Project

Multi-functionality in natural forest ecosystems: biodiversity, tree carbon storage, habitat quality and bee forage provision

Goal: Primary production of biomass and floral resources (nectar and pollen) in forest ecosystems are relevant for availability of timber, carbon storage, beekeeping activities, and pollination-based agriculture. 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 explicit focus is not put on their improvement in future management decisions. The overarching objective of this project is to examine the relationship of biodiversity (taxonomic diversity, functional diversity and functional dominance) with tree biomass and carbon storage, habitat quality, and bee forage provision in natural forest ecosystems.

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Project log

Sylvanus Mensah
added a research item
It is prominently claimed that enhancing forest diversity would play a dual role of nature conservation and climate regulation. While the idea is intuitively appealing, studies show that species richness effects on aboveground carbon (AGC) are not always positive, but instead unpredictable especially across scales and complex terrestrial systems having large‐diameter and tall‐stature trees. Previous studies have further considered structural complexity and larger trees as determinants of AGC. Yet it remains unclear what drives differential diversity‐AGC relationships across vegetation types. Here, we test whether structural complexity and large‐sized trees play an influential role in explaining shifting diversity‐AGC relationships across vegetation types, using a 22.3 ha sampled dataset of 124 inventory plots in woodlands, gallery forests, tree/shrub savannahs and mixed plantations in West Africa. Natural vegetation had greater species richness and structural complexity than mixed plantations, as expected. In addition, AGC was highest in gallery forests and mixed plantations, which is consistent with favorable environmental conditions in the former and high stocking densities and presence of fast‐growing species in the latter. Significant interaction effects of species richness and vegetation on AGC revealed a vegetation‐dependent species richness‐AGC relationship: consistently, we found positive species richness‐AGC relationship in both mixed plantations and woodlands, and nonsignificant patterns in gallery forests and tree/shrub savannah. Further, there was a vegetation‐dependent mediation of structural complexity in linking species richness to AGC, with stronger positive structural complexity effects where species richness‐AGC relationships were positive, and stronger positive large‐sized trees’ effect where species richness‐AGC relationships were neutral. Our study provides strong evidence of vegetation‐dependent species richness‐AGC relationships, which operated through differential mediation by structural complexity of the species richness and large trees’ effects. We conclude that even higher species richness in diversified ecosystems may not always relate positively with AGC, and that neutral pattern may arise possibly as a result of larger dominant individual trees imposing a slow stand dynamic flux and overruling species richness effects.
Sylvanus Mensah
added a research item
Studies on how biodiversity influences ecosystem multifunctionality (EMF) help elucidate ecological mechanisms (e.g. niche complementarity and selection) underlying provision of multiple ecosystem services. While it is acknowledged that biodiversity contributes to EMF, the relative importance of functional traits diversity (niche complementarity) and dominance (selection effects) for EMF needs further investigation. To address this gap, we analysed how tree species diversity influences EMF, using data on species functional traits (specific wood density, specific leaf area and maximum plant height) and four ecosystem functions (carbon storage, habitat quality, forage provision and rockfall protection) in an Afromontane forest in South Africa. We tested the hypotheses that (i) trait diversity rather than dominance would link species richness to EMF; and (ii) diversity rather than species richness effects would increase with the level of EMF. For all possible scenarios of EMF indices, functional trait diversity metrics, especially functional divergence and functional richness correlated positively with EMF. On the other hand, functional dominance also influenced EMF, but played limited role in mediating EMF response to species richness, when compared with functional diversity. Results further revealed that total diversity effects, not species richness effect, generally increased with the level of EMF. In summary, we show that species richness does not fully capture the functional contribution of different species. Compared to dominance, trait diversity had significant advantage in explaining biodiversity-EMF relationship, stressing a greater role of niche complementarity as mechanism underpinning delivery of multiple functions. We argue that functional dominance reflects more the competitive dominance of traits and species within a given community and therefore is more likely to have greater effects on single functions than on multifunctionality.
Sylvanus Mensah
added a research item
Information on forest structure is fundamentally important to track successional vegetation dynamics for efficient forest management. This study reports on vegetation characteristics, dominance patterns and species height growth in a northern mistbelt forest type in South Africa. Common alpha-diversity indices (species richness and Shannon–Weiner diversity), structural vegetation parameters (tree density and basal area), and species importance value index were used. Size class distribution and height–diameter allometry were further examined for the overall stand and most important species. Stem densities (472.0 ± 43.5 and 605.3 ± 28.1 trees ha-1 for >5cm to <10 cm and > 10 cm dbh (diameter at breast height) classes, respectively) and basal area values (1.99 ± 0.19 and 48.07 ± 3.46 m2 ha-1, respectively) are comparable to other Afromontane forests in East Africa. The overall stand showed an inverted-J shaped distribution pattern which is a typical feature of stand size class distribution in most natural forests. Most ecologically important species also exhibited an inverted-J shaped distribution pattern, suggesting good regeneration and recruitment potential. There were significant differences in species on height, reflecting species-specific height growth patterns, possibly a result of intrinsic growth potential and competitive interactions. The present study suggests that conservation and management policies, including protection of surrounding land uses against fire, contribute to maintaining a successful recovery of these forests. However, it should be noted that these forests may be experiencing relatively slow dynamic flux as a result of the overmature state of some trees with several years under relatively strict protection.
Sylvanus Mensah
added a research item
Information on forest structures are fundamentally important to keep track of successional vegetation dynamics for efficient forest management. This study reports on vegetation characteristics, dominance patterns and species height growth in a Northern Mistbelt forest type in South Africa. Common alpha-diversity indices (species richness and Shannon diversity), structural vegetation parameters (tree density and basal area) and species importance value index were used. Size class distribution and height-diameter allometries were further examined for the overall stand and dominant species. Stem densities (472±43.46 and 605.33±28.10 treesha-1 for 5–10 cm and ≥10 cm dbh classes, respectively) and basal area values (1.99±0.19 and 48.07±3.46 m2ha-1, respectively) are comparable to other Afromontane forests in East Africa. The overall stand showed an inverted-J shaped distribution pattern which is typical feature of stand size class distribution in most natural forests. Most ecologically important species also exhibited an inverted-J shaped distribution pattern, suggesting good regeneration and recruitment potential. There were significant effects of species on tree height, reflecting species-specific patterns in height growth, probably a result of intrinsic growth potential and competitive interactions. The present study suggests that conservation and management policies, including protection of surrounding land uses against fire, contributed to maintain a successful recovery of these forests. However, it should be taken into account that these forests may be experiencing relatively slow dynamic flux as a result of the over-mature state of some trees with several years under relatively strict protection.
Sylvanus Mensah
added a research item
Forest stratification plays a crucial role in light interception and plant photosynthetic activities. However, despite the increased number of studies on biodiversity-ecosystem function, we still lack information on how stratification in tropical forests modulates biodiversity effects. Moreover, there is less investigation and argument on the role of species and functional traits in forest layers. Here, we analysed from a perspective of forest layer (sub-canopy, canopy and emergent species layers), the relationship between diversity and aboveground biomass (AGB), focusing on functional diversity and dominance, and underlying mechanisms such as niche complementarity and selection. The sub-canopy layer had the highest species richness and diversity, while the emergent layer had the highest AGB. Species richness–AGB relationship was positive for each forest layer, but stronger for sub-canopy layer than for canopy and emergent layers. Total AGB was strongly correlated with functional diversity, leaf and wood traits of species in the sub-canopy and canopy layers. This suggests that sub-canopy and canopy species are major drivers of stand diversity-AGB relationship, and that resource filtering by canopy or emergent trees may not reduce the strength of diversity–AGB relationship in the sub-canopy layer. We argue that complementary resource use by sub-canopy species that supports niche complementarity, is a key mechanism driving AGB in natural forests. Selection effects are most evident in emergent species and niche complementarity effects for sub-canopy and canopy species, supporting arguments that AGB is affected by sub-canopy species efficient use of limited resources despite competition from emergent species.
Sylvanus Mensah
added a research item
Exploring taxonomic, functional, and structural diversity can provide additional insights into our understanding of diversity responses to environment. Using altitude, slope, and relative radiation index as well as floristic and functional data from a South Africa Afromontane forest, we examined how taxonomic, structural, and functional diversity varied with local environmental variation. Taxonomic and structural diversity were quantified through species richness- and diameter class-based Shannon index and evenness, respectively. Skewness and coefficient of variation of diameter distribution were additionally computed for structural diversity. As for functional diversity, we used functional richness, evenness, divergence, and dispersion based on functional traits. Data were analyzed using multimodel inference and subset regression. We found little evidence of environmental effects on local-scale taxonomic diversity patterns. In contrast, structural and functional diversity metrics varied significantly along environmental gradients. Accordingly, diameter class-based Shannon evenness declined with increasing slope while skewness and coefficient of variation of diameter distribution increased with increasing slope. Functional evenness and divergence decreased with increasing altitude and radiation, respectively, while functional richness and dispersion increased with increasing slope. The results showed that taxonomic diversity patterns were less responsive to local-scale topographical variation than structural and functional diversity. Lower functional diversity on lower slope sites suggests weak environmental filtering effect promoting competitive exclusion and dominance of species with acquisitive traits. On higher slope sites, environmental filtering associated with slope gradient seems to favor coexistence of species with conservative traits and adapted to harsh conditions.
Sylvanus Mensah
added a research item
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.
Thomas Seifert
added 3 research items
Biomass allocation is closely related to species traits, resources avail- ability and competitive abilities, and therefore it is often used to capture resource utilisation within plants. In this study, we searched for patterns in biomass alloca- tion 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 bio- mass 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 bio- mass 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.
Ecosystem services (ESs) underpin human livelihoods around the world. Understanding how socio-environmental aspects influence stakeholders’ perceptions and use of ESs, is important for decision-making processes that target the social expectations. In this study, face-to-face interviews were conducted with eighty-six householders in four villages of Limpopo province (South Africa), to assess the importance and use of ESs. Descriptive rank analysis, ordered logistic regression and Poisson generalised linear mixed-effects models were used. Supporting and provisioning ESs were rated the most important, followed by regulating and cultural ESs. Among the provisioning ESs, timber, firewood and edible plants were the most important, the most cited and used. Age, gender, income and prior recreational experiences played important roles in householders’ perceptions. The frequency of collection of provisioning ESs declined with increasing distance to the forest and presence of foothills in landscape, which formed natural barriers. The results further revealed that employed householders benefited more from these services than unemployed householders. However, there was no significant effect of income variable on the use of the provisioning ESs, suggesting that the collection is more likely oriented towards a domestic usage. The implications of the results were discussed in a context of local development planning.
Honey bees play a vital role in the pollination of flowers in many agricultural systems, while providing honey through well managed beekeeping activities. Managed honey bees rely on the provision of pollen and nectar for their survival and productivity. Using data from field plot inventories in natural mistbelt forests, we (1) assessed the diversity and relative importance of honey bee plants, (2) explored the temporal availability of honey bee forage (nectar and pollen resources), and (3) elucidated how plant diversity (bee plant richness and overall plant richness) influenced the amount of forage available (production). A forage value index was defined on the basis of species-specific nectar and pollen values, and expected flowering period. Up to 50% of the overall woody plant richness were found to be honey bee plant species, with varying flowering period. As expected, bee plant richness increased with overall plant richness. Interestingly, bee plants’ flowering period was spread widely over a year, although the highest potential of forage supply was observed during the last quarter. We also found that only few honey bee plant species contributed 90 percent of the available forage. Surprisingly, overall plant richness did not significantly influence the bee forage value. Rather, bee plant species richness showed significant and greater effect. The results of this study suggest that mistbelt forests can contribute to increase the spatial and temporal availability of diverse floral resources for managed honey bees. Conservation efforts must be specifically oriented towards honey bee plant species in mistbelt forests to preserve and enhance their potential to help maintain honey bee colonies. The implications for forest management, beekeeping activities and pollination-based agriculture were discussed.
Sylvanus Mensah
added 2 research items
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.
Sylvanus Mensah
added 2 research items
Novel species-specific equations for the estimation of height and aboveground biomass were established for four dominant tree species (Syzygium gerrardii Burtt Davy, Combretum kraussii Hochst., Trichilia dregeana Sond. and Croton sylvaticus Hochst.), in the Northern Mistbelt Forests of South Africa. A non-destructive sampling methodology was applied, which was based on measuring standing trees, selecting smaller branches and taking core samples. The species-specific aboveground biomass equations were fitted using predictor variables such as diameter at breast height (DBH) and total height (H). The relative error of estimation was used to examine the accuracy of a pantropical biomass equation versus our established specific model. Biomass values were afterwards up-scaled from tree to stand level for each species, based on the selected models and the forest inventory data. As expected, the DBH–height relationship varied among studied species. The incorporation of both DBH and H in the biomass models significantly improved their precision. A model with DBH² × H as a single variable was suitable for three out of the four studied species, with more than 98% of explained variance. An existing pantropical biomass equation for moist forests showed larger relative error of estimation, especially in the upper range of tree diameter. The estimated aboveground biomass density varied significantly among studied species, with the highest values recorded for S. gerrardii (87.7 ± 15.4 Mg ha⁻¹), followed by T. dregeana (29.4 ± 14.7 Mg ha⁻¹), C. sylvaticus (24.3 ± 11.5 Mg ha⁻¹) and C. kraussii (20.1 ± 6.7 Mg ha⁻¹). It is also found that species-specific production of biomass at the tree level is not always sufficient to reflect the stand-level biomass density. The results from this study contribute to accurately predict aboveground biomass, thereby improving the reliability of the estimates of forest biomass and carbon balance.
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.
Sylvanus Mensah
added a project goal
Primary production of biomass and floral resources (nectar and pollen) in forest ecosystems are relevant for availability of timber, carbon storage, beekeeping activities, and pollination-based agriculture. 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 explicit focus is not put on their improvement in future management decisions. The overarching objective of this project is to examine the relationship of biodiversity (taxonomic diversity, functional diversity and functional dominance) with tree biomass and carbon storage, habitat quality, and bee forage provision in natural forest ecosystems.