1. Understanding tree growth and carbon sequestration

Beyond the objectives of the GeoTrees component of the OFV Initiative—to improve carbon stock estimates using LiDAR—one of the original aims of this mission was to gain a better understanding of the growth mechanisms of tropical trees and their role in the carbon cycle.

As part of this, the team re-scanned around a hundred trees fitted with dendrometers (a device attached to the tree that measures subtle movements in the bark – daily growth, as well as swelling and contraction linked to hydration) using ground-based LiDAR; these trees had already been measured during a previous campaign. This approach allows for the comparison of precise 3D models of tree structure at different dates in order to quantify:

• secondary growth (thickening of the trunk and branches)
• primary growth (elongation of the stems)
• the spatial distribution of biomass and its evolution over time
   

These measurements were carried out by a team including Olivier Martin (CIRAD), Mathilde Millan (postdoctoral researcher at OFVI CNRS) and Danièle Bikie Mindang, a colonel in the Water and Forests Corps and lecturer in geomatics at the National School of Water and Forests in Mbalmayo.

A multi-scale observatory of forest dynamics

The Mbalmayo site is also part of the canobs.net network, which combines RGB and LiDAR drone observations to monitor canopy dynamics and seasonal variations in vegetation. Under the supervision of Pierre Ploton and Nicolas Barbier (IRD-AMAP), staff from ENEF and the National Cartographic Institute (Imma Tcheferi) have been conducting fortnightly flights over the site for several years, thereby building up a unique dataset to track seasonal changes tree by tree across hundreds of hectares. In parallel with the structural measurements, several complementary studies have been carried out:

1. Ecophysiological measurements of leaves as part of the Coforfunc project. In particular, Australian postdoctoral researcher Kali Middelby studied the effect of leaf ageing on their physiological properties and their response to temperature.
2. Measurements of functional traits and properties related to water stress across numerous species.
3. Detailed soil analyses, conducted by Gilles Dauby and David Bauman (AMAP), to understand spatial variations in edaphic properties and their influence on stand dynamics (tree growth and survival).
4. Dynamics of these stands (monitoring of mortality and growth of over 9,500 trees distributed across contrasting forest types, ranging from periodically flooded forests with flood depths of up to 2.5 m to the famous forests dominated by Limbali (Gilbertiodendron dewevrei), whose functioning and dynamics remain a mystery).
5. Dynamics of trees in the understorey, with the establishment of permanent monitoring plots in 20 m by 20 m quadrats for trees with breast-height diameters between 2 and 10 cm, thereby enabling the documentation of these often-neglected forest strata where both emerging canopy species (regeneration) and undergrowth species rarely or even never found in conventional plots.

These measurements of biological characteristics and forest inventories will be fed into the Cafriplot.net database, which aims to improve the management and consistency of botanical data in Central Africa, using tools that all partners can adopt.

These detailed ecological surveys are made possible thanks to the work of a team of experienced climbers trained over many years by IRD-AMAP, comprising ENEF eco-guards and members of local communities, including the exceptional ‘Tonton Alex’ from Kompia.

A research and training centre run in partnership

The project also involved several Cameroonian students from the University of Yaoundé I and the ENS in Professor Bonaventure Sonké’s LaboSystE laboratory. Among them were Aicha Mfout Vessah — already known to the OFVI community following her interview last year — and Fabrice Nzoyeuem Djonko, a PhD student holding an IRD ARTS grant who is studying the effect of flood gradients and topography on the composition of plant communities. Thanks to measurements from the Mbalmayo observatory, Ninon Besson (an OFVI PhD student) is also able to conduct her research into the architecture and functioning of trees, whilst Colonel Danièle Bikie Mindang is preparing to defend her Master’s thesis in plant biology.

This research benefits from the support of numerous Cameroonian partners, notably the ENEF in Mbalmayo and its director Germain Mbock, as well as the Ministry of Forests and Wildlife (MINFOF) and the Ministry of Research and Higher Education (MINRESI).

A natural laboratory facing environmental pressures

The Mbalmayo site also provides an ideal setting for analysing the pressures exerted on tropical forests near urban centres: agricultural expansion, sand extraction from the riverbed, illegal logging and slash-and-burn farming.

The presence of Georges Eyenga, a sociologist of science from the University of Dschang, during the mission also enabled the social and institutional dimensions of the collaborative research to be explored.

Next steps

The data collected during this mission will now be analysed to quantify variations in tree growth more accurately and to improve models linking tree structure, biodiversity and carbon dynamics in tropical forests.

This work contributes to OFVI’s goal of better understanding and monitoring the functioning of tropical forests in order to inform conservation policies and the fight against climate change.

It will also enable the calibration of satellite-based approaches and climate-vegetation models, in order to extrapolate these observations to a larger scale and better anticipate the effects of climate change.

Finally, this type of observatory fosters collaboration with other disciplines, notably Earth and water sciences, to establish water and biogeochemical balances at the catchment scale, and social sciences, to better understand the impact of forest resource management and use practices.