Response of yam varieties to soil moisture regime in Southwestern Nigeria

A field experiment was conducted on varietal response of white yam to moisture regime in Abeokuta. The experiment comprised three varieties of yam (Efuru, Ise-osi and Oniyere), three mulching options (grass, polythene and unmulched), and two planting dates (early and late). Treatments were replicated three times using RCBD lay-out. Model for selecting planting date involved relating potential evapotranspiration (PE) to precipitation (P) in the form of 0.1PE<P < 0.5PE, partitioned for attaining optimal planting date into early {T1= Σ(P-0.1PE) ≤ 0} and late {T2 = Σ(P-0.5PE) ≤ 0}, respectively. For humid period defined by P> PE, the physiological parameters and moisture agro-climatic indices measured during phenological stages of yam grown were analyzed with respect to treatments. Result showed that T1 defined as Σ(P-0.1PE) ≤ 10 mm appeared as the best model that significantly (P < 0.05) influenced emergence rate, phenological growth and tuber yield. All yam varieties evaluated were suitable for planting with respect to yield. Efuru and Ise-osi synchronized perfectly with Actual Water Availability and produced good vegetative growth with LAI of 1.08 and 0.91 leading to higher tuber yield of 12 and 11.64 tonnes ha-1, respectively. Grass mulch had tuber yield, 4-6 tonnes ha-1greater than the polythene and unmulched plots in all varieties. Mulching significantly (P< 0.05) increased tuber yield, 6-8 tonnes ha-1than the unmulched. Conclusively, early planting with grass mulch increased tuber yield.

The “Green Sahara” Left Behind Fossil Rivers

Reconstruction reveals how people living along the banks of the Nile may have relocated as climate changed and flooding increased during the African Humid Period.

Effect of relative humidity, temperature, light and plant age on infection of lowbush blueberry by Sphaerulina vaccinii

Leaf spot and stem canker caused by Sphaerulina vaccinii is associated with premature defoliation in lowbush blueberry resulting in reduced yields. In this study, we investigated the impact of free water, relative humidity (RH), temperature, light, and plant age on leaf infection under controlled conditions. On potato dextrose agar, germination of conidia was usually polar. Growth was minimal at 5 and 10°C, increased at 15 and 20°C, was maximal at 25°C and decreased at 30°C. Percentage of germinated conidia on inoculated blueberry leaves incubated in dark controlled-humidity chambers for 3 days (25°C) was 86.0, 90.5, 81.3 and 28.3% in free water, 100, 97.5 or 95% relative humidity (RH), respectively. Germination did not occur at 90 or 85% RH. Infection of inoculated plants, however, was not favored by free water, but rather by high RH (>95%) and a 14-h photoperiod (180 µmol/m2/s). Infection failed in continuous darkness, continuous light or continuous darkness followed by 4, 8 or 12 h of light. Light and scanning electron microscopy showed that hyphal penetration into stomata on abaxial leaf surfaces was strongly tropic. When germ tubes grew in close proximity to a stomate, a penetration hypha formed at about 90° angles to the germ tube and took the closest path to the stomate. Stomatal penetration was usually direct, but occasionally appressorium-like hyphal swellings formed over stomatal openings. When inoculated plants were exposed to high RH (>95%) at various temperatures, infection occurred after 4 days at 10°C, after 3 days at 15°C and after 1 day at 20 and 25°C. Infection failed to occur at 30°C. Disease severity also increased with duration of the humid period. When leaves were examined microscopically, those that had been incubated for 6 days showed a substantially greater network of epiphytic growth with more stomatal penetrations compared to those incubated for 3 days. Infection was substantially reduced when the humid period was interrupted by alternating days of low RH (60%). Two-week-old leaves were 2.7 times more susceptible than 8-week-old leaves.

Archaeology of Early Pastoralism in East Africa

The first East African pastoralists arrived at the shores of Lake Turkana soon after the end of the African Humid Period, about 5,000 years ago. In the preceding millennia of the Holocene, fishing economies characterized East Africa. The domestic animals of the early pastoralists were not indigenous to East Africa, nor did they spread through the region simultaneously. Early pastoralist archaeological sites around Lake Turkana comprise settlements and remarkable monumental cemeteries. The expansion of pastoralists further south through East Africa was a two-stage process, probably because of the challenges posed by the presence of diseases fatal to livestock. First, caprines spread south and appear to have been integrated into existing forager subsistence systems. Then, starting toward the end of the 2nd millennium bce, specialized pastoralism began to be established across central and southern Kenya and into northern Tanzania. While analysis of lipid residues on potsherds has demonstrated that these Pastoral Neolithic (PN) peoples milked their animals, the question of whether agriculture was also practiced remains unresolved. Analyses of ancient DNA have shown there were at least two episodes of demic diffusion associated with the spread and establishment of the PN in East Africa. Considerable diversity is present in the PN, with three distinct cultures generally recognized across East Africa south of Lake Turkana. Moreover, there is even greater diversity observed in the decoration and shapes of ceramics. However, this cultural diversity is not matched by human genetic diversity, at least among the analyzed skeletons from two of the three cultures—the Elmenteitan and the Savanna Pastoral Neolithic.

The remote response of the South Asian Monsoon to reduced dust emissions and Sahara greening during the middle Holocene

Previous studies based on multiple paleoclimate archives suggested a prominent intensification of the South Asian Monsoon (SAM) during the mid-Holocene (MH, ∼6000 years before present). The main forcing that contributed to this intensification is related to changes in the Earth's orbital parameters. Nonetheless, other key factors likely played important roles, including remote changes in vegetation cover and airborne dust emission. In particular, northern Africa also experienced much wetter conditions and a more mesic landscape than today during the MH (the so-called African Humid Period), leading to a large decrease in airborne dust globally. However, most modeling studies investigating the SAM changes during the Holocene overlooked the potential impacts of the vegetation and dust emission changes that took place over northern Africa. Here, we use a set of simulations for the MH climate, in which vegetation over the Sahara and reduced dust concentrations are considered. Our results show that SAM rainfall is strongly affected by Saharan vegetation and dust concentrations, with a large increase in particular over northwestern India and a lengthening of the monsoon season. We propose that this remote influence is mediated by anomalies in Indian Ocean sea surface temperatures and may have shaped the evolution of the SAM during the termination of the African Humid Period.

Early Holocene greening of the Sahara requires Mediterranean winter rainfall

The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 y ago, is arguably the largest climate-induced environmental change in the Holocene; it is usually explained by the strengthening and northward expansion of the African monsoon in response to orbital forcing. However, the strengthened monsoon in Early to Middle Holocene climate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. Here, we present an 18,500-y pollen and leaf-wax δD record from Lake Tislit (32° N) in Morocco, which provides quantitative reconstruction of winter and summer precipitation in northern Africa. The record from Lake Tislit shows that the northern Sahara and the Mediterranean region were wetter in the AHP because of increased winter precipitation and were not influenced by the monsoon. The increased seasonal contrast of insolation led to an intensification and southward shift of the Mediterranean winter precipitation system in addition to the intensified summer monsoon. Therefore, a winter rainfall zone must have met and possibly overlapped the monsoonal zone in the Sahara. Using a mechanistic vegetation model in Early Holocene conditions, we show that this seasonal distribution of rainfall is more efficient than the increased monsoon alone in generating a green Sahara vegetation cover, in agreement with observed vegetation. This conceptual framework should be taken into consideration in Earth system paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity.

Rock varnish record of the African Humid Period in the Lake Turkana basin of East Africa

Rock varnish is a manganiferous dark coating accreted on subaerially exposed rocks in drylands. It often contains a layered microstratigraphy that records past wetness variations. Varnish samples from latest Pleistocene and Holocene geomorphic features in the Lake Turkana basin, East Africa display a regionally replicable microstratigraphy record of Holocene millennial-scale wetness variability and a broad interval of wetter conditions during the African Humid Period (AHP). Three major wet pulses in the varnish record occurred during the generally wet interval of the early Holocene (11.5–8.5 ka) when the lake attained its maximum high stand (MHS) at 455–460 m. A >23 m drop from the MHS occurred between 8.5 and 8 ka. Subsequently two additional wet pulses occurred during the early to middle Holocene (8–5 ka) when the lake occupied its secondary high stand at 445 m. Collectively, these five wet phases represent an extended wet interval coincident with the AHP in the region. One moderate wet phase occurred during the subsequent climatic transition from the humid to arid regime (5–4.3 ka) after the lake level dropped rapidly from 445 m to <405 m. Five minor wet phases took place during the overall arid period of the late Holocene (4.3–0 ka) when the lake level oscillated below 405 m. These findings indicate that the AHP terminated rapidly around 5 ka in the Turkana basin in terms of lake level drop, but the regional shift in relative humidity from the AHP mode to its present-day condition lagged for about 700 years until 4.3 ka, hinting at a gradual phasing out in terms of moisture condition. These findings further suggest that Lake Turkana overflowed intermittently into the Nile drainage system through its topographic sill at 455–460 m during the early Holocene and has become a closed-basin lake thereafter for the past 8 ky.

Establishment and functioning of the savanna marshes of the Lopé National Park in Gabon since the termination of the African humid period and the arrival of humans 2500 years ago

Holocene paleoecological studies in tropical Africa are rare because most lakes either dried out at the termination of the African Humid Period or have since filled up. However, tropical sedge marshes can be an alternative to perform long-term ecological studies. The Lopé National Park (LNP) in Gabon is a mosaic of forest and savanna enclosed in the equatorial forest, where open areas facilitated the development of peat marshes accumulating several-meter-thick sediment. In order to reconstruct the historical dynamic in these marshes through a local and regional point of view, we compared sedimentological, continuous X-ray fluorescence, and stable isotopic analyses on sediment cores from six herbaceous marshes in the LNP. A reliable chronological frame was based on 50 14C dates, over the last 2500 years in most sites, and reaching 9000 years in one marsh. We show that the origin of these marshes is a major hydrological change, 3450 and 2300 years ago, that affected the entire region, almost concomitantly with the diffusion of Iron Age population. The sedimentation within marshes is homogenous with low intra-site variability. In contrast, high inter-sites variability evidences that the functioning of the marsh itself exerts a much more significant influence than in lakes. However, a regional event is recorded between 1400 and 800 years ago, concurrently with an archeological trace hiatus throughout the forest hinterland of West Central Africa.

The Holocene African Humid Period from Tibesti mountains (Chad): Contribution of the fossil assemblage and the oxygen isotopic composition from lacustrine diatoms

&lt;p&gt;During the Holocene (0-11.7 ka BP), the subtropical regions of Africa were characterized by very significant climatic changes. These changes were evidenced by variations in lake levels which reflect the balance between precipitation and evaporation (P-E) at the watershed scale. These climatic conditions are mainly associated with the dynamics of the African summer monsoon in relation with the location of the Intertropical Convergence Zone (ITCZ), which is modulated by summer insolation induced by the Earth's orbital parameters. This mechanism explains the wetter conditions observed from 11 ka BP to 5.5 ka BP in the Sahelo-Saharan zone. This period, called &quot;Green Sahara&quot; or &quot;African Humid Period&quot;, was characterized by a green landscape, covered by grasslands and trees, dotted with numerous lakes, and incised by large river networks.&lt;br&gt;Despite the numerous studies carried out on the African Humid Period, there is a scarcity of data for quantifying source and origin of precipitation. The Lake Chad Basin (BLT) is a key region for paleoclimatic research because of its position reflecting main tropical atmospheric mechanisms and its endorheic morphology amplifying climatic signals. More particularly, the crater palaeolakes of Trou au Natron (Pic Toussid&amp;#233;) and Era Kohor (Emi Koussi) in the Tibesti mountains offer unique sedimentary archives that may record the climatic history of the Sahara.&lt;br&gt;This work aims to reconstruct the evolution of these crater palaeolakes, thanks to the oxygen isotopic composition of diatoms (&amp;#61540;18Odiatom), from the termination of the last deglaciation until the African Humid Period. The identification of the fossil diatom assemblages combined with the &amp;#61540;18Odiatom values should give insights on the evolution of the limnological parameters, the relative depth, the chemistry, and the water isotopic composition of these palaeolakes.&lt;br&gt;The measurement of &amp;#61540;18Odiatom is carried out using the IR-laser fluorination-isotope ratio mass spectrometry technique at the CEREGE stable isotope laboratory, after dehydration under a flow of nitrogen gas. The diatoms are purified at the CEREGE micropaleontology laboratory after decarbonation and organic matter oxidation. The taxonomic determination of diatoms is carried at CEREGE (France) and at the University of N&amp;#8217;Djamena (Chad).&lt;br&gt;Preliminary results from the two Tibesti mountains records cover the Holocene wet period. They show significant variations in the d18Odiatom values and a distinct evolution of ecosystems as demonstrated by taxonomic assemblage of fossil diatoms. These results are compared with a reconstruction of the d18Odiatom from Lake Chad for the same period. This comparison evidenced substantial data for the reconstruction of the Holocene wet period, in terms of origin of water inflows in the basin, eg Tropical versus Mediterranean and lowland versus mountainous atmospheric processes, and on the reconstruction of the migration and the position of the ITCZ. These two questions are still speculative and will certainly provide data for global climate circulation models which are struggling to reproduce the climate in Saharan latitudes.&lt;/p&gt;

Nile floods during the last African Humid Period: a seasonal record from the deep-sea fan

&lt;p&gt;Seasonal floods are life-supporting events in the Nile Valley and have been crucial to the development of complex societies. Present populations depend on their occurrence but the alteration of fluvial dynamics under climate change remains elusive. In order to better understand how fluvial dynamics respond to climatic changes, we explore past flood dynamics of the Nile River using a unique finely laminated sequence from the Nile deep-sea fan.&lt;/p&gt;&lt;p&gt;Today, floods occur during the summer, when monsoonal rainfall hits the Ethiopian highlands and feeds the Blue Nile. Core P362/2-33 covers the past 9.5 ka BP and is ideally located to record changes in fluvial dynamics during periods of stronger monsoon activity such as the Saharan Humid Periods. The absence of oxygen in the Mediterranean bottom waters during the last Saharan Humid Period (during sapropel S1 deposition) allowed to preserve the laminated structure between 9.5 and 7.5 ka BP.&lt;/p&gt;&lt;p&gt;We focus here on examining the nature of the laminations in order to 1) understand the deposition mechanism and 2) obtain a reconstruction of past fluvial dynamics at seasonal resolution.&lt;/p&gt;&lt;p&gt;Microfacies analysis and elemental micro-XRF scanning indicate that couplets of alternating dark- and light-coloured layers represent seasonal deposits of Nile discharge and marine hemipelagic sedimentation, respectively. Preliminary lamination counts suggest that couplets were deposited at an annual rate for most of the record. Increases in layer thickness is observed around 9.5 and 9.1 ka BP, followed by a gradual decrease until 8 ka. Careful examination of lamination structure and time-series analysis of layer counts will permit to further explore sub-annual changes in flood dynamics during the Saharan Humid Period. Finally, due to its high temporal resolution, our record has the potential to link reconstructions of Nile discharge to other regional archives of hydrological changes (e.g., speleothems, lakes) and thereby identify overarching forcing mechanisms.&lt;/p&gt;