Savanna Transition Research Articles

Soil carbon stock and nutrient characteristics of forest–savanna transition: Estimates from four land use systems in Ghana

Soil carbon stock and nutrient characteristics of forest–savanna transition: Estimates from four land use systems in Ghana

Current Forest–Savanna Transition in Northern South America Departs from Typical Climatic Thresholds

The forest–savanna transition is the most widespread ecotone in the tropics, with important ecological, climatic, and biogeochemical implications at local to global scales. However, the factors and mechanisms that control this transition vary among continents and regions. Here, we analyzed which factors best explain the transition in northern South America (Llanos ecoregion and northwestern Amazon), where common thresholds on typical environmental factors (for example, mean annual precipitation (MAP), wet season precipitation) fail to predict it. For instance, savannas in the Llanos occur at MAP levels (> 1500 mm) which are typical of forests in other tropical regions. We examined the transition’s climate features, soils, and disturbance (fire frequency) spaces using remotely sensed data. We used logistic generalized linear models to assess the effect of seasonal (season length) and intra-seasonal (daily precipitation frequency and intensity) precipitation metrics during the dry season, soil silt content, and fire frequency, on the transition using canopy cover, tree cover, and the maximum Plant Area Volume Density as vegetation structure descriptor variables. Fire frequency and precipitation frequency were the most important variables explaining the transition. Although most fires occur in savannas, we found that a significant percentage of savanna pixels (46%) had no fires. This study indicates that the transition should be characterized regionally in response to biogeographic differences (for example, climatic space) among regions and continents. Our results highlight the importance of fire frequency and intra-seasonal precipitation in determining the transition in northern South America. Furthermore, future studies should consider regional differences in the climatic space of forest and savanna to improve projections of global change impacts on these highly diverse ecosystems.

Weather-Informed Recommendations for Pest and Disease Management in the Cashew Production Zone of Côte d’Ivoire

Poorly informed pest and disease management strategies can have detrimental effects on the environment, crop quality, crop yield, farmers’ income, and the overall sustainability of agriculture. For this reason, integrated pest and disease management (IPDM) draws on knowledge from various research fields to effectively manage risks of bio-aggressor outbreaks. However, many agricultural sectors of Sub-Saharan African countries lack such necessary knowledge, including the epidemiology of bio-aggressors in relation to the increased climate variability. The objective of this work is to provide weather-based guidance for the development of sustainable pest and disease control strategies in cashew cultivation areas of Côte d’Ivoire, the second most important cash crop of the country. Leveraging the bioclimatic knowledge in the literature about fungi and insects, we explored four-year hourly data (2017–2020) of 34 sites of the cashew production zone. The outputs showed potentially conducive weather events for fungi and insects throughout the entire cashew production areas, with the forest–savanna transition zone being the most critical. These outputs were used to elicit recommendations for pest and disease management in consultation with a multidisciplinary stakeholder task force. They recommended that effort for disease prevention in the forest–savanna transition zone, the new cashew production zone, should be an incrementation of the one already recommended in the north savanna. Weather-based disease monitoring, entomopathogenic-fungi-based pest control, and the promotion of early-maturing cashew genotypes are also recommended, especially in the forest–savanna transition zone.

Differential Impacts of Cropland Expansion on Soil Biological Indicators in Two Ecological Zones

Agricultural expansion in Sub-Saharan Africa is characterized by different farm ages in smallholder communities. This study investigated changes in microbial indices broadly (i) at the reconnaissance survey level in four agro-ecological zones and (ii) in different farms at the forest (Dompem) and forest–savanna transition (Adansam) zones, as influenced by the duration of cultivation. Soils from one-year (first cultivation of cleared forest/fallow), three-year, five-year, and ten-year farms were analyzed for basic soil properties, active or labile carbon (POXC), basal respiration (BR), microbial biomass (Cmic) using permanganate oxidizable C, alkali trap, and chloroform fumigation incubation. In both study levels, POXC content was <1% of soil organic carbon (SOC) in all zones, higher in the wet agro-ecological zones, and positively correlated with SOC (r = 0.70, 0.81; p < 0.01, p < 0.001). Dompem SOC and BR declined by 1–23% and 6–25% (p < 0.001), respectively, in the first three years; Cmic (p = 0.002) and %Cmic/SOC (p = 0.610) decreased from three-year farms onwards. Conversely, the Adansam SOC, BR, Cmic, and %Cmic/SOC rather had irregular trends. The microbial indices were influenced by exchangeable acidity, the sum of exchangeable bases, and effective cation exchangeable capacity negatively or positively, followed by SOC, pedogenic compounds, particularly dithionite-citrate iron (Fed), oxalate iron (Feox), and lastly, soil pH. Therefore, understanding the degree, direction, and changing aspects of these drivers of soil ecosystem services is necessary for sustainable soil management practices in different agro-ecological zones.

Assessing the driving forces of Guinea savanna transition using geospatial technology and machine learning in Old Oyo National Park, Nigeria

Savannization and de-savannization are two land cover transition processes that are yet to be understood at a local scale in sub-Saharan Africa. We studied the patterns across different ecological, anthropogenic and climatic factors to infer the trends of savannization and de-savannization in Old Oyo National Park, Nigeria. Geospatial technology and two machine learning methods were used to determine the pattern, dynamics, and importance of savannization and de-savannization with a set of thirteen eco-climatic variables. The savannized areas with total land cover masses of 934.27 km2 in 1986–2003 decreased to 901.01 km2 in 2003–2019, and vice versa for de-savannized areas. The thirteen eco-climatic variables contributed to savannization and de-savannization within the two transition intervals in varying degrees. Relevant stakeholders should employ mild burn severity to manage savannization at locations close to host communities, rivers, and roads within elevations above 450 m and low rainfall in the dry season below 6 mm.

Plant Parasitic Nematodes Associated with Okra in the Forest Savanna Transition and Semi-Deciduous Forest Agro-Ecologies of Ghana

Okra is an important vegetable crop in Ghana. Plant-parasitic nematodes infect okra, reducing the quality and quantity of fresh immature and dry fruits. This study was carried out to determine diversity, density and distribution of plant parasitic nematodes attacking okra in the Forest savanna transition and Semi-deciduous forest agro-ecologies of Ghana. In each agro-ecology, soils from okra rhizosphere were sampled from 15 farms for plant-parasitic nematodes assessment. Nematodes were extracted using the modified Baermann nematodes extraction method. Six nematode genera were identified; Meloidogyne, Pratylenchus, Xiphinema, Helicotylenchus, Rotylenchulus and Scutellonema. Meloidogyne occurred with 64.9% relative abundance and 100% frequency. Rotylenchulus, Pratylenchus, and Helicotylencus nematodes also occurred with 100% frequency. Xiphinema were the least abundant (0.05%) and the least frequent (15%). The findings have shown that a large number of nematodes exist on okra plants and therefore there is the need for educational campaigns to be stepped up by Agricultural Extension Services of the Ministry of Food and Agriculture on plant parasitic nematode infections in okra production.

Variation in Tuber Dry Matter Content and Starch Pasting Properties of White Guinea Yam (Dioscorea rotundata) Genotypes Grown in Three Agroecologies of NIGERIA

The primary objective of this study was to assess the effects of genotype (G), location (L), and G × L interaction on tuber quality traits (dry matter content and starch pasting parameters) in white Guinea yam (Dioscorea rotundata Poir.). Variability in tuber dry matter and starch pasting properties was examined using 18 advanced breeding lines and two dominant landrace cultivars of white Guinea yam grown in three different agroecological zones (forest‒savanna transition, southern Guinea savanna, and rainforest) in Nigeria. The starch pasting properties were evaluated using a Rapid Visco Analyser. Our results show that the G × L interaction effect was low compared to the genotype and growing location effects on the variability of key starch properties. In addition, the repeatability of trait performance across locations was high and relatively uniform for key traits, suggesting that any of the three locations used in this study can be employed for their evaluation. Furthermore, TDr1100873 had a higher dry matter content than the dominant landrace cultivars (Amula and Meccakusa) but was similar to them in starch pasting properties. Hence, TDr1100873 is considered a suitable variety for future breeding activities.

Influence of Biochar on Soil Physicochemical Properties, Erosion Potential, and Maize (Zea mays L.) Grain Yield under Sandy Soil Condition

ABSTRACT Degraded soils in humid tropics are characterized by low fertility and high soil erosion potential. Hence, field experiments were conducted during 2018 and 2019 cropping seasons on an Alfisol at Owo in the forest–savanna transition zone of southwest Nigeria to evaluate the effects of the application of biochar (B) produced from waste wood of trees on physicochemical characteristics, erosion potential, and maize yield. Four treatments considered were control (no biochar application, 10 t ha−1 B, 20 t ha−1 B and 30 t ha−1 B). The treatments were laid out in a randomized complete block design with three replications. Results in both years indicated that B application significantly improved grain yield of maize and soil chemical properties, and reduced bulk density (BD) and penetration resistance (PR), and increased porosity (PO), moisture content (MC), and the mean weight diameter (MWD) of soil aggregates. Pooled over 2 years, B applied at 10, 20 and 30 t ha−1 increased grain yield of maize by 44.4%, 66.7%, and 88.9%, respectively, compared with the control. Averaged over 2 years, incorporating B into the soil significantly reduced soil loss (SL) by 32.3%, 56.9%, and 69.7% at 10, 20, and 30 t ha−1 application rates, respectively, compared with the control. The formation of macroaggregates in the B-amended soils is the critical factor to improve soil erosion potential. Therefore, the application rate of 30 t ha−1 B is considered as suitable for severely degraded soil because this application rate efficiently improves maize yield and soil properties and reduces SL.

The dynamics of property and other mechanisms of access: The case of charcoal production and trade in Ghana

An analysis of the dynamics of access to resources is important for a fuller understanding of rural livelihoods especially with changing times and technology. This paper uses benefits from charcoal production and trade in the forest savanna transition zone of Ghana as a case to explore the dynamics of access in time and space and the mechanisms various social actors apply in order to benefit. The study applies an analytical framework combining the revised property rights framework with “A Theory of Access”. Participatory rural appraisal methods, stakeholder meetings, document reviews and interviews were conducted with social actors along the charcoal commodity chain in Ghana. The paper shows that the ability to benefit from charcoal is more dependent on property than other mechanisms of access, and both customary and statutory institutions are involved in mediating access to charcoal in the area. The realization of economic benefits of charcoal led to a contestation between chiefs and family heads over rights to trees on family lands in some communities and changes in the mode of payment for trees used in producing charcoal. Scarcity and concern over sustainability of trees have also driven the dynamics of the access mechanisms. Chiefs have strengthened their authority in charcoal-producing communities and the low presence of statutory institutions with de jure mandate to regulate the charcoal sector in those communities has implication on plans by the state to formalize and regularize charcoal production and trade in the country. We envisage four adverse effects from the government’s attempt to formalize the charcoal sector – elite capture, exclusion of vulnerable and marginalized people, criminalization of livelihoods and loss of rights. We recommend that the planned formalization should be devoid of directives that would make charcoal production and trade prohibitive for low-income households.

Changing Agricultural Practices and Indigenous Food Crops in the Upper Afram Basin of Ghana

The Forest Savanna Transition zone within which the Upper Afram basin lies, account for a wide variety of food crops and plant species. This study examined the effects of changing ag­ricultural practices on cultivation and availability of indigenous food crops within the basin. Qualitative data obtained from 240 farmers through individual interviews and Focus group dis­cussions were analyzed thematically. The study revealed that even though the farmers employed the traditional mixed cropping system, the widespread use of agro-chemicals in the study area was limiting the practice of intercropping. This accounts for the current dominance of mono cropping system in the study area. These changes have affected the cultivation of a wide variety of indigenous foods within the basin. The uncontrolled use of agro-chemicals constituted one of the major agronomic constraints affecting the cultivation of these crops. Adequate technical support services to effectively control agrochemical abuse among the local farmers will, there­fore, be required. Increased sensitization on the need for local farmers to refocus and maintain some of the traditional agricultural practices that ensured the cultivation of a wide variety of indigenous food crop varieties (NUCS) in their farms is also suggested.

Assessment of an Automated Calibration of the SEBAL Algorithm to Estimate Dry-Season Surface-Energy Partitioning in a Forest–Savanna Transition in Brazil

Evapotranspiration ( E T ) provides a strong connection between surface energy and hydrological cycles. Advancements in remote sensing techniques have increased our understanding of energy and terrestrial water balances as well as the interaction between surface and atmosphere over large areas. In this study, we computed surface energy fluxes using the Surface Energy Balance Algorithm for Land (SEBAL) algorithm and a simplified adaptation of the CIMEC (Calibration using Inverse Modeling at Extreme Conditions) process for automated endmember selection. Our main purpose was to assess and compare the accuracy of the automated calibration of the SEBAL algorithm using two different sources of meteorological input data (ground measurements from an eddy covariance flux tower and reanalysis data from Modern-Era Reanalysis for Research and Applications version 2 (MERRA-2)) to estimate the dry season partitioning of surface energy and water fluxes in a transitional area between tropical rainforest and savanna. The area is located in Brazil and is subject to deforestation and cropland expansion. The SEBAL estimates were validated using eddy covariance measurements (2004 to 2006) from the Large-Scale Biosphere-Atmosphere Experiment in the Amazon (LBA) at the Bananal Javaés (JAV) site. Results indicated a high accuracy for daily ET, using both ground measurements and MERRA-2 reanalysis, suggesting a low sensitivity to meteorological inputs. For daily ET estimates, we found a root mean square error (RMSE) of 0.35 mm day−1 for both observed and reanalysis meteorology using accurate quantiles for endmembers selection, yielding an error lower than 9% (RMSE compared to the average daily ET). Overall, the ET rates in forest areas were 4.2 mm day−1, while in grassland/pasture and agricultural areas we found average rates between 2.0 and 3.2 mm day−1, with significant changes in energy partitioning according to land cover. Thus, results are promising for the use of reanalysis data to estimate regional scale patterns of sensible heat (H) and latent heat (LE) fluxes, especially in areas subject to deforestation.

Impact of cashew induced land use change on vegetation carbon stocks in the forest savanna transition zone

Impact of cashew induced land use change on vegetation carbon stocks in the forest savanna transition zone

Dissolved organic N in shallow tropical lakes and reservoirs: Contribution to total dissolved N and relationships with eutrophication

AbstractDissolved organic nitrogen (DON) plays important biogeochemical and ecological roles in aquatic systems. However, little is known about its importance in small tropical lakes and reservoirs. We hypothesized that (a) DON is higher in these systems than dissolved inorganic N (DIN), and that (b) relative contributions of DON and DIN to the total dissolved nitrogen (TDN) pool decrease and increase, respectively, with trophic status. To test these hypotheses, we measured DON and DIN in six lakes and two reservoirs at the Atlantic forest‐Cerrado savanna transition in Southeast Brazil, in the rainy and the dry season. With one exception in each season, all oligo‐ and mesotrophic systems had DON:DIN ratios >1. DON:DIN ratios were negatively correlated with total phosphorus (TP). This pattern was due to high DIN, mainly ammonium, in eutrophic and hypertrophic systems. DON:DIN ratios <1 occurred at TP higher than 103 and 104 µg L−1 in the rainy and dry season, respectively. We found a general dominance of DON in near‐natural lentic systems at the Atlantic forest‐Cerrado savanna transition, which was more pronounced than that reported for other regions. Human alterations to the TDN pool may affect biological communities and ecosystem functioning of these systems.

Vegetation and climate change in the Pro-Namib and Namib Desert based on repeat photography: Insights into climate trends

This paper presents empirical evidence of historical vegetation and climate change in the arid Pro-Namib and hyper-arid Namib Desert spanning the late 19th century to the present based on one hundred archival landscape photographs that have been re-photographed or ‘matched’. Each photo site was evaluated for changes in woody cover and taken together serve as a proxy for how climate has changed in the region. Vegetation change was related to values for precipitation and temperature derived from the Global Land Data Assimilation System for the period 1948–2017 as well as the number of fog days expected at a site. The resulting analysis reveals a trend of increased vegetation cover associated with increased precipitation (fog and rain) in the coastal Fogbelt and the inland Savanna transition with a shrinking of the hyper-arid Minimum zone between the two. These findings accord with projected effects of global warming on the Benguela upwelling system but are at variance with regional climate model forecasts that project widespread aridification. In the absence of long-term climate data, the results of this research are an important contribution of evidence-based knowledge of past climate trends and their relationship to future climate change scenarios for the region.

Assessment of Some Production Practices on the Quality of Cocoa Beans (Theobroma cacao): A Case Study of Forest Savanna Transition and Semi-Deciduous Rainforest Zones of Ghana

Assessment of Some Production Practices on the Quality of Cocoa Beans (Theobroma cacao): A Case Study of Forest Savanna Transition and Semi-Deciduous Rainforest Zones of Ghana

Effects of 3-year cultivation on the soil nutrient status in a tropical forest and savanna of Central Africa, as determined by the microbial responses to substrate addition

ABSTRACTThe forest–savanna transition zone is widely distributed on nutrient-poor Oxisols in Central Africa, and a population explosion has led to the rapid cultivation of these vegetation types in this zone. To reveal and compare the effects of short-term (3 years) cultivation on the soil nutrient status of the forest and savanna vegetation in this area, we evaluated microbial nutrient limitation and availability by conducting hourly measurements of soil microbial respiration after the addition of glucose in combination with nitrogen (N) and/or phosphorus (P) to soils that were collected from a forest site (FOR), a savanna site (SAV), as well as cropland for 3 years derived from a forest (Crop-F) and a savanna (Crop-S), in eastern Cameroon. The N addition had little effect on the pattern of microbial respiration rate for the FOR and Crop-F sites, indicating N rich for microbes. In contrast, N addition resulted in the increases in maximal respiration rates after the exponential increase for the SAV and Crop-S sites, indicating microbial N limitation, and cultivation accelerated the soil N depletion. Furthermore, we observed that P addition resulted in the increase in the maximal respiration rates, indicating microbial P limitation for all sites, except for FOR site. Since the cultivation significantly affected the microbial properties only in the forest ecosystem, such as the increase in the microbial specific growth rate and the decreased microbial C:N and C:P ratios, these changes would induce the P limitation for Crop-F. These results indicate that (1) the FOR site was a N-rich ecosystem for soil microbes, and 3 years of cultivation in the Crop-F site did not alter the high soil N status but induced microbial P limitation, with the changes in the microbial properties, and that (2) the SAV site was N and P limited for soil microbes, and 3 years of cultivation clearly decreased the soil N availability.

Influence of climate variability, fire and phosphorus limitation on vegetation structure and dynamics of the Amazon–Cerrado border

Abstract. Climate, fire and soil nutrient limitation are important elements that affect vegetation dynamics in areas of the forest–savanna transition. In this paper, we use the dynamic vegetation model INLAND to evaluate the influence of interannual climate variability, fire and phosphorus (P) limitation on Amazon–Cerrado transitional vegetation structure and dynamics. We assess how each environmental factor affects net primary production, leaf area index and aboveground biomass (AGB), and compare the AGB simulations to an observed AGB map. We used two climate data sets (monthly average climate for 1961–1990 and interannual climate variability for 1948–2008), two data sets of total soil P content (one based on regional field measurements and one based on global data), and the INLAND fire module. Our results show that the inclusion of interannual climate variability, P limitation and fire occurrence each contribute to simulating vegetation types that more closely match observations. These effects are spatially heterogeneous and synergistic. In terms of magnitude, the effect of fire is strongest and is the main driver of vegetation changes along the transition. Phosphorus limitation, in turn, has a stronger effect on transitional ecosystem dynamics than interannual climate variability does. Overall, INLAND typically simulates more than 80 % of the AGB variability in the transition zone. However, the AGB in many places is clearly not well simulated, indicating that important soil and physiological factors in the Amazon–Cerrado border region, such as lithology, water table depth, carbon allocation strategies and mortality rates, still need to be included in the model.

Soil phosphorus of stable fraction differentially associate with carbon in the tropical forest and savanna of eastern Cameroon

ABSTRACTThe forest–savanna transition zone, which contains nutrient-poor soils (Oxisols), is found throughout central Africa. To evaluate the effect of deforestation on soil phosphorus dynamics, which regulate the plant growth in this area, we quantified the relationship between phosphorus (P) and carbon (C) in different fractions and compared their relationship to forest and savanna (deforested vegetation) in eastern Cameroon. We analyzed the P, C, and nitrogen (N) contents of soil using the physical fractionation method (0.25–2.0 mm as macro-particulate organic matter [M-POM]; 0.053–0.25 mm as micro-POM; and <0.053 mm as Clay+silt) in different land management (young and old forests and annual and perennial grass savannas at 100-cm soil depth). We found larger soil P stock in forests (4.7–4.9 Mg P ha−1) than that in savannas (3.4–4.0 Mg P ha−1), though soil C and N stocks were similar between the vegetation. We also observed lower soil P stock in the active fraction (M-POM) with its higher C:P and lower C:N ratio in forest surface layer (0–10 cm), indicating that forests have lower available soil P. By using the regression analysis, we found a clear relationship between P and C in the stable fraction (Clay+silt) of the upper layer (0–40 cm) for each land management, and the coefficient of the regression was clearly different between the forest and savanna. It indicates that a more chemically complex organic P form of the stable fraction exists in forest soil than in savanna soil. These results indicate that the deforestation (savannazation) affect the active and stable P dynamics and it should cause the lower soil P stock of the upper layer in savanna than in forest.

Organic matter dynamics in a savanna transition riparian zone: input of plant reproductive parts increases leaf breakdown process

&lt;p&gt;The dynamics of coarse particulate organic matter (CPOM) drives the functioning of most low order freshwater ecosystems. We evaluated plant litter input, litterfall, leaf litter breakdown rates, and the aquatic invertebrate community over the course of one year in a stream situation in a transition zone between savannah and Atlantic forest. Total organic matter input (litter fall) was 335 g m&lt;sup&gt;-2&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;, which was mainly composed of leaves (50%). Higher values are found in the transition from dry to rainy season (September and October), probably in response to water stress. The remaining leaf mass (65% on average) was lowest in May (49%) and highest in August (79%). CPOM productivity in this transition riparian zone is higher than in other savannah systems, but lower than in Atlantic forest. The higher leaf litter breakdown in May was likely accompanied by environmental enrichment due to increase in fruits and flowers (high energy resources). The coefficient of decomposition was classified as fast (k= -0.016) and showed a positive relationship with water flow (increase in physical abrasion). Collected invertebrates were classified as 6% shredders and 12% scrapers. The remaining leaf mass showed a negative relationship with scraper abundance, highlighting the importance of these trophic groups in tropical streams. We can conclude that climate factors (e.g., rainfall) directly affect the input and quality of CPOM and, consequently, leaf decomposition in savanna/Atlantic forest transition zones. &lt;/p&gt;

Winners and losers: tropical forest tree seedling survival across a West African forest-savanna transition.

Forest encroachment into savanna is occurring at an unprecedented rate across tropical Africa, leading to a loss of valuable savanna habitat. One of the first stages of forest encroachment is the establishment of tree seedlings at the forest–savanna transition. This study examines the demographic bottleneck in the seedlings of five species of tropical forest pioneer trees in a forest–savanna transition zone in West Africa. Five species of tropical pioneer forest tree seedlings were planted in savanna, mixed/transition, and forest vegetation types and grown for 12 months, during which time fire occurred in the area. We examined seedling survival rates, height, and stem diameter before and after fire; and seedling biomass and starch allocation patterns after fire. Seedling survival rates were significantly affected by fire, drought, and vegetation type. Seedlings that preferentially allocated more resources to increasing root and leaf starch (starch storage helps recovery from fire) survived better in savanna environments (frequently burnt), while seedlings that allocated more resources to growth and resource‐capture traits (height, the number of leaves, stem diameter, specific leaf area, specific root length, root‐to‐shoot ratio) survived better in mixed/transition and forest environments. Larger (taller with a greater stem diameter) seedlings survived burning better than smaller seedlings. However, larger seedlings survived better than smaller ones even in the absence of fire. Bombax buonopozense was the forest species that survived best in the savanna environment, likely as a result of increased access to light allowing greater investment in belowground starch storage capacity and therefore a greater ability to cope with fire. Synthesis: Forest pioneer tree species survived best through fire and drought in the savanna compared to the other two vegetation types. This was likely a result of the open‐canopied savanna providing greater access to light, thereby releasing seedlings from light limitation and enabling them to make and store more starch. Fire can be used as a management tool for controlling forest encroachment into savanna as it significantly affects seedling survival. However, if rainfall increases as a result of global change factors, encroachment may be more difficult to control as seedling survival ostensibly increases when the pressure of drought is lifted. We propose B. buonopozense as an indicator species for forest encroachment into savanna in West African forest–savanna transitions.