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International audience; Perennial grasses are the main source of fuel during fires in savannas. The demography of these grasses likely varies between species although they have the same general architecture and coexist in savannas. However, very few studies compare their demography. Similarly, their demography likely varies between years because of the variability in weather condition and fire intensity. We described and compared the demography and life-cycle of the four dominant perennial grass species (Andropogon canaliculatus, Andropogon schirensis, Hyparrhenia diplandra and Loudetia simplex) of the Lamto savanna (Ivory Coast) and assessed the influence of their demographic features (stasis, fecundity, growth, fragmentation and retrogression) on this demography. Grass species were monitored over three consecutive years (2015-2016, 2016-2017 and 2017-2018) on three 5 m × 10 m plots. We used a size-classified matrix model with 5 circumference classes (3-10 cm, 10-20 cm, 20-35 cm, 35-50 cm and >50 cm). Results showed differences in the age-based parameters, asymptotic growth rates (λs) and elasticities of the λs of the grass species. The population of A. canaliculatus, A. schirensis and H. diplandra were slowly declining while L. simplex was significantly declining. There were noticeable year-to-year variations in the demography of these four species. The most important demographic parameter influencing λs was the stasis in all species, while retrogression and fragmentation contributed to a relative homogeneity of ages between size-classes 2 to 5. This study provides new insights about the demography of Guinean savanna grasses that could be used to describe the mechanisms of their coexistence, and inform fire management policies.

Background: Biotic and abiotic disturbances such as frequent wildfires and herbivory contribute to maintain trees and grasses coexistence in savanna ecosystems. In comparison to stems and leaves, exposed to fire and herbivory, the roots, protected by being belowground, are less affected by these disturbances. Therefore, indirect estimation of belowground biomass (BGB) of savanna trees from simple allometric relations based on stem measurements can lead to major biases. Aims: In this study we explored how the Leaf ontogenetic change index (LOCI), a quantitative index based on leaf heteroblastic development, can provide an accurate estimate of BGB in Cussonia arborea, a widespread species in West African humid savannas. Methodology: We examined leaf morphometrics on post-fire resprouts of 40 individuals to assess whether LOCI can inform on plant age. We then analyzed by log-level regressions the variation of LOCI in relation to plant stem volume. Subsequently, we studied the variation of BGB according to stem volume, and as a function of both stem volume and LOCI, which allowed us to evaluate the contribution of LOCI to BGB estimation. BGB was obtained destructively by digging up roots and weighing total dry mass of 25 individuals including small and large trees. Statistical analyses were done with the R software. Place and Duration of Study: Study was performed in the Lamto Scientific Reserve, Côte d’Ivoire, between May 2020 and June 2021. Results: Using the stem volume as single explanatory variable of BGB, the regression model provided an adjusted R2 of 0.71. Association of the stem volume with LOCI increased the adjusted R2 from 0.71 to 0.90. Conclusion: Combining LOCI with a measure of stem size provides better estimate of BGB in C. arborea compared to estimate based on stem size only. Since a large proportion of woody species in frequently disturbed environments exhibit an overall strategies promoting persistence, future works should evaluate how these strategies are modulated during ontogeny and can explain biomass variation over time.

Biomass burning has become more frequent and widespread worldwide, with a significant proportion occurring in tropical Africa. Fire dynamics have been generally studied at global or regional scales. At local scale, however, fire impacts can be severe or catastrophic, suggesting local analyses are warranted. This study aimed to characterise the spatio-temporal variations of vegetation fires and identify the main fire hotspots in Côte d’Ivoire, a country of West Africa, one of the world’s burn centres. Using MODIS-derived fire data over a 10-year period (2007–2016), the number of fire days, active fires and fire density were assessed across the entire country. In the southern part dominated by forests, fire activity was low. Three main fire hotspots were identified between 2°30’–8°30’W and 7°00’–10°30’N in the North-West, North-East and Central areas all dominated by savannas. In these areas, Bafing, Bounkani and Hambol regions recorded the highest fire activity where fire density was 0.4±0.02, 0.28±0.02 and 0.18±0.01 fires/km²/year, respectively. At national scale, the annual fire period stretched from October to April with 91% of fires occurring between December and February, with a peak in January. Over the decade, there was a decreasing trend of fire activity. Fire density also was negatively correlated with rainfall >1000 mm for the synchronic analysis, whereas fire density was positively correlated with rainfall in the previous years. Results suggest that the positive relationship between the previous year’s rainfall and fire activity could operate on a cycle from 1 to 4 years. Significance: Three fire hotspots were found primarily in savanna vegetation, which burns more regularly than forestdominated vegetation. The fire season occurs over 7 months, the majority of active fires (91%) occurring in just 3 months (December-January-February) with a peak in January (39%). Fire activity has declined over the past decade with a return time of above-average fires from 1 to 4 years. Fire density is positively correlated to the amount of rainfall in preceding years, whereas fire density and rainfall of the same year were negatively correlated in the region of rainfall >1000 mm.

Prescribed yearly mid-dry season fire is generally used in the Guinean savanna (a humid savanna of West Africa) as a vegetation management tool. Despite this fire, bush encroachment can be observed due to climate change and high sapling recovery rates with increasing CO2 emissions. There is therefore a need to analyze other fire seasons. We analyzed fire behavior during a 5-year field fire in the Guinean savanna of Lamto reserve in Ivory Coast. Early (EDS), mid (MDS), and late (LDS) dry season fires were tested. Nine 0.5 ha plots were burnt annually to determine the rate of spread, fire intensity and the residence time above 60 °C. Fuel characteristics and weather conditions were measured to assess their impact on fire behavior. Recruitment from resprout to adult tree stage were assessed for an estimation of these fires’ severity. Understory grass height, fuel load, and moisture content had greater values in EDS than in MDS and LDS. The rate of spread and intensity of both MDS and of LDS were greater than those of EDS fires. The best predictors for fire behavior were fuel moisture content and air humidity. In dry conditions, fires were faster and more intense. With climate change predicting increasingly longer and drier periods, we expect more and more intense fires. A longer time of exposure to lethal temperatures (>60 °C) and the phenological state of trees in the late dry season explains the higher severity of this fire (there was no recruitment to adult stage under LDS fire). In some years, LDS helped to reduce bush encroachment in the subject region. This data provides important insights into fire behavior and its severity in the Guinean savanna, informing fire management policies and procedures.

To maintain savanna vegetation, mid-seasonal fire has been applied since 1961 in the Lamto Savanna (Côte d'Ivoire). However, this prescribed fire has not impeded tree encroachment during recent years, nor have its effects on insect assemblages been documented. Also the impact of tree intrusion on insect assemblages is poorly studied in savanna. To prevent tree density increasing, a change in fire regime might be a solution. In this study, we examined the effect of different fire regimes (early, mid-seasonal and late fires) on leaf-litter ant assemblages in order to suggest appropriate measures for preventing tree invasion without having an effect on insect communities. Sampling was implemented by combining pitfall trapping and leaf-litter sampling before and after three different fire regimes, early, mid-seasonal and late fires. While the ant species richness declined after the passage of early and mid-seasonal fires, significantly more species were found in the burnt savanna after the late fire. However, the losses or gains of species due to different fire regimes did not cause severe changes in the ant species composition. Of the functional groups identified, only the generalists and specialist predators were respectively strongly affected by the early and mid-seasonal fires, certainly due to micro-habitat modification. Based on the trends observed in the present study, we suggest sampling other invertebrate fauna in similar savanna plots to find out if other insect groups have similar reactions to the applied fire regimes.

The objective of the study conducted in the Lamto Guinean savannah situated at 165 km northwest of Abidjan, Cote d'Ivoire consisted to assess the changes in soil mite abundance, diversity and community structure specific to the second fire cycle applied in 2015, as well as the inter-annual variation between the two fire cycles (2014 and 2015). Three study sites (Salty marigot, Plateau and North piste) were selected in shrub savannah, where on each, three adjacent stands of 100 m x 50 m formerly delimited were considered. The three fire regimes (early, mid-season, and late fire) were respectively applied on the three sites and stands. Thus, 135 soil cores (5 soil cores x 3 sampling periods x 3 fire regimes or stands x 3 sites) were used for mite extraction. 108 soil cores were taken at two upper layers (0-5 and 5-10 cm) for determination of the bulk density and water content. Whatever the fire regimes, the mean density of soil mites decreased after the fire application. The highest value of density was observed through the early fire (1,715 ± 327 ind.m-2) whereas the lowest value was recorded during the mid season fire (1,433 ± 153 ind.m-2). 41 species had been recorded along the three fire regimes and distributed as follows: early fire 34 species, mid season fire 20 species, and late fire 13 species. The mean species richness of soil mites changed significantly across the fire regimes, and reduced after the fire application, except for the mid season fire. The Simpson diversity index was significantly modified across the fire regimes, and increased after the fire application. Beyond to 24 specialist species, over 50% of the species observed before the burns were rediscovery after the fire application, and could explain this variation. The inter-annual variation of soil mites showed that the density (early fire, mid season fire, and late fire), mite richness (early fire), and diversity (early fire and late fire) increased whereas the mite richness (mid season fire and late fire), and diversity (mid season fire) decreased, respectively, during 2015-burn compared to the previous cycle (2014-burn). The rebound of soil mite parameters during the second fire cycle could be assigned (i) to litter and woody debris, which burn in a mosaic, reflecting local fire intensity, (ii) improving of stand complexity and canopy structure, and (iii) fire tolerance of mites.

SUMMARYBiomass burning has links with a number of global concerns including soil health, food security and climate change. In central Côte d'Ivoire (West Africa), we conducted a field study to compare nutrient losses, soil fertility and yam yield in slash-and-burn versus slash-and-mulch agriculture. Trials involved five sites established in the dominant Chromolaena odorata fallows of the region, each consisting of paired plots: slash and burnt biomass (SB) versus slashed and unburnt biomass, but left to serve as mulch (SM). Carbon and five elemental nutrients were assessed in the aboveground biomass prior to burning and in ash after fires; losses were assessed by subtraction. The greatest proportions of loss occurred with C (95%), N (95%) and K (74%), corresponding to losses into the atmosphere of 3532 ± 408, 200 ± 36, 132 ± 36 kg ha−1. Six weeks after the fire, soil properties were assessed: soil organic C, total N and Mg2+ were higher in SM than in SB sites. At final harvest, yam tuber yield was twice as large in SM as in SB (18 ± 4 vs. 9 ± 2 Mg ha−1) with soil C, total N and K+ as the main influential soil parameters. The key finding was that the elements lost in greatest proportion during burning were those mostly influencing yam yields. Because a clear negative relationship between biomass burning and yam production has been established the promotion of the more productive, alternate slash-and-mulch system compared to slash-and-burn system, is warranted. The findings of our research can be used in support of developing a sustainable yam production system in the region and in West Africa more generally.

Questions Perennial tussock grasses represent the principal fuel source for savanna fires; however, basic information about the impact of fire on their demography remains scarce. Do dominant perennial grass species differ in their demographic parameters? What is the overall impact of tussock circumference and fire regimes on grass demographic parameters? Do grass species differ in the sensitivity of their demographic parameters to fire regimes and tussock circumference? Location The study site is located in the savanna of the Lamto reserve, in the center of Ivory Coast (6 degrees 9 ' to 6 degrees 18 ' N, 5 degrees 15 ' to 4 degrees 57 ' E). Methods Data on the nine dominant species of tussock grasses were sampled over one year under four fire treatments (early fire, mid-season fire, late fire and no fire). We studied the impact of these fire treatments and tussock circumference on five demographic parameters: mortality, fragmentation (the division of a tussock into several smaller ones), growth, retrogression (the decrease in tussock size) and fecundity. Results All demographic parameters varied among species and across the fire regimes. Late fire had the largest negative effect on all parameters except fecundity. Schizachyrium platyphyllum was the most disadvantaged species by fire in terms of mortality, retrogression and growth, whilst Andropogon ascinodis was the most prone to fragmentation. Hyparrhenia smithiana and Sorghastrum bipennatum produced the most seeds and Brachiaria brachylopha produced the most new tussocks. The circumference of perennial grasses impacted all demographic parameters. Small individuals had the highest mortality and growth rates, while large individuals showed the highest fragmentation and retrogression rates, and produced the most seeds. Conclusions This study showed that perennial grass species differed in their sensitivity to fire treatments. Nevertheless, the mid-season fire could be advised to managers for the sustainable management of the Lamto savanna.

This study assesses the impact of four fire treatments applied yearly over 3 y, i.e. early fire, mid-season fire, late fire and no fire treatments, on the grass communities of Lamto savanna, Ivory Coast. We describe communities of perennial tussock grasses on three replicated 5 × 5-m or 10 × 5-m plots of each fire treatment. Tussock density did not vary with fire treatment. The relative abundance of grass species, the circumference of grass tussocks and the probability of having a tussock with a central die-back, varied with fire treatment. Mid-season fire had the highest proportion of tussocks with a central die-back while the late fire had the smallest tussocks. Tussock density, circumference, relative abundance and probability of having a central die-back varied with species.Andropogon canaliculatusandHyparrhenia diplandrawere the most abundant of the nine grass species. They had the largest tussocks and the highest proportion of tussock with a central die-back.Loudetia simplexwas the third most abundant species but was very rare in no fire plots. The distribution of tussock circumferences was right skewed and dominated by small tussocks. The proportion of the tussocks with a central die-back strongly increased with circumference, which could lead to tussock fragmentation. Taken together, this study suggests that fire regimes impact grass demography and that this impact depends on grass species and tussock size.

La mesure directe de l’intensite du feu n’etant pas toujours possible car fastidieuse et couteuse, les indices post-feu substituent celle-ci et permettent de mieux comprendre les caracteristiques du feu et son impact sur la vegetation. Cette etude a ete menee dans la reserve de Lamto (Cote d’Ivoire) dans le but d’identifier les indices post-feu qui sont de bons indicateurs de l’intensite du feu. Trois regimes de feu ont ete experimentes de 2013 a 2017: le feu precoce, le feu de mi-saison, et le feu tardif. L’intensite du feu a ete determinee durant chaque feu. Ensuite, les indices post-feux que sont : la hauteur des flammes, l’efficacite de combustion, le volume d’eau evaporee, et les fractions de cendre fine et grossiere ont ete mesures, et la surface couverte par le feu estimee. La hauteur des flammes est le meilleur indicateur de l’intensite du feu. Plus le feu est intense plus la zone d’action des flammes est haute, et plus il est susceptible de reduire le recrutement des jeunes ligneux. Cette etude propose en savane Guineenne un indice facile a mesurer, qui permettrait aux gestionnaires des aires protegees en savane de mieux evaluer l’intensite du feu et son impact sur la vegetation. Mots cles : Hauteur des flammes, indices post-feu, intensite du feu, savane de Lamto, regime de feu.

International audience; African humid savannas are highly productive ecosystems, despite very low soil fertility, where grasses and trees coexist. Earlier results showed that some perennial grass species are capable of biological nitrification inhibition (BNI) while trees likely influence differently on nitrogen cycling. Here we assessed the impact of the dominant grass and tree species of the Lamto savanna (Ivory Coast) on soil nitrifying and denitrifying enzyme activities (NEA and DEA, respectively) and on the abundances of archaeal and bacterial ammonia oxidizers (AOA and AOB, respectively) and nitrite reducers. This is one of the first studies linking nitrifying and denitrifying activities and the abundances of the involved groups of microorganisms in savanna soils. NEA was 72-times lower under grasses than under trees while AOA and AOB abundances were 34- and 3-times lower. This strongly suggests that all dominant grasses inhibit nitrification while trees stimulate nitrification, and that archaea are probably more involved in nitrification than bacteria in this savanna. While nitrite reducer abundances were similar between locations and dominated by nirS genes, DEA was 9-times lower under grasses than trees, which is likely explained by BNI decreasing nitrate availability under grasses. The nirS dominance could be due to the ferruginous characteristics of these soils as nirS and nirK genes require different metallic co-enzymes (Fe or Cu). Our results show that the coexistence of grasses and trees in this savanna creates a strong heterogeneity in soil nitrogen cycling that must be considered to understand savanna dynamics and functioning. These results will have to be taken into account to predict the feedbacks between climate changes, nitrogen cycling and tree/grass dynamics at a time when savannas face worldwide threats.

International audience; The feeding preferences of termites in a Guinean savanna (Lamto, Côte d'Ivoire) were studied in the field to better understand the incidence of termite damages observed on adult trees, which, on interaction with fire, affect the dynamics of the tree community. Two separate multiple-choice test experiments were conducted on the six dominant tree species, aiming at determining preferences for the species and for wood conditions naturally encountered and likely to affect termite consumption behaviour. The preferences of termites were determined by their occurrence on standard-sized wood pieces and their behaviour during the first attack. Two fungus-growing termites, Ancis-trotermes and Microtermes were frequently encountered on wood pieces, whereas wood feeders were absent. The two genera have different preferences, and they do not compete for the same food. Wood density and water content did not explain termite preference. Holes in the bark seem to facilitate termite entry into pieces of wood, whereas the action of fire on wood apparently makes it less attractive to termites. In the field, the decision by termites to forage on a given tree species seems to be more driven by habitat and accessibility differences than real food quality differences.

Savanna trees often display significant hollows due to the combined action of fire and termites (N'Dri et al., J Trop Ecol 27:269-278, 2011). Observations have shown that internal cavities caused by termites in tree stems often result in external hollows under annual fire regimes, and it is quite possible that such hollows/fire interaction may increase the probability of trunk or big branch breakage and/or tree mortality. A study of individual tree and branch mortality as a function of prior termite and fire damage was undertaken in a West African savanna (Lamto, Cte d'Ivoire) where most of the trees naturally have hollows in their stems due to termite and fire interaction. Our goal was to examine the dynamics of hollowing and to determine whether hollowing significantly affected tree mortality. Branch and whole plant mortality were quantified for dominant tree species according to their initial hollow state and height. Four different responses were obtained depending on tree species: (1) mortality increased with cavity severity and tree size (Piliostigma thonningii), (2) mortality depended on tree size only (Bridelia ferruginea), (3) no mortality even after being hollowed by termites and externally damaged by fire (Crossopteryx febrifuga, the species with the highest proportion of individuals with hollows yet the greatest background survival time, 14 +/- A 2 years) and (4) high mortality, but few hollow trees suggesting a weak resistance to hollowing (Cussonia arborea which was insensitive to all the factors examined in this study). For species resistant to hollowing, tree mortality was rare; alternatively, for species prone to hollowing, whole trees died quickly and before the most severe hollow classes could be observed. Long-term demographic data yielded population-level mortality estimates of adult trees at least four times lower in fire-exclusion zones than that in fire-prone areas. Because hollow dynamics interact with fire in affecting adult mortality of some dominant tree species, fire management is important for a sustainable woody component of these savannas.