Aboveground forest biomass varies across continents, ecological zones and successional stages: refined IPCC default values for tropical and subtropical forests

For monitoring and reporting forest carbon stocks and fluxes, many countries in the tropics and subtropics rely on default values of forest aboveground biomass (AGB) from the Intergovernmental Panel on Climate Change (IPCC) guidelines for National Greenhouse Gas (GHG) Inventories. Default IPCC forest AGB values originated from 2006, and are relatively crude estimates of average values per continent and ecological zone. The 2006 default values were based on limited plot data available at the time, methods for their derivation were not fully clear, and no distinction between successional stages was made. As part of the 2019 Refinement to the 2006 IPCC Guidelines for GHG Inventories, we updated the default AGB values for tropical and subtropical forests based on AGB data from >25 000 plots in natural forests and a global AGB map where no plot data were available. We calculated refined AGB default values per continent, ecological zone, and successional stage, and provided a measure of uncertainty. AGB in tropical and subtropical forests varies by an order of magnitude across continents, ecological zones, and successional stage. Our refined default values generally reflect the climatic gradients in the tropics, with more AGB in wetter areas. AGB is generally higher in old-growth than in secondary forests, and higher in older secondary (regrowth >20 years old and degraded/logged forests) than in young secondary forests (20 years old). While refined default values for tropical old-growth forest are largely similar to the previous 2006 default values, the new default values are 4.0-7.7-fold lower for young secondary forests. Thus, the refined values will strongly alter estimated carbon stocks and fluxes, and emphasize the critical importance of old-growth forest conservation. We provide a reproducible approach to facilitate future refinements and encourage targeted efforts to establish permanent plots in areas with data gaps

Estimating aboveground net biomass change for tropical and subtropical forests: Refinement of IPCC default rates using forest plot data

© 2019 The Authors. Global Change Biology Published by John Wiley & Sons Ltd As countries advance in greenhouse gas (GHG) accounting for climate change mitigation, consistent estimates of aboveground net biomass change (∆AGB) are needed. Countries with limited forest monitoring capabilities in the tropics and subtropics rely on IPCC 2006 default ∆AGB rates, which are values per ecological zone, per continent. Similarly, research into forest biomass change at a large scale also makes use of these rates. IPCC 2006 default rates come from a handful of studies, provide no uncertainty indications and do not distinguish between older secondary forests and old-growth forests. As part of the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, we incorporate ∆AGB data available from 2006 onwards, comprising 176 chronosequences in secondary forests and 536 permanent plots in old-growth and managed/logged forests located in 42 countries in Africa, North and South America and Asia. We generated ∆AGB rate estimates for younger secondary forests (≤20 years), older secondary forests (>20 years and up to 100 years) and old-growth forests, and accounted for uncertainties in our estimates. In tropical rainforests, for which data availability was the highest, our ∆AGB rate estimates ranged from 3.4 (Asia) to 7.6 (Africa) Mg ha−1 year−1 in younger secondary forests, from 2.3 (North and South America) to 3.5 (Africa) Mg ha−1 year−1 in older secondary forests, and 0.7 (Asia) to 1.3 (Africa) Mg ha−1 year−1 in old-growth forests. We provide a rigorous and traceable refinement of the IPCC 2006 default rates in tropical and subtropical ecological zones, and identify which areas require more research on ∆AGB. In this respect, this study should be considered as an important step towards quantifying the role of tropical and subtropical forests as carbon sinks with higher accuracy; our new rates can be used for large-scale GHG accounting by governmental bodies, nongovernmental organizations and in scientific research

Maximizing tree diversity in cocoa agroforestry: taking advantage of planted, spontaneous, and remnant Trees

International audienceIntroduction: Cocoa production is recognized as a main factor of forest loss and biodiversity declined in west Africa[3]. Thus, agroforestry is being promoted to restore a minimum forest cover, to conserve biodiversity, and to reinstate key ecosystem services in agricultural landscape. This introduced the Cocoa Forest Initiative in Ivory Coast and Ghana and several certifications for adequate forest cover in cocoa plantations while ensuring cocoa production[4]. Consequently, millions of trees are distributed in cocoa fields. However, it has largely failed, as very few trees have survived and developed properly[5]. Paradoxically, most forest tree species found in cocoa fields today are from natural regeneration, selected by farmers[5]. Three distinct cohorts of trees associated with cocoa plantations[5]: (1) trees spared during forest clearing (remnants), (2) transplanted or planted trees (planted), and (3) spontaneously recruited trees (spontaneous) after or during the creation of the cocoa plantation. The objective of this study is to understand the structure of tree diversity in Ivorian cocoa plantations and identify the main determinants.Materials and Methods: Across 150 cocoa plots, we inventoried all trees with a diameter at breast height (DBH) ≥ 10 cm present on each plot. Twelve socio-environmental variables were measured/estimated for each plot.(1) We calculated the Shannon’s Alpha and Beta diversity for each cohort and each pair of cohorts respectively and then expressed them in Hill numbers. (2) We assessed the effects of twelve socio-environmental variables on the alpha diversity of each cohort with a log-normal likelihood model.Results: Alpha diversity in these fields comes from spontaneous and remnant trees, while planted trees exhibit very low Alpha diversity. However, since planted trees show high complementarity (resulting in high Beta diversity) with spontaneous and remnant trees. Several socio-environmental factors explain these different levels of observed diversity. These factors exhibit different effects by cohort.Discussion: Planted or transplanted trees are typically fruit trees[6], Occasionally, commercial timber species are found[5] explained the low diversity observe. The remnant trees which are spared to provide shade for young cocoa plants[4] reflect the level of diversity of the forest preceding the cocoa plantation, a level that is very high[7]. Spontaneous trees mainly come from propagules from remnant trees[1]. That explained the high diversity of remnants and spontaneous trees. Planted trees and remnant/spontaneous are very complementary and can be explained by the exotic or non-indigenous nature of many planted species, particularly fruit trees[6]. Planted, spontaneous and remnants trees have different ontogenetic development Consequently, it is expected that performance trajectories and the factors modulating these trajectories will be very different among cohorts[2].References:1.Amani, B. H. K., N’Guessan, A. E., Van der Meersch, V., Derroire, G., Piponiot, C., Elogne, A. G. M., Traoré, K., N’Dja, J. K., & Hérault, B. (2022). Lessons from a regional analysis of forest recovery trajectories in West Africa. Environmental Research Letters, 17(11). https://doi.org/10.1088/1748-9326/ac9b4f2.Aubry-Kientz, M., Rossi, V., Boreux, J. J., & Hérault, B. (2015). A joint individual-based model coupling growth and mortality reveals that tree vigor is a key component of tropical forest dynamics. Ecology and Evolution, 5(12), 2457–2465. https://doi.org/10.1002/ece3.15323.Barima, Y. S. S., Kouakou, A. T. M., Bamba, I., Sangne, Y. C., Godron, M., Andrieu, J., & Bogaert, J. (2016). Cocoa crops are destroying the forest reserves of the classified forest of Haut-Sassandra (Ivory Coast). Global Ecology and Conservation, 8, 85–98. https://doi.org/10.1016/j.gecco.2016.08.0094.Kemper, L., Sampson, G., Larrea, C., Schlatter, B., Luna, E., Dang, D., & Willer, H. (2023). The State of Sustainable Markets 2023: Statistics and Emerging Trends.5.Kouassi, A. K., Zo-Bi, I. C., Aussenac, R., Kouamé, I. K., Dago, M. R., N’guessan, A. E., Jagoret, P., & Hérault, B. (2023). The great mistake of plantation programs in cocoa agroforests – Let’s bet on natural regeneration to sustainably provide timber wood. Trees, Forests and People, 12(February). https://doi.org/10.1016/j.tfp.2023.1003866.Laird, S. A., Awung, G. L., & Lysinge, R. J. (2007). Cocoa farms in the Mount Cameroon region: Biological and cultural diversity in local livelihoods. Biodiversity and Conservation, 16(8), 2401–2427. https://doi.org/10.1007/s10531-007-9188-07.Maney, C., Sassen, M., & Hill, S. L. L. (2022). Modelling biodiversity responses to land use in areas of cocoa cultivation. Agriculture, Ecosystems and Environment, 324, 107712. https://doi.org/10.1016/j.agee.2021.10771

24 years to start harvesting timber in West African cocoa agroforestry systems with spontaneous trees demonstrating clear advantages

In West Africa, where over 80% of original forests have been lost to agriculture, finding alternative timber sources is critical for regional needs and sustainability. The widespread development of agroforestry could be a promising source of timber wood, but the production potential of trees in agricultural fields cannot be directly transferred from natural forests or dedicated plantations due to different biophysical environments. Our study assesses the timber production potential of trees in 150 cocoa agroforestry systems (AFS) in Côte d'Ivoire. To achieve this, we: (i) modelled the diameter growth of forest tree species with timber potential in cocoa AFS; (ii) developed specific allometric models for trees in cocoa AFS to estimate their volume at minimum logging diameter (MLD); and (iii) evaluated the effect of tree origin (natural regeneration vs. plantation) on growth trajectories, allometry, and bole volumes. Our results show that, on average, species reach a 50 cm diameter (the smallest MLD) in 33 years, with an average bole height of 8.1 m at this diameter. Depending on species identity, trees reach MLD between 24 and 93 years. Spontaneous trees grow 10% faster annually than (trans)planted trees, reaching MLD 3.7 years earlier on average. For a given bole height, spontaneous trees are 41% larger in volume than (trans)planted trees. These findings highlight that natural regeneration is a more efficient and effective strategy than plantation for renewing trees in cocoa AFS. Natural regeneration results in higher growth rates and greater timber volumes compared to planting. Therefore, natural regeneration shows great potential for (i) sustainable forestry management in agroforestry systems and (ii) significantly contributing to meeting regional timber demands

Drivers of biomass recovery in a secondary forested landscape of West Africa

International audienceThe rapidly growing human population in West Africa has generated increasing demand for agricultural land and forest products. Consequently 90% of the original rainforest cover has now disappeared and the remainder is heavily fragmented and highly degraded. Although many studies have focused on carbon stocks and fluxes in intact African forests, little information exists on biomass recovery rates in secondary forests. We studied a chronosequence of 96 secondary and old-growth forest fragments (0.2 ha each) where 32.103 trees with Diameter at Breast Height > 2.5 cm have been censused. We modelled the biomass recovery trajectories in a time explicit Bayesian framework and tested the effect on recovery rates of a large set of covariates related to the physical environment, plot history, and forest connectivity. Recovery rate trajectory is highly non-linear: recovery rates accelerated from 1 to 37 years, when biomass recovery reached 4.23 Mg.ha(-1).yr(-1), and decelerated afterwards. We predict that, on average, 10%, 25% and 50% of the old-growth forest biomass is respectively recovered 17, 30, and 51 years after abandonment. Recovery rates are strongly shaped by both the number of remnant trees (residuals of the former old-growth forest) and the previous crop cultivated before abandonment. The latter induced large differences in the time needed to recover 50% of an old-growth forest biomass: from 38 years for former Yam fields up to 86 years for former rice fields. Our results emphasize (i) the very slow recovery rates of West African forests, as compared to Neotropical forests (ii) the long-lasting impacts of past human activities and management choices on ecosystem biomass recovery in West African degraded forests

What motivate West African cocoa farmers to value trees? Taking the 4Ws approach to the heart of the field

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