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Savanna vegetation is controlled by bottom-up (e.g. soil and rainfall) and top-down (e.g. fire and herbivory) factors, all of which have an effect on biodiversity. Little is known about the relative contribution of these factors to biodiversity, particularly the long-term effects of top-down disturbance on patterns of woody plant composition. The aim of this study was to identify if various degrees of disturbance regimes create distinct woody species community assemblages. Data were collected over 1820 plots across Kruger National Park, South Africa. Woody species were identified and categorized into one of three height classes: shrub (0.75-2.5 m), brush (2.5-5.5 m), and tree (>5.5 m). Species richness and composition were calculated for each site and height class. A combination of long-term fire and elephant density data were used to delineate areas with varying degrees of top-down disturbance (i.e. low, medium and high). Using these degrees of disturbance, species composition was identified and community assemblages constructed according to each disturbance regime. Our results suggest that areas with similar disturbance regimes have similar species composition. Shrub composition was mainly responsive to the number of fires between the years 1941-1990, while tree composition was more responsive to elephant disturbance. A few dominant species were found equally under all degrees of disturbance at all height classes, while others were more regularly found under specific disturbance regimes at particular height classes. This study highlights that while species richness does not appear to be influenced by long-term, top-down disturbance regimes, species community composition may be responsive to these disturbances. Most species and structural classes persisted across all disturbance regimes, but the long-term effects of top-down disturbances can influence compositional and structural biodiversity. This information provides context for management policies related to artificial water provision, elephants and fire. [ABSTRACT FROM AUTHOR]

Increasing rainfall variability is widely expected under future climate change scenarios. How will savanna trees and grasses be affected by growing season dry spells and altered seasonality and how tightly coupled are tree-grass phenologies with rainfall? We measured tree and grass responses to growing season dry spells and dry season rainfall. We also tested whether the phenologies of 17 deciduous woody species and the Soil Adjusted Vegetation Index of grasses were related to rainfall between 2019 and 2023. Tree and grass growth was significantly reduced during growing season dry spells. Tree growth was strongly related to growing season soil water potentials and limited to the wet season. Grasses can rapidly recover after growing season dry spells and grass evapotranspiration was significantly related to soil water potentials in both the wet and dry seasons. Tree leaf flushing commenced before the rainfall onset date with little subsequent leaf flushing. Grasses grew when moisture became available regardless of season. Our findings suggest that increased dry spell length and frequency in the growing season may slow down tree growth in some savannas, which together with longer growing seasons may allow grasses an advantage over C 3 plants that are advantaged by rising CO 2 levels., (© 2024 The Authors. New Phytologist © 2024 New Phytologist Foundation.)

Modeling fire spread as an infection process is intuitive: An ignition lights a patch of fuel, which infects its neighbor, and so on. Infection models produce nonlinear thresholds, whereby fire spreads only when fuel connectivity and infection probability are sufficiently high. These thresholds are fundamental both to managing fire and to theoretical models of fire spread, whereas applied fire models more often apply quasi-empirical approaches. Here, we resolve this tension by quantifying thresholds in fire spread locally, using field data from individual fires ( n = 1,131) in grassy ecosystems across a precipitation gradient (496 to 1,442 mm mean annual precipitation) and evaluating how these scaled regionally (across 533 sites) and across time (1989 to 2012 and 2016 to 2018) using data from Kruger National Park in South Africa. An infection model captured observed patterns in individual fire spread better than competing models. The proportion of the landscape that burned was well described by measurements of grass biomass, fuel moisture, and vapor pressure deficit. Regionally, averaging across variability resulted in quasi-linear patterns. Altogether, results suggest that models aiming to capture fire responses to global change should incorporate nonlinear fire spread thresholds but that linear approximations may sufficiently capture medium-term trends under a stationary climate.

Savannas cover a fifth of the land surface and contribute a third of terrestrial net primary production, accounting for three-quarters of global area burned and more than half of global fire-driven carbon emissions 1-3 . Fire suppression and afforestation have been proposed as tools to increase carbon sequestration in these ecosystems 2,4 . A robust quantification of whole-ecosystem carbon storage in savannas is lacking however, especially under altered fire regimes. Here we provide one of the first direct estimates of whole-ecosystem carbon response to more than 60 years of fire exclusion in a mesic African savanna. We found that fire suppression increased whole-ecosystem carbon storage by only 35.4 ± 12% (mean ± standard error), even though tree cover increased by 78.9 ± 29.3%, corresponding to total gains of 23.0 ± 6.1 Mg C ha -1 at an average of about 0.35 ± 0.09 Mg C ha -1  year -1 , more than an order of magnitude lower than previously assumed 4 . Frequently burned savannas had substantial belowground carbon, especially in biomass and deep soils. These belowground reservoirs are not fully considered in afforestation or fire-suppression schemes but may mean that the decadal sequestration potential of savannas is negligible, especially weighed against concomitant losses of biodiversity and function., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Widespread woody encroachment is a prominent concern for savanna systems as it is often accompanied by losses in productivity and biodiversity. Extensive ecosystem-level work has advanced our understanding of its causes and consequences. However, there is still debate over whether local management can override regional and global drivers of woody encroachment, and it remains largely unknown how encroachment influences woody community assemblages. Here, we examined species-level changes in woody plant distributions and size structure from the late 1980s to the late 2000s based on spatially intensive ground-based surveys across Kruger National Park, South Africa. This study region spans broad gradients in rainfall, soil texture, fire frequency, elephant density, and other topographic variables. Species-level changes in frequency of occurrence and size class proportion reflected widespread woody encroachment primarily by Dichrostachys cinerea and Combretum apiculatum, and a loss of large trees mostly of Sclerocarya birrea and Acacia nigrescens. Environmental variables determining woody species distributions across Kruger varied among species but did not change substantially between two sampling times, indicating that woody encroachers were thickening within their existing ranges. Overall, more areas across Kruger were found to have an increased number of common woody species through time, which indicated an increase in stem density. These areas were generally associated with decreasing fire frequency and rainfall but increasing elephant density. Our results suggest that woody encroachment is a widespread but highly variable trend across landscapes in Kruger National Park and potentially reflects an erosion of local heterogeneity in woody community assemblages. Many savanna managers, including in Kruger, aim to manage for heterogeneity in order to promote biodiversity, where homogenization of vegetation structure counters this specific goal. Increasing fire frequency has some potential as a local intervention. However, many common species increased in commonness even under near-constant disturbance conditions, which likely limits the potential for managing woody encroachment in the face of drivers beyond the scope of local control. Regular field sampling coupled with targeted fire management will enable more accurate monitoring of the rate of encroachment intensification., (© 2021 by the Ecological Society of America.)

Global change has resulted in chronic shifts in fire regimes. Variability in the sensitivity of tree communities to multi-decadal changes in fire regimes is critical to anticipating shifts in ecosystem structure and function, yet remains poorly understood. Here, we address the overall effects of fire on tree communities and the factors controlling their sensitivity in 29 sites that experienced multi-decadal alterations in fire frequencies in savanna and forest ecosystems across tropical and temperate regions. Fire had a strong overall effect on tree communities, with an average fire frequency (one fire every three years) reducing stem density by 48% and basal area by 53% after 50 years, relative to unburned plots. The largest changes occurred in savanna ecosystems and in sites with strong wet seasons or strong dry seasons, pointing to fire characteristics and species composition as important. Analyses of functional traits highlighted the impact of fire-driven changes in soil nutrients because frequent burning favoured trees with low biomass nitrogen and phosphorus content, and with more efficient nitrogen acquisition through ectomycorrhizal symbioses. Taken together, the response of trees to altered fire frequencies depends both on climatic and vegetation determinants of fire behaviour and tree growth, and the coupling between fire-driven nutrient losses and plant traits.

Dimensions of tree root systems in savannas are poorly understood, despite being essential in resource acquisition and post-disturbance recovery. We studied tree rooting patterns in Southern African savannas to ask: how tree rooting strategies affected species responses to severe drought; and how potential rooting depths varied across gradients in soil texture and rainfall. First, detailed excavations of eight species in Kruger National Park suggest that the ratio of deep to shallow taproot diameters provides a reasonable proxy for potential rooting depth, facilitating extensive interspecific comparison. Detailed excavations also suggest that allocation to deep roots traded off with shallow lateral root investment, and that drought-sensitive species rooted more shallowly than drought-resistant ones. More broadly across 57 species in Southern Africa, potential rooting depths were phylogenetically constrained, with investment to deep roots evident among miombo Detarioids, consistent with results suggesting they green up before onset of seasonal rains. Soil substrate explained variation, with deeper roots on sandy, nutrient-poor soils relative to clayey, nutrient-rich ones. Although potential rooting depth decreased with increasing wet season length, mean annual rainfall had no systematic effect on rooting depth. Overall, our results suggest that rooting depth systematically structures the ecology of savanna trees. Further work examining other anatomical and physiological root traits should be a priority for understanding savanna responses to changing climate and disturbances., (© 2020 The Authors. New Phytologist © 2020 New Phytologist Trust.)

Although studies have shown that mammalian herbivores often limit aboveground carbon storage in savannas, their effects on belowground soil carbon storage remain unclear. Using three sets of long-term, large herbivore exclosures with paired controls, we asked how almost two decades of herbivore removal from a semiarid savanna in Laikipia, Kenya affected aboveground (woody and grass) and belowground soil carbon sequestration, and determined the major source (C 3 vs. C 4 ) of belowground carbon sequestered in soils with and without herbivores present. Large herbivore exclusion, which included a diverse community of grazers, browsers, and mixed-feeding ungulates, resulted in significant increases in grass cover (~22%), woody basal area (~8 m 2 /ha), and woody canopy cover (31%), translating to a ~8.5 t/ha increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54% increase (20.5 t/ha) in total soil carbon to 30-cm depth, with ~71% of this derived from C 4 grasses (vs. ~76% with herbivores present) despite substantial increases in woody cover. We attribute this continued high contribution of C 4 grasses to soil C sequestration to the reduced offtake of grass biomass with herbivore exclusion together with the facilitative influence of open sparse woody canopies (e.g., Acacia spp.) on grass cover and productivity in this semiarid system., (© 2020 by the Ecological Society of America.)

Increasingly frequent and severe droughts under climate change are expected to have major impacts on vegetation worldwide. However, research to date has focused on tree vulnerability to drought in forests. Less is known about trees and drought in savannas, where a sparse tree layer coexists with grass. These tree-grass interactions (often mediated by fire and herbivory) shape savanna tree ecology, and confound predictions of how strongly drought might affect trees. On the one hand, drought is physiologically stressful, which could harm trees and be exacerbated by herbivore impacts; on the other hand, trees adapted to semiarid savannas might be relatively drought tolerant, and the considerable impacts of drought on grass could even benefit trees via reduced grass competition and fire risk, especially in the year following a drought. Here, we sought to understand the net effects of severe drought on the savanna tree layer, and how fire and herbivory mediate these effects. We monitored tree growth, mortality, and community structure for 2 yr within existing long-term fire and herbivory experiments across a drought-severity contrast, following a major drought in Kruger National Park, South Africa. Overall, severe drought was a major stressor for trees. Tree mortality rates in most species increased by an order of magnitude in the year following drought, and slower growth rates for some persisted for 2 yr. At the community level, this translated into substantial decreases in tree densities. Herbivory and fire did little either to mitigate or exacerbate drought effects on trees, and overall, drought swamped effects of herbivory and fire that have otherwise been observed. However, species differed in their responses to drought, with some dominant encroaching species especially vulnerable. We suggest that increasing drought frequency and severity could drastically alter savanna vegetation by repeatedly killing off trees., (© 2019 by the Ecological Society of America.)

Southern African savannas are commonly polarised into two broad types based on plant functional types and defences; infertile savannas dominated by broad-leaved trees typically defended by nitrogen-free secondary compounds and fertile savannas dominated by fine-leaved trees defended by structural defences. In this study, we use trait and other data from 15 wooded savanna sites in Southern Africa and ask if broad-leaved and fine-leaved species dominate on nutrient-poor and nutrient-rich soils, respectively. We then test if there is there any evidence for trade-offs in chemical (i.e., condensed tannins and total polyphenols) vs. structural defences on different soil types. We did not find strong evidence for a general divide in fine- vs. broad-leaved savannas according to soil fertility, nor for a simple trade-off between chemical and structural defences. Instead, we found savanna species to cluster into three broad defence strategies: species were high in leaf N and either (A) highly defended by spines and chemicals or (B) only structurally defended, or (C) low in leaf N and chemically defended. Finally, we tested for differences in browser utilisation between soil types and among plant defence strategies and found that browsing by meso-herbivores was higher on nutrient-rich soils and targeted species from groups A and B and avoided C, while browsing by elephants was mostly not affected by soil type or defence strategy. We propose a framework that can be used as a basis for asking strategic questions that will help improve our understanding of plant defences in savannas.

Predator-prey size relationships are among the most important patterns underlying the structure and function of ecological communities. Indeed, these relationships have already been shown to be important for understanding patterns of macroevolution and differential extinction in the terrestrial vertebrate fossil record. Stable isotope analysis (SIA) is a powerful remote approach to examining animal diets and paleodiets. The approach is based on the principle that isotope compositions of consumer tissues reflect those of their prey. In systems where resource isotope compositions are distributed along a body size gradient, SIA could be used to reconstruct predator-prey size relationships. We analyzed stable carbon isotope distributions amongst mammalian herbivores in extant and Plio-Pleistocene African savanna assemblages, and show that the range of δ 13 C values among mammalian prey species (herbivores and rodents) increases with body mass (BM), because C 4 plant feeding (essentially grazing) is more common among larger taxa. Consequently, δ 13 C values of mammalian carnivores in these systems are related to species' BM, reflecting a higher average C 4 prey component in the diets of larger-bodied carnivores. This pattern likely emerges because only the largest carnivores in these systems have regular access to the C 4 prey base, whereas smaller carnivores do not. The δ 13 C-BM relationship observed in mammalian carnivores is a potentially powerful approach for reconstructing and parameterizing predator-prey size relationships in contemporary and fossil savanna assemblages, and for interpreting how various behavioral, ecological and environmental factors influence prey size selection., (© 2017 International Society of Zoological Sciences, Institute of Zoology/Chinese Academy of Sciences and John Wiley & Sons Australia, Ltd.)

Background: Older adults are encouraged to participate in regular physical activity to counter the age-related declines in physical and cognitive health. Literature on the effect of different exercise training modalities (aerobic vs resistance) on these health-related outcomes is not only sparse, but results are inconsistent. In general, it is believed that exercise has a positive effect on executive cognitive function, possibly because of the physiological adaptations through increases in fitness. Indications are that high-intensity interval training is a potent stimulus to improve cardiovascular fitness, even in older adults; however, its effect on cognitive function has not been studied before. Therefore, the purpose of this study was to compare the effects of resistance training, high-intensity aerobic interval training and moderate continuous aerobic training on the cognitive and physical functioning of healthy older adults., Methods: Sixty-seven inactive individuals (55 to 75 years) were randomly assigned to a resistance training (RT) group ( n  = 22), high-intensity aerobic interval training (HIIT) group ( n  = 13), moderate continuous aerobic training (MCT) group ( n  = 13) and a control (CON) group ( n  = 19) for a period of 16 weeks. Cognitive function was assessed with a Stroop task and physical function with the Timed-Up-and-Go (TUG) and submaximal Bruce treadmill tests., Results: No significant GROUP x TIME interaction was found for Stroop reaction time ( P  > .05). The HIIT group showed the greatest practical significant improvement in reaction time on the information processing task, i.e. Stroop Neutral (ES = 1.11). MCT group participants had very large practical significant improvements in reaction time on the executive cognitive tasks, i.e. Stroop Incongruent and Interference (ES = 1.28 and 1.31, respectively). The HIIT group showed the largest practically significant increase in measures of physical function, i.e. walking endurance (ES = 0.91) and functional mobility (ES = 0.36)., Conclusions: MCT and RT proved to be superior to HIIT for the enhancement of older individuals' executive cognitive function; whereas HIIT were most beneficial for improvement in information processing speed. HIIT also induced the largest gains in physical function.

Purpose: To determine if a cerebral oxygenation response during cortical activation is influenced by exercise training mode., Methods: Sixty-seven individuals (55-75 years old) volunteered for this 16-week intervention study. Participants were randomized into a resistance training (RT) group (n = 22), high-intensity interval training (HIIT) group (n = 13), moderate continuous training (MCT) group (n = 13) and a control (CON) group (n = 19). Near-infrared spectroscopy was used to measure cerebral oxygenation during the Stroop task. A submaximal Bruce treadmill test was used to measure changes in walking endurance., Results: The GROUP × TIME interaction for reaction time on the naming and executive Stroop conditions were not significant (P > 0.05). At post-test, the CON group showed increased brain activation, with significantly higher relative oxy-haemoglobin (O 2 Hb) values during the naming Stroop condition compared to pre-test (P = 0.03), while their increased relative O 2 Hb on the complex condition showed a distinct trend toward significance (P = 0.09). MCT and HIIT participants exhibited decreased brain activation during the Stroop task, with MCT showing a significant increase in relative deoxy-haemoglobin (HHb) compared to pre-test during the naming and executive Stroop conditions (P < 0.05). The HIIT group improved significantly in walking endurance (P = 0.04)., Conclusion: Sixteen weeks of exercise training resulted in more efficient cerebral oxygenation during cortical activation compared to a no-exercise control group. Furthermore, HIIT and MCT were superior to RT for task-efficient cerebral oxygenation and improved oxygen utilization during cortical activation in older individuals.

Background: It is generally recognised that the physical functioning of older adults is enhanced with resistance exercise. The aim of this study was to investigate the time course of changes in upper and lower body muscle strength and physical function in older individuals following a 16 week resistance training (RT) programme and a similar duration detraining (DET) period., Methods: Forty-one inactive individuals (55 to 75 years) were randomly allocated in a RT group (n = 22; three sessions per week) and a control (CON) group (n = 19). Muscle strength was assessed with 10RM leg and bench press tests, while the Timed-Up-and-Go (TUG) test was used to measure functional mobility. The Bruce treadmill test determined the participants' submaximal endurance capacity. Data were analysed using mixed model repeated measures ANOVA and P < 0.05 was considered statistically significant., Results: Main treatment effects were found for muscle strength (P < 0.001) and functional mobility (P < 0.05). Upper and lower body strength generally showed a statistically significant improvement after every 4 weeks in RT (the increase after 16 weeks being 7.3 ± 4.9 kg and 86.6 ± 44.4 kg, respectively; P < 0.001) while TUG performance (-0.2 ± 0.4 s; P < 0.05) and submaximal endurance capacity (0.7 ± 0.9 min; P < 0.001) only improved after 16 weeks. Although muscle strength decreased after DET, it was still better than at baseline. No significant improvements in any performance variable were observed in CON directly after the intervention period (0-16 weeks) (P > 0.05)., Conclusion: A 16-week RT programme has positive effects on both muscular and physical function in older adults, although the time course of these adaptations is different. While the gains in muscle strength and submaximal endurance capacity were not totally lost after DET, functional mobility was completely reversed. Older adults can be reassured that if the need arises to discontinue RT for a certain period they will still retain a large amount of their acquired muscle strength, as well as a degree of physical function such as submaximal endurance capacity. The association between leg strength and submaximal endurance capacity strengthens the notion that RT should be incorporated in training and rehabilitation programmes of ageing and frail older adults.