Botanic Gardens Trust, Sydney, Australia

Monitoring impacting events - fire, rabbits, rainfall, seasonality

Our research aim at the Australian Botanic Garden, Mount Annan has been to understand the ecology of the woodland as it recovers from nearly two centuries of rural use, and apply this knowledge to conservation management. Our main performance indicator has been the ongoing survival and expansion of all the native plant species present in the woodland, while reducing the abundance of exotic species. Treatments used have been ecological burning, exclosure of herbivores and weed eradication. No plantings of any species have been made.

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 though 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. 

Ecology of Cumberland Plain Woodland

Woodland fire 2002
Woodland fire September 2002

After fire 2005
Grassland after fire September 2005 

Eco burns
Map of Mount Annan Botanic Garden Conservation area burns.

  • Pale pink 1991 & 2001
  • Pink 2001
  • Red 2002
  • Orange 2005

Native species in unburnt and burnt sites

Exotic species in unburnt and burnt sites

TABLE

The Australian Botanic Garden, Research Woodland: Seedlings observed October 2001-August 2002 in areas burnt (September 2001) or unburnt, before and/or after good rain in February-March 2002.

? = Seedling identiy uncertain

X = low-medium abundance

XX = high abundance

 

Seedlings after fire
Seedling germination after fire and significant rainfall

Species - Natives

Burnt

Unburnt,

 

before rain

after rain

after rain

Acacia implexa?

X

   

Chloris ventricosa?

   

X

Cotula australis

 

X

X

Cymbonotus lawsonianus

 

X

X

Cyperus gracilis

 

XX

 

Desmodium brachypodum

 

X

X

Desmodium varians?

 

X

 

Dichondra repens

XX

XX

XX

Dodonaea viscosa

 

X

X

Einadia hastata

 

XX

XX

Einadia nutans

 

XX

XX

Eremophila debilis

 

XX

X

Erodium crinitum

 

X

 

Eucalyptus crebra?

 

X

 

Euchiton sp.

 

X

X

Fimbristylis dichotoma

   

X

Galium propinquum

 

X

 

Geranium homeanum

X

X

 

Glycine tabacina

 

XX

X

Hypericum gramineum?

 

X

 

Mentha satureioides

 

X

 

Microlaena stipoides?

 

X

 

Oxalis perennans

 

XX

XX

Phyllanthus virgatus

 

X

 

Plectranthus parviflorus

 

X

X

Poranthera microphylla

 

X

 

Sida corrugata

 

XX

X

Solanum prinophyllum

 

XX

XX

Solenogyne dominii

   

X

Wahlenbergia gracilis

 

X

X

TOTAL NATIVES 30

3

26

18

 

Species - Exotics

Burnt

Unburnt

 

Before rain

After rain

After rain

*Anagallis arvensis

 

XX

XX

*Ciclospermum leptophyllum

 

X

X

*Cirsium vulgare

X

X

X

*Gomphocarpus fruticosus

 

X

 

*Heliotropium amplexicaule

 

XX

XX

*Hypericum perforatum

 

X

 

*Hypochaeris microcephala

 

X

 

*Hypochaeris radicata

   

X

*Medicago lupulina?

 

XX

 

*Modiola caroliniana

X

X

 

*Phytolacca indica

 

X

X

*Plantago lanceolata

 

XX

X

*Senecio madagascariensis

 

XX

XX

*Setaria gracilis

   

X

*Sida rhombifolia

XX

XX

XX

*Solanum nigrum

X

XX

XX

*Sonchus oleraceus

 

X

X

*Trifolium (incl. 3 species)

 

XX

XX

*Verbena bonariensis

   

X

TOTAL EXOTICS 18-21

4-5

16-19

14-17

Unidentified dicotyledons

 

3 species

 

Unidentified monocotyledons

 

3+species

 

 

Rabbit impacts - Exclosure plot observations

In pre-european times grazing and browsing animals included wallabies, kangaroos, possums, bandicoots, as well as birds and invertebrates. Present today, though in smaller numbers are some Swamp wallabies and Wallaroos. We have seen no signs of bandicoots or possums, and we have no idea of what changes have occurred as far as bird and invertebrates are concerned.

Apart from the past impact of domestic sheep, cattle and horses, the main introduced grazing species is now the rabbit. Rabbits were not of concern in our Research Woodland in the 1980s, as the groundcover was much more open and there was little cover for rabbits, to allow populations to build up. By 1991 however the impact of rabbits, particularly diggings and dunghills were obvious enough for us to have the fenced, rabbit exclosure plots set up.

Our initial expectations were that in the fenced plots there would be obvious establishment and growth of species grazed by rabbits elsewhere, in particular of lilies and rarer herbs. However these expectations were not met.

In general the appearance of the fenced plots has remained similar to surrounding unfenced areas. Native plant species richness in fenced plots did not differ significantly from unfenced plots over an 8 year period (see Figure  below). Annual recordings were made in November.

However for exotic species, though there was no difference at 4 years, there was a significant difference at 8 years with 40% more exotic species within the fenced plots (see Figure below).

One plot showing a marked response to the absence of grazing is Plot 10. Plot 10 becomes dominated by dense growth of the exotic grasses *Lolium perenne and *Bromus catharticus in spring, particularly in wet years, the herbage dying off in summer; the dead grass cover preventing establishment of native species later in the season. *Lolium perenne was widespread and common in all plots at the beginning of the monitoring having presumably been encouraged as a pasture species. However by the early 1990s it had disappeared from most plots, but was still present in two of the fenced plots 5 years later. It would appear that rabbit activity helped remove *Lolium perenne but that in their absence, and with the right weather conditions, it can dominate the groundlayer.

Our general observations are that rabbit browsing appears to be random, but some native species particularly Calotis lappulacea and Stackhousia viminea, are heavily affected at certain times. There are only a few plants of Calotis lappulacea in the woodland; these are clustered and were severely chewed by rabbits during dry conditions in 2002. Some were covered with wire netting to protect them. Unprotected plants were eaten down to ground level, some plants subsequently died. All fruits on these plants were eaten. Seed survived on protected plants but no new seedlings have been observed. Inflorescences are commonly chewed off Stackhousia viminea, and while this does not kill the plant it does not allow for seedling recruitment.

A prominent rabbit activity is digging holes 5-10 cm deep, probably associated with digging for roots. It is more noticeable in dry periods. There does not appear to be any particular concentration on particular species, though the digging is generally in open areas, as though the rabbits are using tracks. This digging provides localised patches of bare soil that may be colonised by seedlings of native and exotic species.

Some rabbit activity tends to concentrate their droppings on bare soil areas that become small dunghills. Seedings of some species appear to recruit in these sites, particularly Solanum prinophyllum, *Solanum nigrum  in autumn. With winter rainfall, seedlings of Cotula australis, Crassula sieberiana, *Lepidium species and *Stachys arvensis are common.

Fenced plot 11
Fenced plot during a good season
 Rabbit diggings
Rabbit diggings

Fenced plot in drought
Fenced rabbit exclosure Plot 10 showing seasonal growth of *Lolium perenne.

 Rainfall 01-05

Monthly rainfall 2001-2005 Mount Annan area

 

Native species in unfenced and fenced sites

Exotic species in unfenced and fenced sites

Notes from the 2002 drought

The drought in 2002 occurred at just the right time in our recording sequence. Our monthly recording had begun in 2001 and had given us a year of average year effects. This was followed by the drought year - there was virtually no rain from the end of March to December 2002. Some rain came in late Summer and Autumn 2003, with really good rain in May. (Rainfall 2001: 631 mm, 2002: 524 mm, 2003: 644 mm, longterm average 828 mm)

  • Native species we had classified as annuals - and considered lost as we had not seen them since 1988 - reappeared! This was really exciting! These species were behaving, not as conventional annuals, that go through their life cycle regularly every year, but like ephemerals, species in the semi-arid areas of Australia that only appear every few years after good rainfall.
  • Daucus glochidiatus, a native species in the carrot family, reappeared patchily, but was abundant in those patches. Most plants emerged in areas that hadn’t been burnt. We couldn’t have missed it if it had been present in other years. It is a herb that grows to 50 cm tall in our woodland, and, like a carrot, produces small fruits covered in adhesive hairs.
  • Ranunculus sessiliflorus, an even smaller herb up to 25 cm high, reappeared widely throughout the woodland, in both burnt and unburnt areas. It is in the buttercup family, and, though its flowers are tiny, it flowered profusely.
  • Both are small plants that don’t need a lot of resources to grow to maturity and produce seed for the next generation.
  • Also interestingly, these two species appeared at different times after the breaking of the drought. The Ranunculus sessiliflorus appeared in May, while the Daucus glochidiatus was not seen until October, so the two species have different dormancy characteristics in their seeds. Even though they both reappeared after drought and rain, they did it differently in relation to the seasons. This is an example of how plants behave in a similar fashion - but in slightly different ways. We believe this is a factor that enables coexistence of many species together, as they are not competing directly with each other.
  • A third species behaving as an ephemeral appeared for the first time, a tiny herb in the chenopod family, barely 5 cm tall - Chenopodium carinatum. We found only a few of these tiny plants, so they could easily have been missed before. Being tiny, they completed their life cycle very quickly after the rain, flowering in May and setting seed in June.
  • Ironically, although it was the drought that seemed to prepare these species for germination, ephemerals need periodic episodes of high rainfall, like that in May, to enable production of good seed crops to replenish their soil seedbanks.

^ back to top

The importance of rainfall

While we were well aware that rainfall was important for the successful growth of woodland species we had not truly realised its importance in the initiation of growth and particularly seedling growth events. We had always assumed that rainfall was distributed more or less throughout the year and was not generally a limiting factor, rather that triggering factors like fire or increasing temperatures were most important in regulating growth and recruitment. Rainfall in the late 1980s and early 1990s was generally average or above. However since 2000 there have been a series of dry and sometimes hot years. 2002 was a particularly dry and hot year, with record high temperatures in December.

In the area burnt in our 2002 burn there was little regeneration because of the lack of rainfall, so in summer 2002-2003 we took soil samples and watered them in the glasshouse to see what might be in the soil seedbank. What came up in the soil samples was an impressive array of seedlings many of which we had never seen in the field, though we were familiar with the adult plants there. We also got seedlings of one species Ranunculus sessiliflorus, which we had recorded once back in 1990, and, but for the fact that we had kept a small specimen of it, we would have regarded as an error of identification. As it was, we regarded the species as likely to be extinct in the woodland. Subsequent rain in March 2003 (drought-breaking) resulted in the germination of many seedlings in the woodland, including Ranunculus sessiliflorus and Daucus glochidiatus which has also been recorded only once before. Seedlings of both species, and many others, were recorded in both burnt and unburnt areas, indicating that it was particular site conditions not the effect of fire that allowed seedling establishment, though the open conditions in some burnt areas may have helped by providing locally higher soil temperatures or reduced competition from established plants.

Table

Number of woodland species with seedlings recorded in burnt (September 2001) or unburnt sites before and/or after good rain (February-March 2002), the Australian Botanic Garden, Mount Annan .

Seedlings observed October 2001-August 2002

Species

Burnt

 Burnt  Unburnt

before rain

 after rain  after rain

NATIVES      

3

 26  18

EXOTICS      

4-5

 16-19  14-17

Unidentified dicots

 3  

Unidentified monocots

 3+  


Short-lived ephemeral species

We have been able to compile a list of species, almost all short-lived ephemerals, that recruited well in drought-breaking conditions, ie substantial rain following a long hot dry period. The list includes Ranunculus sessiliflorus, Daucus glochidiatus,*Conyza sumatrensis. While some of these species will occur at other times, generally in smaller numbers, we suggest that factors relating to drought conditions such as temperature may play a part in their dormancy breaking mechanisms. Further work is needed.

As a result of these observations we are convinced that the main force driving processes in the woodland is rainfall rather than fire or temperature. The decreased rainfall events expected as climate change takes hold may have a significant impact on the long term floristic composition of the woodland.

^ back to top

Rain
Rain

Sida rhombifolia seedling
Seedlings of *Sida rhombifolia

Seasonality - annual and short-term changes 2001-2002

Any patch of ground in Cumberland Plain Woodland is likely to change the way it looks during the year. The herbs and grasses grow during favourable weather and die back when itís hot and dry, or when itís too cold. In recording species presence in our plots, the number of species has varied from as low as 2 per square metre during a very dry time to 27 after rain has stimulated growth. The number of species one can find varies over space as well as time. In our 5 m x 5 m plots, the total number of species recorded as present at some time has varied from 34 to 53.

Changes over a two year period 2001-2002, based on individual species abundance (frequency) from the monthly recording of Plots 4 and 9, show the following features:

Our picture of the Cumberland Plain Woodland flora at the Australian Botanic Garden is of a flora of species with characteristics that enable persistence in the face of fluctuating seasonal rainfall and temperature conditions. Most species are perennial and able to survive severe dry periods at ground level as rootstocks or soil stored seed. These characteristics also allow them to survive fire events. Growth cycles are aligned with longterm annual rainfall patterns, with most species being autumn-winter growing and flowering. Under good rainfall conditions growing and flowering will extend into spring with a slight mid Winter hiatus in very cold years. This is different to the Sandstone flora where spring flowering and growth is the main response.

Some difficulties in monitoring Cumberland Plain  Woodland

  • Dry conditions seasonally and even daily
  • Tiny species make location and identification difficult, particularly if they are also short lived
  • Similar closely-related species

^ back to top

Value of long-term monitoring

Given the fragmented nature of Cumberland Plain woodland remnants we see the primary aim of managers of particular remnants is to maintain in the longer term the complete set of species that have persisted on that site and to manage the area over time so that all these native species are maintained. Our research has been focused on documenting the biota of the reserve and to developing strategies to direct change in appropriate directions to benefit long-term survival of as many native species as we can. Some species will need particular attention. With 140 native species in the woodland we still have a long way to go.

Despite the gaps and question marks, our records since 1988 are very valuable because they provide a picture of change over the medium to longer term - a picture that is normally very hard to acquire. So often these days, funding for plant ecological research is provided only on a short term basis, for one to three years, the life of a postgraduate project. And yet extrapolations made from short term research can prove to be mistaken over the longer term. For example, if two out of three years of a project are exceptionally wet, one is likely get very different results from those when two out of three years are drier than average - extrapolating from either extreme may give a misleading picture of what really happens.

An example of where out work has be able to give direction to management has been our documenting of the rate and spread of the exotic woody shrub African olive *Olea europea subsp. cuspidata.The increase in Olea has been met by a program of cutting and poisoning by a team of staff and volunteers. This was begun in 2001, and has lead to the removal of most of the Olea from the woodland. Further invasion will continue particularly when wet weather allows seedling establishment, so there will continue to be a need to remove seedlings. Work with fire has however established that periodic burns may kill juvenile plants and this may help keep the Olea in check.

Field monitoring over 14 years has allowed us to develop the following very important points on vegetation dynamics in Cumberland Plain Woodland:

  • many woodland species occur only in localised patches, and do not spread widely or quickly
  • for most species seed dispersal is local rather than long-distance
  • the force driving processes in the woodland appears to be rainfall rather than temperature
  • growth and flowering of many species tends to be opportunistic in relation to rain, rather than fixed in response to day length
  • periodic drought, rather than fire, appears to have been a driving evolutionary force for individual species
  • for most species persistence is via rootstocks, evolved in response to periodic dryness, but also enabling survival after fire
  • recruitment from seed is episodic, related to rain after drought (or after fire if it has the same effects as drought), therefore colonisation of new areas is slow

What this means for management and conservation

  • Today’s remnants need to be managed to conserve all species at each site
  • Patchy plant distributions mean remnants vary in their species composition, and all remnants, not just the large ones, are likely to be needed to conserve all species
  • Episodic recruitment and opportunistic behaviour need a variety of treatments in management processes such as fire
  • The number of species visible at a site fluctuates in response to seasonal variables, particularly rainfall, and multiple surveys may be needed to record all species present.

For more information on the ecology of Cumberland Plain Woodland go to: Woodland Ecology.

Olive removal
Removal of African Olive by cutting and poisoning

Bush regeneration volunteers 
Bush regeneration volunteers