Fire has a different dynamic in Cumberland Plain Woodland compared with shrubby woodland vegetation on sandstone. Differences in fuel loads between the two are crucial. We discovered that most species responded to subsequent rain in the bare space created by fire, rather than to the direct effects of fire - heat, smoke or ash.
Some effects of fire
There were no signs of any recent burning in our woodland in 1988, and the area had evidently remained unburnt at least since the early 1980s, and probably much longer, as stock grazing presumably kept the grass low. However there was vigorous regrowth taking place in the early years of our recording, and little evidence of aging or senescent shrubs. Aging or senescent shrubs are often conspicuous in Sydney Sandstone vegetation that has remained unburnt for long periods.
However we assumed that fire would be important in Cumberland Plain Woodland ecology, and did want to try out treatments that might remove weed species and benefit native species. With this as an aim we burnt part of the woodland in September 1991.
Three plots were burnt in the September 1991 fire. Average native plant species richness dropped slightly immediately after the fire, and then returned to prefire levels (see Figure below). However native species richness at unburnt plots showed similar drops, indicating that this was weather-related rather than fire-related (1992 and 1993 had below average rainfall). Indeed although there was a difference in species richness between burnt and unburnt sites before the fire, these differences were reduced after the fire.
Average exotic species richness increased significantly immediately after the fire by over 50% compared with prefire levels 3 years earlier (see figure below), but in the next year it dropped to levels similar to that of unburnt sites and remained similar to them for the next 8 years.
In general we did not note any obvious long-term changes in native species composition resulting from the burn and within 3 years the burned area was not noticeably different from adjacent unburnt areas. The main result of the 1991 fire however was that we were able to document the fire responses for a large number of species and demonstrate that the majority of these species could resprout after fire.
Ten years later in September 2001 we burnt the same area again, and the following year September 2002 we burnt an adjacent area. Both fires were again Spring burns and of low intensity.
Fire has a different dynamic in Cumberland Plain Woodland compared with shrubby woodland vegetation on sandstone. Differences in fuel loads between the two are crucial. In our woodland there is not the heavy fuel build-up that comes from the characteristic shrub-dominated component in woodland on sandstone. In Cumberland Plain Woodland most sticks and persistent leaves come from the widely-spaced eucalypts, shrubs are sparse and make little contribution, and herbs and grasses have soft leaves that decompose faster than the hard leaves of sandstone-growing shrubs. Thus in Cumberland Plain Woodland, fires move rapidly through the groundlayer when it is dry enough, but do not generate high temperatures. These observations were confirmed in our 2005 fires where fires in long unburnt woodland were more patchy and of lower intensity than in adjacent areas of grassland.
A surprise from the fires
A major finding from the fires surprised us. Immediate responses were slow because of dry conditions, but we discovered that most species responded to subsequent rain in the bare space created by fire, rather than to the direct effects of fire - heat, smoke or ash. We found almost as many different types of seedlings emerging in bare spaces in unburnt areas after rain, as came up in the burnt areas.
Map of Mount Annan Botanic Garden Conservation area burns (below):
- Pale pink 1991 & 2001
- Pink 2001
- Red 2002
- Orange 2005