Soil-vegetation type, stem density and species richness influence biomass of restored woodland in south-western Australia

•Biodiverse carbon plantings are increasing within agricultural landscapes.•At one site, we test factors influencing biomass variation at multiple scales.•Stem density and vegetation association explained significant biomass variation.•Species richness was positively related to biomass after controlling other factors.•We provide evidence for compatibility between restoration and carbon goals.

Woody plantings are increasing across the globe to satisfy ecosystem service markets for carbon and ecological restoration. Assessments of these complex woody systems typically use coarse-scale parameters, based on the climate and soil type of a region, and/or remotely assessed vegetation cover, to estimate carbon in their above- and belowground biomass. However it remains poorly known what factors influence their biomass at finer scales. Here, we investigated biomass variability after five years across a 250 ha environmental planting on a former agricultural property in south-western Australia. We aimed to understand which factors may influence observed biomass variability. The dominant canopy tree, Eucalyptus occidentalis, was planted as seedlings, and other woody species were direct sown in vegetation associations, according to soil type and landscape position, to reflect historic native assemblages. Results from 42 survey plots stratified across these associations showed variable seedling establishment from the seed mix, and that the amount of above- and belowground biomass varied widely (Coefficient of variation = 60%). A site mean and standard error were inadequate to capture biomass distribution. Instead, two modes were evident within the distribution at approximately 5 Mg ha−1 and 15 Mg ha−1 with variation primarily associated with differential seedling establishment and growth across the vegetation associations. Additionally, multiple regression analysis showed that stem density explained a significant amount of biomass variation whilst greater species richness was associated with increased biomass once stem density had been accounted for – models combining soil-vegetation association, number of individuals, and species richness explained between 60% and 80% of biomass variation depending on the response variable (total or live biomass) and choice of allometric equations to predict biomass. There was some evidence for a role of nitrogen-fixing species in determining biomass variation. There was no evidence for biomass variation being explained by the proportional contribution of the dominant canopy tree (E. occidentalis) or eucalypts in general once number of individuals had been accounted for, despite their large contribution to plot biomass. The substantial variation we show across the site has implications for carbon accounting practices and cost-benefit analyses guiding investment and regulation of the sector. Our results add weight to emerging evidence that restoring woody plant diversity can be compatible with efforts to maximize biomass and show the potential for diverse restored woodland assemblages to meet developing market demands for carbon.