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Assessment of vegetation condition of grassy ecosystems in the Australian Capital Territory. Sarah Sharp, Environment ACT, PO Box 144, Lyneham, ACT 2602, Australia. Email: sarah.sharp@act.gov.au.

Key words: botanical significance, conservation value, disturbance, endangered ecological community, grassland, woodland.

Introduction.  A protocol for determining vegetation condition has been developed by Environment ACT to support a strategic approach for the protection and management of woodlands and grasslands in the Australian Capital Territory (ACT) (ACT Government 2004, 2005). It is particularly useful when considering a threshold for defining ecological communities for legal purposes (such as the listed Yellow Box (Eucalyptus melliodora)/Red Gum (Eucalyptus blakelyi) Grassy Woodland and Natural Temperate Grassland endangered ecological communities), to identify conservation values of sites and to prioritize actions that are required to maintain or enhance the ecological condition of a site.

The vegetation condition rating is based on: (i) the cover, richness and rarity of plant species or Botanical Significance Rating (BSR); and (ii) an assessment of vegetation structure of a survey unit. The rating is applied to polygons (survey units that are homogenous in terms of plant composition and structure, but which vary in size from 0.5 to 50 ha). There may be multiple polygons across a ‘site’ (which may be a property, a block or a discrete remnant). Other data recorded include habitat, disturbance, functional and landscape attributes. These are used to identify other biodiversity values present in sites, including fauna habitat and functional roles, but are not used in the assessment of vegetation condition.

Botanical significance rating.  The BSR of each polygon is derived from the abundance of herbaceous and subshrub species classified in terms of their rarity within the region, together with overall native species richness and cover of the vegetation (ACT Government 2004, 2005).

The sampled frequency of species in the ground layer in woodlands and grasslands of the Southern Tablelands (New South Wales and ACT), together with documented information about the disturbance sensitivity of some species were used to classify the relative rarity of each species. The classification took account of species that are naturally rare and others that are uncommon as a result of anthropogenic disturbance. To determine the relative rarity of each species, data on about 600 native herbaceous and subshrub species (less than 0.5 metre in height) were analysed from over 650 surveys in grassy woodlands and grasslands in the Southern Tablelands (R. Rehwinkel, D. Eddy and S. Sharp pers. comm., 1999). Species were initially classified into groups according to frequency of occurrence and then the classification was reviewed in the light of any bias in the sampling strategy and other studies. For example, as sampling was biased towards higher quality remnants, the apparent frequency of species from high quality grasslands was increased. Conversely, fewer occurrences of species most commonly occurring in low quality remnants were recorded. Because many species had high variability in their frequency throughout the Southern Tablelands Bioregion, the region was divided into four subregions to better reflect those differences, the ACT being separated out into a separate subregion. Studies documenting disturbance sensitivity of species (e.g. Prober & Thiele 1995; Dorrough 2001; Hamilton 2001) were used to review the classification. Each species is grouped according to its relative rarity, ranging from the least common and the most disturbance sensitive species to the most common and most disturbance tolerant species. Examples of these species are presented in Table 1.

Table 1.  Characteristics used to identify Botanical Significance Rating (BSR)
 Cover of native perennial speciesNative species richnessVery common speciesRelatively common speciesLess common native speciesUncommon species
Frequency of occurrence  Occur in > 75% of sites and/or very tolerant of disturbanceOccur in 25–75% of sites and/or moderately tolerant of disturbanceOccur in < 25% of sites and/or sensitive to disturbanceOccur in < 10% of sites and/or highly sensitive to disturbance
Characteristic species  Chenopodium albumRumex browniiWahlenbergia spp.Chrysocephalum apiculatumAustrodanthonia spp.Bothriochloa macraLeptorhynchos squamatus Drosera peltata Triptilodiscus pygmaeus Brachyscome heterodonta Convolvulus erubescensBulbine bulbosaWurmbea dioicaDichopogon spp.Arthropodium spp.Thysanotus spp.Themeda triandraDiuris spp.Caladenia spp.Thelymitra spp.
BSR 1 (very high)> 50%Moderate to very highSeveral to manySeveral to manySeveral to manySeveral to many
BSR 2 none (high)> 50%Moderate to very highSeveral to manySeveral to manySeveral to manyVery few to none
BSR 3 (moderate)> 50%Moderate to very highSeveral to manyManyVery few to noneNone
BSR 4 (low)> 50%LowFewVery few to noneNoneNone
BSR 5 (very low)> 50%Very low, mainly grassesVery fewVery fewNoneNone
Exotic< 50%Very low to noneVery fewVery fewNoneNone

Species richness within a polygon is used to assist in the classification of sites according to their botanical significance. Generally, a site with very low species richness (less than eight forb species) is given a low rating, although a site that contains a low species richness, but more than scattered individuals of species classified as uncommon is rated higher.

The characteristics that define the BSR classification are presented in Table 1. Sites need to have greater than 50% native perennial cover to be given a rating. Sites with a very high to moderate botanical significance (BSR 1, 2 and 3) similarly have a moderate to high native species richness of grasses, forbs and subshrubs but are differentiated by the presence/absence of species identified as uncommon. Sites with very high botanical significance (BSR 1) contain very uncommon species, generally orchids and some lilies. Sites with BSR 3 (moderate botanical significance) contain a moderate to high species richness, but only species that are common or those known to be tolerant to disturbance. Sites with a low botanical significance (BSR 4) contain a lower native species richness (less than eight forb species) and only very common species or those known to increase as a result of disturbance. Sites with a very low botanical significance (BSR 5) contain few native species other than grasses.

Vegetation structure.  The patterns of cover of trees, shrubs, grasses and other herbaceous species, presence of tree regeneration classes and how the life forms are stratified are recorded for each polygon. Structure is classified into groups based on the complexity of the structure:

  • 1
    Complex structure. Woodland with 2–50% canopy cover. Groundlayer has a diverse structure represented by different species and plant families. Tree, shrub and herbaceous regeneration occurring.
  • 2
    Reduced complexity. Dominant strata present. Groundlayer of grassland and woodland may be more structurally homogenous. Tree regeneration in woodlands may be limited.
  • 3
    Highly simplified structure. Tree canopy cover reduced to < 2% (derived grassland). Groundlayer of grassland and woodland reduced to homogenous structure. Tree regeneration limited by lack of mature trees.

Applying vegetation condition ratings to sites.  Five scales of modification (based on McIntyre & Hobbs 1999) are applied to polygons.

  • 1
    Unmodified and grassy vegetation is believed to no longer exist in the ACT, given the intensity of use of areas containing these vegetation communities and evidence from extensive surveys. However, the assumed pre- 1750 grassland and woodland communities have been described (ACT Government 2004, 2005) and are used as benchmarks against which existing vegetation can be compared.
  • 2
    Partially modified vegetation has a complex structure, high to moderate native species richness and contain uncommon species and species known to be highly sensitive to disturbance (high botanical significance, BSR 1–2).
  • 3
    Moderately modified vegetation has a reduced structural complexity and fewer uncommon species (moderate botanical significance, BSR 3). Sites in which the tree stratum has been removed (derived grasslands) but retain a moderate to very high botanical significance (BSR 1–3) are also classified as moderately modified.
  • 4
    Highly modified grasslands are characterized by low species richness represented by very common species (low botanical significance, BSR 4) and a relatively homogenous structure. Woodlands that retain a tree canopy, and have more than 50% cover of native perennial species in the groundlayer and have a BSR of 4–5 are classified as highly modified.
  • 5
    Substantially and severely modified vegetation has a very simple structure and is species depauperate. These include natural grasslands and secondary grasslands with more than 50% cover of native perennial species but few or no forbs remaining (BSR 5) and woodlands with a predominantly perennial exotic groundlayer. Such areas are not considered viable examples of an ecological community, as they are unlikely to have retained the ability to recover because of a depleted store of native seed in the soil, or to maintain a level of resilience (McIvor & McIntyre 2002) to future disturbance episodes. This vegetation may however contain particular ecological values such as key threatened species habitat, old growth trees with hollows for fauna habitat or connecting habitat for animal movement.

Discussion.  The condition classification for grasslands and woodlands is applied in the ACT to support a strategic and transparent approach to prioritizing protection and conservation for each grassland and woodland area. Vegetation condition classes, together with habitat, landscape and functional attributes are applied independently to identify core conservation sites (sites with high conservation value). The multiattribute approach applied in the ACT recognizes that different combinations of attributes provide habitats suitable for different species or groups of species, which may therefore have a range of conservation values. A site may, for example, have a depauperate plant species composition, but may provide excellent habitat for birds, because of the presence of dead timber or old growth trees, especially if the site is large or well connected across the landscape. Conversely, very small sites containing uncommon and/or threatened plant species may be of very high importance for the conservation of flora, but limited value for many animals. Combining habitat and vegetation scores for these sites would result in an average to low score, without recognizing the particular values of each site.

The methodology needs to be tested to determine if quantitative thresholds for vegetation condition classes can be applied to quadrats of a defined size within polygons to improve the rigour, transparency and repeatability of the approach. Such quantitative measures would result in a simpler application: however, reliance on data from the quadrats (i.e. subsamples of the polygon) may result in loss of information guiding conservation outcomes.

The method requires that pre-existing information is available regarding the frequency of occurrence of species within each vegetation community. Therefore, application to other communities that are less well surveyed may not be possible until larger sample of sites within each vegetation community are surveyed.

Acknowledgements.  Nick Webb, Sue McIntyre, Rainer Rehwinkel and David Shorthouse provided very constructive comments on the draft paper. David Shorthouse, Murray Evans and Nick Webb were involved in developing the process for identifying conservation values applied in the lowland woodland and grassland conservation strategies. Suzanne Prober and another anonymous reviewer and Phil Gibbons provided valuable comments to improve clarity and accuracy of the paper.

References

  1. Top of page
  2. References
  • ACT Government (2004) Woodlands for Wildlife: ACT Lowland Woodland Conservation Strategy. Action Plan No. 27. Environment ACT, Canberra.
  • ACT Government (2005) A Vision Splendid of the Grassy Plains Extended: ACT Lowland Grassland Conservation Strategy. Action Plan No. 28. Environment ACT, Canberra.
  • Dorrough J. (2001) The Impact of Grazing and Exotic Invasion on the Persistence of Native Grassland. PhD Thesis, Australian National University.
  • Hamilton S. D. (2001) Impacts of agricultural land use on the floristics, diversity and life-form composition of a temperate grassy woodland. Pacific Conservation Biology 7, 169184.
  • McIntyre S. and Hobbs R. (1999) A framework for conceptualising human effects on landscapes and its relevance to management and research models. Conservation Biology 13, 12821292.
  • McIvor J. G. and McIntyre S. (2002) Understanding grassy woodlands ecosystems. In: Managing and Conserving Grassy Woodlands (eds S.McIntyre, J. G.McIvor and K. M.Heard), pp. 125. .CSIRO Publishing, Melbourne.
  • Prober S. M. and Thiele K. R. (1995) Conservation of the grassy white box woodlands: relative contributions of size and disturbance to floristic composition and diversity of remnants. Australian Journal of Botany 43, 349366.