Cumberland Plain 2005

Fire is a key ecological process in a grassy woodland

In 2005 I completed a PhD in fire ecology. From 2001 to 2005 I studied effects of different frequencies of fire in Western Sydney’s woodlands. Mostly I worked in Cumberland Plain Woodland (CPW), a threatened ecosystem where most plant species are little things: grasses, lilies, herbs. This is Darug country, and was once open grassy woodland. In the 1820s, Dr Peter Cunningham noted that “you might, generally speaking, drive a gig in all directions without any impediment in the shape of rocks, scrubs, or close forest.” The Western Sydney landscape has changed dramatically since then. Less than 8% of CPW is left.

Recording data in a 2 x 2 m plot along a 100 m transect

For my main study I identified recently-burnt areas that had had different frequencies of fire, ranging from very frequent (every 1-3 years), to moderately frequent (4–10 years), to infrequent (20-50 years between the last two fires). I had nine sites in this study, and surveyed twelve 100 metre-long transects in each site, measuring and counting trees, shrubs and grasses. In other studies I looked at flowering at different post-fire ages, at post-fire seedling establishment, and at ground layer plants growing in different woodland microhabitats. I measured how rapidly fuel accumulated.

While most people see Western Sydney through the lens of its spreading suburbs, I spent my time there in the bush – and came to appreciate and value grassy woodland. CPW grows on clay soil and has most of its plant diversity in the ground layer. It is very different to the shrubby heaths and woodlands north, east and south of Sydney, which grow on infertile sandy soils.

Ropes Ck w kangaroos 10 mth pf ready.jpg

Ropes Creek, one of three frequently burnt sites in my study. Frequently burnt sites were open and grassy, with few shrubs. Note the kangaroos.

What did I find?

Fire plays several significant roles in CPW. It keeps shrub species in balance, invigorates the dominant grass, keeps weeds at bay, and safeguards open patches favoured by some herbs. Crucially, fire maintains CPW's grassy woodland structure.

Trees and fire in CPW. Trees, which were almost all eucalypts, were not greatly affected by the wide range of fire regimes I surveyed. Density and basal area of adult trees did not differ significantly between fire frequencies, and nor did the density of juveniles or saplings.

Shrubs and fire in CPW. While most of the plant species in CPW are in the ground layer, there’s also a number of shrub species, particularly peas and wattles. Most shrubs were most abundant under the moderately frequent fire regime of one or two fires per decade. This was particularly the case for obligate seeders, which grew and flowered rapidly, taking advantage of summer rainfall that occurred in each of my study years, despite the prevailing Millennium drought. However one native shrub, the prickly Blackthorn, was most abundant where fire frequency had been low, to the point where it dominated the landscape. Exotic shrubs (weeds) were also recorded most often where fire had been rare.

Blackthorn (Bursaria spinosa) at Orchard Hills, an infrequently burnt site.

Grasses and fire in CPW. Grass cover was high across all my sites, however the originally-dominant Kangaroo Grass was much less abundant at low fire frequency than where fire had occurred more often.

Herbs and fire in CPW. Native herb species richness (the number of species in a given area, in this case ~10 square m), didn’t differ with either fire frequency or microhabitat (around trees, under Blackthorn bushes, out in the open away from trees and Blackthorn). Some species were more abundant in particular microhabitats or at particular fire frequencies. Non-native herbs (weeds) were three times more common at low fire frequency than where fire frequency was high.

Fuel in CPW. Fuel load developed quite rapidly, but the maximum values reached were low relative to those in Sydney’s shrubby woodlands. Infrequently burnt sites had more shrub fuel, and less grass fuel, than frequently burnt ones.

Above: Another frequently burnt site, at Holsworthy. This photo was taken 12 months after a wildfire. The bronze colour in the understorey is profusely flowering Kangaroo Grass (Themeda triandra).

Right: This site, in Plumpton Park, had burnt moderately frequently in patchy arson fires. Once a rubbish dump, rescued by local residents, this small park had an impressive diversity of grasses, herbs and shrubs. The light green grass in the foreground is the native Weeping Grass (Microlaena stipoides).

Photo by Annie Storey.

Thanks to my PhD supervisors Assoc Prof Charles Morris (Western Sydney University), Prof Ross Bradstock (University of Wollongong) and Dr Grahame Douglas (Western Sydney University). Thanks also to many hardy field assistants.

Here’s a short summary conference paper.

Published papers are here and here.

You can access the entire thesis from here (click the thesis title above my name).

This final thesis chapter (PDF) pulls together findings and their implications for management.

Some herbs from herb-rich Cumberland Plain Woodland. L to R: Blue Trumpet (Brunoniella australis), Early Nancy (Wurmbea dioica), Native Leek (Bulbine bulbosa), Bears-ear (Cymbonotus lawsonianus), Austral Bugle (Ajuga australis).