Hotter Soils, Faster Flux: Conifer Plantation CO2 Shows a Greater Temperature Sensitivity than Broadleaf Woodland

Woodland soils play a central role in the terrestrial carbon cycle, acting as both a major carbon sink and source of atmospheric CO₂. Rates of soil organic carbon accrual and CO₂ efflux vary widely among woodland types and differ due to their sensitivities to abiotic drivers. Quantifying woodland responses to these drivers is therefore critical for predicting how different forests respond to future climate change and to inform woodland management.

CO₂ fluxes were continuously monitored for 12 months across four woodland types: a broadleaf woodland, a managed hazel coppice, and two Norway Spruce plantations, one with standard spacing and the other densely planted with a closed canopy. Concurrent measurements of abiotic variables and soil chemistry were also taken.

The highest annual median CO₂ efflux was from the standard spacing planted conifer woodland, followed by the broadleaved woodland (BW), hazel coppice (HC), and then the densely spaced conifer woodland. There was no statistically significant difference in soil carbon. CO2 efflux varied most in the hazel coppice and regularly spaced conifer plantation whereas the broadleaf woodland and densely planted conifers showed the lowest seasonal variation. Wavelet Coherence Transform (WCT) revealed temporal shifts in responses to rising temperatures, with CO2 fluxes rising in the standard spacing conifer woodland prior to warming and fluxes rising in the broadleaf woodland after warming.

Our findings suggest that climate change induced soil warming could accelerate carbon losses via increased soil respiration across all woodland types, with the greatest temperature sensitivity observed in conifer plantations. This is notable given that these non-UK native species, are adapted to cooler, more northerly climates and may therefore be particularly responsive to warming. There is no evidence for soil organic carbon loss, suggesting that higher fluxes are mainly attributable, at present, to faster carbon cycling and increased autotrophic (root) respiration.