Eastern Tropical Africa Research Articles

Xylopia nilotica (Annonaceae) in Ethiopia, Sudan, South Sudan, and Uganda, with a review of the genus in North Eastern Tropical Africa

Based on written records and specimens of the new species, Xylopia nilotica D. M. Johnson & N. A. Murray (2018), described from Uganda and west of the Nile in Sudan and South Sudan, we have mapped the species. We find that it also occurs east of the Nile in South Sudan and in western Ethiopia. Its habitats range from open rain forest to open woodland. Modelling the potential distribution of X. nilotica suggests that it occurs in the periphery of the upper Nile basin, but it is probably undercollected. The potential distributions of X. longipetala (a Guineo-Congolian species) and X. holtzii Engl. (an East African coastal species), distinct but previously considered conspecific with X. nilotica, only marginally overlap with that of X. nilotica, which is endemic to the countries Uganda, South Sudan, Sudan, and Ethiopia, apparently a unique pattern of endemism. The new information does not change the assessment of X. nilotica as Vulnerable (VU). Other species of Xylopia in North Eastern Tropical Africa are X. rubescens Oliver and X. thomsonii Oliver in the Equatoria region of South Sudan, and X. rubescens, X. staudtii Engl. & Diels, and X. aethiopica (Dunal) A. Rich. in Uganda. A record of X. aethiopica in Ethiopia is based on false label information on the only known specimen.

Closure of tropical seaways favors the climate and vegetation in tropical Africa and South America approaching their present conditions

The tectonically induced closure/constriction of the Central American Seaway (CAS) and Indonesian Seaway (Indo) during the early to mid-Pliocene has been associated with many climatic events, such as the onset of Northern Hemisphere glaciation, the intensification of the Atlantic meridional overturning circulation (AMOC) and Australian aridification. However, studies on how the closure/constriction of tropical seaways affects the tropical climate system are still sparse and not systematic. Previous studies have linked the constriction of Indo to the aridification over East Africa and discussed the role of CAS closure in affecting the moisture supply over South America, but the underlying mechanism and combined effect of both tropical seaways are not well studied. In this study, we evaluate the impacts of tropical seaways' closure/constriction and distinguish the relative roles of CAS and Indo on climate in tropical Africa and South America using the NorESM-L Atmosphere-Ocean General Circulation Model (AOGCM) and a dynamic vegetation model (LPJ-GUESS). Our results show that the closure of the CAS leads mainly to aridification in northeastern Brazil, resulting in an expansion of tropical xerophytic shrubland and savanna in this region. The narrowing of the Indo mainly leads to enhanced aridification in eastern tropical Africa, and reduces the extent of tropical forests in eastern and northern tropical Africa, which is generally consistent with the data. The closure/narrowing of the two tropical seaways results in a superposition of the individual seaway's effect, particularly over northeastern Brazil region, which exhibits enhanced aridification compared to the closure of the individual CAS. The seaways' changes are shown to be pivotal for the evolution of climate and vegetation over East Africa and northeastern South America to contemporary conditions.

African Mountain Thistles: Three New Genera in the Carduus-Cirsium Group.

The floras on the highest mountains in tropical eastern Africa are among the most unique floras in the world. Despite the exceptionally high concentration of endemic species, these floras remain understudied from an evolutionary point of view. In this study, we focus on the Carduus-Cirsium group (subtribe Carduinae) to unravel the evolutionary relationships of the species endemic to the tropical Afromontane and Afroalpine floras, aiming to improve the systematics of the group. We applied the Hyb-Seq approach using the Compositae1061 probe set on 190 samples (159 species), encompassing representatives of all genera of Carduinae. We used two recently developed pipelines that enabled the processing of raw sequence reads, identification of paralogous sequences and segregation into orthologous alignments. After the implementation of a missing data filter, we retained sequences from 986 nuclear loci and 177 plastid regions. Phylogenomic analyses were conducted using both concatenated and summary-coalescence methods. The resulting phylogenies were highly resolved and revealed three distinct evolutionary lineages consisting of the African species traditionally referred to as Carduus and Cirsium. Consequently, we propose the three new genera Afrocarduus, Afrocirsium and Nuriaea; the latter did notably not belong to the Carduus-Cirsium group. We detected some incongruences between the phylogenies based on concatenation vs. coalescence and on nuclear vs. plastid datasets, likely attributable to incomplete lineage sorting and/or hybridization.

Europeans in British Administered East Africa, A Biographical Listing 1888–1910, 4th Edition

With the interest and passion of a collector and through the methods of archival investigation, North presents the results of several decades of enquiry into the written material and photographs that identified and placed Europeans who came in contact with the lands of eastern Africa administered by Britain. Since the 3rd edition (North, 2005), the 4th edition presents over 300 additional pages and is spread across two hardback volumes that totals over 8000 biographical entries of Europeans present in primary historical sources. The author begins with an overview of the early colonial history and key archival sources of documented names and biographical details of European settlers, travellers and employees across tropical eastern Africa — from the Indian Ocean coast inland to the lakes and from Zanzibar. The author describes his journey from initial interest during the 1960s in Nairobi to delving into archives in East Africa and Britain, travels to cemeteries, and later online research, to build biographies and source new material. Biographical entries contain names and known aliases, nationalities, and employment roles; and listing of dates, locations, sources, and recorded activities of the individuals. Black and white bust and head shot photographs of individuals are reproduced when possible. One limitation is that not all the photographs are referenced to the original source(s) or format(s). Names of companion Europeans are also cross-referenced in bold font if there is a corresponding entry in the volumes. The author acknowledges previous efforts to catalogue many of the Europeans present across the region, such as the East Africa Hand Book and Dictionary of East African Biography, as well as online resources. The history of these products and the dispersed nature of cited sources are listed by North for future researchers.

Hydrologic Controls on Individual Ostracode Stable Isotopes in a Desert Lake: A Modern Baseline for Lake Turkana

AbstractStable carbon (δ13C) and oxygen (δ18O) isotope measurements in lacustrine ostracodes are widely used as paleolimnological indicators of past climate change. Previous work has used individual ostracode valves to resolve seasonal and subdecadal paleohydrological signals, yet replicated assessments of co‐occurring specimens from modern samples are rare. Here we focus on ostracodes from sediments collected in 1979 from Lake Turkana, an alkaline desert lake in tropical eastern Africa. Sourced from the Ethiopian highlands, the Omo River is the dominant inflow into this lake. We present individual ostracode valve analyses (IOVA) of δ13C and δ18O measurements (n = 329) of the extant benthic ostracode Sclerocypris clavularis from 17 sites spanning the entire lake (navg = ∼19 specimens/site). We demonstrate that the pooled statistics of individual valve measurements at each site are shaped by lake hydrology, superimposed over inter‐individual isotopic variations. Within‐site variance in IOVA‐δ13C is larger (∼60%) than that of IOVA‐δ18O. Yet, site‐based averages exhibit a systematic pattern, with higher δ values toward the southern part of the lake, away from Omo River inflow. We suggest that the latitudinal δ13C gradient may arise from low riverine δ13C and low organic matter δ13C as a productivity response to nutrient‐rich Omo River inflow toward the north. The δ18O pattern may be explained by the diminishing influence of Omo River inflows and more evaporation driving higher IOVA‐δ18O values toward the windier, southern basin. We conclude that pooled IOVA statistics in Omo‐Turkana sediments can aid interpretations of past regional paleohydrology and its variability in this basin.

Spatial and temporal variations of gross primary production simulated by land surface model BCC_AVIM2.0

Gross primary production (GPP) is the largest flux and a crucial player in the terrestrial carbon cycle and has been studied extensively, yet large uncertainties remain in the spatiotemporal patterns of GPP in both observations and simulations. This study evaluates the performance of the second version of the Beijing Climate Center Atmosphere−Vegetation Interaction Model (BCC_AVIM2.0) in simulating GPP on multiple spatial and temporal scales in the Coupled Model Intercomparison Project Phase 6 (CMIP6) experiments. Model simulations driven by two meteorological datasets were compared with two observation-based GPP products covering 1982–2008. Spatial patterns of annual GPP show a significant latitudinal gradient in each dataset, increasing from cold (tundra) and dry (desert) biomes to warm (temperate) and humid (tropical rainforest) biomes. BCC_AVIM2.0 overestimates GPP in most parts of the globe, especially in boreal forest regions and Southeast China, while underestimating GPP in subhumid regions in eastern South America and tropical Africa. The four datasets broadly agree on the GPP seasonal cycle, but BCC_AVIM2.0 predicts an earlier beginning of spring growth and a larger amplitude of seasonal variations than those in the observations. The observation-based datasets exhibit slight interannual variability (IAV) and weak GPP linear trends, while the BCC_AVIM2.0 simulations demonstrate relatively large year-to-year variability and significant trends in the low-latitudes and temperate monsoon regions in North America and East Asia. Regarding the possible relationships between annual means of GPP and climate factors, BCC_AVIM2.0 predicts more extensive regions of the globe where the IAV of annual GPP is dominated by precipitation, especially in mid-to-high latitudes of the Northern Hemisphere and tropical Africa, while the observed GPP in the above regions is temperature- or radiation-dominant. The positive GPP biases due to earlier spring growth in boreal forest regions and negative GPP biases in off-equator tropical areas in the BCC_AVIM2.0 simulations imply that cold stress on biomes in boreal mid-to-high latitudes should be strengthened to restrain plant growth, while drought stress in low-latitude regions might be eased to enhance plant production in the future version of BCC_AVIM.

Integrating spatial and legacy data to understand archaeological sites in their landscape. A case study from Unguja Ukuu, Zanzibar

AbstractSpatial analysis is paramount for understanding, monitoring, and conserving ancient settlements and cultural landscapes. Advancing remote sensing and prospection techniques are expanding the methodological frame of archaeological settlement analysis by enabling remote, landscape‐scale approaches to mapping and investigation. Whilst particularly effective in arid lands and areas with sparse or open ground cover, such as vegetation and buildings, these approaches remain peripheral in tropical environments because of technical and contextual challenges. In tropical Eastern Africa, for example, scales, resolution and visibility are often compromised by thick vegetation cover, inadequate access to, if not lack of, imagery resources and technologies, and the availability of comparative archaeological data for interpretation. This paper presents the initial results of spatial analysis, using historic landscape characterisation, remote sensing, published and legacy data, and a pilot ground survey to examine the earliest settlement of Zanzibar, Unguja Ukuu. Comparing multiple strands of evidence in a Geographic Information System (GIS), we use each as a test on the others to draw out the strengths and weaknesses of each technique in the context of tropical and coastal Eastern Africa. Drone photogrammetry, geophysical prospection, and ground survey were compared with legacy remote sensing resources and the results of a coring survey conducted across the site during the 1990s into a GIS platform to produce multi‐phase hypothetical maps of the archaeological site in the context of its potential resource landscape. These were then tested against the results of recent excavations. The discussion highlights the challenges and potential of combining these techniques in the context of Eastern Africa and provides some suggested methods for doing so. We show that remote sensing techniques give an insight into current landscapes but are less useful in understanding or modelling how sites would have fitted into their surroundings in the past, when conditions were potentially very different.

A synoptic revision of the golden glories, genus Decalobanthus (Convolvulaceae)

Eighteen species of Decalobanthus are recognized in this concise revision of the genus. Two new combinations are made and one new name is proposed to accommodate a change in rank. The genus is centered in Southeast Asia and Malesia, with outlier species in the tropical Pacific and one species that reaches the Neotropics; a single wide-ranging species extends from eastern Tropical Africa and Madagascar in the west to the Hawaiian Islands in the east. Each species is provided with a diagnosis, summary of the species' ecology, compiled vernacular names, and uses. Distributions are mapped based on herbarium voucher specimens. Where appropriate, typifications and nomenclatural problems are explained and comments on morphological variation and the need for further study are highlighted. Selected species are illustrated with colour photos of living plants. An index of numbered collections examined is provided to aid in specimen identification and herbarium curation.

Nutraceutical Potentials of Azanza garckeana (Snot Apple): A Review

Background: Nutraceuticals are products derived from natural sources and are used as therapeutic agents for the management of various health disorders. Several nutraceuticals have been produced from fruits, including wild edible fruits. Azanza garckeana is an example of wild edible fruit belonging to the Malvaceae family. It is a semi-deciduous flowering plant that grows in savan-na-wooded grasslands and open forests. It can be found in tropical Eastern Africa, Southern Africa, and some countries of Western Africa. All parts of the plant are useful for medical and economic purposes. There are various traditional claims on the application of various parts of the plant in the management and treatment of diabetes, infertility, aches and pains, hemorrhage, liver and cardiac diseases, etc. Objective: This article presents a review of the plant Azanza garckeana regarding its botanical de-scription, its traditional relevance economically and medically, and its established nutritional as well as pharmacological relevance based on reported in vitro and in vivo experimental investigations. Conclusion: Experimental reports on the nutritional and pharmacological potentials of Azanza garckeana are sparse. Therefore, there is a need for extensive research to further establish the plant as a potential candidate for a pharmacological or nutraceutical agent for the management of oxida-tive stress-related diseases and other human health disorders.

Nonuniform Late Pleistocene glacier fluctuations in tropical Eastern Africa

Today's ice caps and glaciers in Africa are restricted to the highest peaks, but during the Pleistocene, several mountains on the continent were extensively glaciated. However, little is known about regional differences in the timing and extent of past glaciations and the impact of paleoclimatic changes on the afro-alpine environment and settlement history. Here, we present a glacial chronology for the Ethiopian Highlands in comparison with other East African Mountains. In the Ethiopian Highlands, glaciers reached their maximum 42 to 28 thousand years ago before the global Last Glacial Maximum. The local maximum was accompanied by a temperature depression of 4.4° to 6.0°C and a ~700-m downward shift of the afro-alpine vegetation belt, reshaping the human and natural habitats. The chronological comparison reveals that glaciers in Eastern Africa responded in a nonuniform way to past climatic changes, indicating a regionally varying influence of precipitation, temperature, and orography on paleoglacier dynamics.

Climate, vegetation, and weathering across space and time in Lake Tanganyika (tropical eastern Africa)

Climate and vegetation influence weathering rates and processes; however, evaluating the effects of each and feedbacks between systems, has yet to be accomplished for many types of landscapes. A detailed understanding of how these processes interact to shape landscapes is particularly crucial for reconciling future scenarios of changing climate, where profound alterations to both the biosphere and geosphere are anticipated. In the tropics, ecosystem services, such as soil and water quality, are linked to both vegetation and weathering processes that form a strong control on natural resources that are the foundation of many communities’ daily subsistence. This understanding is further complicated by intensifying land-use within tropical watersheds, which decouples vegetation change from climate; it is yet unclear what the direct effects of vegetation change may be on erosion and weathering when operating independent of climate. Long term observational records tracking changes to the critical zone do not exist in tropical Africa, however, sedimentary paleo-records from lakes are often of sufficient length and resolution to record the impact of bioclimatic variability on surface processes. Here, we use a novel approach combining long (60ka) and intermediate-length (400yrs) lake sediment records along with historical repeat photography from Lake Tanganyika (Tanzania) to document changes and relationships among climate, vegetation, and weathering at multiple scales. These records illustrate that glacial-interglacial climate change did not significantly alter weathering intensity. Instead, we observe chemical and physical weathering responses only when the vegetation becomes more open beginning at the transition to the Holocene. Also, the largest change in chemical weathering intensity occurs only within the last ∼3ka. This is consistent with a major reorganization of vegetation and is directly attributable to Iron Age human activity, rather than climate. Furthermore, anthropogenic landscape alteration as early as ∼2.5ka, in addition to well-documented comparisons of historical land-use, suggest widespread responses of both chemical weathering intensity and enhanced soil erosion to human activity. This shows that changes in vegetation structure induced by anthropogenic activity, decoupled from climate change, generate a disproportionately large weathering response.

Taxonomic revision of the Afrotropical Agabus raffrayi species group with the description of four new species (Coleoptera, Dytiscidae).

We revise the Afrotropical Agabus raffrayi species group, motivated by the discovery of new diversity in Kenya and South Africa. Whilst Agabus is mainly a holarctic genus, the Agabus raffrayi group is restricted to high altitude regions of eastern Africa and temperate parts of South Africa, from where we describe the southernmost Agabus in the world. The following new species are introduced: Agabus anguluverpussp. nov. from Mount Kenya in central Kenya, Agabus austellussp. nov. a widespread species in South Africa, Agabus riberaesp. nov. from the Kamiesberg and northeastern Cederberg ranges in the Northern and Western Cape Provinces of South Africa and Agabus agulhassp. nov. from the Agulhas Plain, Western Cape Province, South Africa. We provide a distribution map, a determination key for males, quantitative measurements of diagnostic characters, habitus photos and detailed photos of male genitalia for all described species in the group, as well as images of diagnostic characters and habitats. The presence or absence of an elongated section between the subapical broadening and the base of the apical and subapical teeth of the male aedeagus is a useful novel character, first revealed by our study. In contrast with the most recent revision of Afrotropical Agabus, we show that Agabus ruwenzoricus Guignot, 1936 is restricted to eastern Africa; South African records of this species having been based on misidentifications, no species of the group being common to southern and eastern Africa. We speculate that the raffrayi group may display phylogenetic niche conservatism, being restricted, as an originally temperate taxon, to higher elevations in tropical eastern Africa, but occurring at lower altitudes in temperate South Africa.

Two new species of Rhipidoglossum (Orchidaceae: Angraecinae) from Tanzania and Zimbabwe

Two new species of angraecoid orchid from East and South-central Africa are described and illustrated. Rhipidoglossum lucieae is known from the Eastern Arc Mountains of Tanzania (West Usambaras) and has been misidentified as R. pulchellum in previous floristic treatments. Rhipidoglossum philatelicum is reported from the Eastern Highlands of Zimbabwe (Bvumba and Chimanimani Mts) and has been misclassified under R. subsimplex. Distribution maps and the conservation status of these two novelties are provided. Future directions in epiphytic orchid species discovery in eastern tropical Africa are proposed.

Late Quaternary paleoenvironmental changes in tropical eastern Africa revealed by multi–proxy records from the Cherangani Hills, Kenya

There is paucity in the proxy records available for the reconstruction of vegetation and climate histories of tropical eastern Africa, especially north of the equator. Here, we present a new paleoenvironmental record based on a 3.5-m sediment core from the Kapkanyar mire, Cherangani Hills, Kenya. Using analyses of pollen, microscopic charcoal, and loss-on-ignition data, we examine the vegetation and climate history of tropical eastern Africa over the past ∼23 Kyrs. Our findings show that high levels of dry montane forest vegetation and high frequencies of Poaceae and Chenopodiaceae/Amaranthaceae (Cheno/Am) pollen types dominate the LGM period (∼23.5–18 Kyrs). The last deglaciation (∼18–12 Kyrs) is marked by a decrease in Afromontane vegetation such as Ulmaceae, Combretaceae, and Podocarpus and resurgence in Cheno/Am pollen with a minimal representation of Olea pollen. The presence of Combretaceae pollen alongside increased percentages of Cheno/Am pollen indicates that cold and dry climate likely prevailed during the last deglaciation period. The record shows that the eastern African region was generally wet over most of the Holocene, albeit with increased frequency of drought episodes. The first imprints of anthropogenic activities in the record appear between ∼4 and 3.5 Kyrs.

Genetic structuring, dispersal and taxonomy of the high-alpine populations of the Geranium arabicum/kilimandscharicum complex in tropical eastern Africa.

The scattered eastern African high mountains harbor a renowned and highly endemic flora, but the taxonomy and phylogeographic history of many plant groups are still insufficiently known. The high-alpine populations of the Geranium arabicum/kilimandscharicum complex present intricate morphological variation and have recently been suggested to comprise two new endemic taxa. Here we aim to contribute to a clarification of the taxonomy of these populations by analyzing genetic (AFLP) variation in range-wide high-alpine samples, and we address whether hybridization has contributed to taxonomic problems. We identified only two genetic groups. One corresponded to G. kilimandscharicum, which has been reported as exclusively high-alpine and confined to the eastern Rift mountains in East Africa. The other corresponded to G. arabicum, reported from lower altitudes on the same mountains as well as from a wide altitudinal span in Ethiopia and on the western Rift mountains in East Africa. The four populations analyzed of a recently described species from the Bale Mts in Ethiopia were admixed, indicating that they result from recent long-distance dispersal of G. kilimandscharicum from East Africa followed by hybridization with local G. arabicum in naturally disturbed habitats. Some admixture between the two genetic groups was also inferred on other mountains, supporting earlier suggestions of introgression based on morphology. We did not find support for recognition of the recently suggested new subspecies of G. arabicum in Ethiopia. Interestingly, the high-alpine G. kilimandscharicum lacked clear geographic structuring, suggesting a recent history of colonization of the different mountains or extensive intermountain gene flow.

Northern Hemisphere control of deglacial vegetation changes in the Rufiji uplands (Tanzania)

Abstract. In tropical eastern Africa, vegetation distribution is largely controlled by regional hydrology, which has varied over the past 20 000 years. Therefore, accurate reconstructions of past vegetation and hydrological changes are crucial for a better understanding of climate variability in the tropical southeastern African region. We present high-resolution pollen records from a marine sediment core recovered offshore of the Rufiji River delta. Our data document significant shifts in pollen assemblages during the last deglaciation, identifying, through changes in both upland and lowland vegetation, specific responses of plant communities to atmospheric (precipitation) and coastal (coastal dynamics and sea-level changes) alterations. Specifically, arid conditions reflected by a maximum pollen representation of dry and open vegetation occurred during the Northern Hemisphere cold Heinrich event 1 (H1), suggesting that the expansion of drier upland vegetation was synchronous with cold Northern Hemisphere conditions. This arid period is followed by an interval in which forest and humid woodlands expanded, indicating a hydrologic shift towards more humid conditions. Droughts during H1 and the shift to humid conditions around 14.8 kyr BP in the uplands are consistent with latitudinal shifts of the intertropical convergence zone (ITCZ) driven by high-latitude Northern Hemisphere climatic fluctuations. Additionally, our results show that the lowland vegetation, consisting of well-developed salt marshes and mangroves in a successional pattern typical for vegetation occurring in intertidal habitats, has responded mainly to local coastal dynamics related to marine inundation frequencies and soil salinity in the Rufiji Delta as well as to the local moisture availability. Lowland vegetation shows a substantial expansion of mangrove trees after ~ 14.8 kyr BP, suggesting an increased moisture availability and river runoff in the coastal area. The results of this study highlight the decoupled climatic and environmental processes to which the vegetation in the uplands and the Rufiji Delta has responded during the last deglaciation.

A phenetic study of the Emilia coccinea complex (Asteraceae, Senecioneae) in Africa

Emilia coccinea complex is a widespread and morphologically variable species in tropical and subtropical Africa. Jeffrey’s (Kew Bull 52:205–212, 1997) revision of the African Emilia species with large capitula resulted in a complex of eight species with E. coccinea sensu stricto restricted to eastern and southern tropical Africa and characterised by long-appendaged style branches and bright orange flowers. To evaluate the delimitations within this complex, a morphological phenetic study based on 134 herbarium specimens spanning the geographical range of the E. coccinea complex was undertaken using cluster analysis and ordination (principal coordinates analysis and non-metric multidimensional scaling). Five of the eight species (E. emilioides, E. jeffreyana, E. praetermissa, E. subscaposa, and E. vanmeelii) formed distinct phenetic groups, whereas two species (E. caespitosa and E. coccinea) were indistinguishable because of variability in some key characters (viz., cypsela indumentum and shape of cauline leaves) suggesting that they are possibly one heterogeneous species. Emilia lisowskiana is not supported as a distinct species as three E. coccinea specimens group with it in the cluster analysis. Emilia emilioides with mostly long, narrow cauline leaves, narrow capitula, and unappendaged style branches apices is the most distinct taxon in all analyses. Univariate analyses of ten selected characters revealed that the reproductive features are able to distinguish some species, as well as a few vegetative ones. The application of various species concepts to this species complex is discussed. A key to the species in this complex is provided.

Discovering Karima (Euphorbiaceae), a New Crotonoid Genus from West Tropical Africa Long Hidden within Croton.

Croton scarciesii (Euphorbiaceae-Crotonoideae), a rheophytic shrub from West Africa, is shown to have been misplaced in Croton for 120 years, having none of the diagnostic characters of that genus, but rather a set of characters present in no known genus of the family. Pollen analysis shows that the new genus Karima belongs to the inaperturate crotonoid group. Analysis of a concatenated molecular dataset combining trnL-F and rbcL sequences positioned Karima as sister to Neoholstia from south eastern tropical Africa in a well-supported clade comprised of genera of subtribes Grosserineae and Neoboutonieae of the inaperturate crotonoid genera. Several morphological characters support the relationship of Karima with Neoholstia, yet separation is merited by numerous characters usually associated with generic rank in Euphorbiaceae. Quantitative ecological data and a conservation assessment supplement illustrations and descriptions of the taxon.

Resolving Contrasting Regional Rainfall Responses to El Niño over Tropical Africa

Abstract The evolution of El Niño can be separated into two phases—namely, growth and mature—depending on whether the regional sea surface temperature has adjusted to the tropospheric warming in the remote tropics (tropical regions away from the central and eastern tropical Pacific Ocean). The western Sahel’s main rainy season (July–September) is shown to be affected by the growth phase of El Niño through (i) a lack of neighboring North Atlantic sea surface warming, (ii) an absence of an atmospheric column water vapor anomaly over the North Atlantic and western Sahel, and (iii) higher atmospheric vertical stability over the western Sahel, resulting in the suppression of mean seasonal rainfall as well as number of wet days. In contrast, the short rainy season (October–December) of tropical eastern Africa is impacted by the mature phase of El Niño through (i) neighboring Indian Ocean sea surface warming, (ii) positive column water vapor anomalies over the Indian Ocean and tropical eastern Africa, and (iii) higher atmospheric vertical instability over tropical eastern Africa, leading to an increase in the mean seasonal rainfall as well as in the number of wet days. While the modulation of the frequency of wet days and seasonal mean accumulation is statistically significant, daily rainfall intensity (for days with rainfall > 1 mm day−1), whether mean, median, or extreme, does not show a significant response in either region. Hence, the variability in seasonal mean rainfall that can be attributed to the El Niño–Southern Oscillation phenomenon in both regions is likely due to changes in the frequency of rainfall.

Northern Hemisphere control of deglacial vegetation changes in the Rufiji uplands (Tanzania)

Abstract. In tropical eastern Africa, vegetation distribution is largely controlled by regional hydrology, which has varied over the past 20 000 years. Therefore, accurate reconstructions of past vegetation and hydrological changes are crucial for a better understanding of climate variability in the tropical southeastern African region. We present high-resolution pollen records from a marine sediment core recovered offshore of the Rufiji River delta. Our data document significant shifts in pollen assemblages during the last deglaciation, identifying, through changes in both upland and lowland vegetation, specific responses of plant communities to atmospheric (precipitation) and coastal (coastal dynamics and sea-level changes) alterations. Specifically, arid conditions reflected by a maximum pollen representation of dry and open vegetation occurred during the Northern Hemisphere cold Heinrich event 1 (H1), suggesting that the expansion of drier upland vegetation was synchronous with cold Northern Hemisphere conditions. This arid period is followed by an interval in which forest and humid woodlands expanded, indicating a hydrologic shift towards more humid conditions. Droughts during H1 and the shift to humid conditions around 14.8 kyr BP in the uplands are consistent with latitudinal shifts of the intertropical convergence zone (ITCZ) driven by high-latitude Northern Hemisphere climatic fluctuations. Additionally, our results show that the lowland vegetation, consisting of well-developed salt marshes and mangroves in a successional pattern typical for vegetation occurring in intertidal habitats, has responded mainly to local coastal dynamics related to marine inundation frequencies and soil salinity in the Rufiji Delta as well as to the local moisture availability. Lowland vegetation shows a substantial expansion of mangrove trees after ~ 14.8 kyr BP, suggesting an increased moisture availability and river runoff in the coastal area. The results of this study highlight the decoupled climatic and environmental processes to which the vegetation in the uplands and the Rufiji Delta has responded during the last deglaciation.