Pterostylis gibbosa (R.Br.) Illawarra Greenhood Orchid Recovery Plan

NSW National Parks and Wildlife Service, September 2002
ISBN 0 731 36905 X

6 Biology and ecology

6.1 Life Cycle and Habit

Pterostylis gibbosa is a deciduous, perennial plant that dies back to an underground tuberoid during the summer season. During late summer or autumn the tuberoid produces a flat rosette of leaves up to 100 mm in diameter. In winter the flower spike develops and grows up to 45 cm tall. After flowering in September - October, the rosette withers as the fruits develop. The basal rosette of leaves often withers as the flower stalk emerges but may persist if there are cool, moist conditions in spring. The original tuberoid dies each year with a replacement tuberoid being formed on a small stolon or 'dropper'. Each year the plant will emerge in a slightly different location (but within a centimetre or so), depending on the length of the stolon to the new tuberoid. Time of emergence and time of desiccation are very dependent on the prevailing climatic conditions. In wet seasons, the plant emerges earlier and survives longer above ground (Recovery Team pers. comm.).

6.2 Phenology

Flowering occurs from late August and can last until early December in favourable seasons. Usually up to nine or ten (although up to 12 have been observed by Taylor (1999)) apple-green, hooded, waxy flowers are formed per spike. The lowest flowers open first, with opening occurring progressively up the spike as the lower flowers close and wither.

6.3 Reproductive Biology

6.3.1 Vegetative reproduction

Pterostylis gibbosa does not spread vegetatively to any great extent. Under exceptionally favourable conditions, a single plant can produce more than one new tuberoid in the same winter season. Additionally, an extra tuberoid could be produced in response to damage to the new tuberoid as a replacement to the damaged one (G. Bradburn, ANOS pers. comm.). In cultivation more than one tuber can be formed by removing new tubers prematurely. The plant responds by developing a new tuber from the rosette and by producing a new rosette from the tuber. This procedure can be successful more than once in the one season with optimised growing conditions in cultivation or occasionally in the field if climatic conditions allow.

6.3.2 Breeding System

Pterostylis species are generally pollinated by male gnats of the genus Mycomya. These insects are called fungus gnats, as one species has larvae that live in silken tubes on the underside of bracket fungi. Only two or three other genera are implicated in the pollination of Pterostylis, but these are regarded as being of minor significance (D. Colless, CSIRO pers. comm.). The male Mycomya are deceived into attempting to copulate with the labellum. The labellum is located on the end of a sensitive elastic strap that flips upwards in response to disturbance to partially block the opening of the hood.

In order for pollination to occur, the pollinator lands on the labellum with its dorsal side towards the flower. This triggers the labellum to spring back and trap the insect with its front end towards the top of the hood and against the column, where the stigma and anthers with the pollinia are positioned. If the right pollinator is trapped it will be able to crawl out through the opening at the top of the hood passing the stigma, where any pollen from previously visited flowers will rub off and then past the anthers where the pollinia will adhere to its head or back. If insects smaller than the pollinator become trapped they may crawl out without pollination occurring or, if larger than the pollinator, they may stay trapped until the labellum opens again.

Nothing is known of the habitat requirements of Mycomya. They are probably very common, but are not often seen due to their small size (D. Colless, CSIRO pers. comm.). Abundance is believed to peak in September, which is the main month of flowering of P. gibbosa.

Movement of the labellum has also been observed without any apparent triggering by an insect (Whelan & Kohler 1991). During one observation period, the labellum of selected flowers in one grouping covering several hectares were all closed prior to sunrise but began to open after sunrise. The labella of most of the observed orchids did not close again until well after dusk.

During a study by Zedler (1994), one insect was found with orchid pollinium attached and is thought to be a potential pollinator of P. gibbosa. It was sent to the CSIRO Division of Entomology for identification. This insect has not previously been described but is a Mycerophilid (fungus gnat) of the genus Heteropterna in the family Keroplatidae.

A study conducted by Whelan & Kohler (1991) at Yallah showed that, in 1990, only 8% of the flowers had pollinia removed. Darwin (1886) reported that in a New Zealand Pterostylis species the flower-to-capsule ratio was "much less than a quarter", which may indicate that the pollination rate should perhaps not be expected to be very high if this is true for other Pterostylis species. Only 11% of the flowers set fruit. Artificially self and cross-pollinated flowers both set seed, however it is not known if the seed produced by artificial self-pollination was viable (Whelan and Kohler, 1991). Genetic investigations in the same study confirmed that the orchid reproduces by outcrossing (R. Peakall, Australian National University pers. comm.), as genetic variability within the Transgrid populations was consistent with random mating, not selfing or clonal spread.

In an Honours project by Taylor (1999) on herbivory and pollination of Pterostylis gibbosa in four populations the flower-to-capsule ratio (pollination rate) was found to be 19% at Nowra, 14% at Milbrodale and 5% for both Yallah and Albion Park. The density of plants at Nowra was lower than at other sites yet it has the highest pollination rate. This may indicate that there may be factors other than density such as pollinator availability that are important for pollination success. The Nowra and Milbrodale sites adjoin surrounding forests, which is potential pollinator habitat. In contrast, the Yallah and Albion Park sites are surrounded by agricultural and urban land use. The limited potential pollinator habitat and use of pesticides and other chemicals may have an impact in reducing the number of available pollinators for these two sites and thereby lowering the pollination rate.

6.3.3 Fruiting

The fruit of P. gibbosa is a dry, dehiscent, obovoid capsule. When the fruit is mature, six longitudinal slits form through which the seed is shed. The seed is minute and each capsule contains thousands of seeds. The testa of the seed is elongate, giving the seed a high surface to volume ratio. This shape is found in other plant families and is characteristic of wind dispersal (Rasmussen 1995).

6.3.4 Seed biology and ecology

Orchid seeds do not have specialised storage tissue, but contain tiny lipid bodies within the embryo that serve as a food source. Nearly all species are reliant on a mycorrhizal fungus to support the first stages of growth after germination.

Pterostylis gibbosa sheds seed in spring and is dormant until the following autumn. The presence of fungi in the leaf litter, as for all terrestrial orchids, is needed for successful germination. P. gibbosa, like other orchids in the Rufa group, require the presence of a specific strain of Ceratobasidium cornigerum for successful germination (Mark Clements, Centre for Plant Biodiveristy and Research pers. comm.). This fungus is widespread and associated with leaf litter.

In autumn, when cooler temperatures and moist conditions stimulate the growth of the fungus, seeds of P. gibbosa imbibe. If infected by a specific fungus, the seed germinates and forms a tiny corm-like structure known as a protocorm. This develops into a tuberoid that stays dormant until the next growing season. In some cases, the protocorm formed between the fungus and the seed can remain dormant for up to two seasons. Seeds have the ability to exclude other fungi for about a month. However if they have not come in contact with C. cornigerum within this time they will be invaded and killed by other fungi (Quality Environmental Management 1994).

Sharma et al. (2000) found a high mean seed viability of 76% in a selection of seeds collected from 23 P. gibbosa plants (15 from Milbrodale, three from Yallah, three from Albion Park and two from Nowra).

6.3.5 Seedling growth

Seedlings appear to take several seasons before reaching sexual maturity. Depending on the growing season, the seedlings may form one or two leaves one season and then re-emerge and form three or more leaves the next season.

Seedlings first appear as a plant of one or two very small leaves. As the growing season continues these leaves will increase in size. New seedlings will continue to appear throughout the growing season. Therefore, in any one year some seedlings will be significantly larger than the newly appearing seedlings.

A seedling in its second year will appear as a relatively large and vigorous two leaf seedling. (A mature plant that has been severely affected by adverse conditions will have the same appearance as a second year seedling that has developed rapidly under ideal conditions.)

In the third year seedlings will vary greatly depending on attributes and environment. Some will remain as two leaf plants while others will have three or more leaves and will be counted as a non flowering mature plant. Some of these may flower.

6.4 Population Structure

Pterostylis gibbosa tends to occur in clumps with individuals scattered in clusters. Outside these clumps, only scattered individuals or small groups are found. The smaller clusters within the patches usually consist of one or several mature flowering plants with a scatter of more numerous small rosettes that do not flower (Whelan & Kohler 1991).

6.5 Response to Disturbance

6.5.1 Response to fire

P. gibbosa is capable of surviving fire, due to the regenerative capacity of its tuberoid. Occasional fire may be necessary to provide suitable conditions for the establishment of seeds and seedlings and to maintain habitat suitable for the survival of the species (Recovery Team pers. comm.).

If a fire occurs after the orchid has emerged (winter or spring), the above ground parts of the plant are destroyed and the orchid will generally remain dormant until the following season. Fires at this time clearly preclude seed set in that season.

In the long term, frequent fire at this time of year will probably eliminate the population since the plant does not propagate itself vegetatively to any great extent (see sections 2.5 and 6.3.1). Even though the tuber replaces itself each year, it is not known how long it can keep doing this.

Summer fires are assumed to have the least impact on P. gibbosa, as the species remains dormant underground over summer, provided such fires are not of a sufficient intensity to destroy the underground tuber. Observations following bushfires in East Gippsland's Croajingalong National Park, Victoria in March 1983 and the effects of Ash Wednesday, February 1983, in the Adelaide Hills on orchids (The Orchadian, 1984) indicate that high intensity fires can destroy some orchid tubers completely.

The Croom Road site at Albion Park experiences frequent fires that are the result of arson. Despite this, the P. gibbosa population appears to have increased in numbers (G. Bradburn, ANOS pers. comm.). This appears to argue against the supposition that high frequency fire is a key threatening process for P. gibbosa.

It must be noted that the fires at Croom Road are generally localised and it is unknown how frequently and at what time of year areas of the site that contain P. gibbosa are being burnt. It does appear however, that burning areas adjacent to P. gibbosa populations provides ideal conditions for recruitment (G. Bradburn, ANOS pers. comm.).

Monitoring in 1999 where plants were tagged in four areas, two burnt in December and two that remained unburnt, indicated that there was no difference in the number of leaves and rosette diameter between burnt and unburnt areas. Due to a fire in the unburnt areas in August/September, it was not possible to compare flowering or pollination data between the burnt and unburnt areas.

Any prescribed fires in areas containing P. gibbosa should be undertaken after flowering and prior to emergence of the rosette. This period will vary with climatic conditions but generally occurs between December and February. Monitoring should be undertaken to ensure that rosettes emerge after any prescribed fires, and that they set seed and recruit.

6.5.2 Response to drought

In seasons that become dry after the rosette has appeared, individuals may not flower or may even die back to the tuberoid. If conditions later improve, the plant may re-emerge but in these circumstances flowering is rare (G. Bradburn and J. Riley, ANOS pers. comm.). In exceptionally severe seasons the plant may not emerge at all.

Generally, the species is quite flexible in its response to climatic conditions with plants dying back in poor conditions and growing quickly in good times. Additionally, the plant may die back and reappear within a season.

6.5.3 Response to grazing

Grazing can have a beneficial or unfavourable effect on Pterostylis gibbosa, depending on its timing and intensity. The removal of adjacent vegetation cover may benefit the orchid by increasing light levels and allowing the growth of P. gibbosa where vegetation cover is dense. The extensive removal of adjacent vegetation cover may be unfavourable as it will reduce moisture levels and increase exposure of the orchid to heat and wind.

Grazing causing the removal of P. gibbosa plant parts is likely to have a direct effect on the growth potential and reproductive success since each plant produces only one flowering stem in a season. When damage is inflicted it completely denies the plant the chance to reproduce until the following season. If herbivory levels remain high in subsequent seasons this may be detrimental to the long term reproductive success and survival of the species (I. Taylor pers. comm.).