Precipitation gradients drive high tree species turnover in the woodlands of eastern and southern Africa

Savannas cover one-fifth of the Earth's surface, harbour substantial biodiversity, and provide a broad range of ecosystem services to hundreds of millions of people. The community composition of trees in tropical moist forests varies with climate, but whether the same processes structure communities in disturbance-driven savannas remains relatively unknown. We investigate how biodiversity is structured over large environmental and disturbance gradients in woodlands of eastern and southern Africa. We use tree inventory data from the Socio-Ecological Observatory for Studying African Woodlands (SEOSAW) network, covering 755 ha in a total of 6780 plots across nine countries of eastern and southern Africa, to investigate how alpha, beta, and phylogenetic diversity varies across environmental and disturbance gradients. We find strong climate-richness patterns, with precipitation playing a primary role in determining patterns of tree richness and high turnover across these savannas. Savannas with greater rainfall contain more tree species, suggesting that low water availability places distributional limits on species, creating the observed climate-richness patterns. Both fire and herbivory have minimal effects on tree diversity, despite their role in determining savanna distribution and structure. High turnover of tree species, genera, and families is similar to turnover in seasonally dry tropical forests of the Americas, suggesting this is a feature of semiarid tree floras. The greater richness and phylogenetic diversity of wetter plots shows that broad-scale ecological patterns apply to disturbance-driven savanna systems. High taxonomic turnover suggests that savannas from across the regional rainfall gradient should be protected if we are to maximise the conservation of unique tree communities

Precipitation gradients drive high tree species turnover in the woodlands of eastern and southern Africa

DATA AVAILABILITY STATEMENT : Data are available from the Dryad Digital Repository: https://DOI.org/10.5061/dryad.k98sf7mcb (Davies et al. 2023).Savannas cover one-fifth of the Earth’s surface, harbour substantial biodiversity, and provide a broad range of ecosystem services to hundreds of millions of people. The community composition of trees in tropical moist forests varies with climate, but whether the same processes structure communities in disturbance-driven savannas remains relatively unknown. We investigate how biodiversity is structured over large environmental and disturbance gradients in woodlands of eastern and southern Africa. We use tree inventory data from the Socio-Ecological Observatory for Studying African Woodlands (SEOSAW) network, covering 755 ha in a total of 6780 plots across nine countries of eastern and southern Africa, to investigate how alpha, beta, and phylogenetic diversity varies across environmental and disturbance gradients. We find strong climate-richness patterns, with precipitation playing a primary role in determining patterns of tree richness and high turnover across these savannas. Savannas with greater rainfall contain more tree species, suggesting that low water availability places distributional limits on species, creating the observed climate-richness patterns. Both fire and herbivory have minimal effects on tree diversity, despite their role in determining savanna distribution and structure. High turnover of tree species, genera, and families is similar to turnover in seasonally dry tropical forests of the Americas, suggesting this is a feature of semiarid tree floras. The greater richness and phylogenetic diversity of wetter plots shows that broad-scale ecological patterns apply to disturbance-driven savanna systems. High taxonomic turnover suggests that savannas from across the regional rainfall gradient should be protected if we are to maximise the conservation of unique tree communities.The UK Natural Environment Research Council.http://www.ecography.orgam2024Plant Production and Soil ScienceSDG-15:Life on lan

Pantropical variability in tree crown allometry

Aim Tree crowns determine light interception, carbon and water exchange. Thus, understanding the factors causing tree crown allometry to vary at the tree and stand level matters greatly for the development of future vegetation modelling and for the calibration of remote sensing products. Nevertheless, we know little about large‐scale variation and determinants in tropical tree crown allometry. In this study, we explored the continental variation in scaling exponents of site‐specific crown allometry and assessed their relationships with environmental and stand‐level variables in the tropics. Location Global tropics. Time period Early 21st century. Major taxa studied Woody plants. Methods Using a dataset of 87,737 trees distributed among 245 forest and savanna sites across the tropics, we fitted site‐specific allometric relationships between crown dimensions (crown depth, diameter and volume) and stem diameter using power‐law models. Stand‐level and environmental drivers of crown allometric relationships were assessed at pantropical and continental scales. Results The scaling exponents of allometric relationships between stem diameter and crown dimensions were higher in savannas than in forests. We identified that continental crown models were better than pantropical crown models and that continental differences in crown allometric relationships were driven by both stand‐level (wood density) and environmental (precipitation, cation exchange capacity and soil texture) variables for both tropical biomes. For a given diameter, forest trees from Asia and savanna trees from Australia had smaller crown dimensions than trees in Africa and America, with crown volumes for some Asian forest trees being smaller than those of trees in African forests. Main conclusions Our results provide new insight into geographical variability, with large continental differences in tropical tree crown allometry that were driven by stand‐level and environmental variables. They have implications for the assessment of ecosystem function and for the monitoring of woody biomass by remote sensing techniques in the global tropics

Multiscale assemblage of an ectomycorrhizal fungal community: the influence of host functional traits and soil properties in a 10-ha miombo forest

Ectomycorrhizal fungi (EMF) are highly diversified and dominant in a number of forest ecosystems. Nevertheless, theirscales of spatial distribution and the underlying ecological processes remain poorly understood. Although most EMF areconsidered to be generalists regarding host identity, a preference toward functional strategies of host trees has never beentested. Here, the EMF community was characterised by DNA sequencing in a 10-ha tropical dry season forest—referred to asmiombo—an understudied ecosystem from a mycorrhizal perspective. We used 36 soil parameters and 21 host functionaltraits (FTs) as candidate explanatory variables in spatial constrained ordinations for explaining the EMF communityassemblage. Results highlighted that the community variability was explained by host FTs related to the ‘leaf economicsspectrum’ (adjusted R2 = 11%; SLA, leaf area, foliar Mg content), and by soil parameters (adjusted R2 = 17%), notably totalforms of micronutrients or correlated available elements (Al, N, K, P). Both FTs and soil generated patterns in thecommunity at scales ranging from 75 to 375 m. Our results indicate that soil is more important than previously thought forEMF in miombo woodlands, and show that FTs of host species can be better predictors of symbiont distribution thantaxonomical identity.info:eu-repo/semantics/publishe

Tree growth, recruitment, and survival in a tropical dry woodland: The importance of soil and functional identity of the neighbourhood

Understanding the processes that foster species coexistence is a longstanding goal in community ecology. Trade-offs in species performances for different ecological functions have been proposed as a general mechanism of species assemblage. The nature of these trade-offs can be revealed by analysing the demographic characteristics of species combined with their functional traits (FT) and the surrounding environment. The respective roles of soil, stand structure, and the functional dissimilarity of the neighbourhood, to the variations of growth, recruitment, and survival were determined for 19 most frequent tree species examined at the individual level in a tropical dry woodland (miombo; Democratic Republic of the Congo). Four functional groups can be recognised by contrasting life-history strategies and mainly related to tree stature, leaf phenology, leaf area (LA), leaf nutrient content, ectomycorrhizal status, and dispersal mode. Growth is regulated by the functional dissimilarity of the neighbourhood, local stand structure, and soil conditions. Recruitment is comparatively less impacted by functional neighbourhood but is mainly influenced by stand basal area and mean DBH as well as available calcium. At the community level, survival is mainly explained by physical and chemical soil variables and by both negative and positive effects of traits dissimilarity with the neighbourhood. Functional dissimilarity of the neighbourhood has the greatest positive effect on growth and survival, whereas competitive hierarchy is detrimental for growth of Caesalpinioideae species in particular, and for survival to a lesser extent. The general involvement of foliar traits and stand basal area on demographic performances indicates that competition for light is a key axis of species niche differentiation along the fast-slow continuum, even in this tropical dry woodland. Our results also pointed out a trade-off between water resources and phosphorus availability or aluminium avoidance, which separates growth strategies and leads to habitat specialisation. Multiple trade-offs allow species coexistence, with distinct drivers for tree growth, recruitment, and survival. Combining demographic and trait-based approaches provides unique key insights to understand species coexistence mechanisms.info:eu-repo/semantics/publishe

Fine-scale habitats influence tree species assemblage in a miombo forest

Aims: Relationships between local habitat heterogeneity and tree communities in miombo woodlands have been very little studied. While some studies have addressed this topic at broad scales and based on few environmental parameters, this study aims at 1) detecting fine-scale habitats (≤ 10 ha) on the basis of a detailed characterisation of soil explicitly considering past anthropogenic disturbances, and an exhaustive census of the tree community, and at 2) searching for indicator tree species corresponding to the resulting habitats.Methods: The study was carried out in the miombo woodland of Mikembo Forest Reserve, Upper Katanga, The Democratic Republic of the Congo. A complete census of the tree community was conducted in a 10-ha forest dynamics plot comprising 160 adjacent quadrats of 25 × 25 m, with a total of 4604 trees (DBH > 10 cm). Thirty-six physicochemical soil parameters were measured. Studying the frequency distribution of soil charcoal content allowed identifying local signature of past human agriculture in the soil. Two strategies were used to define habitats: 1) a combination of PCA on soil variables and Ward clustering, and 2) Multivariate Regression Trees (MRT) to search for key soil parameters allowing the best prediction of species composition. Tree-habitat associations were tested by means of a robust statistical framework combining the IndVal index and torus randomisations.Important findings: The forest contained 82 tree species and a significant proportion of wet miombo species (e.g. Marquesia macroura). We detected a strong east-west edaphic gradient driven by soil texture; most chemical soil parameters followed this pattern. Five habitats were identified based on soil factors and floristic composition. Nine indicator species of these habitats were found. The key soil factors discriminating habitats were total calcium, available forms of phosphorus, and clay content. Even though past agricultural practices were successfully detected in soils, they did not display any significant influence neither on habitat differentiation nor on the associated tree communities. Based on an unprecedented large number of soil parameters, fine-scale soil heterogeneity and niche partitioning were shown to contribute to the variability of the floristic composition in this forest. Our results indicated that considering the most variable environmental parameters, as in PCA, is a poor manner for defining habitats. In contrast, combining MRT with the IndVal index and torus randomisation has proved to be a much more robust and sensitive approach to highlight tree-habitat associations at this scale. The common dichotomous viewpoint of considering deterministic and neutral effects as acting at broad and fine scales, respectively, is not confirmed when measuring suitable environmental variables, even in a case where the physical environment does not exhibit strong heterogeneity.info:eu-repo/semantics/publishe