The Blind Spot in active tropical forest restoration: unknown impacts on soil carbon

Restoring formerly degraded ecosystems is a promising nature-based solution to mitigate climate change and ensure the provisioning of ecosystem services. Consequently, ecosystem restoration is prominent on both governmental and private agendas (e.g., the Bonn Challenge, airline carbon off-sets by planting trees). Two opposing strategies are employed to promote forest restoration: active versus passive (e.g. natural regeneration) restoration. Assessing how these two approaches influence biodiversity hot spots such as tropical rainforests is uniquely important, but the benefits and limitations of these two techniques have not been thoroughly compared.

Among all tropical moist forests globally, forests of Asia-Oceania have experienced the highest disturbance rates in the past three decades, among which Sabah, Malaysian Borneo, contains forests with past managements to strategically assess long-term forest recovery following active and passive restoration strategies.

How overall forest carbon balance, including carbon storage in the soil, is affected by active versus passive restoration, remains a blind spot not only at this site, but also globally. Given that up to half of the total carbon stored in secondary tropical rainforests can be stored belowground, and that this carbon has slower turn-over rates than above-ground vegetation, Sabah is a perfect testing ground to examine how common forest restoration influences below-ground carbon dynamics and total forest carbon balance.

To address this, we collected soil samples in 15 actively restored and 15 naturally regenerating forest plots in INFAPRO, a restoration project in Sabah. This site was severely, selectively logged for two decades and then actively restored by planting (mainly) Dipterocarpacaea (i.e., diptertocarps) seedlings more than 20 years ago. These trees associate with ectomycorrhizal fungi that mediate important soil biochemical cycles as root-inhabiting tree symbionts.

At this restoration site, active restoration enhanced tree diversity, promoted rare species, and increased above-ground carbon density in living vegetation in comparison to natural regeneration. We hypothesize that active restoration, including the planting of diptertocarps, further enhances the presence of ectomycorrhizal fungi, leading to a suppression of free-living microbial decomposition of plant litter inputs (i.e., the Gadgil effect), and an increase in total soil carbon storage. While this may increase total soil carbon storage, the more persistent fraction that is mineral-associated may decrease. This is due to slowed plant litter decomposition and thus less production of compounds that absorb onto mineral surfaces in addition to less microbial necromass inputs sticking to minerals due to the lower growth efficiency by ectomycorrhizal fungi compared to free-living microbes.

This knowledge on soil carbon storage and its persistence is a much needed contribution to holistic assessments of active restoration compared to natural regeneration. Empirical results on soil carbon analyses will be generated by the time of the EGU conference.