Savanna Species Research Articles

Population development of three sympatric dove species in African acacia savanna following a drought

The breeding densities of three dove species: Laughing Dove Spilopelia senegalensis, Ring-necked Dove Streptopelia capicola, and Mourning Collared Dove Streptopelia decipiens were studied by means of the territory mapping method in 400 ha of acacia savanna in northern Namibia. About half of the study area comprised cultivated fields, orchards, sport fields and human settlements. The study was conducted in February–June 2020, after a prolonged drought in 2017–2019. During the drought, most doves vacated their territories and ceased to breed, but breeding populations recovered rapidly. The overall population density of all species was the highest in February/March (30.3 pairs / 100 ha), intermediate in April (27.8), and lowest in May/June (23.5). Population densities were higher in the man-modified than in natural savanna. While the population densities in natural savanna significantly increased with the advance of the dry season, in man-modified savanna the densities decreased slightly.

Co‐occurring bunchgrasses are associated with different plant species in dry pine savannas

Grass species are often included during restoration to reinstate feedbacks with fire. In biodiverse pine savannas of the southeastern United States, wiregrass (Aristida beyrichiana) is the preferred candidate for restoring low‐intensity fires through the understory. Less common but locally co‐dominant in dry pine savannas, pineywoods dropseed (Sporobolus junceus) is another bunchgrass candidate for restoration. If two bunchgrass species associate with different suites of plants in the understory, restoring both could support a twofold goal of restoring fire regimes and plant biodiversity. To guide restoration goals, we tested the hypothesis that in natural (i.e. not recently restored) dry pine savannas in North Florida where wiregrass and dropseed are co‐dominant, they are associated with different understory plant species richness, occurrence, and abundance under or adjacent to their canopies. Species richness of associated plants did not differ between bunchgrass species or with distance from bunchgrass (i.e. under vs. adjacent). Some associated plant species, however, were more likely to occur and had greater abundance with one or the other bunchgrass species irrespective of distance. Our results suggest that including more than one bunchgrass species in restoration plantings could increase dry pine savanna biodiversity while also reinstating plant‐fire feedbacks.

Size reduction and skull shape parallelism following the evolutionary forest-to-savanna transition in Platyrrhini monkeys.

There are strong physiological and behavioral differences that allow animals to live in forests versus savannas. For example, terrestrial forest-dwelling mammals tend to be small compared to species living in savannas. Robust capuchin monkeys (genus Sapajus) are widespread in tropical South America, occurring in both forest and savanna environments, with forest species considered basal in an evolutionary context. Whether or not skull shape and size variations are associated with variation in resource use remains unknown, particularly for the two species living in savanna (Sapajuslibidinosus and S. cay). Here we show that savanna species present convergent size and skull characteristics that may facilitate living in this new environment. Geometric morphometric methods were used to assess skull size and shape variation for 184 Sapajus specimens distributed across South America. We used phylogenetic generalized least squares to test size against environmental variables and multivariate morphological trajectories/partial least square analyses on the skull shape to detect shape differences in specimens between forest and savanna biomes. Our findings reveal Sapajus size reduction in the evolutionary transition from forest to savanna, a process related to increasing seasonality. Moreover, we found morphological parallelism in the skull (e.g., muzzle shortening) and (large) molars in the two savanna species, features that may facilitate the processing of harder food such as fallback resources. We associate these phenotypic differences to the evolutionary process of colonizing the savannas by primates (including early hominins), leading to morphological adaptations to tolerate stressful, seasonal environments, such as body size reduction and ingestion and mastication of tough foods.

Exotic Grasses Reduce Infiltration and Moisture Availability in a Temperate Oak Savanna.

Biological invasions represent one of the most urgent conservation challenges. Oregon white oak (Quercus garryana) savannas, a complex of grassland and transitional forest, are especially sensitive to these invasions. These ecosystems have been severely degraded and fragmented over the past century and are being encroached by conifers, and oak seedlings are failing to emerge from the understory at many locations. Understanding competitive interactions between Oregon white oak and associated native and exotic vegetation would provide insight into forest-grassland dynamics and the role of exotic grasses in the decline of native species, the processes that maintain temperate savanna ecosystems, and the role of soil water uptake by individual savanna species in contributing to overall species assemblages. In this study, we quantified the soil moisture budget for invaded and uninvaded oak-associated ecosystems. From February to October 2007 we used a split paired plot experiment in Duncan, British Columbia, Canada to measure soil moisture on treatment sites where exotic grasses were removed with herbicide and control plots where they were not, using three depths (5, 20, and 35 or 50 cm) in the soil profile. Our results show that the plots that contained exotic vegetation had a faster rate of soil drying following precipitation events at the 5 cm depth than plots with the predominantly native species. We attribute this difference to the capacity of exotic vegetation to exploit soil moisture more rapidly than native vegetation at times of the year when native vegetation cannot. These results provide insight into one mechanism by which exotic grasses affect associated native plants and could help guide restoration efforts.

Historical and current distribution and movement patterns of large herbivores in the Limpopo National Park, Mozambique

This study provides a first attempt to describe the historical distribution and movement patterns of selected large herbivore (LH) species in Limpopo National Park (LNP), an area in Mozambique today connected to a network of transboundary conservation areas. Between 1976 and the early 2000s, most LH species were absent in this area following the civil war in Mozambique followed by intense poaching due to weak law enforcement capacity. Through the reconstruction of the historical and current distribution and movement patterns of seven LH species in five periods, we investigate possible changes in distribution and movement patterns over time. Data collection is based on a systematic literature search, censuses reports, online databases, dung count transects, and camera trap surveys. We mapped all LH observations and movements using ArcGIS 10.1. Our results reveal a dramatic collapse of LH populations between the peak of the colonial period and the post-colonial/civil war period (1800–2001), followed by a slight recovery from the post-proclamation of Great Limpopo Transfrontier Park to the current period (2002–2021). While LH population decline applied to all seven species, there are species-specific differences in the process of restoration: African elephant (Loxodonta africana), African buffalo (Syncerus caffer), and plains zebra (Equus quagga) appear to recover to a greater extent than giraffe (Giraffa camelopardalis), eland (Tragelaphus oryx), blue wildebeest (Connochaetes taurinus), and white rhino (Ceratotherium simum). We found evidence of the functioning of proposed wildlife corridors in the LNP. The results give reason to assume that restoration of populations of LH is still in a very early and vulnerable state and that further efforts are necessary to strengthen the slowly increasing populations of LH. Our results highlight the importance of combining past and current data as a guide for the restoration of threatened species in African savannas impacted by human activities.

Tree survival and resprouting after wildfire in tropical dry and subhumid ecosystems of Chiquitania, Bolivia

Lack of understanding the negative and positive effects of wildfires poses a challenge to the restoration and management of tropical ecosystems. Although fire can have negative impacts on tropical forests, it is known that many tropical savanna and woodland species are adapted to fire. The objective of this study was to determine the resilience of tree species to wildfire in a seasonally dry region of eastern lowland Bolivia. The study was carried out in four tropical dry Chiquitano ecosystems (Abayoy, Cerrado, Chiquitano dry forest, and sub-humid forest), in the department of Santa Cruz, 6–18 months after wildfires occurred in July - October 2019. At 45 sampling points, in both burned and unburned areas, we established 50-m x 5-m transects to determine tree species composition, number of individuals, resprout types, survival, diameter, and height of individuals that were ≥1 cm diameter at breast height (DBH). We calculated the percentages of surviving and dead stems by resprout categories and used logistic regression to determine the probability of survival and probability of resprouting as a function of pre-burn size. In the four ecosystems, 35–71% of the individuals sampled survived (crown and basal resprouting) after fire, while an additional 18–59% of individuals had only basal resprouting. Tree mortality (no reprouts) ranged from 5 to 11%. In burned areas, larger-diameter trees had a greater probability of survival. In contrast, the smaller-diameter trees had a higher probability of resprouting from the base. In conclusion, the Chiquitano tropical dry ecosystems have a high resilience to wildfire in terms of survival and resprouting. Resprouting strategy appears to be an important survival mechanism in trees, especially small trees, and must be considered in the passive restoration of burned forests.

Mean height increase in saplings of a keystone woody savanna species over 15 years similar to that over a single season

AbstractWoody structure, particularly that created by large trees, is crucial to savanna biodiversity pattern and process. We assessed changes in vertical and horizontal canopy over 15 years of saplings of Vachellia (Acacia) erioloba (camelthorn), a keystone species and one of the only trees to grow large in arid savanna on low rainfall areas of Kalahari sands, southern Africa. We also assessed whether the fastest growing saplings over the 2003–2004 growing season were also the fastest growing over the following 15 years. We measured height, canopy extent, and diameter of largest stem at ground level of 120 saplings of V. erioloba in November 2003, remeasuring these in March 2004. In November 2019, we were able to find 67 of these saplings, and we measured them again to assess growth between 2004 and 2019. We modeled interindividual explanatory variables for growth (i.e., measures taken in 2004 of height, diameter, number of stems, evidence of fire, and largest stem diameter). We also compared residuals of growth between 2003 and 2004 with those between 2004 and 2019, to assess whether individuals showed similar progress relative to their peers over the two periods considered. Height increase was extremely variable, but mean increase over 15 years was only 45.4 mm, similar to that measured for the 2003–2004 growing season (43.1 mm). Mean horizontal canopy showed far greater growth (mean 120.2 mm over 15 years, compared with 7.12 mm over 2003–2004) than vertical growth. Individuals that had shown the fastest increase in horizontal canopy in 2003–2004 tended to be the same individuals that grew fastest over the following 15 years, but even the highest “achievers” showed increases of only 47.2 mm and 37.5 mm year−1 in average horizontal canopy extent and height, respectively. This study demonstrates how crucial long‐term studies are, particularly in arid systems. Many individuals may be trapped in the “sapling” class and could take decades to become large trees. Woody sapling growth was not cumulative, and modeling studies in savannas should be cautious in extrapolating growth from one season to several years.

Estimating mistletoe biomass in a semi-arid savanna woodland, southwest Zimbabwe

Despite the biological importance of mistletoes in forests and woodlands globally, mistletoe biomass is rarely quantified, particularly in tropical regions. Thus, we constructed allometric models to predict the biomass of three mistletoe species (Erianthemum ngamicum, Plicosepalus kalachariensis, and Viscum verrucosum) in semi-arid savanna, southwest Zimbabwe. We destructively sampled 30 individuals of each mistletoe species and recorded their broad haustorium diameter, narrow haustorium diameter, and the width, breadth, vertical depth, and percentage of leaves of the whole plant. After measuring the dry biomass of each mistletoe, we fitted linear regression models – selected and compared based on corrected Akaike Information Criterion (AICc) minimization. Mistletoe volume (r2 = 0.95) best predicted biomass in E. ngamicum, while mistletoe size was the best predictor for P. kalachariensis (r2 = 0.90) and V. verrucosum (r2 = 0.95). Models using haustorium diameter had the lowest predictive power in all species. V. verrucosum and P. kalachariensis had greater total mean dry biomass than E. ngamicum. Stems constituted the larger proportion of total biomass than leaves in E. ngamicum (75 vs. 23%) but the reverse is true for P. kalachariensis (40 vs. 58%). Of the total biomass, fruits and flowers constituted 6% in V. verrucosum and E. ngamicum and 2% in P. kalachariensis. The leaf to stem biomass contribution decreases in E. ngamicum but increases in P. kalachariensis as the plant grows. Models using mistletoe biometry (width, breadth, and vertical depth of the whole plant), rather than host size (height, diameter at breast height, and canopy area), best predicted total mistletoe biomass. Not only does this enable estimation of mistletoe biomass beyond the scale of infected individuals, but the contribution of mistletoes to local biomass, carbon stock, and ecosystem processes (e.g., nutrient cycling) can also be quantified at different spatial scales helping elucidate their keystone role in semi-arid savanna.

Multi-temporal spectral reflectance of tropical savanna understorey species and implications for hyperspectral remote sensing

• Spectra of savanna understorey vegetation species over phenological seasons. • Separability of functional groups, and species, depended on season. • Informs species separation and timing for drone-based hyperspectral captures. • VNIR drone spectral range excludes foliar traits such as cellular water absorption. The relationship between species phenology and spectral separability is essential to determine the optimal remote sensing sampling period to maximise spectral separability of vegetation species. However, this relationship in many native grasses, introduced grasses and herbs, shrubs, and vine species in tropical savannas is unknown. We measured the in-situ hyperspectral response of monospecific vegetation stands of several understorey species representing different functional groups over phenological stages (throughout dry and wet seasons) using a non-imaging spectrometer. We present a spectral library of both native and introduced species of a tropical savanna environment. We analysed the data using continuum removal to highlight absorption features. Most understorey species displayed a photosynthetic spectral response with increased greenness at the end of the wet season that progressively declined as vegetation dried out. For some species, there were seasonally dependent differences in absorption features with spectral differences between the late wet and early dry season, and late dry and early wet seasons. We resampled the data to the spectral range typical of drone-mounted hyperspectral sensors (i.e., 150 bands between 400 and 1000 nm), which omits the water absorption features of the SWIR. These findings suggest an ideal sampling period for measuring outdoor canopy reflectance of understorey species, which will promote methodological improvements of hyperspectral data capture. The use of a VNIR only hyperspectral sensor will exclude the important regions of the spectrum for plant spectral identification including non-pigment bands for water, nitrogen and cellulose. The results have implications for determining the completeness of rehabilitation assessing reestablishment among indigenous species across mine sites under rehabilitation.

Three years of warming and rainfall reduction alter leaf physiology but not relative abundance of an annual species in a Mediterranean savanna

Increasing air temperatures and decreasing rainfall can alter Mediterranean ecosystems, where summer heat and drought already limit plant regeneration. Manipulative field studies can help to understand and anticipate community responses to climate changes. In a Mediterranean oak wooded pasture, we have investigated the effects of warming (W, via open-top chambers increasing 1.4 °C mean air temperature), reduced rainfall (D, via gutters removing 33% of rainfall) and the combination of both factors (WD) on the winter-annual Geranium dissectum L. We measured reproductive phenology and output, leaf physiology during the reproductive phase, and plant relative abundance. Warming had a positive effect on plant height and little effects on leaf physiology. Rainfall reduction enhanced leaf water use efficiency. However, the most noticeable effects occurred in WD plants, which exhibited lower leaf predawn water potential and earlier flowering phenology in the first year of treatment, and a higher ratio of leaf dark respiration (R) to net CO2 assimilation (Pn) at comparable temperatures in the third year, compared to control plants. Leaf R at ambient temperature was similar across climatic treatments. The relative abundance of G. dissectum decreased by 23% over three years, but similarly across treatments. A short life cycle helps G. dissectum to escape severe late-spring heat and drought stress. Moreover, stomata closure and thermal acclimation of R can attenuate plant stress impact on reproduction. Adaptability of the short-lived annual G. dissectum could mitigate climate change impact on community composition over short periods (e.g. three years); however, a reduction in net carbon gain could eventually affect its reproductive success and persistence in the community.

Sympatric finches increase trophic and spatiotemporal partitioning to enable coexistence through periods of low resource availability

Abstract Ecological niche theory dictates that sympatric species cannot occupy the same ecological niche at the same time. Sympatric granivorous finch species in tropical savannas appear to contradict this theory by moving in mixed‐species flocks and feeding together upon the same resources. Here, we explored this contradiction by tracking individuals from three finch species (Gouldian—Chloebia gouldiae, long‐tailed—Poephila acuticauda and masked finch—P. personata) often seen feeding together, while simultaneously determining the dietary composition during periods of high and low resource abundance. Dietary composition was characterised using stable isotope ratios of δ13C and δ15N within the blood, and foraging areas were determined from continuous detection using passive VHF radio telemetry. Individuals were sampled early in the dry season when grass seeds were abundant, in the middle of the dry season and at the end when seeds were very scarce. The results showed plasticity in ecological niche overlap among the three finch species depending on grass seed availability within the landscape. In the early dry season, the species foraged together on the abundant grass seed. As the dry season progressed and grass seed availability declined, masked and long‐tailed finches exhibited greater spatial partitioning, masked and Gouldian finches increased dietary partitioning, and long‐tailed and Gouldian finches increased both dietary and spatial partitioning. Our study showed that sympatric granivorous finches utilise trophic and spatiotemporal partitioning to enable coexistence through periods of low resource availability in tropical savannas. Gouldian finches increased activity and ranging behaviour, while long‐tailed and masked finches diversified their diet. Read the free Plain Language Summary for this article on the Journal blog.

The roles of functional traits in canopy maintenance along a savanna/seasonally dry tropical forest gradient in northeastern Brazil

Vegetation gradients, such as those between savanna and seasonally dry tropical forest (SDTF) vegetations, may experience nearly identical macroclimatic conditions but still differ because of local ecological filters selecting for distinct plant functional aspects related to water storage and use. We examined how leaf phenology, water potential, wood density, and wood saturated water content varied seasonally along a savanna, transition, and SDTF vegetation gradient at the eastern border of the Chapada Diamantina Highlands, Brazil. We monitored the leaf phenologies of 523 individuals of 48 woody species (20 savanna, 14 transition, and 14 SDTF species) for two years. We identified four phenological groups: brevideciduous, deciduous, evergreen (EG) plants having continuous growth, and evergreen plants having seasonal growth. Deciduous species were found throughout the gradient, while EG species accounted for more than 80% of relative density in savanna areas. Precipitation was negatively related to leaf fall for all phenological groups, and positively related for leaf flushing in deciduous species. More than 80% of all species exhibited wood densities between 0.50 and 0.91 g cm−3. The first principal component explained 77.64% of the observed variance, associated with wood saturated water content, water potential, and wood density. All of the savanna species were distributed along the positive axis of the principal component analysis as compared to SDTF species. We demonstrated that the effects of water limitations along the gradient were critical to the selection of functional traits associated with water-use strategies and expressive deciduousness in SDTF vegetation, interpreted as drought avoidance strategies.

Variations in bark structural properties affect both water loss and carbon economics in neotropical savanna trees in the Cerrado region of Brazil

Abstract Even after complete stomatal closure, plants lose water through the leaf cuticles and bark. This residual water conductance of leaves (gleaf‐res) and stems (gbark) can negatively impact plant water balance and affect plant survival in seasonally dry environments. However, little is known about the costs and benefits associated with such water leaks, especially on stem level. Here, we characterized the structural and functional determinants of the variability in gbark across tropical savanna species to elucidate how variations in this trait are related to contrasting growth strategies. The high variability in gbark across species was associated with morphoantomical properties of the outer bark (thickness, density and lenticel investment), and such characteristics influenced both stem transpiration and respiration, suggesting the existence of a trade‐off between water conservation and oxygen permeability, which reflected contrasting growth and dehydration tolerance strategies. For instance, species with higher gbark and gleaf‐res presented a fast resource acquisition strategy but were more prone to drought‐induced mortality by hydraulic failure. However, model simulations revealed that the relative contribution of gleaf‐res and gbark to overall water balance depended on whether leaves were less or more resistant to cavitation than the stems. Synthesis. By combining correlative studies, experimental results and a modelling exercise, we provide a new understanding of the costs and benefits associated with the variability in gbark across tropical savanna species and a new perspective for studies of water relations and carbon economics in species from a hyperdiverse savanna.

Ecological Drivers of Habitat Use by Meso Mammals in a Miombo Ecosystem in the Issa Valley, Tanzania

Vast stretches of East and Southern Africa are characterized by a mosaic of deciduous woodlands and evergreen riparian forests, commonly referred to as “miombo,” hosting a high diversity of plant and animal life. However, very little is known about the communities of small-sized mammals inhabiting this heterogeneous biome. We here document the diversity and abundance of 0.5–15 kg sized mammals (“meso-mammals”) in a relatively undisturbed miombo mosaic in western Tanzania, using 42 camera traps deployed over a 3 year-period. Despite a relatively low diversity of meso-mammal species (n = 19), these comprised a mixture of savanna and forest species, with the latter by far the most abundant. Our results show that densely forested sites are more intensely utilized than deciduous woodlands, suggesting riparian forest within the miombo matrix might be of key importance to meso-mammal populations. Some species were captured significantly more often in proximity to (and sometimes feeding on) termite mounds (genus Macrotermes), as they are a crucial food resource. There was some evidence of temporal partitioning in activity patterns, suggesting hetero-specific avoidance to reduce foraging competition. We compare our findings to those of other miombo sites in south-central Africa.

CO2‐stimulation of savanna tree seedling growth depends on interactions with local drivers

Abstract Woody encroachment in southern African savanna has been partly attributed to rising atmospheric [CO2] fertilising the growth of C3 trees but less so that of competing C4 grasses. However, growth conditions (resource availability, competition, rooting space and herbivory) must be suitable for the effects of elevated CO2 (eCO2) to be realised. This research investigated the interactions between the positive effect of eCO2 on tree seedling growth and limitations imposed by drought, herbivory and competition with C4 grasses. Seedlings of the prolific encroaching C3 tree Vachellia karroo were grown at ambient (400 ppm) or eCO2 (800 ppm) in Open‐Top Chambers and exposed to a variety of stresses typical of savanna systems. Photosynthetic, growth and allocation responses to eCO2 and other treatments were determined. Unsurprisingly, we show strong growth and water‐saving responses of V. karroo seedlings to eCO2 when in the absence of competition and herbivory. However, the addition of either grass competition or simulated herbivory in the first season of growth significantly moderated this stimulation, while neither drought nor shading diminished the eCO2 effect relative to similarly treated plants grown at ambient [CO2]. Synthesis. We demonstrate that eCO2‐induced C3 stimulation in encroaching savanna species such as V. karroo will be inconsistent across time and space. This research does not detract from the suggestion that increasing atmospheric CO2 is implicated in woody encroachment, but rather that eCO2 benefits to C3 tree seedlings are only realised when growth conditions are suitable. Inconsistencies in eCO2 response will translate into spatial and temporal variation in seedling responses to eCO2 and CO2‐driven woody encroachment, explaining some of the variability observed in woody encroachment across geographical regions and resource and herbivore gradients.

Mechanisms of dietary resource partitioning in large‐herbivore assemblages: A plant‐trait‐based approach

Abstract Sympatric large mammalian herbivore species differ in diet composition, both by eating different parts of the same plant and by eating different plant species. Various theories proposed to explain these differences are not mutually exclusive, but are difficult to reconcile and confront with data. Moreover, whereas several of these ideas were originally developed with reference to within‐plant partitioning (i.e. consumption of different tissues), they may analogously apply to partitioning of plant species; this possibility has received little attention. Plant functional traits provide a novel window into herbivore diets and a means of testing multiple hypotheses in a unified framework. We used DNA metabarcoding to characterize the diets of 14 sympatric large‐herbivore species in an African savanna and analysed diet composition in light of 27 functional traits that we measured locally for 204 plant species. Plant traits associated with the deep phylogenetic split between grasses and eudicots formed the primary axis of resource partitioning, affirming the generality and importance of the grazer–browser spectrum. A secondary axis comprised plant traits relevant to herbivore body size. Plant taxa in the diets of large‐bodied species were lower on average in digestible energy and protein, taller on average (especially among grazers), and tended to be higher in tensile strength, zinc, stem‐specific density, and potassium (and lower in sodium, stem dry matter content, and copper). These results are consistent with longstanding hypotheses linking body size with forage quality and height, yet they also suggest the existence of undiscovered links between herbivore body size and a set of rarely considered food–plant traits. We also tested the novel hypothesis that the leaf economic spectrum (LES), a major focus in plant ecology, is an axis of resource partitioning in large‐herbivore assemblages; we found that the LES was a minor axis of individual variation within a few species, but had little effect on interspecific dietary differentiation. Synthesis. These results identify key plant traits that underpin the partitioning of food–plant species in large‐herbivore communities and suggest that accounting for multiple plant traits (and trade‐offs among them) will enable a deeper understanding of herbivore–plant interaction networks.

Fire stimulates grass flowering in the Cerrado independent of season

AbstractQuestionsPost‐fire flowering is an underestimated plant trait of many fire‐prone ecosystems and is rarely considered for tropical savannas. Therefore, we analyzed the effects of fire season and history on post‐fire flowering of grass species in a tropical savanna (Cerrado), by evaluating the number of species, frequency of individuals and number of reproductive tillers after wet‐ and dry‐season fires, and under different fire histories.LocationsOpen savannas (campo sujo) of the Cerrado in central Brazil (Reserva Natural Serra do Tombador [RNST], 13°35–13°38′ S, 47°45′–47°51′ W).MethodsWe sampled flowering individuals in 10 Poaceae species and counted flowering/vegetative tillers of grass species in 1 m × 1 m plots after wet‐ and dry‐season fires and under different fire histories (3, 18 and 72 months post fire).ResultsSome of the studied species responded differently to fire according to season but most flowered after fire events. Moreover, the exclusion of fire led to a decrease in the number of flowering individuals and reproductive tillers of grass species.ConclusionsWe describe a unique event for tropical savannas: beside rapid resprout after fire events, grasses can allocate resources for both vegetative and reproductive tiller production, even during the dry season. If fire is excluded from the system, fewer grass species and individuals will flower and there will be a lower percentage of reproductive tillers, showing the importance of fire for grass flowering in these tropical savannas, independently of fire season.

Termites have wider thermal limits to cope with environmental conditions in savannas.

The most diverse and abundant family of termites, the Termitidae, evolved in African tropical forests. They have since colonised grassy biomes such as savannas. These open environments have more extreme conditions than tropical forests, notably wider extremes of temperature and lower precipitation levels and greater temporal fluctuations (of both annual and diurnal variation). These conditions are challenging for soft‐bodied ectotherms, such as termites, to survive in, let alone become as ecologically dominant as termites have.Here, we quantified termite thermal limits to test the hypothesis that these physiological limits are wider in savanna termite species to facilitate their existence in savanna environments.We sampled termites directly from mound structures, across an environmental gradient in Ghana, ranging from wet tropical forest through to savanna. At each location, we quantified both the Critical Thermal Maxima (CTmax) and the Critical Thermal Minima (CTmin) of all the most abundant mound‐building Termitidae species in the study areas. We modelled the thermal limits in two separate mixed‐effects models against canopy cover at the mound, temperature and rainfall, as fixed effects, with sampling location as a random intercept.For both CTmax and CTmin, savanna species had significantly more extreme thermal limits than forest species. Between and within environments, areas with higher amounts of canopy cover were significantly associated with lower CTmax values of the termite colonies. CTmin was significantly positively correlated with rainfall. Temperature was retained in both models; however, it did not have a significant relationship in either. Sampling location explained a large proportion of the residual variation, suggesting there are other environmental factors that could influence termite thermal limits.Our results suggest that savanna termite species have wider thermal limits than forest species. These physiological differences, in conjunction with other behavioural adaptations, are likely to have enabled termites to cope with the more extreme environmental conditions found in savanna environments and facilitated their expansion into open tropical environments.

Woody plant encroachment constrains regeneration of ground‐layer species in a neotropical savanna from seeds

AbstractWoody plants have been encroaching into savannas on a global scale. Global climatic changes, along with changes to disturbance regimes, are considered to be the main drivers of this process. Particularly, many formerly open physiognomies of the Brazilian savanna have become encroached by woody species over the course of a few decades under fire suppression. In this scenario, many typical savanna species of the ground‐layer are reducing their above‐ground presence, possibly due to change in environmental filtering as a result of encroachment. Consequently, at encroachment sites, changes to microhabitats occur, such as less incident light to the soil surface, reduced red:far‐red ratios and soil surface temperatures, and increased litter deposition. These changes may hinder the regenerative capacity of ground‐layer savanna species. This study investigated the role of encroachment‐induced environmental changes as filters for the recruitment ability of ground‐layer savanna species. We examined the germination and seedling emergence of 12 species under controlled conditions, simulating natural aspects of encroached and non‐encroached sites of the Brazilian savanna. The germination and/or seedling emergence of all species examined were/was negatively affected by the simulated environmental filtering changes. Increased litter deposition reduced seedling emergence more than temperature and light affected germination. Filtering changes caused by woody encroachment represent a bottleneck for the regeneration from seeds of ground‐layer savanna species. Filtering changes by woody encroachment is one of the underlying mechanism explaining changes in species presence in savannas, and it ultimately leads to positive feedback loops wherein woody encroachment begets more woody encroachment.

Trade-offs between xylem hydraulic efficiency and mechanical strength in Chinese evergreen and deciduous savanna species.

Evergreen and deciduous species coexist in tropical dry forests and savannas, but differ in physiological mechanisms and life-history strategies. Hydraulic conductivity and mechanical support are two major functions of the xylems of woody plant species related to plant growth and survival. In this study, we measured sapwood-specific hydraulic conductivity (Ks), leaf-specific hydraulic conductivity (KL), modulus of rupture (MOR) and elasticity (MOE), xylem anatomical traits and fiber contents in the xylems of 20 woody species with contrasting leaf phenology (evergreen vs deciduous) in a Chinese savanna. Our results showed that deciduous species had significantly higher Ks and KL but lower MOR and MOE than evergreen species. Evergreen species experienced more negative seasonal minimum water potential (Pmin) than deciduous species during the dry season. Furthermore, we found trade-offs between xylem hydraulic efficiency and mechanical strength across species and within the evergreen and deciduous groups, and these trade-offs were modulated by structural and chemical traits. Both Ks and KL were significantly related to hydraulic weighted vessel diameter (Dh) across all species and within the deciduous group. Both MOR and MOE were significantly related to wood density, neutral detergent fiber and acid detergent fiber across species and within evergreen and deciduous groups. Our findings demonstrated that Chinese evergreen and deciduous savanna species diverged in xylem hydraulic and mechanical functions, reflecting conservative and acquisitive life-history strategies for evergreen and deciduous species, respectively. This study provides new information with which to understand the hydraulic and biomechanical properties and ecological strategies of savanna species in long-term dry-hot environments.