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Rural Industries Research & Development Corporation
A report for the RIRDC/Land and Water Australia/FWPRDC Joint Venture Agroforestry Program Supported by the Natural Heritage Trust
by A. C. Borsboom, J. Wang, N. Lees, M. Mathieson and L. Hogan
May 2002
(02/044 QDN-6A)
Executive Summary
The broad objectives of this study were to expand knowledge of the importance of farm forestry to biodiversity in south east Queensland and to commence assessment of how biodiversity can be enhanced within plantations. The broad objectives rely on the premise that management and design options recommended for enhancing biodiversity values in farm forestry plantations are practical and ecologically sustainable and can be successfully incorporated into farm forestry practices without significantly compromising timber production costs.
Specific objectives of this study were to:
The field assessment in this project was conducted
between June 1999 and January 2001. Fauna surveys and associated habitat
assessments were conducted over two winter and two summer periods.
Eighteen survey sites were established; 3 on cleared, grazed, improved pasture; 11 in various age Gympie messmate (Eucalyptus cloeziana) plantations; and 4 in selectively logged eucalypt forests.
The age classes of plantations surveyed were 0.3 to 1.8, 2.1 to 3.8, 15 to 16.5 and 38 to 40.5 years of age. Mean plantation area was 7 hectares (range 1.5 to 10.5 ha). Vertebrate fauna, both native and introduced, were surveyed using a range of systematic techniques. Opportunistic fauna records were also kept. Fauna surveyed included mammals (bats, arboreal and ground species), birds, reptiles and frogs. Habitat measurements were taken on the study sites, and included plant species (native and introduced), ground cover, vegetation structure, plantation tree growth, logs and stumps.
A total of 205 vertebrate species were recorded during the survey period, of which 175 were found in the Gympie Messmate plantations. The 175 species from the plantations represent a third of the vertebrate species occurring in the bioregion provinces in which the survey sites were located.
Biggest contributors to vertebrate species richness in the plantations were birds with 100 species. All other vertebrate groups contributed to the remaining 75 vertebrate species, the arboreal mammals contributing least with three species. Greatest vertebrate species richness was in the plantations 38 to 40.5 years of age with a mean vertebrate diversity of 63.7 species, which was not significantly different from the mean of 69.7 for the selectively logged eucalypt forest sites. Species composition changed with plantation age, 66 species restricted to plantations less than 16. 5 years of age and 38 restricted to the 38 to 40.5 year old plantations. Mean number of vertebrate fauna species recorded was higher in the four plantation age classes than on the cleared, grazed, improved pasture sites.
Two bird species and the introduced cane toad were the only species recorded in all 11 plantations surveyed. Five of the six introduced vertebrate species recorded were found in plantations. A rare frog, lizard and blind snake species and one threatened frog species were recorded in the 38 to 40.5 year old plantations. Seven of the eight rodent species occurring in the plantations are known major or minor pests in crops, orchards or food/grain storage facilities. Two of the potentially most serious rodent pest species (house mouse - Mus musculus; grassland melomys - Melomys burtoni), showed a clear preference for plantations with a thick, grassy understorey. A further five vertebrate species recorded in the plantations are potential pests in eucalypt plantations.
Except for birds as a group and a few individual vertebrates species (e.g. some rodents), there was insufficient data to compare vertebrate abundance between the four plantation age classes, selectively logged eucalypt forest and cleared, grazed, improved pasture. Bird abundance in the plantations 38 to 40.5 years old was significantly higher than for cleared, grazed, improved pasture and the plantations 0.3 to 1.8 years of age. Bird abundance was split into diet guilds and showed that birds with diets of either vegetable matter, fruit and invertebrates or seed and invertebrates were either absent or in low numbers on all plantation sites. Nectivorous birds were significantly more abundant in the 38 to 40.5 year old plantations compared to the plantations 0.3 to 1.8 years of age.
Comparing various plantation habitat attributes with vertebrate species richness found the best positive correlation was with percentage vegetation cover above 1.8 m (correlation of 0.81). Other plantation habitat attributes with positive correlations with vertebrate species richness were plant density above 5 m (0.78), plantation tree trunk diameter at 1.3 m [dbh] (0.74), plantation tree height (0.73), plant density above 8 m (0.72) and plantation age (0.68). There was no correlation between species richness and plantation area within the range of 1.5 to 10.5 hectares. Vertebrate species richness was negatively correlated (-0.67) with the number of grass, sedge and herb species present.
The 38 to 40.5 year old plantations, where the greatest number of vertebrate species were recorded, were significantly more complex in habitat structure than the younger plantations. The plantations 38 to 40.5 years of age had significantly more stumps and logs, higher plant densities in a number of height classes, more leaf litter, less grass species and less combined grass, sedge and herb species.
They were also not grazed, had good connectivity to native forest, had no evidence of recent fire and had minimal plantation maintenance since initial planting, tending and thinning. The relative importance of each of these habitat attributes to high species richness in the 38 to 40.5 year old plantations was not determined in this study.
The results of this study show vertebrate diversity was low on cleared, grazed, improved pasture and that single species eucalypt plantations of 10 hectares or less with connectivity to some form of natural habitat, even without any special management to enhance biodiversity, will support more vertebrate species. There should therefore be continuing effort to encourage plantation establishment on cleared lands. With a clear benefit to vertebrate diversity through establishment of single species eucalypt plantations, it is worth investigating the benefit of multi-species eucalypt plantings to vertebrate diversity.
This study clearly shows all vertebrate groups, not just birds, will benefit from eucalypt plantations and that even rare and threatened species may use these plantations. Rare and threatened species should be encouraged, but the grower’s rights to cut timber should be protected. Unless it is planned to replant plantations after timber harvesting, farm forestry plantations will only provide a short to medium term gain to vertebrate diversity on the farm. The impact on vertebrate fauna of timber harvesting and subsequent replanting is unknown and requires further investigation. Where more than one plantation block is planned, establish adjacent to each other and stagger harvest times by at least four years. Where pest species can potentially use habitat within a plantation to breed and threaten adjacent crops, etc, management of the plantation understorey or groundcover may be required. Such habitat management could potentially affect vertebrate diversity.
With highest vertebrate diversity recorded in the 38 to 40.5 year old plantations, it is recommended that the siting, establishment and management of new eucalypt plantations incorporate habitat features found in these older plantations. These features include good connectivity to native forest, the retention of logs and stumps, suppression of a thick grassy groundcover, a good litter layer, encouragement of a layered understorey of shrubs and small trees and exclusion of grazing animals and hot fires. For practical purposes, retention of stumps and logs and development of a complex shrubby understorey should occur every second planting row. To provide reasonable time for development and use of a complex shrubby understorey by vertebrates, maximize time between planting and harvest by growing trees for longer term products such as pole timbers. It is recommended measuring the biodiversity benefits of establishing a complex shrubby understorey beneath plantation trees, by using commercial, shade-tolerant, native species that can be harvested for foliage, flower, and other products.
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