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Characterizing Congo Basin Drought resilience: an Integrative Modelling approach

Periodic Reporting for period 1 - COBADIM (Characterizing Congo Basin Drought resilience: an Integrative Modelling approach)

Reporting period: 2019-03-19 to 2021-03-18

African rainforest, is the second largest on Earth and provides important ecosystem services through the absorption of atmospheric carbon, mitigating climate change. Both remote sensing data and tree ring stable isotope measurements have recently shown that these forests are currently being subjected to a long-term drying trend, raising fears over the drought resilience of tropical tree species and the persistence and magnitude of this carbon uptake.

Drought events in tropical rainforests have a strong potential to alter forest structure and tree function, providing large feedbacks to the climate system. Slow-growing commercial tropical tree species with rotations of >50 years, put additional pressure on sustainable forestry practices and necessitate a forward looking approach with supporting policy. Yet, the needed long-term data and detailed physiological growth responses of tropical tree species to drought remain poorly measured. As such, there is a to obtain novel information on the function of tropical forests in the Congo Basin under a (drier) future climate in order to assess the ecosystem climate feedbacks and guarantee sustainable forest management practices.

Limited research has been completed to characterise changes in forest structure and function in African rainforests. Predictions of future growth responses and carbon budgets of tropical forests are thus restricted by an incomplete understanding of how different species are limited by various resources. Connecting leaf phenological and wood anatomical traits together in a new comprehensive dataset can be used to inform ecosystem models, as current models lack the phenological response to drought required to simulate robustly annual and diurnal changes in primary productivity.

The overall scientific goal is to enhance model estimates of carbon and water exchange and growth in response to drought in tropical tree species. This will be achieved using an interdisciplinary approach integrating (historical) trait based measurements and a novel [Ca] X-Ray Fluorescence (XRF) based tracer to drive a data-informed models. Here, we have shown that we used historical and retrospective (wood) data to provide important and novel ecological and botanical information, with the potential to expand this framework to other tropical species or regions. In particular, we used historical aerial footage and phenology data to provide important insights into the Land-Use and Land-Cover change and forest regrowth and recovery. Historical phenology data proved to be key in assessing climate responses across a large set of species. These data complement recent [Ca] XRF scans of central African wood cores (dating back to <1850). We could confirm the correspondence between seasonal rings and a strong [Ca] response and how to provide insight into seasonal to decadal changes in tree growth from ringless tropical trees.
During the action we covered both experimental and computational work. Custom growth chamber were build to measure drought responses in seedlings of both tropical and temperate species during the spring of 2020. The chambers consist of two stand alone growth chambers outfitted with precision scales to measure drought responses using a mass balance methodology (see attached image). Data were collected by a bespoke data logger and open hardware and software. Due to COVID disturbance further experimental results are lacking, but retrospective XRF scans were made at the SOLEIL/INRAE synchrotron platform using co-funding. Preliminary data and results of the measured wood cores were communicated during a vPICO session at the 2021 European Geoscience Union (EGU) meeting.

We characterized the historical context of forest growth and regrowth in the central Congo Basin using historical aerial photographs from the late 1950s. Given the impact of tropical forest disturbances on atmospheric carbon emissions, biodiversity, and ecosystem productivity, accurate long-term reporting of Land-Use and Land-Cover (LULC) change in the pre-satellite era (<1972) is an imperative. Our analysis therefore provides crucial spatial information to inform on all these processes. A comparison with contemporary LULC data showed a shift from previously highly regular industrial deforestation of large areas to discrete smallholder farming clearing, increasing landscape fragmentation and providing opportunties for substantial forest regrowth.

Furthermore, data recovery and synthesis of historical phenology records was successful. A first study, covering leaf phenology across tropical species and their relation to climate has been submitted to the Journal of Ecology. We analyze the leaf phenological patterns of 129 species (4706 individual observation years, 91.2 percent basal area, 94 evergreen, 35 deciduous). We illustrated the divergent behaviour within and across species, the variability of climate-phenology relationships and underscore the importance of accounting for constituent signals in canopy-wide scaling, and the interpretation of remotely-sensed phenology signals, with strong contributions of annually deciduous species but also the relative importance of large and/or abundant trees. Our results provide a baseline reference for future remote sensing and modelling studies aiming to establish and predict tropical forest ecosystem dynamics.
During our historical aerial footage analysis we estimated above-ground carbon gains through reforestation to range from 811 to 1592 Gg C, partially offsetting historical deforestation (2416 Gg C), in our study area. Our analysis provides methods and insights into key spatial and temporal patterns of deforestation and reforestation at a multi-decadal scale, providing a historical context for past and ongoing forest research in the area. Both the publication as the supporting data are freely available for consultation and re-use. These results will provide both supporting information and critical context for further study and policy.

Phenological data described during the action will be published as an open access dataset, and generally all publications within the context of the action are foreseen to be openly accessible by both scientists, the public and other stake-holders. To ensure reproducibility protocols used in the drought experiment are public and should facilitate other experiments both within the host lab but also elsewhere. The setup is a cost effective means to address small scale eco-physiological experiments. The open hardware description should therefore accelerate research by removing barriers in setup and data acquisition. Recent data acquisitions from XRF measurements at SOLEIL are made publicly accessible to foster international collaborations and provide access to this unique dataset. A manuscript based upon these measurements is currently being written and will inform subsequent modelling efforts.

The project had a wide societal impact due to the application of citizen science and the development of tools for the scientific community. Software infrastructure reached a scientific public of more than 60 000 user, with other components cited >30 times over the course of the action. Citizen outreach through an annotation project had ~2300 registered contributors. The Virtual Forest outreach programme, raising awareness about seasonality in forest and forest dynamics still reaches ~500 unique monthly visitors, many returning to follow progress.