Effect of climate warming on dead organic matter carbon pools in Irish forests on mineral soils

With over 60% of Ireland’s forests on mineral soils, understanding how litter, deadwood, and soil organic matter change under warming conditions is essential for national GHG inventories and for informing sustainable management practices in commercial forests. While growth responses to climate change remain uncertain for managed forests, decomposition is expected to accelerate. In this context, we quantified how warming scenarios alter dead organic matter (DOM) and soil carbon pools. Using the CBM-CFS3 modelling framework, we simulated Sitka spruce (Picea sitchensis (Bong.) Carr.) stands on mineral soil under baseline climate and +1, +2, +3 °C (mean annual temperature) sensitivity scenarios over a 40-year rotation. Growth curves and management matrices were identical across scenarios, with only temperature controls on decay parameters adjusted. Annual pool time series were aggregated to pool group totals, and differences to baseline, per °C slopes, and end-of-rotation outcomes were calculated. Warming produced almost linear losses in DOM carbon stocks. At end-of-rotation under the +3 °C scenario, the difference relative to baseline was ca. −5.9 tC/ha for litter, −1.1 tC/ha for deadwood, and −0.4 tC/ha for soil organic matter, with a total of ca. −7.4 tC/ha. Per-degree sensitivities at end-of-rotation were ca.−1.98 (litter), −0.35 (deadwood), and −0.14 tC/ha °C (soil), totalling ca.−2.47 tC/ha °C. Litter pools diverged from the baseline within ca. 15-20 years, whereas soil changes appeared later and were smaller. Very fast and fast surface pools showed the largest responses to warming, whereas slow soil pools were comparatively stable and sometimes slightly offsetting. For Ireland’s mineral-soil forests, short-term warming mostly reduces carbon in the soil surface (fast-cycling pools, e.g. litter and fine debris), while changes in stabilised soil carbon stay low over one rotation period when biomass growth is constant. These findings suggest greater value in retaining residues and maintaining forest floor inputs. Moreover, they support tracking litter, deadwood, and soil separately in national inventories, and provide per °C sensitivity estimates that can be used to reflect climate response in projections without adding uncertainty from aboveground biomass growth assumptions.