The midstorey is a critical component of the structure of many kinds of forest globally. We constructed statistical models of the factors influencing the percentage cover of two dominant Acacia spp. (Montane Wattle [Acacia frigiscens]) and Silver Wattle [Acacia dealbata]) in the midstorey of Mountain Ash (Eucalyptus regnans) forests in mainland south-eastern Australia. We modelled the influence on the percentage cover of two these two species of Acacia of : (1) the age of the overstorey eucalypts (which corresponded to the time elapsed since the last major stand-replacing disturbance), and (2) environmental drivers (slope, aspect, elevation, and topographic wetness).

Stand age was an important factor influencing the percentage cover of both Montane Wattle and Silver Wattle. We found evidence of a non-linear, humped-shaped percentage cover-stand age relationship for the percentage cover of Montane Wattle, with the highest values in stands of Mountain Ash that were 30-60 years old. There were no differences in percentage cover among other age classes. The highest values for the percentage cover of Silver Wattle were for stands regenerating after the 2009 fire with markedly lower levels of cover in other age classes. There were no differences in cover between other age classes. Although our data contained evidence of inter-specific differences between Montane Wattle and Silver Wattle in their response to stand age, both species persisted as a midstorey component in old growth Mountain Ash forest.

 No environmental covariates influenced the percentage cover of Montane Wattle or Silver Wattle. Both tree species occur well beyond our study region and the set of environmental conditions we modelled may therefore not be limiting the occurrence of these tree species. We suggest that disturbance is the key driver of site occurrence of the Montane Wattle and Silver Wattle in the Mountain Ash forests of the Central Highlands of Victoria.

Vegetation surveys

We completed detailed vegetation surveys at each of our 178 long-term sites. We measured the projective (percentage) foliage cover of each Acacia spp. across six 10 m x 10 m plots located at 20 m increments along a 100 m transect on each of our sites. We completed measurements of percentage cover in summer 2019-2020.

Environmental and other variables

We calculated values for a suite of covariates for each of our 178 field sites for subsequent use in constructing statistical models. We assigned the age of the forest at each site to one of five age classes: 1 = old-growth dominated by trees that germinated before 1900 (13 sites), 2 = 1939 regrowth (dominated by trees that regenerated as a result of the 1939 wildfires) (85 sites), 3 = 1960–1990s regrowth (i.e. trees that regenerated between 1960 and 1990) (15 sites), 4 = sites were regenerated after the 2009 wildfire (31 sites), and 5 = mixed-aged forest (in which there were two or more distinct age cohorts of trees in the stand) (31 sites). Our age class classification was based on the dominant age cohort of living overstorey trees in a stand. However, we note that the vast majority of the mixed-aged stands supported an old-growth component with a number of individual large old living trees.

We interrogated a 20 m resolution Digital Elevation Model to extract data on slope, aspect and elevation for the centroid of each site. We also calculated values for a Topographic Wetness Index (TWI) (Moore and Hutchinson 1991) for each site. TWI which is a measure of relative position in the landscape and thus potential water distribution. Calculation of TWI requires a Digital Elevation Model (DEM) that has hydrological integrity, and we used the ANUDEM algorithm (Hutchinson 2011) to generate a DEM of our study region at a grid resolution of 20 m. For each cell, the size of the catchment that flows to it was divided by its width, adjusted geometrically by the aspect of inflow direction. This ‘specific catchment’ was then divided by the cell’s local slope. Lower values indicate ridges and upper slopes that have little to no contributing catchment, with values increasing for lower slopes, valley bottoms, and drainage lines.