Grevillea obtusiflora subsp. obtusiflora and subsp. fecunda Recovery Plan

NSW National Parks and Wildlife Service, September 2001
ISBN 0 731 36281 0

6 Biology and ecology

6.1 Reproductive Biology

6.1.1 Vegetative reproduction

Grevillea obtusiflora subsp. obtusiflora

Personal observations indicate that G. obtusiflora subsp. obtusiflora is likely to be a clonally reproducing taxon with each plant capable of spreading over a wide area. Makinson (1997) noted that suckering stems may arise at more than approximately 1.5m from a parent ramet. G. obtusiflora subsp. obtusiflora is apparently wholly dependent on root suckering for reproduction (Makinson 1997). The term 'root suckering' is here used to indicate the initiation of daughter ramets at some distance from the parent plant. It is likely, but not yet confirmed, that the tough wandering "roots" that form inter-ramet connections are in fact rhizomes (modified subterranean stems) rather than true roots. The suckers become effectively independent of the parent plant once established. The rate of recruitment is unknown.

The clonal root system was found to be much more substantial in G. obtusiflora subsp. obtusiflora than G. obtusiflora subsp. fecunda, with roots extending for many metres (pers. obs.).

Root suckering species respond favourably to mechanical soil disturbance and often quickly recolonise roadside scrapes. Several Grevilleas have developed vegetative reproduction by root suckers to the complete exclusion of sexual reproduction (Olde & Marriott 1994).

Makinson (1997) has suggested that the Clandulla sub-population of G. obtusiflora subsp. obtusiflora comprises a very few clonal lines.

Grevillea obtusiflora subsp. fecunda

Makinson (1997) found that G. obtusiflora subsp. fecunda also reproduces through root-suckering. The roots of one plant are capable of spreading over a wide area with numerous genets above ground (pers. obs.). The extent of clonality in this taxon is not known.

6.1.2 Breeding system

Grevillea obtusiflora subsp. obtusiflora

G. obtusiflora subsp. obtusiflora flowers sparsely in winter and spring with flowering peaking in September (Figure 7). G. obtusiflora subsp. obtusiflora's floral morphology indicates it is predominantly pollinated by birds, with bees being potential secondary pollinators (Makinson pers. comm.). The potential reduction or elimination of a specific pollinator may be another cause in failure to set fruit in this taxon. Plant patch structure and interpatch distances may affect pollination within and among patches and therefore affect fruit set (Eriksson and Bremer 1993). G. obtusiflora subsp. obtusiflora is thought to not produce seed and to only reproduce vegetatively.

G. obtusiflora subsp. obtusiflora flower
Figure 7 G. obtusiflora subsp. obtusiflora flower (detail).

It is not known whether the failure to produce seed is the result of inbreeding depression or self-incompatibility (Makinson 1997).

G. obtusiflora subsp. obtusiflora flowers and styles on some plants have been noted to wither, and in some cases detach from the plant. Ants were also recorded in some flowers. Flower dissection has not revealed physical factors that may detrimentally affect the ability of G. obtusiflora subsp. obtusiflora to produce seedpods and seed, additionally, swelling of the ovary was not detected in any G. obtusiflora subsp. obtusiflora flower (pers. obs.).

Makinson (1997) stated that in most cases, when a Grevillea species exhibited a partial or total 'abandonment' of sexual reproduction, correlated with minor morphological differences, a variably expressed capacity for vegetative reproduction occurred in the 'parental' populations. Makinson (1997) also suggested that an inability to set fruit in clonal populations may result from self-incompatibility or compounded inbreeding depression within populations that may ancestrally have been preferentially out-breeding.

Other biological causes of failure to produce seed may include pollen unviability, stigma non-receptivity or shrunken pollen grains.

Ramets of some clonal plant species alter their rhizome lengths in response to their microenvironment, i.e. they utilise favourable habitat, this is termed 'clonal foraging' (Cain et al. 1996). Personal observations indicate that G. obtusiflora subsp. obtusiflora appears to undertake this type of foraging as stems were variably spaced and limited excavation supported the extent of clonality.

The lack of regular new genetic input from sexual reproduction, may also indicate that the genetic diversity of the population is low.

Grevillea obtusiflora subsp. fecunda

G. obtusiflora subsp. fecunda flowers abundantly and sets copious amounts of fruit (Makinson 1997) (Figure 8). Seed has been collected by the Society for Growing Australian Plants (SGAP). It is possible, in common with other Grevillea species, that the seeds are rapidly gathered and dispersed by ants, as the seeds have elaiosomes that may encourage harvesting by ants.

Due to the particular flower morphology, G. obtusiflora subsp. fecunda is predominantly pollinated by birds, with bees being potential secondary pollinators (pers. obs.). Birds have been shown to be particularly attracted to the colour red, birds seen feeding on the nectar of G. obtusiflora subsp. fecunda include the Yellow-tufted Honeyeater and the Eastern Spinebill (pers. obs.).

G. obtusiflora subsp. fecunda flower
Figure 8 G. obtusiflora subsp. fecunda flower (detail).

6.1.3 Seed Production, Phenology and Fecundity.

Grevillea obtusiflora subsp. obtusiflora

Fruits, seeds and seedlings have not been recorded for G. obtusiflora subsp. obtusiflora.

Eriksson (1993) suggested that many clonal plants do not show signs of senescence and as a consequence possess almost unlimited fecundity.

Lack of success in seed production limits the distribution of a species, including the establishment of new sites, as long distance dispersal is reduced without seed production, additionally seeds are the common initiator for new populations (Eriksson 1993).

Grevillea obtusiflora subsp. fecunda

Grevillea obtusiflora subsp. fecunda flowers profusely and virtually all flowers go on to produce follicles (pod-like fruit) (Figure 9). Although some follicles will not set seed, most will set two seeds. Relative to the number of flowers, seed set is low, a tendency reflected in other Grevilleas (Olde & Marriott 1994). Flower heads in both G. obtusiflora subsp. obtusiflora and G. obtusiflora subsp. fecunda were bagged in an attempt to gather seed. A total of 12 seeds were collected from G. obtusiflora subsp. fecunda.

Grevillea obtusiflora subsp. fecunda follicle
Figure 9 - Grevillea obtusiflora subsp. fecunda follicle

6.1.4 Seed viability, dormancy and germination factors

Grevillea obtusiflora subsp. obtusiflora

As mentioned previously G. obtusiflora subsp. obtusiflora has not been observed to set seed.

Grevillea obtusiflora subsp. fecunda

The dormancy mechanisms, if any, used by G. obtusiflora subsp. fecunda have not been established.

In Grevillea, dormancy strategies can include chemical barriers, and physical barriers such as durable seed coats and waxy or corky layers. The dormancy mechanism varies from species to species, and Grevillea seed generally requires some combination of the following circumstances to germinate; weathering, stratification, soaking/leaching, removal of elaiosome, and fire (Langkamp 1987, Olde & Marriott 1994).

6.1.5 Seed dispersal, seedling establishment and growth

Grevillea obtusiflora subsp. fecunda

Most fruits in Grevillea fall soon after dehiscence, but in some may persist to the next flowering season and beyond (Olde & Marriott, 1994). Seed dispersal in many Grevillea species is thought to be carried out by ants who carry the seed into nests and eat the elaiosome or waxy wing attached to the seed, this process is termed myrmecochory (Olde and Marriott, 1994). In other Grevillea species germination of seedlings has been noted emerging from ant nests, this process has not been observed in G. obtusiflora subsp. fecunda to date. Seed in this taxon is most likely dispersed directly below the plant and is distributed by wind, water and ants.

G. obtusiflora subsp. fecunda is a pioneer taxon and shows a definite association with mechanically disturbed ground, such as found along the road at one of the sites (Makinson 1997). However, it is not known whether the response to disturbance is the result of seed germination or vegetative reproduction.

Seedlings have been observed at Site F2 in an area of disturbance and after a period of heavy rain, indicating that the seeds of this taxon are viable.

6.2 Population Structure

Grevillea obtusiflora subsp. obtusiflora

The number of plants is estimated at approximately 1400 ramets at nine sites within the Clandulla State Forest. Additionally, several plants have been found on freehold land adjacent to the Clandulla State Forest. It is difficult to establish the exact number of genets within the population due to the degree of clonality exhibited by the taxon. There is evidence of recent vegetative recruitment with young suckers (pers. obs.).

Grevillea obtusiflora subsp. fecunda

The number of plants is estimated at nine hundred adult plants at three separate sites. The roadside sites tend to be in long 'strips' along the roads, with plants growing in clusters within suitable habitat. Numbers remained stable during the survey period. Seedlings have been recorded at site F2 (pers. obs.).

6.3 Herbivory and Seed Harvesting

Grevillea obtusiflora subsp. obtusiflora

No herbivory on this plant has been observed to date, despite rabbit droppings being abundant at one of the sites. Feral goats and pigs are also expected to be present in the general locality.

Grevillea obtusiflora subsp. fecunda

In Grevillea, seed harvesting is often carried out by ants (Olde and Marriott 1994). The ants are thought to consume the seed's waxy wing or elaiosome, which may assist in overcoming one of the seed's dormancy mechanisms.

As many of the country roads in the locality act as routes for stock transport, there is some potential for the plants to be trampled or grazed. However, herbivory on this taxon has not been recorded.

6.4 Fire Ecology

Grevillea obtusiflora subsp. obtusiflora

Within the genus Grevillea there are a range of mechanisms in species that allow persistence after a fire event. G. obtusiflora subsp. obtusiflora's root suckering habit would assist its survival of a fire event. Although the overall effect of fire on the survival of this taxon is not known, numerous suckers resulted from a burn in 1996 at site O2. The main site in Clandulla State Forest (O1) is thought to have burnt approximately 30 years ago (Ken Sampson pers. comm.). Makinson (pers. comm.) suggests that a fire interval in the order of 10 - 15 years is likely to favour the taxon.

Grevillea obtusiflora subsp. fecunda

G. obtusiflora subsp. fecunda appears to have two strategies to assist in surviving a fire event; root suckering, and possibly the burying of seed by ants. Seed that has been buried is likely to have a degree of protection from the high temperatures that are generated in a fire. It is possible that an ash bed response assists germination after fire with the influx of nutrients, and possibly dormancy-breaking smoke compounds into the soil. As it is thought that it would take four to five years after a fire for an adult plant to produce flowers, accordingly, a fire interval in the order of 10-15 years is likely to provide an opportunity for plant maturation and a period of seed production.

The rural areas in which the largest sites of Grevillea obtusiflora subsp. fecunda are found have not been burnt for approximately 30 years (Bruce Bell, Rylstone Shire Council).