- University: AgroParisTech
- Place of defence: Montpellier
Abstract
Biodiversity loss and climate change are critical challenges of the 21st century, driven largely by land-use conversion and agricultural intensification. Passive restoration through natural colonisation offers a cost-effective pathway to biodiversity recovery, yet its ecological role in intensive agricultural landscapes remains unclear. We quantified and compared total wood biomass, soil organic carbon (SOC) stock, ecosystem carbon stock, soil nutrient concentrations, and flying insect biomass across four land-use types in Denmark—naturally colonized forests, plantation forests, mature forests, and agricultural land. The study included 120 plots for tree biomass (10 sites), 84 plots for SOC and ecosystem carbon stock (7 sites), and 96 plots for flying insect biomass (8 sites). We analysed tree biomass following Danish National Forest Inventory (NFI) methods, analysed soil samples (0–25 cm) for SOC stock, ecosystem carbon stock, and soil nutrient concentrations, and determined flying insect biomass after collecting specimens using malaise traps. Tree community composition differed significantly among habitats (PERMANOVA, p = 0.001), with naturally colonized forests supporting a higher proportion of native species. Mature forests stored the greatest total wood biomass i.e., above-ground, below-ground, and deadwood combined (AGB + BGB + DWB = 311.46 ± 46.59 tons/ha), significantly more than plantations (117.27 ± 9.14 tons/ha) and naturally colonized forest (7.51 ± 2.31 tons/ha). For ecosystem carbon stocks, we analysed only plantations, naturally colonized forests, and agricultural land (mature forests were excluded because SOC data were unavailable). Ecosystem carbon stock was defined as biomass-derived carbon plus SOC for the same plots. Under this definition, plantations had the highest ecosystem carbon stock (116.8 ± 6.63 tons/ha), significantly exceeding agricultural land (57.91 ± 6.69 tons/ha) and naturally colonized forests (55.61 ± 2.74 tons/ha). SOC stocks did not differ significantly (p = 0.974) between land-use types. Agricultural land had the highest flying insect biomass (4.78 ± 2.84 g), followed by naturally colonized forests (3.55 ± 2.46 g), mature forest (1.90 ± 1.39 g), and plantation forest (1.71 ± 0.98 g). BGB and DWB showed a negative correlation with soil pH and phosphorus, but a positive correlation with total carbon (TC), total nitrogen (TN), soil organic carbon (SOC) stock, and the C: N ratio. Our findings reveal trade-offs between carbon storage and flying insect biomass, highlighting the need for integrated management that considers both carbon sequestration and insect communities.
