buxijugum and Zieria parrisiae

NSW National Parks and Wildlife Service, 2002
ISBN 0 731 36496 1

3. Species Information

3.1 Description and Taxonomy

Figure 1. Zieria formosa

Figure 1. Zieria formosa

Zieria formosa is a dense rounded shrub that grows up to 2 m in height. The leaves are arranged in clusters of three leaflets (trifoliolate) on a common stalk, with the leaf clusters occurring in opposite pairs long the branchlets. Both surfaces of the leaves are covered with a dense velvety layer of short stellate (star-shaped) hairs, giving the foliage a pale greyish-green appearance. The upper surface is covered with numerous small warts (tubercles) whilst the lower surface is sparsely warty. The flowers are pale pink and are arranged in large, 26-45 flowered clusters. The flower clusters arise from the leaf axils near the ends of the branchlets and protrude slightly beyond the leaves. Individual flowers are about 6-9 mm across with four obovate petals. Flowering occurs in September-October. The fruit is a four chambered capsule about 5 mm across. Each chamber contains one or rarely two elliptical dark-brown seeds 2-2.5 mm long. See Figure 1 for photograph of flowering branchlet.

Zieria buxijugum is an erect shrub growing to 3.5 m in height. The leaves are arranged in clusters of three leaflets (trifoliolate) on a common stalk, with the leaf clusters occurring in opposite pairs long the branchlets. The leaves are strongly aromatic when crushed, and both surfaces of the leaf are covered with a dense, velvety layer of stellate (star-shaped) hairs and numerous conspicuous warts (tubercles). The central leaflet is linear to narrow oblanceolate in shape, 15-30 mm long and 2-3 mm broad, with the edges rolled under. The outer leaflets in the cluster are similar, but only about three quarters the size. The leaves are dull grey-green above and pale green beneath. The flowers are white and are arranged in clusters of up to 28. The flower clusters arise from leaf axils near the ends of the branchlets on a common stalk which is up to 1.5 cm long. The individual flowers are 6-7 mm across with four ovate-elliptic petals. Flowering occurs in September. The fruit is a four chambered capsule about 5 mm across. The capsule is red-brown when young, becoming greenish at maturity. Each chamber contains one or rarely two elliptical dark-brown seeds 2-2.5 mm long. (See cover photograph for illustration of leaves, flowers and very young fruit).

Figure 2. Zieria parrisiae

Figure 2. Zieria parrisiae

Zieria parrisiae is a bushy shrub growing to 4.2 m in height. The leaves are arranged in clusters of three leaflets (trifoliolate) on a common stalk, with the leaf clusters occurring in opposite pairs long the branchlets. The leaves are strongly aromatic when crushed, and the young stems and both surfaces of the leaf are covered with a moderately dense, layer of short stellate (star-shaped) hairs and numerous conspicuous warts (tubercles). The leaf edges are slightly toothed (dentate) due to the warts and are also slightly rolled under. The leaves are opposite and comprised of three leaflets (trifoliolate). The central leaflet is narrow-lanceolate in shape, 25-30 mm long and 3-4 mm wide, with the edges rolled under. The outer leaflets are similar, but only three quarters the size. The flowers are white and occur in clusters of up to 33. The flower clusters are arranged on a common stalk 10-15 mm long that arises from the axils of leaves near the ends of the branchlets. Individual flowers are 7-9 mm across with four ovate-elliptic petals. Flowering occurs between September and November. The fruit is a warty four-chambered capsule about 5mm across. Each chamber contains one or rarely two elliptical grey-brown seeds about 2 mm long. See Figure 2 for photograph of flowering branchlet.

Note: The above descriptions are a simplification of the descriptions for these species as presented in Armstrong (2002) and Briggs & Leigh (1990).

3.2 Distribution

Only a single population of each species is known. The three populations occur on private properties located about six and 10 km west of Pambula in the far South Coast area of NSW(see Figure 3). The species do not co-occur. Numerous searches of similar habitat in the region by several botanists in the 1980s failed to locate other populations (Briggs and Leigh, 1990).

Figure 3. Zieria parrisiae

Figure 3. Distribution of Zieria buxijugum, Z. formosa and Z. parrisiae.

Zieria formosa is known from only one population on a rhyolite rock outcrop located at Lochiel, six kilometres south-west of Pambula. In 1987 there were 125 'adult' plants in the population (Briggs and Leigh, 1990 ). A resurvey of the population in April 2001 found that the adult population (plants >1 m in height) had reduced to 38 plants, but there were also another 700 smaller plants (see Tables 1 & 2). The population is spread over approximately 1 ha.

Zieria buxijugum is known from only one population on an ignimbrite rock outcrop on Box Range Farm, approximately 10 km west of Pambula. In 1987 there were only 68 plants in the population (Briggs and Leigh, 1990 ). When the species was resurveyed in February 1999, the Z. buxijugum population was found to have increased to 121 individuals over 1 m in height. However, by April 2001 this had reduced to only 32 plants over 1 m in height, although there were another 273 smaller plants (see Tables 1 & 2). The population is restricted to an area of about 0.4 ha.

Zieria parrisiae is also known from only one population on an ignimbrite rock outcrop on Box Range Farm, approximately 10 km west of Pambula.

In 1987 there were only 40 plants (4 adults and 36 plants resprouting from near their bases) in the population (Briggs and Leigh, 1990). In April 2001 the population was resurveyed and the adult population (plants over 1 m in height) had increased to 36, with another 185 plants also present (see Tables 1 & 2). The population is split into two main patches located about 200 m apart along a small gully. A third, smaller patch is located between the two larger patches.

3.3 Land Tenure and Zoning

Most of the population of Z. formosa occurs on land zoned Rural Residential, and a small number of plants are on land zoned Rural 1a. The land on which Z. buxijugum and Z. parrisiae occur is zoned Rural 1a. The primary objective of the Rural 1a zone is to enable the continuation of traditional forms of rural land use and occupation. Neither zoning has any special requirements for the conservation of flora and fauna.

3.4 Habitat

Zieria formosa occurs on the north-east aspect of an upper, moderately steep slope of a 'break-away' area above a small valley. The soil is a skeletal, grey, sandy loam. The site is strewn with broken ignimbrite rocks and boulders and there is much exposed surface rock. The vegetation is a shrub-dominated community and includes Black Wattle (Acacia mearnsii), Blackfellows' Hemp (Commersonia fraseri), Large-leaf Hop-bush (Dodonaea triquetra), Snowy Mint-bush (Prostanthera nivea), Sweet Pittosporum (Pittosporum undulatum), White Kunzea (Kunzea ambigua), Yellow Tea-tree (Leptospermum flavescens), Nodding Blue Lily (Stypandra glauca), Cockspur Flower (Plectranthus parviflorus), Rock Lily (Dendrobium speciosum), Rock Fern (Cheilanthes tenuifolia), Shrubby Platysace (Platysace lanceolata) and Tree Violet (Hymenanthera dentata).

Zieria buxijugum occurs on a steep slope with an easterly aspect on an ignimbrite rock outcrop. The soil is a skeletal brown loam with a high organic matter content. There is much exposed ignimbrite rock. The vegetation is a shrub community dominated by Bracelet Honey-myrtle (Melaleuca armillaris) and has a sparse shrub layer including Shiny Cassinia (Cassinia longifolia), Cockspur Flower, (Plectranthus parviflorus), Violet Daisy-bush (Olearia iodochroa), Shrubby Platysace (Platysace lanceolata) and Rock Lily (Dendrobium speciosum). The shrub community is surrounded by Silvertop Ash (Eucalyptus sieberi) - Yellow Stringybark (E. muelleriana) open-forest.

Zieria parrisiae occurs in a gully on a north-north-east-facing mid slope of a steep hillside above a small creek. The soil is a skeletal, grey loam overlying rhyolite rock. The site is strewn with broken rhyolite rocks and boulders. The species occurs in an intergrade zone between a shrub community and open-forest. The shrub community is dominated by Bracelet Honey-myrtle (Melaleuca armillaris), White Kunzea (Kunzea ambigua) and Black Wattle (Acacia mearnsii). Other associated species include Shiny Cassinia (Cassinia longifolia), Cockspur Flower, (Plectranthus parviflorus), Blackfellows' Hemp (Commersonia fraseri), Shrubby Platysace (Platysace lanceolata) and Rock Lily (Dendrobium speciosum). The adjacent open-forest is dominated by Yellow Stringybark (Eucalyptus muelleriana).

3.5 Ecology

General

The mature plants of all three species flower prolifically and high levels of fruit set were observed in 1986/87 (Briggs, pers. comm.). Seedlings of varying size are present at all sites (see Table 2), suggesting some recruitment probably would occur in most years in the absence of browsing.

The plants of both Z. formosa and Z. buxijugum grow mainly in full sun, although some of the Z. buxijugum population receives part shade that is provided by associated taller shrubs. Conditions on these sites during summer would be extremely hot, and often very dry. Zieria parrisiae occurs as a midstorey shrub and grows in partial to moderately heavy shade. Heavy frosts are uncommon at the three sites, and the driest conditions normally occur during winter.

Competition to seedlings and young plants from other native understorey species appears relatively low, as there is much exposed rock at the three sites and there are few other native sub-shrubs or herbaceous species present.

Following the 1997/98 drought there has been substantial death of the co-dominant shrub (mainly Yellow Tea-tree and White Kunzea) at the Z. formosa site, leaving few remaining live adult shrubs on that part of the slope which supports Z. formosa.

On the Z. parrisiae sites many of the large White Kunzea and Bracelet Honey-myrtle shrubs have become senescent and have either died or have been blown over in storms in the past couple of years. This has resulted in a substantial thinning of the shrub overstorey canopy and has consequently increased light penetration to the understorey. This change in the micro-environment may well be beneficial to the survival and growth of the numerous young Z. parrisiae plants that currently comprise the major portion of the population of this species.

Life Cycle

Zieria parrisiae commences flowering in late September and flowering extends through to early November. Z. buxijugum and Z. formosa flower mainly in September, with flowering in Z. formosa extending into October. The three species are almost certainly insect pollinated, and native bees, hover flies and blowflies have been observed visiting the flowers. Fruit at all stages through to maturity can be found on plants still producing flowers in October. The fruits develop and ripen rapidly, and seed shed appears to be mostly complete by the end of December.

Seven plants (representing three genotypes) of Z. formosa, eight plants (one genotype) of Z. buxijugum and 27 plants (representing five genotypes) of Z. parrisiae are still growing at the Australian National Botanic Gardens (ANBG) from cuttings taken in 1986. These plants have survived 15 years in cultivation in the relatively harsh climate of Canberra where winters are much colder than those that would be experienced in the natural habitat of these species.

Individuals have not been regularly monitored in the field, but the four large, relatively undamaged plants of Z. parrisiae observed in 1986 were still healthy in April 2001. Given the large size to which Z. parrisiae grows and the slow growth that has taken place over the past 15 years, it would appear that this species might live for 50 years, or more. The other two species probably live to at least 20 to 30 years old, but there are little data on which to base these estimates.

Table 1 Number of plants (adults) of Z. formosa, Zieria buxijugum and Z. parrisiae in 1987.
  Z. formosa Z. buxijugum Z. parrisiae
No. of adults(1 m + high) 125 68 4 (plus 36 resprouting stumps)
Table 2 Population structure of Zieria formosa, Z. buxijugum and Z. parrisiae in 2001.
Species Plant height class Total population
  0.2 - 0.5 m 0.5 - 1 m > 1 m  
Zieria formosa          
Section (Su)* 95 135 153 37 420
Section (St)* 219 82 11 0 312
Section (Po)* 3 1 0 1 5
Total 317 218 164 38 737
% of Total 43% 30% 22% 5%  
           
Zieria buxijugum          
Total 78 131 64 32 305
% of Total 26% 43% 21% 10%  
           
Zieria parrisiae          
Sub-popltn lower 39 16 16 21 92
Sub-popltn middle 2 0 1 4 7
Sub-popltn upper 77 23 11 11 122
Total 118 39 28 36 221
% of Total 53% 18% 13% 16%  

Population Structure and Population Trends

The number of adult plants (>1 m in height) recorded in 1987 by Briggs & Leigh (1990) for the three species is presented in Table 1. Height-class data for each of the species, as measured by Briggs and Wright (unpublished data) in April 2001, are presented in Table 2.

The 70% reduction in the adult population of Z. formosa (plants over 1 m in height) from 125 plants in 1987 to 38 in 2001 was primarily due to death of adults during a drought in 1997/98. At the time of a survey of the population in 1999 by Briggs & Smith (unpublished data), 74 dead shrubs of this species were still clearly recognisable, but two years later these have now largely decomposed. Fortunately there has been vigorous seedling regeneration since the 1997/98 drought, and this is reflected in the relatively large number of individuals in the lower height classes.

The 53% reduction in the number of Zieria buxijugum plants from 68 in 1986 to 32 plants over 1 m high in 2001 is a recent event, and has been caused by browsing damage sustained during the winter of 2000. At that time, the stems of many of the Zieria plants were pruned off, or have subsequently died-back following the browsing damage. Prior to that event, Briggs & Smith (unpublished data) recorded 121 plants over 1 m high in 1999. At that time there had been a 78% increase in the population since 1987. It can be seen from Table 2 that there are still numerous individuals in all the smaller height classes, giving the species potential for substantial recruitment into the taller height class and restoring the number of 'adult' plants to a similar or higher number than that recorded in 1999. It is to be noted that currently only five Z. buxijugum plants are over 2 m in height. Because of their height these few plants are less susceptible to heavy browsing than the rest of the population.

There has been a substantial increase (from 4 to 36) in the number of plants of Z. parrisiae over 1 m high since 1987, although it should be noted that only half of these plants are more than 1.5 m in height, and these are not yet likely to contribute substantially to seed production. The four adults of Z. parrisiae recorded by Briggs & Leigh in 1986 are still surviving, and these are now between 3.95 m and 4.2 m in height. The population structure of Z. parrisiae is currently skewed to the smaller height classes, with 53% of the population being seedlings less than 20 cm high.

Disturbance Regimes

Zieria formosa appears to be relatively unpalatable and shows little sign of being browsed by native or introduced animals.

Prior to winter 2000, Z. buxijugum had not shown signs of significant browsing damage, however since then all but the five largest plants have been severely browsed, apparently by Swamp Wallabies. This browsing event had largely ceased during the first half of 2001. Based on observations made in 1986/87 and over the past four years, heavy browsing of Z. buxijugum may only be an intermittent disturbance event, but this may well significantly reduce seed production for more than one year following such activity.

Browsing by goats in the mid 1980s had a major adverse impact on the adult population of Zieria parrisiae, with all but four of the tallest plants having been smashed and browsed to near ground level. Field observations since 1996 indicate that browsing by Swamp Wallabies is having an ongoing adverse effect on the growth and survival of Z. parrisiae seedlings and young plants through the constant 'nipping off' of the seedlings as they produce new growth.

Whilst a drought in 1997/98 had a major impact on the adult population of Z. formosa, this drought did not appear to have a significant affect on the other two species.

There is no direct observation on the effects of fire intensity, seasonality or frequency of fire events on any of these species. However, on the sites where Z. parrisiae and Z. buxijugum occur, the presence of very large individuals of other fire sensitive species indicates that the sites have not been burnt for several decades. It is predicted that these communities would be very slow to recover from fire (Briggs and Leigh 1990).

3.6 Ability of Species to Recover

The current small number of large, reproductively mature plants for each species is a concern, in that production of seed is presently very limited and localised on each of the sites. Unless there are significant levels of soil-stored seed (an unknown factor at present), currently low levels of seed production are likely to limit future recruitment. The development of more plants into large, reproductively mature plants will be important to restore seed production to what is likely to be more normal and desirable levels. Fortunately, the three species currently have substantial numbers of plants in the seedling and lower height classes, indicating all have the ability to recruit given an adequate seed source and favourable conditions. Providing the threats detailed below are removed or adequately controlled there is no strong reason why many of the plants in these lower size classes would not develop into reproductively mature individuals, and a more robust population structure be restored.