Structural complexity across a continuum of woodland establishment methods from planting to natural colonisation

Abstract

Expanding forest and woodland cover is a global strategy to mitigate and adapt to the climate crisis and reverse biodiversity decline. While most woodland in temperate regions is created through tree planting, natural colonisation has been advocated for as an alternative or complementary approach. However, there is limited understanding on how the structural attributes of woodlands created through these different approaches develop through time. To address this knowledge gap, we assessed a suite of structural metrics for 28 woodland sites (aged 13–43 years) that were established along a planted to natural colonisation continuum in England. We used an Uncrewed Aerial System to collect LiDAR data alongside field surveys and calculated metrics relating to above-ground biomass accumulation (canopy height and basal area) and metrics relating to structural complexity (the horizontal and vertical arrangement of canopy) as a proxy for biodiversity potential. Canopy height and basal area were higher in woodlands with larger proportions of planting. Additionally, woodlands with higher proportions of planting displayed greater vertical complexity (canopy stratification) whereas there was weak evidence that woodlands with higher proportions of natural colonisation develop greater horizontal complexity (gap fraction). This suggests that tree planting is the better option when biomass accumulation is the primary goal, whereas natural colonisation or hybrid approaches are likely to be beneficial when the focus is biodiversity or a mix of outcomes. Woodlands created through hybrid approaches that combine planting and natural colonisation offered intermediate values of biomass accumulation and structural complexity.