Comparing the development of naturally colonised and planted temperate forests across a 60-year chronosequence

Tree planting may be preferred for carbon storage and timber production, whereas natural colonisation may be preferred for increasing forest biodiversity.

Abstract

1. Natural colonisation is proposed as a cost-effective alternative to tree planting for combatting climate change and promoting biodiversity. However, in temperate regions, few studies have compared the development of planted and naturally colonised forests across the spatial or temporal scales needed for generalisable results to inform policy and practice.

2. We identified a paired space-for-time chronosequence spanning >60 years of naturally colonised and planted forest patches (forest creation types), all adjacent to mature forest. We compared forest structure across forest ages and examined differences in tree community composition in naturally colonised sites.

3. Field surveys were conducted using 10-m radius plots at varying distances from the mature forests. All trees ≥7 cm diameter were identified and measured to calculate basal area. Using LiDAR data, we extracted tree canopy height, variability and cover as metrics of structural heterogeneity across the forest creation types.

4. Both canopy height and basal area, predictors of above-ground biomass, were consistently higher in planted forest patches, averaged across all age pairings at 8.56 m² ha⁻¹ greater and 2.1 m taller than naturally colonised forests, respectively. Naturally colonised forests had more open canopies (19% on average), while canopy height variation was similar between forest creation types. A greater abundance of willow species (Salix spp.) was associated with younger naturally colonised forests while the community composition of older naturally colonised forests was more comparable to adjacent mature forests.

5. Synthesis and applications: During forest creation in temperate, agriculturally dominated systems, tree planting accumulates above ground biomass more quickly than natural colonisation whereas naturally colonised forests are more open and structurally heterogeneous. Differences in above-ground biomass persist for decades where differences in structural heterogeneity reduce with age. Tree planting may therefore be preferred for carbon storage and timber production, whereas natural colonisation may be preferred for increasing forest biodiversity.

Source: Journal of Applied Ecology