Destructive effect of fire on terrestrial orchid populations at Warrandyte, Victoria.
Fire is an influential and destabilising factor in Australian ecosystems, particularly for orchids with high dependence on associated plants and microclimate. Growth and flowering of terrestrial orchids may be unaffected or depressed after fire, enhanced with synchronised flowering, or fire dependent, e.g. some species of Diuris, Caladenia, Eriochilus, Microtis and Lyperanthus (Dixon 1996). Responses are complex and site dependent, with different effects related to the intensity and frequency of burns, and the soil type and depth. Floral displays after fire may be impressive where there is relatively deep sandy soil: for example, Anglesea in Victoria and south-eastern parts of Western Australia. This can lead to a misperception and popular view that fire is generally beneficial for orchids and, therefore, always useful in the management of nature reserves. There are few reported longerterm, detailed studies of the effects of fire on terrestrial orchid populations, and only brief references to species (Erickson 1965; Duncan 2013). There were significant decreases in the numbers of some terrestrial species and increased growth and flowering of others after fire at Kinglake, Victoria (Barnett 1984).
My observations are from 1960 to 2017. In this more than 50-year observational study of Tindals Wildflower Reserve in Warrandyte, Victoria, the effects of planned burning on terrestrial orchid populations are reported.
The 2.6 ha reserve is 3 km south-west of the Warrandyte township, about 22 km east of Melbourne. It occupies the upper slopes of a hilltop bounded by Tindals Road and Warrandyte-Templestowe Road, and is an isolated patch of natural vegetation now surrounded by rural and residential development. The reserve supports a dry sclerophyll forest dominated by Yellow Box Eucalyptus melliodora, Red Box E. polyanthemos, and Bundy E. goniocalyx, on very skeletal soil overlying weathered Silurian mudstone sediments (Douglas and Ferguson 1976) (Fig. 1). The vegetation is sensitive, even to minimal disturbance, with erosion occurring after heavy rain, and it depends heavily on a significant undisturbed leaf layer. In 1990, 143 plant species, 115 indigenous (80%) and 28 introduced (20%), were listed for the reserve (McMahon et al. 1990). These included around 33 orchid species. Under 'Ecological Objectives' the report stated that prescribed fire will play an important role in maintaining and enhancing the ecological values of the reserve. In the first instance, fire is required for the control of Large Quaking-grass Briza maxima. The rationale for prescribed burning relates to concern by local residents that the vegetation in the reserve increases the likelihood of property damage caused by uncontrolled fire.
Fire history of the reserve
Natural fire has not occurred in the reserve in the past 60 years (pers. obs.), despite several large and destructive fires in the local area in the 1960s. Deliberate burning has been used in autumn as a management tool, in rotating patches in 1992, 1997, 2002 and 2006, with an estimated 70% of the total area burnt over that time. The 2006 burn was moderately hot, destroying most of the litter layer and ground cover including mosses, grasses and herbaceous species.
Observations of the flora, and orchids in particular, have been made repeatedly since 1960 and findings are described below. Two areas are compared for orchid growth: areas unburnt since 1997 and an area burnt in 2006. Observations were made from June/July of 2012 until the end of February 2013 and from June/July 2013 until the end of February 2014 to encompass spring, summer and early autumn flowering. There are now large areas in this reserve with very few orchid species and only scattered colonies. The number of plants in 1 m x 1 m quadrats was recorded, selecting areas where the density of each species was greatest. The scatter of plants was too sparse to use larger quadrats or transect sampling.
Two measures were made for colony-forming species in both burnt and unburnt areas. One was where colonies were most abundant and dense, and another in less dense colonies that were more typical of the area. Non colony-forming species were very scattered and recorded in 1 m x 1 m quadrats where they were most abundant. Populations were surveyed from emergence of leaves and stems bearing flowers from June/July until the end of February to encompass spring, summer and early autumn flowering.
Nomenclature follows VicFlora (2015) except where otherwise indicated.
General Observations 1960-2012
Over the period 1960-2012, the eucalypt dominants have reduced in number and density with progressive die-back and opening of the canopy. The original dry sclerophyll forest was described as woodland, a more open vegetation, in 1990 (McMahon et al. 1990), confirming that a loss of cover from the original vegetation had occurred.
Weed species, especially Briza maxima, have been prominent since the 1960s in some disturbed areas where rubbish was dumped and have increased in recent decades, along with exotic garden species. In areas unburnt for 20 years, the variety of native grasses, legumes, composites and other wildflower species has remained fairly constant. Some old roads and tracks have regenerated understorey and ground layers, with almost full coverage. In the patches burnt since 1992, Cassinia species and Golden Wattle Acacia pycnantha have developed a medium-tall shrub layer, larger in places and shading the ground; species diversity and the moss layer are much reduced here. There are no formal records available of species flowering for the period 1978-1990, before deliberate burning commenced, but my general observations indicate diversity similar to that reported in McMahon et al. (1990).
Most of the orchid species in the reserve were seen until the drought, with its drier summers, in the 1990s, after which there was a marked decline from 33 species to 12 species (Table 1). In particular, the mid-summer and autumn flowering species have been seen rarely, most not at all in the latter period. The areas adjacent to the reserve also were rich in orchids until urban development overtook them in the 1980s and 1990s.
Until 1980 there were large colonies, up to 20 m in diameter, of Pterostylis curta, P. nutans and P. pedunculata, as well as scattered P. longifolia and groups of Glossodia, Caladenia, Diuris, Thelymitra and Microtis. Maximum numbers were seen in July to October, followed by a few late spring-summer species including Calochilus robertsonii, P. rufa and Dipodium punctatum. In the past 10 years, only an occasional flowering plant of these species has been seen. The area of the reserve with the highest orchid density is now along the borders of an old track where fire has not destroyed the previously abundant moss beds.
Over the past 15 years, the size and number of Pterostylis colonies and total numbers of all orchids have progressively reduced. In particular, flowering Caladenia species have reduced from five to one species, Diuris from three to one species and Glossodia major, once common, has not been sighted. Nearly all of the species listed in Table 1 were recorded as abundant or common up to 1979, but now are rated as scattered, uncommon or absent.
In 1980, areas adjacent to the fenced reserve supported an intact ground layer of grasses and low plants, including most orchid species (Table 1). Orchid plants are now rare outside the fence. At west Warrandyte, fires result in extreme desiccation and death of terrestrial tubers in the shallow 2-6 cm topsoil (pers. obs.).
Area burnt prior to 2006
A ground layer of grass and moss is regenerating, and Cassinia and Acacia sp. persist, with die-back and general decline occurring 12-20 years after fire. The commoner orchid species P. curta, P. nutans, P. pedunculata, Diuris maculata and Thelymitra species are scattered, with less dense colonies compared to the larger colonies that occurred prior to 1990.
Area burnt in 2006
The deliberate burn involved a rectangular area approximately 40 x 80 m from the apex of the hill, extending north towards the reserve entrance on the northern fence line. There is a scattered eucalypt overstorey and a shrub layer about 6 m tall, often with a complete canopy dominated by fire-stimulated Cassina sp. and Golden Wattle Acacia pycnantha. The grassy layer with moss, herbs, composites and legumes is almost completely absent and replaced by eucalypt and Cassinia litter. In places, bare burnt soil is still evident around tree bases, indicating that the fire was intense. There are a few scattered plants of Dusky Coral Pea Kennedia rubicunda, Grassland Cranesbill Geranium retrorsum and grasses.
Comparison of orchid populations in burnt areas and areas unburnt since 1997
Orchid populations in unburnt areas (2012-17) Pterostylis nutans is the dominant species in unburnt areas, with a maximum of over 400 individuals m-2 (Table 2). The other colonial Pterostylis species, P. pedunculata with 176 individuals m-2 and P. curta with 176 individuals m-2, each had 30% of plants flowering. Diuris had 22 individuals m-2. The more scattered P. longifolia had 15 individuals m-2, most being in flower. Microtis unifolia occurred in very isolated colonies, 145 plants m-2 and not flowering. Diuris pardina, Caladenia dilatata, Thelymitra aristata and Dipodium punctatum were scattered in low numbers at =10 m-2, most in flower. Genoplesium despectans was not found in the reserve. Most plants in the unburnt areas were associated with a dense moss bed or assemblage of grasses (Figs 2 and 3).
Orchid populations in areas burnt in 2006
In the burnt areas, only infrequent plants of P. pedunculata, P. curta and P. melogramma were found. Pterostylis pedunculata was the only species flowering, with a density of 4m-2 at one site only (Figs. 4-7). Plants were short and in poor condition, with small grey-green leaves in dry leaf litter, usually where moss was beginning to regenerate.
In the periods 2013-14, 2014-15 and 2015-16, general observations and estimates of numbers were made, with very similar results to those presented in Table 2. No flowering plants were seen in the area burnt in 2006. In 2016-17, specifically in August and September, there was significant regeneration of moss beds and scattered plants of P. pedunculata, P. curta and P. nana with only P. pedunculata in flower (six plants in total in 2016). There were scattered groups of very small Pterostylis seedlings in moss; these were not able to be identified to species level. More seedlings were found in 2017, and three plants of P. pedunculata flowered. No additional species were found.
Only 14 species were seen from 2012-16 compared with 33 recorded in 1967-78 (McMahon et al. 1990). This is a significant reduction which is not explained by seasonal variation.
The effects of deliberate burning of Dry Sclerophyll Forest on orchid diversity and populations
Prescribed burning in 2006 and earlier has led to a marked decrease in the number of terrestrial orchid species, in both the total number of plants and the number of plants in flower. Barnett (1984) found a similar result in wet sclerophyll rainforest in Kinglake on deeper and moister clay soils. Following the Black Saturday fires, Duncan (2013) assessed the terrestrial orchids at Kinglake, Lake Mountain, Bunyip and Wilsons Promontory, and found only a small number of species adversely affected. The species at Tindals Wildflower Reserve are different from Duncan's study apart from one, P. longifolia, and the soils are shallower and the rainfall lower. Duncan found that P. longifolia was not adversely affected by fire, unlike at Tindals Wildflower Reserve, where it was almost eliminated in burnt areas. In general, studies have indicated Pterostylis is not inhibited by fire on deeper soils (Backhouse and Jeanes 1995). The findings in this study indicate marked decline in genera and in all Pterostylis species and it is suggested that soil type and depth, and position of orchid tubers in relation to ground level are very important factors in responses to fire.
The observations in August and September 2016-17 indicate the return of significant numbers of seedlings and a few flowering plants of P. pedunculata 10-11 years after the 2006 burn. These may represent the spread of seed from flowering plants in the three unburnt adjacent areas. It is doubtful that regeneration was from tubers surviving over 10-11 dry seasons.
Compact clay soils retain heat longer than non-clay soils and temperatures are high near the surface, reaching 60[degrees]C at a depth of 7-8 cm (Beadle 1940), sufficient to explain the death of tubers at west Warrandyte. Diuris and Pterostylis generally have the largest tubers and are more likely to survive mild to moderate intensity fire. Caladenia, Corybas, Glossodia and Acianthus have smaller tubers and are less likely to survive fire. They have not been observed in the reserve for many years. Burns in the last 15 years are likely to have been hotter due to much drier conditions and greater build-up of leaf litter. Prolonged dryness after 2006 has contributed to poor regeneration of the moss beds, grasses and herbs necessary for orchid populations to thrive. The shading effects of fire-stimulated Cassinia and Acacia also may reduce regeneration.
There are other factors contributing to orchid decline, including grazing by rabbits, trampling by foot traffic, and digging up of tubers by the White-winged Chough Corcorax melanorhamphos (pers. obs. 2012-14) (Fig. 7). There is no buffer zone of orchids around the fenced reserve to aid regeneration. In larger reserves such as Warrandyte State Park, with fewer burnt areas, the smaller species are still found.
The long-term sustainability and management of small reserves in areas of residential development is difficult, and experienced rangers are pessimistic about their viability, particularly after a sequence of dry years (Cam Beardsell, pers. comm. 2014). The objectives of increased diversity and reduction and control of weed species for Tindals Wildflower Reserve (Mc-Mahon et al. 1990) have not been realised. Weed species such as B. maxima and exotic garden escapees, established for 40 years in parts of the reserve, are common and have not declined, despite the deliberate use of fire and hand weeding. Diversity of all strata, especially the ground layer and orchids, has been dramatically reduced.
The reserve has a relatively low fuel load due to the short stature of vegetation and low biomass associated with skeletal and poor soil. The small size of the reserve and relatively low biomass in relation to other areas of natural vegetation do not justify the fears of residents concerning increased fire risk. There has not been a major fire risk or significant fire in the past 60 years, in contrast with nearby areas. Fire has been very destructive to all orchids and many other species at this site, which has very shallow soil. Rotational prescribed burning since 2003 has contributed strongly to decline in all orchid species. To maintain residual colonies, deliberate burning should be avoided. Other management strategies such as manual removal of weed species, restriction of foot traffic to paths, and exclusion of rabbits are recommended.
Cam Beardsell discussed management issues of reserves. Millie Lee provided historical information regarding the reserve and its plants.
Backhouse G and Jeanes J (1995) The Orchids of Victoria. (The Miegunyah Press: Carlton, Victoria)
Barnett JM (1984) The Initial Effects of Fire on Orchids in Stringybark Box Forest. The Victorian Naturalist 101, 188-190.
Beadle N (1940) Soil temperatures during forest fires and their effect on the survival of vegetation. Journal of Ecology 28, 180-192.
Dixon K (1996) Influence of fire on management of Australian orchid habitats. In IUCN/SSC Orchid Specialist Group (1996) Orchids--Status Survey and Conservation Action Plan, p. 20. Eds E Hagslater and V Dumont (IUCN: Gland Switzerland and Cambridge, UK)
Douglas JG and Ferguson JA (eds.) (1976) Geology of Victoria. Geological Society of Australia. Special Publication No. 5.
Duncan M (2013) Response of orchids to the 2009 Black Saturday bushfires. The Orchadian 17, 415-422.
Erickson R (1965) Orchids of the west. 2nd edn. (Paterson Brokensha: Perth)
McMahon ARG, Bedggood SE and Carr GW (1990) The vegetation and management of Tindals Road Wildflower Reserve, Warrandyte, Victoria. Ecological Horticulture Pty. Ltd., Report for the City of Doncaster and Templestowe, Clifton Hill, Victoria.
VicFlora (2015) Flora of Victoria, Royal Botanic Gardens Melbourne <http://data.rbg.vic.gov.au/vicflora>
Peter B Adams
School of Biosciences, The University of Melbourne; National Herbarium of Victoria, Royal Botanic Gardens, Melbourne Email: email@example.com
Received 22 March 2018; accepted 27 September 2018
Table 1. Records of orchid species in Tindals Wildflower Reserve 1967-2017. *Nomenclature follows McMahon et al. (1990). Orchid species recorded 1967-78 Common Name Recorded 2012-15 Acianthus exsertus* Mosquito Orchid Caladenia carnea Pink Fingers Caladenia Caladenia dilatata Green-comb Spider Orchid + Caladenia gracilis Musky Caladenia Caladenia praecox Early Caladenia Calochilus robertsonii Purplish Beard-orchid Corybas diemenicus Stately Helmet-orchid Corybas incurvus Helmet Orchid Cyanicula caerulea Blue Caladenia Cyrtostylis reniformis Gnat Orchid Dipodium punctatum Hyacinth Orchid + Diuris lanceolata* Small Golden Moths Diuris orientis Wallflower Orchid Diuris pardina Leopard Orchid + Diuris x palachila Donkey Orchid Diuris pardina Leopard Orchid Diuris sulphurea Tiger Orchid Eriochilus cucullatus Parson's Bands Genoplesium despectans Sharp Midge-orchid Glossodia major Wax-lip Orchid Microtis unifolia Common Onion-orchid + Pheladenia deformis Bluebeard Caladenia Pterostylis sp. aff. alata Striped Greenhood + Pterostylis curta Blunt Greenhood + Pterostylis melagramma Tall Greenhood + Pterostylis nana Dwarf Greenhood + Pterostylis nutans Nodding Greenhood + Pterostylis parviflora Tiny Greenhood + Pterostylis pedunculata Maroon Hood + Pterostylis plumosa Bearded Greenhood Pterostylis revoluta Autumn Greenhood Pterostylis rufa Rusty-hood Thelymitra aristata Great Sun-orchid + Thelymitra rubra Salmon Sun-orchid Table. 2. Effects of fire on orchid populations in Tindals Wildflower Reserve (1 m survey squares). Species Burnt area Burnt area Unburnt area 16/7/2012 flowering non-flowering flowering Nodding Greenhood Pterostylis nutans Maximum density 0 0 126 Nodding Greenhood Pterostylis nutans Moderate density 0 0 37 Maroon Hood Pterostylis pedunculata Maximum density 4 10 49 Maroon Hood Pterostylis pedunculata Moderate density 0 2 13 Blunt Greenhood Pterostylis curta Maximum density 0 6 40 Blunt Greenhood Pterostylis curta Moderate density - - 3 Tall Greenhood Pterostylis melagramma Maximum density 0 2 11 Tall Greenhood Pterostylis melagramma Moderate density - - 6 Leopard Orchid Diuris pardina 0 0 6 Green-comb Spider Orchid Caladenia dilatata 0 0 5 Dwarf Greenhood Pterostylis nana 0 0 15 Species Unburnt area 16/7/2012 non-flowering Nodding Greenhood Pterostylis nutans Maximum density 310 Nodding Greenhood Pterostylis nutans Moderate density 70 Maroon Hood Pterostylis pedunculata Maximum density 142 Maroon Hood Pterostylis pedunculata Moderate density 24 Blunt Greenhood Pterostylis curta Maximum density 136 Blunt Greenhood Pterostylis curta Moderate density 12 Tall Greenhood Pterostylis melagramma Maximum density 4 Tall Greenhood Pterostylis melagramma Moderate density 3 Leopard Orchid Diuris pardina 0 Green-comb Spider Orchid Caladenia dilatata 1 Dwarf Greenhood Pterostylis nana 7
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|Author:||Adams, Peter B|
|Publication:||The Victorian Naturalist|
|Date:||Dec 1, 2018|
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