Fire as a Catalyst for Carbon Sequestration: Respiration Suppression and Regeneration Feedback in South African Fynbos Shrubland

Wildfires are mainly considered to be CO₂-releasing events, while their long-term impact on biogeochemical carbon sequestration remains a major source of uncertainty. We analysed five years of ecosystem-scale eddy covariance data in a South African Fynbos shrubland that experienced a wildfire in the middle of the measurement period and combined it with leaf-scale ecophysiological measurements to quantify the ecosystem-scale carbon feedbacks and energy flux shifts following wildfire.

Unexpectedly, wildfire doubled the annual net carbon sink from 5.36 to 10.55 t_C ha^-1 yr^-1. This increase was driven by a ca. 50% suppression of ecosystem respiration while ecosystem energy exchange remained stable. These findings reveal a significant missing carbon pool of ca. 110 t_C ha^-1 over the course of the fire return interval of 15-20 years. Likely explanations for this discrepancy are either a below-ground carbon pool protected from volatilization through fire or a potential sink into dissolved carbon, potentially leading to eventual long-term ocean storage.

To identify the biological drivers of this carbon sequestration, we measured gas exchange in the two main regeneration plant types of this fire-dominated ecosystem, i.e. obligate reseeders, whose seedlings must achieve reproduction before the next fire to persist, and resprouting species that invest into fire tolerance traits at the cost of slower growth. Stomatal conductance (g_sw) was the primary trait distinguishing the two strategies. Reseeders initiated photosynthesis earlier in spring and exhibited g_sw that was highly responsive to changes in ambient CO₂ and light, while resprouters exhibited stronger resilience to drought but no response to ambient CO₂ fluctuations. This difference in response to CO₂ suggests that current climate trends may preferentially boost reseeders, potentially partially offsetting the impacts of shortened fire return intervals. Conversely, resprouter resilience may prove crucial under a higher drought intensity and duration scenario.

Our unexpected findings for this Mediterranean-climate shrubland (typically considered to be a low carbon sink ecosystem) underscore the necessity for ground-based ecophysiological data to constrain Earth system models, and challenge biomass-centric climate policies, particularly in fire-prone, naturally tree-free ecosystems.