Botanic Gardens Trust, Sydney, Australia

Grassland remnants and recolonisation

There are large areas of open, periodically mown grassland adjacent to our woodland, and elsewhere in the Australian Botanic Garden, Mount Annan. These areas were formerly woodland but were cleared of trees during farming times, either for cultivation of crops (wheat was an important crop in the Campbelltown area in the early 19th century) or to provide more open areas for sheep and cattle grazing.

The species composition of these grassland sites now depends on the past management history of the site. They range from sites completely dominated by exotic pasture grasses and weeds such as *Paspalum dilatatum, through sites with mixtures of exotic and native species, to remnant sites with persistent native ground species. Native species composition in remnant sites may have similar species to nearby woodland sites.

Because of their longevity, drought-hardiness and persistent rootstocks, many native ground species can persist on cleared or grazed sites for long periods. However tree-clearing, followed by significant soil disturbance, and particularly cultivation and fertiliser addition, generally results in the complete eradication of native vegetation.Remnant sites where there has been little past soil disturbance, cultivation or complete removal of native vegetation, may be recognised by a group of persistent, but poorly-dispersed ‘survivor’ species, a good example being Brunoniella australis.

Where there has been significant soil disturbance, and the former native species have gone, recolonisation by native species is limited to a small group of mainly grass species. These species are easily dispersed by wind or animal vectors and able to recruit in the relatively open conditions. Other native species are unable to colonise the disturbed sites as they only disperse their seeds locally and have very limited abilities for distance dispersal (see Seed dispersal) Native species may also be unable to compete with the rapid growth and dense cover of many exotic species.

Due to absence of seed propagules of most native species, disturbed sites do not develop species-rich groundcovers similar to remnant grassland or woodland sites, at least not in a 20 to 40 year timeframe.

Grassland colonisation model

Grassland /woodland interfaces are currently a focus of some of our ecological studies.

We have developed a schematic model indicating possible species changes that may occur on old pasture areas abutting remnant woodland. Though there is no evidence that it will lead to vegetation similar to remnant Species-rich Woodland, this may provide indications of ways in which regeneration of marginal areas around remnant woodland can be improved, and may be adopted for regeneration of sites with little of no remnant vegetation, providing that natural soils have not been too altered.

Postulated life stages and generalised interactions for native plant species in old pasture on margin of mature remnant Cumberland Plain Woodland vegetation over 40 years assuming average seasonal and periodic events occur.

Relevant to marginal grassland strip 50-100 m wide along woodland margin.

Decade 1, years 1-10

Seed-rain period

  • Main seasonal process is seed-rain from adjacent woodland plants, particularly trees and grasses

Decade 2, Years 11-20

Population clusters develop

  • Main seasonal processes are continuing seed-rain from adjacent woodland plants, and Increased opportunistic seedling establishment with rain.
  • Supressed plants may flower adding seed to soil seedbank.
  • Existing exotic weeds may need to be controlled.

Decade 3 Years 21- 30

Increased Diversity and structure

  • Increased abundance of native grasses and grassland herbs, and occasional shrubs and trees.
  • Main seasonal processes are continuing seed-rain from adjacent woodland plants, and
  • Increased opportunistic seedling establishment with rain.
  • Supressed plants may flower adding seed to soil seedbank.
  • Existing exotics may need to be controlled.

Decade 4 Years 31-40

Woodland structure but species-poor

  • Increasing density of tree canopy.
  • Seed-rain of bird-dispersed species and tree species.
  • Shade requiring species expand.

Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.

Ecology of Cumberland Plain Woodland