What is the carbon balance of tropical managed forests?

Managed forests are a major component of tropical landscapes and almost half of standing primary tropical forests, up to 400 million ha, are designated by national forest services for timber production. However, so far, most of our understanding of the tropical forest carbon cycle yields is from plot networks located in old-growth undisturbed forests while the carbon balance of managed forests at the regional and continental scale remains poorly studied. Here we propose a methodological framework in order to quantify the carbon footprint of selective logging at a regional scale. The yearly balance of a logged forest unit is modeled by aggregating 3 submodels dealing with (i) emissions from extracted wood, (ii) emissions from logging damages and (iii) storage from biomass recovery after logging. Models are parameterized and uncertainties are propagated through a MCMC algorithm. We used the 30-years statistics from the National Forest Service to estimate the carbon balance of managed forests in French Guiana. Over this period, selective logging emitted 0.76 Tg C in the atmosphere. Our results highlight the key role of the local carbon cycle in managed forests for climate regulation at the global scale. (Texte intégral

Key drivers of ecosystem recovery after disturbance in a neotropical forest

Background: Natural disturbance is a fundamental component of the functioning of tropical rainforests let to natural dynamics, with tree mortality the driving force of forest renewal. With ongoing global (i.e. land-use and climate) changes, tropical forests are currently facing deep and rapid modifications in disturbance regimes that may hamper their recovering capacity so that developing robust predictive model able to predict ecosystem resilience and recovery becomes of primary importance for decision-making: (i) Do regenerating forests recover faster than mature forests given the same level of disturbance? (ii) Is the local topography an important predictor of the post-disturbance forest trajectories? (iii) Is the community functional composition, assessed with community weighted-mean functional traits, a good predictor of carbon stock recovery? (iv) How important is the climate stress (seasonal drought and/or soil water saturation) in shaping the recovery trajectory? Methods: Paracou is a large scale forest disturbance experiment set up in 1984 with nine 6.25 ha plots spanning on a large disturbance gradient where 15 to 60% of the initial forest ecosystem biomass were removed. More than 70,000 trees belonging to ca. 700 tree species have then been censused every 2 years up today. Using this unique dataset, we aim at deciphering the endogenous (forest structure and composition) and exogenous (local environment and climate stress) drivers of ecosystem recovery in time. To do so, we disentangle carbon recovery into demographic processes (recruitment, growth, mortality fluxes) and cohorts (recruited trees, survivors). Results: Variations in the pre-disturbance forest structure or in local environment do not shape significantly the ecosystem recovery rates. Variations in the pre-disturbance forest composition and in the post-disturbance climate significantly change the forest recovery trajectory. Pioneer-rich forests have slower recovery rates than assemblages of late-successional species. Soil water saturation during the wet season strongly impedes ecosystem recovery but not seasonal drought. From a sensitivity analysis, we highlight the pre-disturbance forest composition and the post-disturbance climate conditions as the primary factors controlling the recovery trajectory. Conclusions: Highly-disturbed forests and secondary forests because they are composed of a lot of pioneer species will be less able to cope with new disturbance. In the context of increasing tree mortality due to both (i) severe droughts imputable to climate change and (ii) human-induced perturbations, tropical forest management should focus on reducing disturbances by developing Reduced Impact Logging techniques

Modéliser la dynamique post-exploitation de la biomasse des forêts amazoniennes

Along with being home to a huge variety of plants and wildlife, rainforests also play an important role in storing carbon: the Amazon rainforest alone holds around 30% of the total carbon stored in land-based ecosystems, thus playing a major role in climate change mitigation. When Amazonian forests are selectively logged to extract high-value timber, part of this stored carbon is lost, but the loss can be compensated for in the medium to long term if the forest is left to regrow. New trees and trees that survived the logging grow to fill the gaps left by the felled trees. However, it is not clear how differences in the forest (for example, forest maturity), environmental factors (such as climate or soil) and the degree of the disturbance caused by the logging affect the ability of the forest ecosystem to recover the lost carbon. We used computer modeling to analyze data from over a hundred different forest plots across the Amazon rainforest. Our results show that the forest's ability to recover carbon after selective logging greatly differs between regions. For example, the overall amount of carbon recovered in the first ten years is predicted to be higher in a region in the north known as the Guiana Shield than in the south of the Amazonian basin where the climate is less favorable. Our findings highlight the key role the trees that survive selective logging play in carbon recovery

Bilan Carbone de l'exploitation forestière sur le domaine forestier permanent de Guyane française

International audienceWe propose a set-valued controller with a signum multifunction nested inside another one. We prove that the controller is well posed and achieves robust ultimate boundedness in the presence of mismatched, non-vanishing disturbances. Even more, the selected output can be made arbitrarily small. Also, by applying an implicit/explicit Euler scheme similar to the one introduced by Acary and Brogliato (2010) for matched disturbances, we derive a selection strategy for the discrete-time implementation of the set-valued control law. Simulations demonstrate that the discrete scheme diminishes chattering substantially, compared with a fully explicit method

Modelling aboveground biomass dynamics in Amazonian selectively logged forests

Large areas (2 million hectares per year) of Amazonian forests are selectively logged in a polycyclic harvest system. Modeling the post-logging dynamics of these production forests is thus of primary importance to assess their future carbon storage capacity as well as the structural and dynamic features of the forest that will be found in the next logging cycle. In this study, we used a network of 100 permanent sample plots in 10 sites spread across the Amazon basin to model three post-logging biomass fluxes (recruitment, growth and mortality). The temporal evolution of these biomass fluxes (recruitment, growth and mortality) for surviving trees and recruits and their relative importance in explaining biomass recovery through the Amazonian basin were modeled taking into account spatial as well as temporal autocorrelation in a mixed model framework. Incorporating both the environmental variability and the logging characteristics in the developed model indicate that the two key drivers of post-logging biomass fluxes are the relative biomass loss due to logging and the initial aboveground biomass. Overall, environmental factors had little additional weight in explaining the Amazon-wide variations of post-logging biomass fluxes. Our results stress the importance of developing specific modelling frameworks to account for the peculiar carbon cycle in managed tropical forests in order to better recognize their key role for climate regulation at the global scale. (Texte intégral

The main challenges of sustainable forest management in the Amazon: why sustainable forest management in the Amazon should be reinforced?

Since the last 50 years, tropical natural forests have been intensively logged in the tropics to supply the increasing demand of tropical timber. Unplanned logging operations caused forest degradation leading to their conversion due to the loss of their commercial value. Today, natural forests remain a major source of timber and tropical production forests worldwide are estimated to cover around 400 million ha Studies carried out on the long term impact of logging on timber yield showed that at best about only 50% of the timber volume extracted during the first harvest will be available for the subsequent cycles. We estimated the potential of natural production of the Amazonian forest to supply the future wood demand using more than 200 pem1anent plots for monitoring the long term response after logging of the TmFO network. Our results clearly show that natural forests alone will not be able to supply the increasing demand of tropical timber in the region and that significant efforts in forest restoration through plantations as well as intensive silvicultural practices such as enrichment planting in gaps and liberation, must be initiated immediately. Although natural forests will not be able to supply alone the timber market demand, they will play a major role in environmental services (including biodiversity protection and conservation, climate change mitigation rather than solely for timber provision while they will continue to provide necessary income to forest communities that depend on the forests for their livelihoods

Can timber provision from Amazonian production forests be sustainable?

Around 30 Mm3 of sawlogs are extracted annually by selective logging of natural production forests in Amazonia, Earth's most extensive tropical forest. Decisions concerning the management of these production forests will be of major importance for Amazonian forests' fate. To date, no regional assessment of selective logging sustainability supports decision-making. Based on data from 3500 ha of forest inventory plots, our modelling results show that the average periodic harvests of 20 m3 ha−1 will not recover by the end of a standard 30 year cutting cycle. Timber recovery within a cutting cycle is enhanced by commercial acceptance of more species and with the adoption of longer cutting cycles and lower logging intensities. Recovery rates are faster in Western Amazonia than on the Guiana Shield. Our simulations suggest that regardless of cutting cycle duration and logging intensities, selectively logged forests are unlikely to meet timber demands over the long term as timber stocks are predicted to steadily decline. There is thus an urgent need to develop an integrated forest resource management policy that combines active management of production forests with the restoration of degraded and secondary forests for timber production. Without better management, reduced timber harvests and continued timber production declines are unavoidable

Typology and dynamics of agroforestry systems in the mountains of Timor Leste

In Timor-Leste, a country located in the south-east of the Indonesian archipelago, GIZ has initiated a project with the aim of developing agroforestry systems (AFS) that are productive, profitable and preserve natural resources. Since 2020, CIRAD researchers have joined the project wishing to describe the diversity of traditional AFS existing in the country. In the Baucau region, located in the north-east of the country, at altitudes 0-1500 m, with rainfall 1000-2000 mm/year, first inventories and surveys have identified 5 types of AFS which vary greatly in function of their tree density. From the lowest to the highest tree density system: i) Crop system including a Fallow phase (3 months to 10 years), ii) SylvoPastoral system; iii) Young Agroforest; iv) Home Garden; v) Forest Garden. Further biomass inventories, soil observations and sampling, participative mapping activities along with Peeble Score Methods and semi-structured interviews contributed to characterize the AFS at a socio-economic, ecological and agricultural practices level.The AFS evolution hypothesis is that they become denser over time, with an increase in biodiversity. However, it has been observed that home and forest gardens are often managed by older people for varied but self-consumed crops, whereas young people are looking for crops that are more marketable and easier to cultivate in non-agroforestry systems. In order to avoid the clearing of these AFS, it is necessary to look for methods to intensify production, adapt it to the needs of young people and valorise the products. Finally, AFS are also markers of the complex social order between families in the same village or with other villages: concerning the sharing of tasks, ownership and exploitation of land products. It is essential to take these factors into account if continued external support is to be provided