Browsing by Author "Mensah, Sylvanus"

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    Aboveground biomass and carbon in a South African mistbelt forest and the relationships with tree species diversity and forest structures
    (MDPI, 2016) Mensah, Sylvanus; Veldtman, Ruan; Du Toit, Ben; Kakai, Romain Glele; Seifert, Thomas
    ENGLISH ABSTRACT: Biomass and carbon stocks are key information criteria to understand the role of forests in regulating global climate. However, for a bio-rich continent like Africa, ground-based measurements for accurate estimation of carbon are scarce, and the variables affecting the forest carbon are not well understood. Here, we present the first biomass study conducted in South Africa Mistbelt forests. Using data from a non-destructive sampling of 59 trees of four species, we (1) evaluated the accuracy of multispecies aboveground biomass (AGB) models, using predictors such as diameter at breast height (DBH), total height (H) and wood density; (2) estimated the amount of biomass and carbon stored in the aboveground compartment of Mistbelt forests and (3) explored the variation of aboveground carbon (AGC) in relation to tree species diversity and structural variables. We found significant effects of species on wood density and AGB. Among the candidate models, the model that incorporated DBH and H as a compound variable (DBH2 × H) was the best fitting. AGB and AGC values were highly variable across all plots, with average values of 358.1 Mg·ha−1 and 179.0 Mg·C·ha−1, respectively. Few species contributed 80% of AGC stock, probably as a result of selection effect. Stand basal area, basal area of the ten most important species and basal area of the largest trees were the most influencing variables. Tree species richness was also positively correlated with AGC, but the basal area of smaller trees was not. These results enable insights into the role of biodiversity in maintaining carbon storage and the possibilities for sustainable strategies for timber harvesting without risk of significant biomass decline.
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    Patterns of biomass allocation between foliage and woody structure : the effects of tree size and specific functional traits
    (Marin Dracea National Research-Development Institute in Forestry, 2016-02) Mensah, Sylvanus; Kakai, Romain Glele; Seifert, Thomas
    ENGLISH ABSTRACT: Biomass allocation is closely related to species traits, resources availability and competitive abilities, and therefore it is often used to capture resource utilisation within plants. In this study, we searched for patterns in biomass allocation between foliage and wood (stem plus branch), and how they changed with tree size (diameter), species identity and functional traits (leaf area and specific wood density). Using data on the aboveground biomass of 89 trees from six species in a Mistbelt forest (South Africa), we evaluated the leaf to wood mass ratio (LWR). The effects of tree size, species identity and specific traits on LWR were tested using Generalised Linear Models. Tree size (diameter) was the main driver of biomass allocation, with 44.43 % of variance explained. As expected, LWR declined significantly with increasing tree diameter. Leaf area (30.17% explained variance) and wood density (12.61% explained variance) also showed significant effects, after size effect was accounted for. Results also showed clear differences among species and between groups of species. Per unit of wood mass, more biomass is allocated to the foliage in the species with the larger leaf area. Inversely, less biomass is allocated to the foliage in species with higher wood density. Moreover, with increasing diameter, lower wood density species tended to allocate more biomass to foliage and less biomass to stems and branches. Overall, our results emphasise the influence of plant size and functional traits on biomass allocation, but showed that neither tree diameter and species identity nor leaf area and wood density are the only important variables.
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    Selected key ecosystem services, functions, and the relationship with biodiversity in natural forest ecosystems
    (Stellenbosch : Stellenbosch University, 2016-12) Mensah, Sylvanus; Seifert, Thomas; Veldtman, Ruan; Stellenbosch University. Faculty of Agrisciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: Ecosystem services (ES) are the benefits people obtain from ecosystems. Biodiversity is a relevant component in this context, as it supports most ecosystem functions. The worldwide decline in natural ecosystem biodiversity and degradation of ecosystems will likely limit the opportunities to improve the services provided if an explicit focus is not put on their maintenance and improvement in future management decisions. This thesis aims at contributing to the knowledge base of ES provision and its relation with biodiversity in South African Mistbelt forests. First, a socio-empirical study was conducted with households from different localities in Limpopo province to assess the relative importance and use of ES by local population. Questionnaires, descriptive rank analysis, logistic regressions and Poisson mixed effect models were used. Results revealed that provisioning and supporting ES (timber and firewood, edible plants and honey) were the most important, and that age, gender, income as well as prior recreational experiences played important roles in respondents’ perceptions. “Timber and firewood” and “honey” were selected for further investigation in Mistbelt natural forests in Limpopo. Second, the potential of Mistbelt forests as foraging habitat for managed honey bees, and as tree biomass and carbon sinks was investigated. Forest inventory was conducted to collect data on woody plant species (plant density, diameter, total height, pollen and nectar values of honey bee plant species; branch and wood core samples, leaf area, plant maximum height). The diversity of honey bee plant species, temporal availability of honey bee forage and relationship of plant diversity with forage production, were explored, using descriptive analysis and general linear models. As results, 48% of the woody plant species recorded in the inventory were honey bee plant species. The highest potential of forage supply was observed during the last quarter of the year. Bee plant species richness, not overall plant richness, showed the greater effect on forage provision. New multispecies biomass models were established and used to estimate aboveground biomass and carbon stocks, and examine the relationship of aboveground carbon with tree species diversity and structural variables. Allometric models with “DBH2xH” were the best fitting. Biomass and carbon stand values of 358.1 Mg ha-1 and 179.0 Mg ha-1, respectively, were obtained. Basal area of most important species, and of largest trees, as well as plant richness were positively correlated with carbon stock. The relationship of biomass and carbon stock with biodiversity components was the final focus. An analysis scrutinised the effects of taxonomic diversity on carbon storage. Correlations were explored through functional traits-based diversity and dominance, using structural equation and linear mixed-effects models. Results showed that taxonomic diversity effects on carbon stock were positive, and mediated by functional diversity and functional dominance. The effects, however, were greater for functional diversity than for functional dominance. In sum, a novel contribution was made to the ES provision of South African Mistbelt forests, which provides a base for the assessment of these forests and their contribution and underpins the need to protect these ecosystems as a natural heritage.
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    Tree species diversity promotes aboveground carbon storage through functional diversity and functional dominance
    (Wiley Open Access, 2016) Mensah, Sylvanus; Veldtman, Ruan; Assogbadjo, Achille E.; Kakai, Romain Glele; Seifert, Thomas
    ENGLISH ABSTRACT: The relationship between biodiversity and ecosystem function has increasingly been debated as the cornerstone of the processes behind ecosystem services delivery. Experimental and natural field-based studies have come up with nonconsistent patterns of biodiversity–ecosystem function, supporting either niche complementarity or selection effects hypothesis. Here, we used aboveground carbon (AGC) storage as proxy for ecosystem function in a South African mistbelt forest, and analyzed its relationship with species diversity, through functional diversity and functional dominance. We hypothesized that (1) diversity influences AGC through functional diversity and functional dominance effects; and (2) effects of diversity on AGC would be greater for functional dominance than for functional diversity. Community weight mean (CWM) of functional traits (wood density, specific leaf area, and maximum plant height) were calculated to assess functional dominance (selection effects). As for functional diversity (complementarity effects), multitrait functional diversity indices were computed. The first hypothesis was tested using structural equation modeling. For the second hypothesis, effects of environmental variables such as slope and altitude were tested first, and separate linear mixed-effects models were fitted afterward for functional diversity, functional dominance, and both. Results showed that AGC varied significantly along the slope gradient, with lower values at steeper sites. Species diversity (richness) had positive relationship with AGC, even when slope effects were considered. As predicted, diversity effects on AGC were mediated through functional diversity and functional dominance, suggesting that both the niche complementarity and the selection effects are not exclusively affecting carbon storage. However, the effects were greater for functional diversity than for functional dominance. Furthermore, functional dominance effects were strongly transmitted by CWM of maximum plant height, reflecting the importance of forest vertical stratification for diversity–carbon relationship. We therefore argue for stronger complementary effects that would be induced also by complementary light-use efficiency of tree and species growing in the understory layer.