Technical report: effect of vegetation management on woodland communities in the Traprock region

Technical report: effect of vegetation management on woodland communities in the Traprock region

[Summary]: The modification of natural woodland tree densities through tree removal or clearing is one management option used by landholders to increase native grass production for livestock grazing. These variegated landscapes are found in many eastern states where both livestock grazing and tree removal represent significant anthropogenic disturbances on natural woodlands. An understanding of the effects of vegetation management on native vegetation characteristics is an important first step to ensure appropriate management of woodland communities.

The broad aim of this research project was to determine if vegetation management has an effect on floristic composition, species richness and plant cover in the Traprock wool-producing region of southern Queensland. The following specific questions were examined: Is there a difference in floristic composition, species richness and plant cover between mature tree density classes?; Do woody regrowth areas have a different floristic composition and lower species richness than areas without woody regrowth?; and Are there differences in vegetation responses across vegetation types?

Forty-seven sites were sampled across the study area according to vegetation type (ironbark/gum woodland and box woodland), density of mature trees (low [<6 trees/ha], medium [6-20 trees/ha] and high [>20 trees/ha]), and the presence or absence of woody regrowth. Sample sites were established randomly within patches and the composition and relative abundance (frequency) of plant species were determined in a 500 m² quadrat. Stand structural characteristics, including foliage projective cover of distinct strata, and cover of litters, logs and rocks were also determined subjectively within each quadrat. Tree recruitment was determined by measuring the density of juvenile trees in three height classes < 1m, 1-3m and > 3m within each 500 m² quadrat. Management history (from landholder questionnaires), landscape context, the perimeter and area of sampled vegetation patches, and the level of disturbance at each site were also determined for all sites sampled.

Patterns in floristic composition were determined using Indicator Species Analysis (ISA) and non-metric Multidimensional Scaling (nMDS). Analysis of Similarity (ANOSIM) determined whether there were significant differences in floristic composition between mature tree density classes, with or without woody regrowth. nMDS was also used to assess patterns in cover data for growth forms. The relationship between floristic composition and environment was examined by Canonical Correspondence Analysis (CCA). In addition, analysis of variance was used to determine if groups differed significantly for species richness, percentage cover of selected plant functional groups and tree and shrub recruitment.
Distinct patterns in floristic composition were detected by multivariate analysis. The results show that vegetation management has an effect on floristic composition and plant cover. nMDS showed a major gradient in floristic composition from low and medium density no regrowth sites, through mostly low and medium density regrowth, and medium and high density box woodlands to high density ironbark/gum woodlands. Stand structure (cover of strata) showed much the same pattern as floristic composition.
Low density (open paddock) areas were indistinguishable in terms of pre-European vegetation types, although distinct from other treatments in terms of floristic composition. At higher tree densities, a difference in composition between vegetation types was evident. Woody regrowth areas were shown to have a similar floristic composition to that of high density no regrowth areas. While some patterns in species richness (and components) were evident, richness did not reflect patterns exhibited by floristic composition and cover.
While differences in grass cover exist between low and high density treatments, no difference in grass cover was evident between low and medium density no regrowth treatments, suggesting that intermediate tree densities may not adversely impact on pasture production. Tree recruitment was higher in those areas where the understorey was allowed to regrow, and notably highest in low density plots regardless of vegetation type, indicating that the presence of mature trees may inhibit the excessive regrowth by eucalypt seedlings.

This research shows that retaining trees in variegated landscapes provides significant landscape heterogeneity and important habitats for plant species that may be largely excluded from open grassland habitats. Importantly, there is some support for maintaining a medium density of trees in box woodlands that can potentially provide both production and biodiversity benefits. Future studies should separate the effects of sheep grazing from that of tree density to increase understanding of the ecological changes undergone by vegetation in response to management.

File reproduced in accordance with the copyright policy of the publisher/author.