What factors influence the occurrence and abundance of midstorey Acacia in Mountain Ash forests?

Abstract

The midstorey is a critical structural component of many forests globally. Using statistical models, we quantified the influence of two sets of variables on the percentage cover and basal area of two dominant Acacia spp. (Montane Wattle [Acacia frigescens] and Silver Wattle [Acacia dealbata]) in the midstorey of Mountain Ash (Eucalyptus regnans) forests in mainland south-eastern Australia. Specifically, we focused on the influence of (1) the age of the overstorey eucalypts (corresponding to the time since the last stand-replacing disturbance), and (2) environmental drivers (aspect, topographic wetness index, slope, elevation). We found evidence for generally non-linear relationships between stand age and the percentage cover and the basal area of both Silver Wattle and Montane Wattle. Silver Wattle had the highest values for percentage cover, and Montane Wattle the lowest, in stands regenerating from fire in 2009. The basal area of Silver Wattle was highest in stands that regenerated after the 2009 wildfires and after disturbance that occurred between 1960 and 1990s. For Montane Wattle, basal area was lowest in stands that regenerated in 2009 but values did not differ among stands of other ages. Both Acacia species were a midstorey component in old-growth Mountain Ash forest. No environmental covariates influenced the percentage cover of Montane Wattle or Silver Wattle. However, our model for the basal area of Montane Wattle contained evidence of a positive relationship with topographic wetness. The general paucity of environmental drivers in most of the models we constructed is likely due to the fact that both tree species occur well beyond our study region. Hence, the set of environmental conditions modelled may not be limiting the percentage cover or basal area of these midstorey tree species. Disturbance appears to be the key driver of dynamics of Montane Wattle and Silver Wattle in Mountain Ash forests.