Positive biodiversity‐ecosystem function relationships (BEFRs) have been widely documented, but it is unclear if BEFRs should be expected in disturbance‐driven systems. Disturbance may limit competition and niche differentiation, which are frequently posited to underlie BEFRs. We provide the first exploration of the relationship between tree species diversity and biomass, one measure of ecosystem function, across southern African woodlands and savannas, an ecological system rife with disturbance from fire, herbivores and humans. We used >1000 vegetation plots distributed across 10 southern African countries, and structural equation modelling, to determine the relationship between tree species diversity and aboveground woody biomass, accounting for interacting effects of resource availability, disturbance by fire, tree stem density and vegetation type. We found positive effects of tree species diversity on aboveground biomass, operating via increased structural diversity. The observed BEFR was highly dependent on organismal density, with a minimum threshold of c. 180 mature stems ha‐1. We found that water availability mainly affects biomass indirectly, via increasing species diversity. The study underlines the close association between tree diversity, ecosystem structure, environment and function in highly disturbed savannas and woodlands. We suggest that tree diversity is an under‐appreciated determinant of wooded ecosystem structure and function.
Questions Forest-grassland boundaries in the tropics are primarily interpreted as driven by fire and herbivory. So far, frost has received little attention as driver of tropical vegetation boundaries. Here, we study mid-altitudes in south-central Africa and ask the following questions: a) Is there a lower treeline between grasslands and forests on the Angolan Plateau? b) If so, do topoclimatic differences play a role in it? c) What do our results imply for tropical highlands globally? Location The Angolan Plateau (10°-16°S and 13°-21°E). Methods We mapped vegetation by applying a supervised support vector machine based classification on a Landsat satellite image. We used a 1000 m spaced point grid to extract the mean vertical distribution of the vegetation units from a digital elevation model. The topographic position of each point was calculated using a channel network approach. Microclimatic data were collected across topographic gradients at two sites along the Plateau. Analyses focused on the number of nights where temperature dropped below 0°C, median and minimum temperatures, and diurnal temperature ranges. Results Lower treelines separate miombo forests from geoxyle grasslands in all major valley systems. Treelines are located 150–200 m below the headwater divides. Nocturnal outgoing net longwave radiation and resulting cold-air pooling cause frequent frosts in the valleys during the dry season. Topography controls frost patterns, with minima of down to -7.5°C and diurnal temperature amplitudes up to 40°C. Conclusions The results show that vegetation patterns in central Angola are shaped by frost. The processes causing frost depend on air humidity. Drier atmosphere and lower global temperatures indicate a longer frost season, lower minima and frost extending to lower elevations and latitudes for the Pleistocene. Pleistocene forest fragmentations and the rise of endemism-rich grasslands in comparable tropical highlands worldwide may be explained by crossing thermic thresholds via increasing seasonal aridity.
Despite its importance for carbon stocks accounting, belowground biomass (BGB) has seldom been measured due to the methodological complexity involved. In this study, we assess woody BGB and related carbon stocks, soil properties and human impact on two common suffrutex grasslands ( Brachystegia - and Parinari grasslands) on the Angolan Central Plateau. Data on BGB was measured by direct destructive sampling. Soil samples were analysed for select key parameters. To investigate vegetation dynamics and human impact, we used Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) and fire data retrieved via Google Earth Engine. Mean belowground woody biomass of sandy Parinari grasslands was 17 t/ha and 44 t/ha in ferralitic Brachystegia grasslands of which 50% correspond to carbon stocks. As such, the BGB of Brachystegia grasslands almost equals the amount of aboveground biomass (AGB) of neighbouring miombo woodlands. Almost the entire woody BGB is located in the top 30 cm of the soil. Soils were extremely acid, showing a low nutrient availability. Both grassland types differed strongly in EVI and fire seasonality. The Parinari grasslands burnt almost twice as frequent as Brachystegia grasslands in a 10-year period. Our study emphasizes the high relevance of BGB in suffrutex grasslands for carbon stock accounting.
The miombo region in Africa is covered by a mosaic of woodlands and geoxylic grasslands and subject to disturbances like fires, frost and drought, and low nutrient availability. The dominance of Fabaceae Detarioideae species in miombo ecosystems is remarkable but little understood. We therefore compared plant functional traits (PFTs) of common woody species of the Angolan plateau, grouped by life form (trees, geoxyles), lineage (Fabaceae: Detarioideae, non‐Detarioideae) and symbiont association (ectomycorrhiza, rhizobia). PFTs reflect group‐specific adaptations to prevalent environmental conditions. To analyse the impact of environmental drivers, we selected PFTs reflecting ecophysiological aspects of leaf morphology, nutrient content and water transport. Traits were measured following standardized protocols. We found differences in key PFTs between trees and geoxyles reflecting both, life form specific adaptations to environmental conditions and lineage specific strategies to cope with environmental stresses. We interpret higher leaf thickness and higher wood density of geoxyles as responses to harsher open environments. Fabaceae in general and ectomycorrhizal species showed better nutrient status. Symbiotic associations of detarioid legumes with ectomycorrhiza show specific advantages for phosphorous uptake as compared to Rhizobia associated Fabaceae and to non‐Fabaceae and thus may be crucial for the stunning dominance of Detarioideae in miombo landscapes.
Geoxyles and their congeneric tree species are a very common feature in the Zambezian Floristic Region, however , little is known about differing expression of plant functional traits between the two life forms and thus their evolutionary adaptations to prevailing habitat conditions. Therefore, we measured selected plant functional traits and compared them between the geoxyle and tree growth forms. We selected six species pairs that are common in the study area, i.e. Brachystegia russelliae vs. Brachystegia longifolia (FabaceaeÀDetarioideae); Cryptosepalum exfoliatum subsp. suffruticans vs. Cryptosepalum exfoliatum subsp. pseudotaxus (Faba-ceaeÀDetarioideae); Combretum platypetalum vs. Combretum zeyheri (Combretaceae); Parinari capensis vs. Pari-nari curatellifolia (Chrysobalanaceae); Syzygium guineense subsp. macrocarpum (suffrutex form vs. tree form) (Myrtaceae); Uapaca nitida var. suffrutescens vs. Uapaca nitida var. nitida (Phyllanthaceae). For analysis we selected traits reflecting adaptations to environmental conditions such as specific leaf area (SLA), leaf thickness (LT), wood density (WD), leaf nitrogen (LN), leaf carbon (LC), leaf phosphorus (LP), leaf calcium (LCa), leaf magnesium (LMg), leaf potassium concentrations (LP) and leaf dry matter content (LDMC). Additionally, we measured leaf water potential (WP) during dry and wet seasons. The results did not show a general pattern of inter-life-form trait variation among trees and geoxyles. There were significant differences in LA, WD, LN, LC, LMg and LK between geoxyles and trees, but mostly among species pairs. This indicates that the growth form transition is largely decoupled from leaf economics. Thus, the detected differences in plant functional traits among the species pairs are mostly taxon related responses to specific environmental stresses and habitat conditions.
Seasonally dry woodlands are the dominant land cover across southern Africa. They are biodiverse, structurally complex, and important for ecosystem service provision. Species composition and structure vary across the region producing a diverse array of woodland types. The woodlands of the Huíla plateau in southwest Angola represent the extreme southwestern extent of the miombo ecoregion and are markedly drier than other woodlands within this ecoregion. They remain understudied, however, compared to woodlands further east in the miombo ecoregion. We aimed to elucidate further the tree diversity found within southwestern Angolan woodlands by conducting a plot-based study in Bicuar National Park, comparing tree species composition and woodland structure with similar plots in Tanzania, Mozambique, and the Democratic Republic of Congo. We found Bicuar National Park had comparatively low tree species diversity, but contained 27 tree species not found in other plots. Plots in Bicuar had low basal area, excepting plots dominated by Baikiaea plurijuga. In a comparison of plots in intact vegetation with areas previously disturbed by shifting-cultivation agriculture, we found species diversity was marginally higher in disturbed plots. Bicuar National Park remains an important woodland refuge in Angola, with an uncommon mosaic of woodland types within a small area. While we highlight wide variation in species composition and woodland structure across the miombo ecoregion, plot-based studies with more dense sampling across the ecoregion are clearly needed to more broadly understand regional variation in vegetation diversity, composition and structure.
Aim In tropical Africa, savannas cover huge areas, have high plant species richness and are considered as a major natural resource for most countries. There is, however, little information available on their floristics and biogeography at the continental scale, despite the importance of such information for our understanding of the drivers of species diversity at various scales and for effective conservation and management. Here, we collated and analysed floristic data from across the continent in order to propose a biogeographical regionalization for African savannas. Location We collated floristic information (specifically woody species lists) for 298 samples of savanna vegetation across Africa, extending from 18° N to 33° S and from 17° W to 48° E. Taxa We focused on native woody species. Methods We used ordination and clustering to identify the floristic discontinuities and gradual transitions across African savannas. Floristic relationships, specificity and turnover, within and between floristic clusters, were analysed using a (dis‐)similarity‐based approach. Results We identified eight floristic clusters across African savannas which in turn were grouped into two larger macro‐units. Ordinations at species and genus levels showed a clear differentiation in woody species composition between the North/West macro‐unit and the South/East macro‐unit. This floristic discontinuity matches to the High (i.e. N&W) and Low (S&E) division of Africa previously proposed by White (1983) and which tracks climatic and topographical variation. In the N&W savannas, the floristic gradient determined by rainfall was partitioned into the Sudanian (drier) and Guinean (wetter) clusters. Within the highly heterogeneous S&E savannas and woodlands, six clusters were identified: Ugandan, Ethiopian, Mozambican, Zambezian, Namibian and South African. Main conclusions The proposed pan‐African classification of savannas and woodlands might assist the development of coordinated management and conservation policies.