Phenology of Acacia albida trees in Zambezi riverine woodlands

The phenology of Acacia albida trees was studied over eight years in Mana Pools National Park, Zimbabwe. Leaf flush occurred towards the end of the rains. It was followed by flowering, with ripe fruit-fall in the late dry season. Leaf-fall occurred early in the following wet season. After seasons of low rainfall, trees were severely defoliated by caterpillars soon after leaf flush and this prompted a second flush of leaves. In these years there were also two periods of flowering and fruit production. During dry years, the proportion of trees which failed to set fruits after flowering increased, fruit production by those trees which did bear fruit was reduced and fruit-fall started earlier in the year.     

Biomass dynamics of herbaceous vegetation in Zambezi riverine woodlands

Seasonal changes in biomass, productivity and large mammal utilization of annual grasslands on Zambezi alluvium in Mana Pools National Park, Zimbabwe, were recorded over four years and compared with perennial grasslands. Annual grasses grew only in the wet season, but forb productivity peaked at the start of the dry season. Large mammals and termites ate dead annual grasses in the dry season. Production was usually greater in perennial grasslands, than in annual ones, and perennial grasses often grew throughout the year. Large mammal utilization of grasslands varied between 53 and 99% of annual production.

Résumé

L'on rapporte les changements saisonniers de biomasse, productivité et utilisation par les grands mammifères des herbages annuels dans les plaines alluviales du Zambèze au Parc National de Mana Pools, Zimbabwe, pendant 4 ans et on les compare avec ceux des herbages pérennes. Les herbes annuelles ne croissent que pendant la saison des pluies mais la productivité du fourrage était au maximum en début de pérennes que dans les annuels et les herbes pérennes poussaient souvent tout au long de I'année. L'utilisation par les grands mammifères variait de 53 à 99% de la production annuelle.     

The dendrochronological potential of Baikiaea plurijuga in Zambia

Climate has been demonstrated to change at different scales for as far back as we have been able to reconstruct it. However, anthropogenic factors have accelerated and are predicted to cause significant changes in temperature and precipitation around the globe. As a consequence, vegetation is being affected. To understand the historical behaviour of individual tree species and have insight on the potential effects of climate change, tree-ring studies have been applied. In this study, we examined a genus new to dendrochronology, namely Baikiaea plurijuga (Spreng.) Harm that dominates the Zambezi teak forests in Zambia with the objective of determining whether B. plurijuga forms annual rings and if so, whether these rings are cross-datable. We further determined the relationship between ring- width of B. plurijuga and climatic variables with the aim of understanding the potential climate change effects on the growth of these species in Zambia. We collected tree-ring samples from three Zambezi Teak forest reserves: Zambezi, Ila, and Masese located in Kabompo, Namwala, and Sesheke study sites respectively. Our examination of wood anatomical structures reviewed that the wood of B. plurijuga is diffuse porous and forms annual rings which were confirmed with samples of known age. The analysis resulted in three strong tree-ring chronologies of B. plurijuga. These chronologies were correlated with climate data from local weather stations which correlated negatively with evaporation and temperature and positively with rainfall. Our regression analysis indicated that evaporation has the highest influence on tree growth at all the study sites compared to temperature and rainfall alone. Evaporation in November and March, for example, explained almost a third of the radii’s variance at the Namwala and Sesheke sites. The likely future temperature increase and rainfall decrease that are projected by IPCC for Southern Africa are likely to adversely affect B. plurijuga in Zambia.     

Below and above-ground carbon distribution along a rainfall gradient. A case of the Zambezi teak forests,Zambia

Understanding carbon (C) stocks or biomass in forests is important to examine how forests mitigate climate change. To estimate biomass in stems, branches and roots takes intensive fieldwork to uproot, cut and weigh the mass of each component. Different models or equations are also required. Our research focussed on the dry tropical Zambezi teak forests and we studied their structure at three sites following a rainfall gradient in Zambia. We sampled 3558 trees at 42 plots covering a combined area of 15ha. Using data from destructive tree samples, we developed mixed-species biomass models to estimate above ground biomass for small (<5 cm diameter at breast height (DBH, 1.3 m above-ground)) and large (≥5 cm DBH) trees involving 90 and 104 trees respectively, that belonged to 12 species. A below-ground biomass model was developed from seven trees of three species (16–44 cm DBH) whose complete root systems were excavated. Three stump models were also derived from these uprooted trees. Finally, we determined the C fractions from 194 trees that belonged to 12 species. The analysis revealed that DBH was the only predictor that significantly correlated to both above-ground and below-ground biomass. We found a mean root-to-shoot ratio of 0.38:0.62. The C fraction in leaves ranged from 39% to 42%, while it varied between 41% and 46% in wood. The C fraction was highest at the Kabompo site that received the highest rainfall, and lowest at the intermediate Namwala site. The C stocks varied between 15 and 36 ton C ha⁻¹ and these stocks where highest at the wetter Kabompo site and lowest at the drier Sesheke site. Our results indicate that the projected future rainfall decrease for southern Africa, will likely reduce the C storage potential of the Zambezi teak forests, thereby adversely affecting their mitigating role in climate change.     

Effects of annual rainfall and habitat types on the body mass of impala (Aepyceros melampus) in the Zambezi Valley, Zimbabwe

Body mass is often considered as a good indicator of body condition of individuals in ungulates, hence of their fitness, and thus, may be used as an index to monitor the status of populations subject to harvesting schemes. Here, we report the influence of annual rainfall (a proxy for primary production) and habitat on the body mass of impala in a population cropped for meat in a communal area of Zimbabwe. We analyzed the data from 2 contrasted years for rainfall, in two different habitats. In the good year (i.e. high annual rainfall) impala were heavier than in the poor year, and adult females seemed to be less affected than males by variation in primary production. We show that adult males were suffering from a seasonal decrease in body mass, supposedly linked to the rut, particularly in good habitat. Overall, the habitat effect appeared to be dominated by the rainfall effect, and this may be due to the very high animal densities in the good habitat, i.e. fast resource depletion. Our results also suggest that males (juvenile and adult) are more susceptible to changes in food resource abundance and quality than females, which supports previous studies on sexually dimorphic and polygynous species.     

Fish production in Lake Kariba,reconsidered

Synopsis Revised total and available production, yield and mean biomass per ha were calculated for each species in Lake Kariba, and for the whole lake. The revision was undertaken because (1) the original value 1 g for W₀ for each species was too high, (2) B_i+1 was occasionally used instead of B_i in the calculation of mean biomass for an interval i to i + 1, and (3) species' contributions to mean parameter values for the whole lake were not weighted according to their mean standing crop. Revised values are, A = 1224, P = 720, Y_A = 400, Y_{Pp, = 202 and B} = 827 kg ha^–1y–1. These correspond to 38.2%, 66.7%, 107.1% and 25.8% respectively of the values calculated initially.     

Comparative ecology of Serranochromis species (Teleostei: Cichlidae) in the Upper Zambezi River floodplain

The Upper Zambezi River drainage of Zambia contains six Serrunochromis species belonging to the largemouth, piscivorous subgenus Serranochromis and three smallmouth, invertebrate-feeding species of the subgenus Sargochromis. A survey of the Barotse floodplain and associateddrainages revealed that two species, S. (S.) longimanus (Boulenger) and S. (S.) thumbergi (Castelnau), were uncommon on the floodplain. S. thumbergi appeared to be more abundant in the Kafue, Chobe, and Kabompo rivers than the Upper Zambezi proper. Adults of the three largest species (i.e. S. (S) robustus (Günther), S. (S.) altus (Winemiller & Kelso-Winemiller), and S. (Sar.) giurdi (Pellegrin)) inhabit the main channel of the Zambezi River during low water, whereas most adults of the other four species (i.e., S. (S.) macrocephalus (Boulenger), S. (S.)ungusticeps (Boulenger), S. (Sar.) codringtoni (Boulenger), and S. (Sar.) carlottae (Boulenger)) were collected from lagoons, canals and backwaters. Comparisons of population size structure and gonad condition during the falling water (May-August) and low water (September- December) seasons indicated that the seven common Serranochromis initiate spawning near the end of the low water period. Examination of gonad condition during the pre-spawning period indicated that Serrunochromis attain maturity at the following sizes: S. robustus, males (275-300 mm s.l .), females (250-275 mm); S. macrocephalus, both sexes (150-200 mm); S. altus, males (275-325 mm), females (250-300 mm);S. angusticeps, males (250-275 mm), females (175-225 mm); S. giardi, males (275-300 mm), females (225-250 mm); S. codringtoni, males (150 mm), females (125-150 mm); S. carlottue, males (150-175 mm), females (100-125 mm). Age was estimated from scale annuli comprised of closely spaced circuli that corresponded to the period of rising water (December- March). S. robustus, S. altus, S. angusticeps, and S. giurdi matured at approximately 3 years, and S. macrocephalus, S. codringtoni, and S. curlottae matured at approximately 2 years in the Barotse floodplain. The average numbers of mature oocytes per female were as follows: S. robustus- 1165; S. macrocephalus-810; S. altus-920; S. angusticeps-627; S. codringtoni-580; S. carlottue-362. Fecundity exhibited a weak positive relationship with female size. Analysis of stomach contents showed that interspecific diet overlap was low among both immature and mature size classes of Serrunochromis. Adult size classes of S. robustus fed primarily on small Synodontis catfishes, S. mucrocephalus fed mostly on mormyrid and cichlid fishes, S. altus specialized on mormyrids, and S. angusticeps fed primarily on small cyprinid and characid fishes. The three members of the subgenus Sargochromis consumed mostly invertebrates. Molluscs were the most important item in the diet of S. giardi, and aquatic insects and fish scales were predominant in the diet of S. carlottae. Larger size classes of S. codringtoni consumed large amounts of seeds (Graminaceae). If habitat and diet are considered together, niche separation among Upper Zambezi Serrunochromis species appears to be nearly complete.     

Asian aquatic plant moth Parapoynx diminutalis,accidentally introduced earlier,contributes to control of an aquatic weed Hydrilla verticillata in South Africa

The Shire River drains from Lake Malawi to the Lower Zambezi River. Annual flow is dependent mainly on lake level, partially controlled by the operation of a barrage at Liwonde to regulate flows for hydroelectricity generation in the escarpment reaches of the river. Downstream of the escarpment, the floodplains of the Lower Shire River support important subsistence fisheries that yielded on average 5 100 t y^?1 between 1977 and 1993. Catches from the major fishery area, the Elephant Marsh, were positively correlated with mean annual flow for the same year. Since the target fish species (mainly clariid catfishes) were mostly caught in their second and third year, improved recruitment through higher breeding success at higher flow levels is less likely to explain interannual variation in catches than is greater upstream migration of fish from the Lower Zambezi River in high-flow years. In 1978 a sudden closure of Liwonde Barrage resulted in rapid drainage of stagnant water from the Elephant Marsh floodplain vegetation into the river, causing a mass fish mortality and thus a marked decline in catches from 1979, followed by gradual recovery by 1981. The delay in recovery contrasts with the annual direct catch/flow relationship, and raises questions about the most important influences on Lower Shire catch rates.     

Using citizen‐based survey data to determine densities of large mammals: a case study from Mana Pools National Park,Zimbabwe

Since 1993, members of the national wildlife society have undertaken annual surveys of large mammals in the Zambezi alluvial woodlands of Mana Pools National Park, Zimbabwe. Data are collected along 36 systematically‐arranged transects. We provide the first thorough assessment of the data from any survey within this long‐term project. The transect data from 2011 were analysed with DISTANCE software to assess if the data were suitable for determining the densities of large mammals using distance sampling techniques. Successful application of distance sampling depended on observers using printed, large‐scale, georeferenced satellite images onto which they mapped the location of animal groups detected. The assumptions of the distance sampling were well met and thus the 2011 survey provided reliable estimates of the densities of nine species of common large mammals on the Zambezi alluvium during the late dry season. Estimated density in this dry‐season concentration area varied from 3.6 km^?2 for kudu, to 204 km^?2 for impala. The precision of the estimates ranged from a coefficient of variation of 7.9% for elephant, to 25.5% for buffalo. For elephant, warthog and baboon, the morning and afternoon densities differed significantly.