Carbon loss during the early decomposition stages of tree stumps in a post-wildfire Spanish black pine forest

• C loss was estimated by both “wood mass loss” and “in situ CO2 efflux” methods.
• The closed chamber method improved the overall estimations of C loss from stumps.
• C loss depended on post-fire environmental conditions and woody substrate quality.
• C loss was higher in high-burn severity sites than in low-burn severity ones.
• Stumps acted as hot spots of CO2 during their early stages of decomposition.

Post-fire carbon released by the decomposition process of tree stumps in a Spanish black pine (Pinus nigra Arn. ssp. salzmannii) forest of the Cuenca Mountain range (Spain) was assessed during the first three years after felling fire-killed trees. Carbon loss was estimated at high- and low-burn severity sites by two different ways: (1) via wood mass loss (indirect method); and (2) via in situ CO2 efflux measurements (Rstump, direct method). By the indirect method, different aboveground wood decomposition parameters were estimated, i.e. decay rate (ka) and half life period (t0,5). By the direct method, multiple regression models related stump diameter and temperature to instantaneous Rstump. The results indicate that C loss depended on post-fire environmental conditions and woody substrate quality (i.e. stump size). Both methods showed similar C release patterns, with higher values obtained by the direct method for all study sites and tree stump sizes, likely because a portion of the CO2 originating in the belowground part of stump was diffused through the decomposed aboveground part. Using the defined Rstump models for ecosystem upscaling, the annual C loss of the study sites ranged from 0.08 ± 0.01 to 1.33 ± 0.06 Mg C ha−1 year−1. Thereby, stumps could be considered hot spots of CO2 production during their early stages of decomposition, which particularly at post-fire managed areas, with large numbers are left to decompose, can represent a significant and poorly studied part of the total ecosystem respiration.