Assessment of Alluvial Riparian Blackbutt Forest in Wyong LGA against River-Flat Eucalypt Forest EEC

Abstract

Sampling and numerical analysis of Alluvial Riparian Blackbutt Forest, as originally defined and mapped in Bell (2002), revealed four definable forms of this community present within Wyong LGA. While this dataset is small (10 samples), the highly restricted nature of this community, and the extent to which it occurs in degraded sites or on private property, suggests that further collection of data and re-analysis will be limited. Two broad forms of the community (dry & moist) have been delineated, each with two forms largely definable on canopy composition (Dry Blackbutt; Dry Blackbutt-Apple; Moist Blackbutt-Bluegum; Moist Blackbutt-Spotted Gum). Dry Blackbutt forms occur principally in the Wallarah Creek catchment, including Spring Creek, which drains into Budgewoi Lake. Moist Blackbutt forms occur within the Wyong River catchment, including the lower reaches of Ourimbah Creek and Wyong River, and drain into Tuggerah Lake. Buttonderry Creek, which flows into Porters Creek Wetland and thence to Tuggerah Lake, also supports Moist Blackbutt. In keeping with regional trends in vegetation patterns, Alluvial Riparian Blackbutt Forest in the southern parts of Wyong LGA (Wyong River, Ourimbah Creek, etc) support a strong mesic element in the canopy and mid-layers consistent with vegetation upstream from mapped occurrences. Areas along these creeks where Blackbutt (Eucalyptus pilularis) dominates are restricted to the east where they meet Tuggerah Lake. Immediately west, there is a transition to Sydney Bluegum (Eucalyptus saligna) dominated riparian forest and lowland rainforest on alluvial soils, with both canopy species occurring as ecotonal communities. Identical patterns are evident in the southern Lake Macquarie LGA, where major tributaries such as Martinsville and Dora Creeks change from mesic Eucalyptus saligna associations to dryer Eucalyptus pilularis forms. Similarly, within the dryer Wallarah Creek catchment, Eucalyptus pilularis grades upstream into riparian forests of Angophora costata, Eucalyptus resinifera and Eucalyptus robusta, but Eucalyptus saligna are the associated mesic rainforest elements are absent. Assessing all fourteen paragraphs of the Final Determination for River-Flat Eucalypt Forest on Coastal Floodplains shows that most of the key characteristics of this EEC are met by Alluvial Riparian Blackbutt Forest. However, some characteristics are not met by this community. On balance, based on key characteristics there is more support for the inclusion of Alluvial Riparian Blackbutt Forest in River-Flat Eucalypt Forest on Coastal Floodplains EEC than there is for its exclusion. In terms of floristic composition, simply comparing the number of shared species between a target area or type and a listed EEC is inconclusive in determining the presence or absence of a specific EEC. In the current assessment, applying this method using the 145 native plant species recorded from ten sample plots revealed seven other Sydney Basin EECs (1 Rainforest & 7 Forests) as being more likely than River-Flat Eucalypt Forest on Coastal Floodplains to be represented. Similarly, examining such data as an expression of the proportion of shared species is also uninformative: fourteen other alternative EECs (11 Forests, 2 Woodlands & 1 Grassland) were found to support higher percentages of shared species from the 10 plots than River-Flat Eucalypt Forest on Coastal Floodplains. In total, sixteen EECs were more likely than River-Flat Eucalypt Forest on Coastal Floodplains to be represented by the Alluvial Riparian Blackbutt Forest data if based on floristic composition alone. Alluvial Riparian Blackbutt Forest within Wyong LGA is a restricted community occurring on floodplain alluvium, and its inclusion in River-Flat Eucalypt Forest on Coastal Floodplains EEC is warranted despite discrepancies in floristic composition.