Regrowth vegetation in production landscapes: comparisons between remnant and regrowth Brigalow (Acacia harpophylla) communities, Southern Queensland

Regrowth vegetation in production landscapes: comparisons between remnant and regrowth Brigalow (Acacia harpophylla) communities, Southern Queensland

Endangered Brigalow (Acacia harpophylla F. Muell. ex Benth.) dominated ecosystems have been extensively cleared and modified for agricultural production throughout the Southern Brigalow Belt Bioregion of southern Queensland, Australia. Intensification of the agriculture in the region has resulted in significant fragmentation and disturbance of natural vegetation remnants. Rehabilitating previously cleared vegetation (regrowth) is an important conservation alternative where there is inadequate cover of intact remnants, that aims to ensure the persistence of Brigalow communities throughout highly modified landscapes. While legislation is now in place to restrict clearing of remnant and Brigalow regrowth in southern Queensland, little is known of the functioning and dynamics of regrowth communities in comparison to remnant vegetation. This research investigates patterns in floristic composition, stand structure and microenvironment of Brigalow remnants and a number of regrowth communities differing in time since clearing.

Thirty-eight sites in the Darling Downs region, southern Queensland, were sampled encompassing Brigalow remnant (two treatments: 'Remnant‘ (sites with an absence of clearing but continue to experience small disturbances such as grazing) and 'Reserve‘ (sites with an absence of disturbance within state and national reserves)) and different aged Brigalow regrowth (<20 years; 20 – 30 years; 30 – 40 years; >40 years). Floristic composition and projected cover of strata were recorded in a 500m2 quadrat at each site. Patch and surrounding landscape context (spatial) environmental data were also determined for each site. Soil chemical data (including soil, pH, organic content, cation exchange capacity and phosphorus and nitrogen concentrations) were determined from bulked soil samples. Landscape spatial data was derived from remotely sensed imagery. Un-weighted paired group arithmetic averaging (UPGMA), non-metric multidimensional scaling (nMDS) ordination, analysis of similarity (ANOSIM) and analysis of variance (ANOVA) were used to identify patterns in composition, functional group richness and stand structure between remnant and regrowth sites. Canonical Correspondence Analysis (CCA), Kruskall Wallis correlations and linear step-wise regression were used to identify potential environmental drivers of vegetation responses (Appendix I).

Multivariate analyses (UPGMA and nMDS) showed general gradients from recent regrowth through older stages of regrowth to remnant and reserve sites for both floristic composition and stand structure data. There were no differences between remnant and old regrowth (>40 years) in terms of stand structure (ANOSIM, p<0.05) and total species richness (ANOVA, p<0.05), although differences were evident amongst other treatments. Remnant and Reserve sites were compositionally different to older regrowth (ANOSIM, p<0.05). No significant invasion of exotic species was observed within any treatment type (ANOVA, p>0.05).

These results suggest that stand structure in regrowth vegetation returns to that similar to remnant vegetation within 40 years of initial clearing, predominantly driven by the development of the overstorey trees and woody understorey species. However, Brigalow remnant and old regrowth (>40y) remain different in terms of floristic composition. The significance of this finding is discussed in relation to Alternate Stable State Theory.

The proportion of surrounding vegetation and landuse type, as well as patch factors, such as grazing intensity and soil properties, were highly correlated (CCA) with patterns in composition and stand structure observed across the Brigalow treatments. Regrowth sites exhibited higher proportion of regrowth vegetation as well as cropping and grazing in the surrounding landscape compared to remnant sites. Soil nutrients, such as phosphorus, potassium and cation exchange capacity, and stem density and grazing intensity were also higher within regrowth sites. These findings suggest an environmental influence may be responsible for the differential patterns in composition observed between remnant and regrowth communities in the region.

It is concluded that while structure of Brigalow regrowth returns to that of remnant Brigalow, the distinct floristic species composition of older regrowth may indicate a longer time period is needed for equivalence or a possible alternate stable state in these highly modified agricultural landscapes. These results are significant for the management of highly fragmented communities in production landscapes, where management actions may need to be directed towards a more active approach towards regeneration. To ensure the long-term persistence of Brigalow communities, further investigation of the development trajectory of old Brigalow regrowth is required.