DOI: 10.5061/dryad.zw3r2285m
title: What factors influence the extent of midstorey development in Mountain Ash forests?

description: 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.
description: <b><i>Vegetation
 surveys</i></b> We completed detailed vegetation
 surveys at each of our 178 long-term sites. We measured the projective
 (percentage) foliage cover of each <i>Acacia</i> 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. <b><i>Environmental and other
 variables</i></b> 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.