Browsing by Author "Medjibe, VP"

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    Estimates and determinants of stocks of deep soil carbon in Gabon, Central Africa
    (Geoderma, 2019-05-01) Wade, AM; Richter, DD; Medjibe, VP; Bacon, AR; Heine, PR; White, LJT; Poulsen, JR
    © 2019 Despite the importance of tropical forest carbon to the global carbon cycle, research on carbon stocks is incomplete in major areas of the tropical world. Nowhere in the tropics is this more the case than in Africa, and especially Central Africa, where carbon stocks are known to be high but a scarcity of data limits understanding of carbon stocks and drivers. In this study, we present the first nation-wide measurements and determinants of soil carbon in Gabon, a nation in Central Africa. We estimated soil carbon to a 2-m depth using a systematic, random design of 59 plots located across Gabon. Soil carbon to a 2-m depth averaged 163 Mg ha −1 with a CV of 61%. These soil carbon stocks accounted for approximately half of the total carbon accumulated in aboveground biomass and soil pools. Nearly a third of soil carbon was stored in the second meter of soil, averaging 58 Mg ha −1 with a CV of 94%. Lithology, soil type, and terrain attributes were found to be significant predictors of cumulative SOC stocks to a 2-m depth. Current protocols of the IPCC are to sample soil carbon from the surface 30 cm, which in this study would underestimate soil carbon by 60% and underestimate ecosystem carbon by 30%. A nonlinear model using a power function predicted cumulative soil carbon stocks in the second meter with an average error of prediction of 3.2 Mg ha −1 (CV = 915%) of measured values. The magnitude and turnover of deep soil carbon in tropical forests needs to be estimated as more countries prioritize carbon accounting and monitoring in response to accelerating land-use change.
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    Forest structure determines the abundance and distribution of large lianas in Gabon
    (Global Ecology and Biogeography, 2017-04-01) Poulsen, JR; Koerner, SE; Miao, Z; Medjibe, VP; Banak, LN; White, LJT
    © 2016 John Wiley & Sons Ltd Aim: Lianas are an important component of forest structure in the tropics, accounting for up to 45% of total stems. Mounting evidence that tropical forests are undergoing structural changes, with a growing abundance of lianas reducing forest carbon storage potential, imparts a sense of urgency to study the drivers that control liana abundance and biomass, particularly in Africa where data come from a few, small-scale studies. Location: Gabon, Africa. Methods: In the first countrywide study of lianas, we implemented the most ambitious, large-scale forest inventory in tropical Africa to date, quantifying the density, basal area and biomass of large lianas (≥10 cm in diameter) using a systematic, random design of 104 plots located across Gabon. Additionally, we examined the relative importance of environmental variables (mean annual precipitation, mean annual temperature, seasonality, soil nitrogen, soil fertility), disturbance (effect of gaps, forest type) and forest structure (large tree biomass) in driving macroscale variation in the abundance of large lianas. Results: In total, we surveyed 1354 large lianas, and found the density, basal area and biomass of large lianas in Gabon to be comparable to that in other tropical forests. The success of large lianas was positively related to soil N, but most strongly correlated with forest structure, particularly large tree biomass. The strength of the association between large lianas and large trees increased with tree size class. Main conclusions: Forest structure and the availability of large trees may be more important predictors of the abundance and distribution of large lianas in African tropical forests than environmental variables and disturbance. Changing environmental conditions are likely to have little direct effect on large lianas, but climate change, defaunation and land-use activities that diminish forest structure and reduce the number of large trees could have strong indirect effects on large lianas in Central African forests.
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    From town to national park: Understanding the long-term effects of hunting and logging on tree communities in Central Africa
    (Forest Ecology and Management, 2021-11-01) Maicher, V; Clark, CJ; Harris, DJ; Medjibe, VP; Poulsen, JR
    Anthropogenic disturbances are changing the structure and composition of tropical forests worldwide. Multiple disturbances often occur simultaneously in forests. Hunting and logging, for example, are within-forest disturbances that impact vast areas of seemingly intact rainforests. Despite recent work on the individual effects of these disturbances, our understanding of how they interact to influence tree communities is still limited. In northern Republic of Congo, we explored the effects of hunting and logging on tree communities. Over an 8-year period, we monitored 12,552 tree stems (≥10 cm diameter-at-breast height) spread over 30 1-ha plots along a gradient of human disturbance to compare the tree diversity between hunted and logged forest, once-logged forest, and protected forest free of both disturbances. Tree density, species richness, and community composition were affected by both hunting and logging. Forest close to human settlements was richer, more heterogenous, and more dynamic in species composition across censuses. In hunted and logged forest, fast-growing secondary species with low shade tolerance replaced old growth species. Comparatively, the once-logged forest had the greatest stem density and intermediate species richness with an increased density of shade-bearing species over time. Both tree species spatial turnover and tree recruitment were greatly affected by proximity to human settlements. A shift towards abiotically dispersed trees and increasing seed predation by rodents near villages can partly explain the differences in tree recruitment across the forest types. The combination of hunting and logging seems to have a greater impact on tree communities than either single disturbance, especially with nearness to villages.
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    Natural regeneration of selected timber species in the Republic of Congo
    (African Journal of Ecology, 2014) Medjibe, VP; Poulsen, JR; Clark, CJ; Mbani, OA
    © 2014 John Wiley & Sons Ltd.Natural regeneration of timber species is critical to the sustainable management of tropical forests. To understand what determines regeneration success of timber species in the Congo Basin, we evaluated whether seedling recruitment rates differed between forest logged 30 years previously and unlogged forest and determined the environmental factors that influence seedling density, growth and survival. We monitored the fate of 2186 seedlings of seven timber species within 462, 25-m<sup>2</sup> plots located along 21 transects. We characterized seedling plots by light availability, soil nutrient availability and pH, and abundance of mammalian herbivores and then used linear and generalized linear mixed models to evaluate the variables that influenced seedling density, growth and survival. Light availability and canopy openness were 18% and 81% higher in logged than unlogged forest, and concentration of soil nutrients varied between sites. Seedling density was 32% higher in unlogged than logged forest. Taking all species together, seedling survival was positively correlated with calcium and negatively with magnesium and available phosphorus. Rates of seedling growth increased with available light. Taken separately, seedlings of the selected timber species responded differently to abiotic and biotic factors, demonstrating species-specific regeneration requirements.
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    Old growth Afrotropical forests critical for maintaining forest carbon
    (Global Ecology and Biogeography, 2020-10-01) Poulsen, JR; Medjibe, VP; White, LJT; Miao, Z; Banak-Ngok, L; Beirne, C; Clark, CJ; Cuni-Sanchez, A; Disney, M; Doucet, JL; Lee, ME; Lewis, SL; Mitchard, E; Nuñez, CL; Reitsma, J; Saatchi, S; Scott, CT
    Aim: Large trees [≥ 70 cm diameter at breast height (DBH)] contribute disproportionately to aboveground carbon stock (AGC) across the tropics but may be vulnerable to changing climate and human activities. Here we determine the distribution, drivers and threats to large trees and high carbon forest. Location: Central Africa. Time period: Current. Major taxa studied: Trees. Methods: Using Gabon's new National Resource Inventory of 104 field sites, AGC was calculated from 67,466 trees from 578 species and 97 genera. Power and Michaelis–Menten models assessed the contribution of large trees to AGC. Environmental and anthropogenic drivers of AGC, large trees, and stand variables were modelled using Akaike’s information criterion (AIC) weights to calculate average regression coefficients for all p. ossible models. Results: Mean AGC for trees ≥ 10 cm DBH in Gabonese forestlands was 141.7 Mg C/ha, with averages of 166.6, 171.3 and 96.6 Mg C/ha in old growth, concession and secondary forest. High carbon forests occurred where large trees are most abundant: 31% of AGC was stored in large trees (2.3% of all stems). Human activities largely drove variation in AGC and large trees, but climate and edaphic conditions also determined stand variables (basal area, tree height, wood density, stem density). AGC and large trees increased with distance from human settlements; AGC was 40% lower in secondary than primary and concession forests and 33% higher in protected than non-managed areas. Main conclusions: AGC and large trees were negatively associated with human activities, highlighting the importance of forest management. Redefining large trees as ≥ 50 cm DBH (4.3% more stems) would account for 20% more AGC. This study demonstrates that protecting relatively undisturbed forests can be disproportionately effective in conserving carbon and suggests that including sustainable forestry in programs like reduced emissions for deforestation and forest degradation could maintain carbon dense forests in logging concessions that are a large proportion of remaining Central African forests.
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    Reducing Carbon Emissions from Forest Conversion for Oil Palm Agriculture in Gabon
    (Conservation Letters, 2017-05-01) Burton, MEH; Poulsen, JR; Lee, ME; Medjibe, VP; Stewart, CG; Venkataraman, A; White, LJT
    Copyright and Photocopying: © 2016 The Authors. Conservation Letters published by Wiley Periodicals, Inc. Growing demand for palm oil is driving its expansion into the African tropics, potentially leading to significant carbon emissions if tropical forest is converted to palm monoculture. In this first study of a Central African oil palm concession (31,800 ha), we predict that the conversion of 11,500 ha of logged forest to a palm plantation in Gabon will release 1.50 Tg C (95% CI = [1.29, 1.76]). These emissions could be completely offset over 25 years th rough sequestration in planned forest set-asides given a 2.6:1 ratio of logged to converted forest. Using an agricultural suitability model, we find that careful national land-use planning could largely avoid high carbon emissions while meeting goals for palm oil production. We recommend that Gabon adopts a national carbon threshold for land conversion and requires concession-level set-aside ratios that meet no-net emissions criteria as mechanisms for steering plantations away from high carbon forests.