Typical Savanna Research Articles

Fire Occurrence Mediates Small-Mammal Seed Removal of Native Tree Species in a Neotropical Savanna

Seed dispersal and predation are critical processes for plant recruitment which can be affected by fire events. We investigated community composition of small mammals in gallery forests with distinct burning histories (burned or not burned ∼3 years before) in the Cerrado (neotropical savanna). We evaluated the role of these animals as seed removers of six native tree species, potentially mediated by the occurrence of fire. We sampled four previously burned sites and four unburned ones. Seed removal was assessed using two exclusion treatments: exclusive access of small rodents and access of all seed-removing vertebrates. The previous burning changed the structural characteristics of the forests, increasing the density of the understory vegetation and herbaceous cover, which determined differences in species composition, richness, and abundance of small rodents (abundance in the burned forests was 1/6 of the abundance in the unburnt ones). Seed removal rates across the six species were reduced in burnt forests in both treatments and were higher for the “all vertebrates” treatment. Other vertebrates, larger than small rodents, played a significant role as seed removers for five of the six species. The effects of fire were consistent across species, but for the two species with the largest seeds (Hymenaea courbaril and Mauritia flexuosa) removal rates for both treatments were extremely low in the burned forests (≦5%). The observed decline in small rodent seed predation in the burned forests may have medium to long-term consequences on plant communities in gallery forests, potentially affecting community composition and species coexistence in these forests. Moreover, fire caused a sharp decline in seed removal by large mammals, indicating that the maintenance of dispersal services provided by these mammals (mainly the agouti Dasyprota azarae) for the large-seeded species may be jeopardized by the burning of gallery forests. This burning would also affect several small mammal species that occur in the surrounding typical savanna habitats but also use these forests. Fire events have been increasing in frequency and intensity because of human activities and climate changing. This current scenario poses a serious threat considering that these forests are fire-sensitive ecosystems within the Cerrado.

Priority areas for conservation of primates in a threatened Amazonian savanna

Primates are globally recognized as an important component of biodiversity, however, more than half of primate species in the world are threatened and agriculture expansion is one of the main threats. Brazil has one of the largest networks of protected areas (PAs) in the world, but there are some conservation gaps, such as the Amazonian savannas. We aim to identify a network of priority areas to conserve a Brazilian Amazonian savanna highly threatened by agriculture expansion, by using seven primate species and four vegetation types as targets. We constructed species distribution models (SDMs) for the primates and used a Systematic Conservation Planning approach. We defined as a quantitative conservation target the proportion of the distribution of each primate species within the network according to traits related to their vulnerability to extinction. In addition, we set a target of including at least 30% of each savanna type within priority areas. We created a map of potential use of the land for agriculture and another of environmental risk, which were included as costs in the decision process, and together with the SDMs and vegetation types, identified the network of priority areas by using the software Marxan. We evaluated the feasibility of implementing conservation actions, such as establishing Conservation Units (e.g. reserves managed by the government), or implementing community-based conservation actions in each priority area. Additionally, we estimated the economic investment (US$/year) required to establish Conservation Units across the priority areas. Conservation targets for primates and vegetation types were met by protecting 3,240 km2 of the Savannas of Amapá. An investment of approximately US$958,122/year over five years is required to turn these priority areas into Conservation Units, however, we propose other strategies such as conservation on community lands and public policies. All these strategies would allow for protecting forest cover and the heterogeneous environments that are suitable for primates and other biodiversity components.

Methane gas emissions from savanna fires: what analysis of local burning regimes in a working West African landscape tell us

Abstract. Savanna fires contribute significantly to greenhouse gas emissions. While it is recognized that these fires play a critical role in the global methane cycle, there are too few accurate estimates of emissions from West Africa, the continent's most active fire region. Most estimates of methane emissions contain high levels of uncertainty as they are based on generalizations of diverse landscapes that are burned by complex fire regimes. To improve estimates we used an approach grounded in the burning practices of people who set fires to working landscapes. We collected and analyzed smoke samples for 36 experimental fires using a canister method for the early dry season (EDS) and mid-dry season (MDS). We also collected data for savanna type, grass type, biomass composition and amount consumed, scorch height, speed of fire front, fire type, and ambient air conditions for two sites in Mali. We report values for fire intensity, combustion completeness, patchiness, modified combustion efficiency (MCE), emission factor (EF) and methane emission density. Our study found that mean methane EFs ranged from 3.83 g kg−1 in the EDS to 3.18 g kg−1 in the MDS, but the small sample did not provide enough power for this effect to be significant. We found head fires had nearly double the CH4 EF of backfires (5.12 g kg−1 to 2.74), a significant difference. Byram's fire intensity was a significant driver of CH4 EF but with weak effect. Methane emission density increased marginally from 0.839 g m−2 in the EDS to 0.875 g m−2 in the MDS, a difference that was not significant. Head fires, however, had much higher emission densities than backfires – 1.203 vs. 0.708 g m−2 – respectively, a significant difference. We suggest the reason for the higher methane emissions from head fires, which have higher intensity, is the longer flame lengths that burn green leaves on trees, releasing methane. We conclude that policies aimed at shifting the burning regime earlier to reduce methane emissions will not have the desired effects, especially if fire type is not considered. Future research should consider the state and amount of leafy biomass combusted in savanna fires.

Seasonality in aerodynamic resistance across a range of North American ecosystems

Surface roughness – a key control on land-atmosphere exchanges of heat and momentum – differs between dormant and growing seasons. However, how surface roughness shifts seasonally at fine time scales (e.g., days) in response to changing canopy conditions is not well understood. This study: (1) explores how aerodynamic resistance changes seasonally; (2) investigates what drives these seasonal shifts, including the role of vegetation phenology; and (3) quantifies the importance of including seasonal changes of aerodynamic resistance in “big leaf” models of sensible heat flux (H). We evaluated aerodynamic resistance and surface roughness lengths for momentum (z0m) and heat (z0h) using the kB−1 parameter (ln(z0m/z0h)). We used AmeriFlux data to obtain surface-roughness estimates, and PhenoCam greenness data for phenology. This analysis included 23 sites and ∼190 site years from deciduous broadleaf, evergreen needleleaf, woody savanna, cropland, grassland, and shrubland plant-functional types (PFTs). Results indicated clear seasonal patterns in aerodynamic resistance to sensible heat transfer (Rah). This seasonality tracked PhenoCam-derived start-of-season green-up transitions in PFTs displaying the most significant seasonal changes in canopy structure, with Rah decreasing near green-up transitions. Conversely, in woody savanna sites and evergreen needleleaf forests, patterns in Rah were not linked to green-up. Our findings highlight that decreases in kB−1 are an important control over Rah, explaining > 50% of seasonal variation in Rah across most sites. Decreases in kB−1 during green-up are likely caused by increasing z0h in response to higher leaf area index. Accounting for seasonal variation in kB−1 is key for predicting H as well; assuming kB−1 to be constant resulted in significant biases that also exhibited strong seasonal patterns. Overall, we found that aerodynamic resistance can be sensitive to phenology in ecosystems having strong seasonality in leaf area, and this linkage is critical for understanding land-atmosphere interactions at seasonal time scales.

Functional traits as indicators of ecological strategies of savanna woody species under contrasting substrate conditions

Environmental filters and functional adjustments of species to local conditions are major determinants of plant communities in stressing habitats. We set out to understand the role of different substrates in functional trait variation in wood plants by investigating key functional traits at the community and intraspecific levels in different types of Brazilian savanna: two sites of alluvial savannas (lowland savannas that occur in old alluvial deposits in riverbeds with sandy soils and low water-holding capacity), and two sites of typical savannas (upland savannas that occur in interfluvial zones, far from the influence of rivers, in which clay soils with higher fertility and water-holding capacity predominate). We conducted this study in four sites with savanna vegetation in the northeast region of the Cerrado Biome, nearby the transition with the Amazon. In each site we sampled six functional traits for each tree species: leaf area, leaf thickness, specific leaf area (SLA), stem-specific density, relative bark thickness and maximum plant height. We sampled 101 wood species, ranging from 26 to 62 species within each site. In both community and intraspecific levels, alluvial savanna plants showed higher SLA and specific-stem density, and lower leaf area, leaf thickness, bark thickness, and plant height compared with the typical savanna. We also found that, at a given leaf thickness, species from typical savanna exhibited ∼20% higher SLA. In addition, at a given plant height, typical species showed ∼56% higher bark thickness and ∼20% lower specific-stem density. Our findings suggest that savanna tree species growing on alluvial savannas tend to adopt strategies related to water acquisition while species growing on typical savannas invest in both water conservation strategies and protection against high-intensity fires. We conclude that water deficit and fire, both related to substrate properties, play a key role in determining the woody plant communities functioning in savannas.

Facilitation by isolated trees triggers woody encroachment and a biome shift at the savanna–forest transition

Abstract Woody encroachment into grassy biomes is a global phenomenon, often resulting in a nearly complete turnover of species, with savanna specialists being replaced by forest‐adapted species. Understanding the mechanisms involved in this change is important for devising strategies for managing savannas. We examined how isolated trees favour woody encroachment and species turnover by overcoming dispersal limitation and environmental filtering. In a savanna released from fire in south‐eastern Brazil (Cerrado), we sampled woody plants establishing under 40 tree canopies and in paired treeless plots. These trees comprised eight species selected for habitat preference (savanna or forest) and dispersal syndrome (bird dispersed or not). We recorded dimensions of each tree, dispersal syndrome and habitat preference of recruits, and quantified the physical environment within each plot, aiming at a mechanistic understanding of woody encroachment. We found clear evidence that isolated trees cause nucleation and drive changes in functional composition of savanna. Effectiveness as nucleator differed among species, but was unrelated to their functional guilds (habitat preference or dispersal syndrome). The density of saplings in nuclei was partially explained by soil moisture (+), daily temperature amplitude (−) and sum of bases (−). Our results indicate that isolated trees act first as perches, strongly favouring bird‐dispersed species. They then act as nurse trees, considerably changing the environment in favour of forest‐adapted recruits. In the long term, as the nuclei expand and merge, savanna specialists tend to disappear and the savanna turns into a low‐diversity forest. Synthesis and applications. Fire suppression has allowed the nucleation process and consequently the woody encroachment and fast replacement of savanna specialists by forest species in the Cerrado. By elucidating the mechanisms behind woody encroachment, we recommend using prescribed fires to burn forest seedlings and to reduce tree canopy size wherever the management goal is to maintain the typical savanna structure and composition.

Ophidiophobia, myth generation, and human perceptions: Implications for snake conservation in a typical savanna community of northern Ghana

ABSTRACT Snakes have fascinated and terrified humans throughout history. Worldwide, innate fear (ophidiophobia), culturally-founded superstition, and myths have caused pervasive snake persecution, snakebite mismanagement, human injuries, and fatalities, particularly in the tropics. We analyzed 20 common snake myths narrated by 934 respondents inhabiting a typical rural savanna community of northern Ghana. The myths summarized perceived, self-assessed knowledge about snakes and were evaluated in their zoo-ecological contexts versus their folkloristic explanatory origins. Only eight snake myths (~40%) had any justifiable scientific basis, partially representing misinterpretations among predominantly male, less-educated respondents. Contrastingly, 70% of the myths were largely rooted in ophidiophobia, representing a major driver of human-wildlife conflict and indiscriminate snake persecution. To promote wildlife-friendly perceptions and behavior toward snakes and their conservation, we recommend innovative gap-bridging conservation education and public awareness that reconciles myths and realities about snakes, thus reducing snakebite incidences, mortality, and widespread persecution and killing of snakes.

Thirty years of clear-cutting maintain diversity and functional composition of woody-encroached Neotropical savannas

Humans have changed vegetation dynamics in Neotropical savannas by suppressing fires, allowing trees and shrubs to expand into ancient savanna landscapes in a process known as woody encroachment. This woody encroachment drives the loss of biodiversity and modifies the functioning of savanna ecosystems. Here we combine satellite data analysis with an experimental approach to test the hypothesis that long-term management by clear-cutting helps restore the diversity and functional composition of open savannas. First, we used Landsat time series of the Normalized Difference Water Index, to assess changes in vegetation structure, comparing experimental areas with open savannas in the same region. We then obtained field experimental evidence comparing areas managed during 30 years versus unmanaged areas, including data on vegetation structure and composition. Our results from satellite image analyses indicate that, before the first clear-cutting, vegetation structure was similar in managed and unmanaged sites, and both differed from open savanna. When clear-cutting manipulation started, NDWI of managed areas became persistently lower than that of unmanaged control areas. In the field, we found that in managed areas, species diversity and richness of typical savanna species had increased, and that species composition had changed to become more similar to open savannas. We also observed the recovery of savanna functional composition, suggesting that ecosystem processes were restored by clear-cutting management. Our findings reveal that the repeated removal of dominant woody species by clear-cutting has contributed to maintain the diversity and functioning of savannas degraded by forest encroachment.

The type of substrate does not influence the intraspecific population structure of shrub‐tree species in the Brazilian savanna

AbstractLocal environmental conditions influence the occurrence and abundance of populations, and therefore, the contribution of the same species to the ecological structure and processes of communities can vary in different environments. We investigated the influence of the substrate on the intraspecific population structure of four shrub‐tree species from the Brazilian savanna (Aspidosperma tomentosum, Byrsonima pachyphylla, Davilla elliptica, and Ouratea hexasperma) that occur in adjacent areas, but under different environmental conditions: in shallow soils with rocky outcrops and steep relief in Rupestrian Savanna, and in deep soils and flat relief in Typical savanna. We hypothesized that the relief and physical–chemical conditions of the soil in the Rupestrian Savanna limit the establishment and development of species regarding the abundance of individuals and the horizontal and vertical structure of populations. We found that in the Rupestrian Savanna, the soil has higher levels of total cation exchange capacity and phosphorus, while in Typical Savanna, it is associated with higher pH, base saturation, and clay levels. We show that intraspecific variations do not differ between the two environments in number of individuals, average values of trunk diameter, and total plant height. We observed that the same species contributes in different ways to the structuring of communities in both environments. Our hypothesis was rejected because the population structure was not molded by the substrate type. We conclude that the shrub‐tree species seem to find adaptive alternatives to colonize the different types of substrates, for example, establishing in less stressful microhabitats on the rocky environments.

Spatial heterogeneity in nitrification and soil exploration by trees favour source–sink dynamics in a humid savanna: A modelling approach

Abstract Savannas are structured ecosystems characterized by a grass layer interspersed with trees. Trees strongly modify their local environment and favour nutrient accumulation under their canopies. Tree roots can also forage horizontally far beyond the canopy projection to increase nutrient uptake. In the Lamto savanna (Côte d’Ivoire), grasses are able to inhibit nitrification while trees stimulate it. Here, we used a two‐patch model simulating nitrogen (N) dynamics in a humid savanna between an open patch (without tree) associated with a low nitrification rate and a patch of tree clump associated with a high nitrification rate. The model also includes horizontal N fluxes between these two patches corresponding to horizontal soil exploration by tree roots. We analysed the impact of spatial heterogeneity in nitrification and soil horizontal exploration on N budget and plant biomass. Despite high N losses under trees due to nitrification stimulation by trees, our results show that the ability of trees to explore horizontally the open allows them to uptake more nutrients in total. This leads to an asymmetric N flux from the open to tree clumps, which contributes to nutrient enrichment under tree clumps and thereby to tree growth. Although trees have the ability to horizontally explore the soil to accumulate nutrients under their canopy, increasing the surface occupied by tree clumps increases N losses per hectare of savanna due to the increased nitrification under trees and the subsequent increase in leaching. While perennial savanna grasses show a restricted horizontal soil exploration to control nutrient availability, our results predict that the extension of tree roots outside their canopy increases their nutrient acquisition in the Lamto savanna. This study is the first one emphasizing the influence of horizontal exploration of trees and tree cover on savanna N budget and functioning. Overall, the proportion of tree cover and horizontal soil exploration are important factors to consider in savannas characterized by spatial heterogeneity in N cycling created by trees and grasses. These factors appear critical to the functioning of West African humid savannas and should be investigated in other savanna types. A free Plain Language Summary can be found within the Supporting Information of this article.

Small rodent communities (Muridae) in Gabonese savannas: species diversity and biogeographical affinities

Abstract The study contributes to the knowledge of species composition and biogeographical affinities of savannas rodent in Gabon. Unlike small rodents in Gabonese forests, there is little data on the diversity of small rodents in Gabonese savannas. The diversity and distribution of rodent murid communities was studied in four different types of savanna in Gabon: Coastal Basin (South-West), Lopé/Okanda (in the Center), Batéké Plateaux (Southeastern) and Ngougnié/ N’yanga (in the South). A total of 428 individuals representing six species were captured over 11,920 trap nights. Trap success was highly variable (2.2–6.9 %). The most abundant species were Mus minutoides (69%) followed by Lemniscomys striatus (21.5%). Indices of species richness varied from 2 to 5 and diversity (Shannon and Weaver) was low in the four savannas with the highest value at Ngougnié/N’yanga (H′ = 1.2). Species distributions show that Gabonese savanna small rodents conform to four distribution types, with one species known from Zambesian savannas exhibiting austral affinities (Pelomys campanae: occurs in three southern savannas). This new information provides important insight into the biogeography of small rodents at a local and regional level. Moreover, the correspondence analysis highlighted an influence of local ecological factors on population abundance.

Species composition and community structure of dung beetles (Coleoptera: Scarabaeidae: Scarabaeinae) compared among savanna and forest formations in the southwestern Brazilian Cerrado

Although dung beetles are important members of ecological communities and indicators of ecosystem quality, species diversity, and how it varies over space and habitat types, remains poorly understood in the Brazilian Cerrado. We compared dung beetle communities among plant formations in the Serra Azul State Park (SASP) in the state of Mato Grosso, Brazil. Sampling (by baited pitfall and flight-interception traps) was carried out in 2012 in the Park in four habitat types: two different savanna formations (typical and open) and two forest formations (seasonally deciduous and gallery). A total of 5,400 individuals collected comprised 57 species in 22 genera. Typical savanna had the greatest species richness and abundance, followed by open savanna and deciduous forest, while the gallery forest had the fewest species but high abundance. Tunnelers (one of three main nesting behavior guilds) showed the greatest richness and abundance (except in the gallery forest, where one dweller species was extremely abundant) in all plant formations. We found that species richness and abundance of the dung beetle community are influenced by differences among plant formations. Habitat heterogeneity in the different plant formations along with anthropic influences (fire, habitat fragmentation) are cited as important factors that explain guild and species richness and distribution patterns. These results emphasize the importance of protected areas, such as SASP, for the maintenance and conservation of species diversity in the Brazilian Cerrado.

Optimal plot size for carbon-diversity sampling in tropical vegetation

Obtaining high-quality information on vegetation attributes directly linked to ecosystem services in tropical forests, such as carbon stock and biodiversity, is essential to develop ecological information to be used in planning conservation strategies. Defining the optimum vegetation sample plots size is an important part of the process of obtaining information, since is crucial for its quality, and also for its financial costs and time required. In this context, we evaluated whether: (i) the optimum plot size selected in two approaches (basal area/carbon stock vs biodiversity) are different since they are associated with different ecological drivers; (ii) different tropical forests vegetation types present different optimum sample plot size due to their ecological specific attributes, such as species composition, structure and functioning. For this we evaluated the vegetation sample plots optimum size of different vegetation types of tropical savanna and forest of broad occurrence in the tropics (Atlantic Semideciduous Forest – ASF; Cerrado Woodland – CW; Cerrado Forest – CF; and Seasonally Dry Tropical Forest - SDTF), in two approaches: forest basal area and carbon stock, since they are related variables; and biodiversity variables by species composition similarity and heterogeneity in communities. We used 4 forest fragments in Brazil that were sampled in a way that allowed the arrange of subplots to form different plot sizes. In order to evaluate the optimum sample plot size in the basal area/carbon stock approach we used four methodologies, in which the obtained variability was compared to a reference size (1000 m2). For the biodiversity approach we used the floristic similarity and composition heterogeneity between units as metric, evaluating differences in the similarity patterns and possible reductions or increases in variability compared to the reference size. We found there are no significant differences for biodiversity variables between sample plot sizes for all vegetation types, thus the optimum size may be selected using just the basal area/carbon stock approach. In the basal area/carbon stock approach, most of the evaluated methods generally presented lower plot size values compared to the reference size (1000 m2) in all vegetation types, except for the ASF. There are differences between vegetation types that may be grouped in two groups related to the optimum size selected, that are related to their ecological restriction patterns: CW, CF and SDTF with optimum size between 600 and 1000 m2; and the second with ASF, with optimum sample plot size between 1000 m2 and 1400 m2.

Grass competition is more important than fire for suppressing encroachment of Acacia sieberiana seedlings

Understanding the factors controlling tree seedling recruitment is an integral research priority for savanna ecosystems, particularly for the management of woody plant encroachment. We investigated the effects of increased nutrients, fire and grazing on the survival, growth and biomass of Acacia sieberiana using a completely randomized experimental design in the greenhouse. We simulated a typical savanna microhabitat by planting a common savanna grass species, Eragrostis curvula, with seedlings of A. sieberiana. The trees and grasses were allowed to establish for 12 weeks. Thereafter, treatments of fertilizer (100 kg N/ha), simulated grazing and fire were applied. Weekly measurements of relative growth rate (RGRheight and RGRdiameter) were recorded. At the end of 24 weeks, survival, total above- and below-ground biomass, and key functional traits of the trees (specific leaf area and average thorn length) were recorded to assess differences in tree performance. We found that fertilization was beneficial to the grass only, increasing their total biomass. While, the direct removal of grass biomass by grazing in our study revealed no main (statistical) effect on tree seedling growth, we observed decreased survival of tree seedlings with higher grass biomass. Fire increased the tree seedlings’ investment in shoot growth and stem diameter. However, in the absence of grass competition, there were no differences in the post-fire allocation of the resprouting tree seedlings. Higher grass biomass was found to result in decreased specific leaf area of tree seedlings. We conclude that grass competition is important for controlling the encroachment of A. sieberiana.

Water infiltration in the Brazilian tropical savanna: the case of Cerrado típico

The Brazilian savanna, regionally known as Cerrado is an important water source due to its spatial position in the highlands known as Planalto Central of Brazil. Yet, the hydrological functioning of such complex ecosystems and its many variations is poorly understood. In this paper, we characterized the soil surface permeability to water under in one of the most common vegetation types: the Cerrado típico which is a type of savanna. In doing so, we measured infiltration capacity using the Mini-Disk infiltrometer (n = 15) and soil resistance to penetration using the Stolf penetrometer (n = 7) in a Latossolo (oxisol) under Cerrado típico located in Brasília (Planaltina), Distrito Federal, Brazil. Our results indicated a high infiltration capacity under Cerrado típico which is potentially associated with a low penetration resistance. This fact might highlight the importance of the natural undisturbed ecosystems, in this case Cerrado típico, as a land-cover which favors hydrological processes which, in turn, promotes soil-groundwater recharge that are inportant to maintain water bodies and reservoirs in the region.

Fixed or mixed? Variation in tree functional types and vegetation structure in a forest-savanna ecotone in West Africa

AbstractWe analysed thirty-five 400-m2 plots encompassing forest, savanna and intermediate vegetation types in an ecotonal area in Ghana, West Africa. Across all plots, fire frequency was over a period of 15 years relatively uniform (once in 2–4 years). Although woodlands were dominated by species typically associated with savanna-type formations, and with forest formations dominated by species usually associated with closed canopies, these associations were non-obligatory and with a discrete non-specialized species grouping also identified. Across all plots, crown area index, stem basal area and above-ground biomass were positively associated with higher soil exchangeable potassium and silt contents: this supporting recent suggestions of interplays between potassium and soil water storage potential as a significant influence on tropical vegetation structure. We also found an average NDVI cover increase of ~0.15% year−1 (1984–2011) with plots dominated by non-specialized species increasing more than those dominated by either forest- or savanna-affiliated species. Our results challenge the traditional view of a simple forest vs. savanna dichotomy controlled by fire, and with our newly identified third non-specialized species grouping also potentially important in understanding ecotonal responses to climate change.

Exploring tree diversity and stand structure of savanna woodlands in southeastern Sudan

Savanna woodlands in Sudan host great biodiversity, provide a plethora of ecosystem goods and services to local communities, and sustain numerous ecological functions. Although the importance of the Acacia trees in these areas is well known, up-to-date information about these woodlands’ diversity is limited and changes in their woody vegetation composition, density, diversity and relative frequency are not monitored over time. This study explored tree diversity and stand stage structure in Nuara Reserved Forest, a typical savanna woodland ecosystem in southeastern Sudan. A total of 638 circular sample plots (1000 m2 for each) were established using a systematic sampling grid method. The distance between plots was 200 m. In each plot, all living trees with diameter at breast height (DBH) ≥5.00 cm were identified and counted, and their DBH values were recorded. From these data, tree composition, diversity, density and stage structure were assessed. There were 12,259 individual trees representing four species (Acacia seyal, Balanites aegyptiaca, Acacia Senegal and Acacia mellifera) that belong to two families. The dominant species was Acacia seyal. Average tree density was 191 trees/hm2 and the Shannon-Weiner index for trees diversity was 0.204. Overall, young trees comprised 86.30% of the forest. The state of tree richness and density in the study area was low compared to other similar environments in the region and around the world. We recommended adoption of a proper management system that includes monitoring of woody vegetation diversity in this forest, and management actions to enhance tree diversity and sustain ecosystem services to local communities. In addition to care for the dominant Acacia seyal stands, more attention and conservation should be devoted to reestablishing Acacia senegal and Acacia mellifera trees because of their high ecological and economic values for local communities.

Forb community responses to an extensive drought in two contrasting land-use types of a semi-arid Lowveld savanna

Projected increases in the frequency and severity of drought events are expected to impose changes in the ecology of native forb communities in semi-arid ecosystems. We examined the state of forb communities during, and directly after an extreme drought event across two contrasting land-use types, which included a protected area (high diversity of free roaming wild herbivores) and communal rangeland (long history of intensive cattle grazing) in a semi-arid Lowveld savanna of the Gazankulu area, South Africa. Forb floristic data were collected towards the end of the drought and repeated after the drought release a few months later. Forb community composition was significantly different among land-use types. Community changes were not induced by annual forb emergence alone, but through species-specific dominance shifts, which differed among land-use types. Forb richness, diversity and biomass were equally low at both land-use types and increased significantly after the drought release, although the magnitude of response was much stronger in the protected area, whereas drought contributed to a directional change in the protected area with a complete post-drought turnover in both annual and perennial forb species, much less variability was observed in the communal rangeland, which may suggest long-term effects imposed by land-use history.

The role of bud protection and bark density in frost resistance of savanna trees.

Frost events occur with a significant frequency in savannas of the Southern Hemisphere, especially in the Cerrados of Brazil. One of the main strategies to deal with such events is to invest in thick and dense bark, which can insulate internal branch tissues and protect buds, essential to ensure resprouting if frost damage causes plant canopy die-back. Such strategies may be fundamental to determine the persistence of savanna species in regions where low temperatures and frost events are recurrent. Here we describe bud protection and bark strategies of 53 woody species growing in typical savanna vegetation of central Brazil. In addition, we used an experimental approach exposing branches to 0°C to measure temperature variation in internal branch tissue and test its relationship to bud protection and bark properties. We found that the majority of species (69%) showed medium to high bud protection against extreme temperatures; however, the degree of bud protection was not clearly related to bark properties, such as bark thickness and density. Bark density is a fundamental trait in determining protection against low temperatures (0°C), since species with low bark density showed lower temperature variation in their internal branch tissues, independently of the bud protection degree. Bark properties and bud protection are two different (albeit related) strategies for the protection and persistence of savanna trees under extreme environmental temperatures and can explain ecological observations related to savanna tree responses after frost events.

From woody cover to woody canopies: How Sentinel-1 and Sentinel-2 data advance the mapping of woody plants in savannas

Woody vegetation is a central component of savanna ecosystems providing ecosystem services for local livelihoods. Accurate monitoring of woody vegetation in savannas is therefore desirable, yet large scale mapping approaches rely on relatively coarse spatial resolution satellite data, which cannot directly capture the scattered nature of savanna trees. Studies at regional scale thus estimate the fractional cover of woody plants for a given area, whereas binary tree/no-tree estimates are restricted to the use of very high-resolution (VHR) images at local scales. With the launch of the Sentinel satellite systems (Sentinel-1 and Sentinel-2), the spatial resolution of images approaches the size of medium/large tree crowns, providing the opportunity to map the presence/absence of tree canopies, rather than the fraction of woody cover or forested areas. Here, we used a support vector machine (SVM) to classify the presence/absence of woody canopies from Sentinel-1 and Sentinel-2 data at a 10-m spatial resolution for the entire African Sahel. Training samples for the SVM classifier were collected from VHR images provided by Google Earth and Sentinel satellite data were processed in Google Earth Engine. Accuracy assessment was performed based on independent VHR images, showing an overall accuracy of 93% (71% and 98% for producer’s accuracy of woody and non-woody pixels, 91% and 93% for user’s accuracy of woody and non-woody pixels) when combining Sentinel-1 and Sentinel-2 data (overall accuracy of 89% using Sentinel-1 only and 91% using Sentinel-2 only). The combined use proved to perform significantly better (p < 0.05) than the single use of any of the two. A comparison with existing tree cover maps (by aggregating presence/absence of tree canopies into fractional cover) showed noticeable differences, reflecting the need for new woody cover products adapted to the nature of savanna ecosystems. The Sentinel woody canopy map was able to reproduce the general pattern of scattered woody canopies, but generally overestimated the woody coverage (11.37 ± 26.13% (mean ± sd) when aggregating to 250 m resolution) due to the 10 × 10-m spatial resolution which exceeds the crown size of a typical savanna tree. The cloud-based Sentinel-1 and Sentinel-2 analysis presented is a step towards large scale mapping of woody canopy (tree/no-tree) in savannas. Ultimately, such direct assessment of woody canopy areas will allow monitoring of temporal dynamics of woody canopies in future studies as Sentinel time-series expands to multiple years.