Species diversity promotes aboveground carbon stocks
In our study there was a significant and positive effect of species richness on aboveground carbon, after effects of environmental drivers (e.g. Elevation) were accounted for. While this finding in line with some previous studies that controlled for the effects of environmental factors (Ruiz-Jaen and Potvin 2010; Wu et al. 2015; Mensah et al. 2016), it also supports the positive relationship in diverse natural forests; that is, biomass and carbon stocks increase with increasing species diversity. In fact, various local and global studies on forest ecosystems have observed a positive relationship between species richness and forest biomass or carbon (Cavanaugh et al. 2014; Ruiz-Benito et al. 2014; Wu et al. 2015; Mensah et al. 2016). In addition, studies in subtropical forests (Vance-Chalcraft et al. 2010), Spruce dominated forest stands (Wang et al. 2011), Collaborative forests in Terai, Nepal (Mandal et al. 2013), and tropical forests (Barrufol et al. 2013) have also reported increases in biomass productivity with increasing diversity.
It noted that increasing species richness, would be increased carbon storage in forest ecosystems because higher taxonomic diversity would lead to a higher proportion of large trees (Ruiz-Jaen and Potvin 2010; Ruiz-Benito et al. 2014). Some other recent evidences showed that a positive effect of species richness on aboveground carbon might also be explained through the biotic interactions such as facilitation, where by some species could enhance soil fertility (e.g. by fixing nitrogen) for the productivity of other species. This fact is even always used to support the reason why mixed species, communities of plantations are more productive than mono-specific stands (Mensah et al. 2016). Although it might also be well promising that increasing species richness increases the probability of inclusion of highly productive and naturally dominant species (Ruiz-Benito et al. 2014; Mensah et al. 2018).
While our result in the dry evergreen Afromontane forest supports the positive species richness-aboveground carbon relationship, it should be well known that finding of the inverse effect also exists. For example, report by Ruiz-Jaen and Potvin (2011) from natural forest of Barro Colorado Island in Central Panama. Other studies have reported null or negative relationship between aboveground carbon and species diversity in forest ecosystems (Zhang et al. 2014; Whittaker and Heegaard, 2003). These discrepancy findings suggest that the effects of diversity on forest carbon may vary with other factors such as the types, sites and the successional stages of the forests (Lasky et al. 2014; Wu et al. 2015), and also the specific dimension of the diversity measure used (Vance-Chalcraft et al. 2010; Lasky et al. 2014).
Effects of environmental factors on tree aboveground carbon stock
In this study we did not found significant effect of elevation on tree aboveground carbon stock. In line with our finding, Cavanaugh et al. (2014) and Mensah et al. (2016) reported no significant relationship between forest carbon and elevation. However, this finding runs contrary to many previous studies that tested the relationships between elevation and biomass or carbon stock (Ensslin et al. 2015). On the one hand, some scholars found that biomass and carbon stocks could decrease with increasing elevation (Moser et al. 2007). On the other hand, studies observed positive correlation between increasing tree carbon and increasing elevation (Zhang et al. 2018; Zhu et al. 2010). This lack of clarity on the relationship between altitude and forest biomass may be partly due to the variation in the elevation range among studies.
Unlike altitude and aspect, slope showed significant effect, and accounted for 5.51% of aboveground carbon variance, evidencing that variations in carbon stocks can result from topological constraints, particularly difference in slope. Consistent with our results, slope has been identified as an important environmental gradient that affects tree carbon (de Castilho et al. 2006; Chave et al. 2003). This is because, aboveground carbon is inherently related to wood and biomass production, the effect of slope can be seen as prior impacts of environment on availability of resources (Luizao et al. 2004), which in turn influence forest dynamics. For instance, steeper slope will speed up nutrients, water runoff, and constrain trees and will also favor highly water and nutrient efficient species against others. Taking this into account, it follows that tree growth and biomass production can be potentially declined at higher slope areas, as results of moisture and nutrient stress (Clark et al. 2010; Durán et al. 2015), whereas flat and gentle slope areas would allow for more water availability, to which plant would likely respond positively. The significant effect of slope supports the fact that ecosystem functions in general and biomass carbon stock in particular are environment- structured (Wu et al. 2015).
Diversity effects mediated via functional diversity and functional dominance
In the recent decades, interest has increased significantly in determining the multiple biodiversity measures and ecosystems functioning relationships. One of the most commonly examined relationships is that between species richness and productivity (Ruiz-Jaen and Potvin 2010).
The use of different measures of biodiversity component to provide additional mechanisms motivating the effects of biodiversity on carbon stocks has also gained increasing interest in recent years (Cadotte et al. 2011; Conti and Díaz 2013; Finegan et al. 2015; Lasky et al. 2014; Ruiz-Benito et al. 2014; Vance-Chalcraft et al. 2010; Ziter et al. 2013). Accordingly, functional diversity and dominance metrics were also examined in this study. While most of these studies tended to compare the relative effects of species richness and other biodiversity components on aboveground carbon stocks, by assuming that these effects were mediated via functional diversity and functional dominance. In this study the structural equation modeling results confirm this assumption and this is, therefore, supports the need to explore beyond species richness to elucidate the mechanisms that drive relationship between diversity and productivity. The results further support the hypothesis that both niche complementarity and selection effects are not exclusively affecting carbon storage in tropical forests (Cavanaugh et al. 2014; Wu et al. 2017). Therefore, diversity (species richness) predicts carbon stock through effects of both functional diversity and functional dominance, partly because these diversity components are based on specific functional traits, which would reflect functional variations among the species (Di´az and Cabido 2001; Mensah et al. 2016). Indeed, in this finding increased species richness indirectly accounted for differences among species, in terms of ecological niche and resource use.
Functional diversity effects on aboveground carbon stock
The linear mixed models were used to examine the effects of functional diversity measures on tree aboveground carbon in Dindin dy evergreen Afromontane forest of southeast Ethiopia. Out of the four functional diversity metric used in this study, only functional richness and functional dispersion were found to significantly explain variation in aboveground carbon stock. There is a recent scientific evidence for functional diversity effects on biomass and carbon. Yuan et al. (2018) observed a significant effect of functional evenness on aboveground carbon in temperate mixed forests. Similarly, in tropical rain forests of Bolivia, Brazil, and Costa Rica, Finegan et al. (2015) found functional richness among other functional diversity indices as significant predictor for biomass variation. A study by Ziter et al. (2013) in unmanaged forest fragments in Quebec showed significant and positive relationships between functional dispersion and aboveground carbon. Other study by Finegan et al. (2014) reported significant but negative effects of the functional richness on stand biomass in tropical forests. While in this study, these functional diversity indices have their specific biological meaning, the positive effect of functional richness on the aboveground carbon might be due to functional richness being positively correlated with species richness (refer SEM output; Masonet al. 2005).
Functional diversity has been dissected into four relatively independent components: functional richness FRic, evenness FEve, divergence FDiv (Villeger et al. 2008; Mouchet et al. 2010) and dispersion FDis (Laliberte and Legendre 2010). These indices quantify the trait hypervolume of the community (FRic, FDis) and the distribution of abundance or biomass of the species in this volume (FEve, FDiv and FDis). Both these functional characteristics may measure niche complementarity, and therefore increment of ecosystem processes by functional trait variety. The functional would increase carbon stock because species with different traits would differ in resource use, and would more efficiently use the resources available within the community for higher growth and productivity, indicating the importance of niche complementarity effects in facilitating ecosystem processes (Finegan et al. 2015). Unlike functional richness, functional evenness and functional dispersion, functional divergence did not show any relationship with species richness and show no significant influence on aboveground carbon. According to Laliberte and Legendre (2010), the functional dispersion is the mean distance of each species, weighted by its relative abundances, to the centroid of all species in a community. Therefore, both functional richness and functional dispersion relate to the niche space or volumes of niche space, and functional diversity as measured here could reflect some form of “niche differentiation” (Carroll et al. 2011). Niche differentiation is expressed as the beta niche of species, i.e. differentiating species with different optima between communities across environmental gradients (Weiher and Keddy 1995; Silvertown et al. 2006). A greater functional diversity, that is, greater value and range of functional traits, would reflect not only the magnitude of “niche differentiation”, but also the differences in resource utilization by species, thus promoting diversity effects on ecosystem functioning. This is consistent with Zhu et al. (2016) who observed increasing niche difference contributes to species coexistence and positive diversity effects on biomass productivity.
Functional dominance effects on aboveground carbon stock
Community weighted mean (CWM) functional trait values, which are community trait values weighted (selection effect) by species abundances (Muscarella and Uriarte 2016; Thukar and Chawla 2019) used to reflects locally optimal strategies of a community and to predict functional dominance effects. CWM functional traits as functional dominance metric could be used to elucidate on ecological ﬁtness by resource competition ability and environmental ﬁltering (Cornwell and Ackerly 2009; Kraft et al. 2015). Therefore, functional dominance could indicate some aspect of “relative performance or fitness differences” between competitors for limiting resources (Cadotte et al. 2011; Carroll et al. 2011). The dominant functional traits (high wood density and low specific leaf area) in the stressful areas are indicative of a stress tolerant life history strategy (Chapman and McEwan 2018). Moreover, the finding that functional dominance significantly affected tree carbon storage is in line with the previous finding that the magnitude of “relative fitness differences” strengthens the influence of diversity on biomass productivity (Carroll et al. 2011). In this study the functional dominance effects varied with the functional trait. Functional traits are those attributes of an organism or a part of an organism which strongly influences fitness through their effects on the overall structure, function, and diversity of ecosystems (Wieczynski et al. 2019). Particularly, CWM of wood density showed negative and significant effect on aboveground carbon stocks. It is not surprising given that wood density is a potential predictor of tree biomass, which highly correlates with the carbon stock. There are some evidences that CWM of wood density is negatively related to the biomass increment, as being good predictor of individual tree diameter increments (Finegan et al. 2015). After evaluating biomass stocks in tropical forests, some scholars found that AGC-wood density relationship varies from negative to null to positive depending on the forest community and forest identity (Baker et al. 2004; Stegen et al. 2009). In case of the present finding about CWM of wood density indicates that low wood density species grow faster and expect to store more biomass; therefore, it recommends that conserving and planting low wood density species would likely help to increase the carbon stock.
In analysis of combined effects of functional dominance metrics only CWM of specific leaf area and of maximum plant height were retained in the final model, with maximum plant height being the significant predictor. This is most likely tree height is a key factor for species-specific or multispecies biomass regressions. Ali (2015) suggest that strong dominance by tall and conservative species, rather than a set of coexisting species with diverse heights and exploitative nature, results in greatest carbon stocks in natural forest ecosystems. Therefore, the positive and significant relationship between CWM of maximum plant height and carbon stocks indicates the potential importance of characteristics of dominant and adult trees for ecosystem functioning and productivity, thus supporting the selection effects hypothesis.
Functional diversity and functional dominance partly effects aboveground carbon stocks
In testing the proportion of variation explained by the selection effect and niche complementarity mechanisms, functional diversity explained more variation of aboveground carbon than functional dominance (Tables 3 and 4). Unfortunately, this rejects the second hypothesis of this study, and suggests that niche complementarity effects appear to be more important than selection effects. This finding consistent with study by Mensah et al. (2016) supports that functional diversity results in more variance in aboveground carbon than selection effects. In contrary, study by Finegan et al. (2015), Ruiz-Jaen and Potvin’s (2011) revealed that selection effects more important for the aboveground biomass and carbon storage in tropical forests. The reason for this discrepancy is that in this study, functional dominance metrics (community weight mean of functional traits) were estimated using species relative abundance, but a study by Ruiz-Jaen and Potvin (2011) and Finegan et al. (2015) used species relative basal area and species relative biomass, respectively, as weighting variable. As noted by Mensah et al. (2016), the strength of relationship between community weight mean of functional traits and the ecosystem function of interest could depend on the weighting variables. Therefore, community weighted mean values of functional trait weighted by biomass or basal area as weighting variables would likely show stronger relation with biomass and carbon than abundance-based communities mean values.
In examining joint effects, the present finding supports the thought that these two hypotheses (niche complementarity and selection effects) are not mutually exclusive, and can contribute to ecosystem functioning. Previous evidence of both complementarity and selection effects on ecosystem function suggests they can also contribute at different proportions at different times of ecosystem services (Fargione et al. 2007). Both complementarity and selection effects mutually promote species coexistence. As reason out by Mensah et al. (2016), these two hypotheses could even be the outcome of interactions of the “relative fitness differences” and the “niche differences”, whereby some species’ populations could be suppressed by dominant competitors, to allow effective utilization of the available resources. In this study the selection effects are strongly mediated through specific maximum plant height, which indicates the effect of dominant species and suggests a possible competitive exclusion in terms of utilization of resources (refer SEM e.g., sun light).