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Abstract

Tropical savannas, defined as ecosystems formed by a continuous layer of graminoids (grasses and sedges) with a discontinuous layer of trees and/or shrubs, are the most common vegetation type (physiognomy) in the tropics. Tropical savannas are found over a wide range of conditions: rainfall from approximately 200 mm to 1500 mm a year, temperature from subtropical regimes such as the South American Chaco and the South-African savannas with temperature seasonality and cold-month average temperatures below 10 °C, to low-latitude savannas with no temperature seasonality, and soils from volcanic soils such as in parts of the Serengueti plains in Tanzania to dystrophic soils such as in the Brazilian cerrados. The one constant climatic characteristic of tropical savannas is rainfall seasonality. Yet the duration of the dry season can vary from 3 to 9 months, with a mode of 5 to 7 months.
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... Tropical grassy biomes -savannas and grasslands -dominate the tropics (Bourlière and Hadley 1983, Solbrig et al. 1996, Bond and Parr 2010, covering about 20% of the world's land surface (Cole 1986, Scholes and Archer 1997, Franklin 1999, Sankaran et al. 2005, Parr et al. 2014. They are globally important to human economies, supporting a large proportion of the world's human population and most of its rangeland, livestock and biomass of large, wild herbivore (Scholes and Archer 1997, Sankaran et al. 2005, Bond and Parr 2010, Parr et al. 2014, providing important ecosystem services and influencing the earth'satmosphere (Bond 2008, Parr et al. 2014. ...
... They are globally important to human economies, supporting a large proportion of the world's human population and most of its rangeland, livestock and biomass of large, wild herbivore (Scholes and Archer 1997, Sankaran et al. 2005, Bond and Parr 2010, Parr et al. 2014, providing important ecosystem services and influencing the earth'satmosphere (Bond 2008, Parr et al. 2014. Savannas are typical in areas where rainfall is highly seasonal (Johnson and Tothill 1985, Solbrig et al. 1996). ...
... Savannas are the dominant vegetation type of northern Australia (Wilson et al. 1990, Gillison 1994, extending from south-eastern Queensland across the north of the continent (Solbrig et al. 1996), and covering almost a quarter of Australia (Williams and Cook 2001b (Cole 1986). The variation in vegetation types across Australian savannas are driven by rainfall and soil patterns (Williams et al. 1996, Williams andCook 2001b) and, although eucalypts (Eucalyptus spp., Corymbia spp.) and Acacia spp. ...
Thesis
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Thesis Abstract Species all over the world are declining to unsustainable population levels due to habitat destruction, introduced species and pollution. Susceptible species begin to decline prior to noticeable degradation of the environments in which they live. Savannas are the dominant vegetation type in northern Australia and, although considered relatively unmodified, have experienced severe faunal decline, particularly among granivores. Granivorous birds represent 20% of Australian land birds and range-scale declines have been documented for several species. Declines of the granivorous birds of Australian tropical savannas are associated with land use change resulting from European settlement, such as land clearance, changes in fire regime, pastoralism, introduced species and shrub proliferation. Although it is generally recognized that granivorous birds have declined, there are still many gaps in our understanding of the underlying causes. The Black-throated finch southern subspecies (Poephila cincta cincta, herein referred to as BTF) is an endangered, endemic granivore of eastern Australia. BTF have had an estimated range contraction of 80% since the late 1970s. Little is known about BTF ecological requirements, home range size or movement patterns. Increased knowledge of these will greatly assist with management and conservation actions. In this thesis, I investigate the ecology of BTF in north-eastern Australia. I examine (a) home range sizes, movement patterns and habitat use, and (b) habitat requirements, nesting and foraging site selection. Understanding how animals move in the landscape to meet their demands – food, water, and shelter - is a prerequisite for successful conservation outcomes. To acquire more information about BTF movements, I mist netted in eight sites on the Townsville Coastal Plain (TCP) and radio-tracked in two of these sites. I color-banded 102 BTF and estimated the home ranges of 15. More than half of all resightings occurred within 200m of the banding site and within 100 days of capture. Long distance movements (up to 17 km) were recorded for only three individuals. Home range size differed between sites but not between seasons (early dry season and late dry season). BTF home ranges encompassed four broad vegetation types among eight available (Regional Ecosystems), with habitat selection significantly different from random. BTF showed a distinct local daily movement pattern at one site (roosting to feeding area). In this study, BTF maintained home ranges ranging from 25.2 to 120.9 ha over short time scales (e.g. within seasons). Vegetation structure and composition greatly influence the way animals use habitat. Determining key habitat features for a threatened species is paramount for identifying appropriate management actions. To understand these for BTF, vegetation surveys were undertaken at 10 sites on TCP. BTF flocks were most closely associated with higher cover of native grasses, low shrub cover, with the presence of dead trees and high cover of certain grass species, e.g. Eragrostis spp. Small flocks were associated with low percentages of native grasses, high shrub cover and low grass richness. BTF showed a preference for nesting at sites with lower tree and grass diversity, but no preferences for ground cover structural features. Other environmental structural features in the nesting habitat might be more important to the birds. I explored BTF nesting habitat selection by comparing areas around nests (used) with that in the surrounding area (available). Individual nests were used for breeding and/or roosting. Fifty active BTF nests were found during this study. BTF nested in four tree species, preferentially using Eucalyptus platyphylla and Melaleuca viridiflora in areas of low tree density. BTF showed a preference for nesting in sites with lower tree and grass diversity, but no preference for ground cover structural parameters. Other environmental features in the nesting habitat might be more important to the birds as structural features. Specific patches where animals are exploiting resources differ in nature and appearance from the matrix in which they are embedded. Being ground-foragers, BTF are likely to be specific in the micro-structure preferences of foraging patches, such as the presence of bare ground patches. To understand the BTF needs, foraging patches – specific areas where animals were exploiting resources – were examined. Vegetation structure and composition were compared between foraging patches (used) and surrounding (neighboring) and general areas (available). Of the ground cover structural characteristics, nearly all variables were significantly different between used, neighbouring and available areas. BTF foraged preferentially in areas with lower diversity of grasses but nearby areas with high diversity (neighbouring areas). BTF selected specific structural vegetation features for foraging patches, compared with neighboring and available areas, particularly the ground cover features. Foraging patches were less densely vegetated than neighboring and available areas; however, they adjoined areas with high grass structural complexity (higher visual obstruction, higher vegetation density in almost all levels measured, higher vegetation cover, particularly grass cover, and higher number of species per hectare). In summary, foraging BTF require open areas finely interspersed with to grassy areas. In this study, I found that BTF habitat must encompass patches with suitable grasses (e.g. Eragrostis spp.), and patches with bare ground or low vegetation density (ground cover) to allow BTF access to the seed bank. BTF prefer a general absence of shrubs but the scattered presence of a medium strata. Therefore, large homogeneous areas will not usually meet the requirements of BTF populations. Extensive woody thickening could disadvantage BTF, as has been found for other granivorous birds such as the Golden-shouldered parrot (Psephotus chrysopterygius). Instead, they require a mosaic of vegetation within their daily home range: areas with bare ground finely interspersed with areas of suitable grass species, low shrub density, presence of suitable woody plant cover and the presence of species such as Eucalyptus platyphylla and Melaleuca spp.
... The shrubs and grasses are located in the southern and eastern lowlands of the country. The shrubs and grasses are part of the savannas in Africa and are characterized by low cation exchange capacity, very low phosphorus, and nitrogen (Solbrig & Medina, 1996). Evergreen broad-leaf trees are the most productive forest types followed by deciduous broad-leaf trees, grass, and shrubs, respectively. ...
Article
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Global environmental changes have implications for the terrestrial ecosystem functioning, but disentangling individual effects remains elusive. The impact of vegetation responses to increasing atmospheric CO2 concentrations is particularly poorly understood. As the atmospheric CO2 concentration increases, the CO2 acts as a fertilizer for plant growth. An increase in atmospheric CO2 reduces the amount of water needed to produce an equivalent amount of biomass due to closing or a narrowing of the stomata that reduces the amount of water that is transpired by plants. To study the impacts of climate change and CO2 fertilization on plant growth, we analyzed the growing season sensitivity of plant growth to climatic forcing from alpine to semi-desert eco-climatic zones of Ethiopia for various plant functional types over the period of 1982–2011. Growing season 3rd generation Normalized Difference Vegetation Index of Global Inventory Modeling and Mapping Studies (NDVI) was used as a proxy of plant growth, while mean growing season precipitation (prec), temperature (temp), and solar radiation (sr) as the climate forcing. The sensitivities of plant growth are calculated as a partial correlation, and a derivative of NDVI with respect to prec, temp and sr for earliest and recent 15-year periods of the satellite records, and using a moving window of 15-year. Our results show increasing trends of plant growth that are not explained by any climate variables. We also find that an equivalent increase in prec leads to a larger increase in NDVI since the 1980s. This result implies a given amount of prec has sustained greater amounts of plant foliage materials over time due to decreasing transpiration with increasing CO2 concentration as expected from the CO2 fertilization effect on water use efficiency and plant growth. Increasing trends of growth in shallow-rooted vegetation tend to be associated with woody vegetation encroachment.
... The water restriction during the dry season has an important effect in the herbaceous plant community: the aerial biomass of most of the species dries, providing significant amounts of fuel material (Govender et al. 2006), which can lead to fire, an important determinant of savanna vegetation (Solbrig et al. 1996;Lehmann et al. 2014). If fire occurs, plants have resprouting traits allowing them to quickly produce aerial biomass after fire, which give advantage on competition with other plants (Zaloumis and Bond 2011). ...
Article
In seasonal systems, species have specific traits to deal with water shortage. This is the case for plants in Neotropical savannas, where the dry season can endure 3 to 5 months without rain. Ecophysiological strategies to deal with the dry period are well known for savanna trees, however little is known for non-arboreal species such herbs and subshrubs concerning their water use strategies. In this study we analyzed traits related to water and nutrient storage in underground organs and leaves of non-arboreal Cerrado (Neotropical savanna of Brazil) plants. We hypothesized that underground organs are able to retain water and nutrients allowing non-arboreal plants to resprout even during water shortage. We measured wood density (WD) and saturated water content (SWC) in underground organs of 15 species differing in their growth form (5 herbs, 5 subshrubs and 5 shrubs). We also measured macronutrients concentration in underground organs and leaves of these species. We tested relationships between WD and SWC and between concentrations of underground and leaf macronutrients. We found that WD and SWC are negatively related, and that herbs showed the lowest WD, but the highest capacity to store water (higher SWC). We did not find any significant difference concerning nutrient content in underground organs and leaves among the growth forms; however, the leaf nutrient content was positively affected by the nutrient content in underground organs for N, K and Ca. We conclude that underground organs may have an important role on storing water and nutrients in savanna herbaceous species, which will promote their resprout even during the dry period.
... Savannas can be discovered over an extensive variety of climatic situations with annual rainfall of less than 300 mm to extra than 1,500 mm and are generally characterized by the coexistences of trees and grasses (Solbrig et al., 1996). ...
... Mortality of long-lived savanna trees not only affects population dynamics but may modify biodiversity (Huston 1994) and ecosystem functioning (Solbrig et al . 1996). Consequently, extensive dieback may constitute a major ecological perturbation. Tree dieback has increased worldwide in magnitude and extent since the turn of the century, with unprecedented losses of mostly the dominant species (Fensham & Holman 1999) occurring in Australia, North America and Europe (Heatwole & Lowman 1986). The cause ...
Article
The aim of this study was to understand how communities of adult and juvenile (seedlings and saplings) woody plants were impacted by fire and the 2014–2016 El Niño drought in Kruger National Park, South Africa. We used a landscape‐scale fire experiment spanning 2013–2019 in a semi‐arid savanna in the central west of Kruger National Park (mean annual precipitation, 543 mm). Adult and juvenile woody species composition were recorded during and after the drought in 40 plots that experienced a mix of no fire, moderate fire, and frequent fire treatments. Using multivariate modeling, we related community composition in juvenile and adult woody plants to year of sampling and the experimental fire treatments. Post‐drought, there was significant adult woody plant top‐kill, especially in dominant species Dichrostachys cinerea (81% reduction in abundance), Acacia nigrescens (30%), and Combretum apiculatum (19%), but there was no significant change in adult species richness. Two years post‐drought, abundance of all juveniles decreased by 35%, and species richness increased in juveniles in both the frequent fire (7%) and no fire treatments (32%). Counter‐intuitively, the El Niño drought increased species richness of the woody plant community due to the recruitment of new species as juveniles, a potential lasting impact on diversity, and where different fire regimes were associated with differences in community composition. Drought events in semi‐arid savannas could drive temporal dynamics in species richness and composition in previously unrecognized ways. Using woody species abundance data collected in a landscape scale fire experiment in a semi‐arid savanna in the Kruger National Park, South Africa, we related community composition in juvenile and adult woody plants to year of sampling and the experimental fire treatments using multivariate modelling. Drought increased species richness of the woody plant community which is due to the recruitment of new species as juveniles, a potential lasting impact on diversity, and where different fire regimes favoured differences in community composition. Drought events in semi‐arid savannas could drive temporal dynamics in species richness and composition in previously unrecognised ways.
Article
Question Savannas are globally widespread and furnish a variety of ecological services through their structural heterogeneity. Unfortunately, those essential ecosystem services are threatened by climate changes including expected increases in duration of drought and nitrogen deposition. The objective of this study was to determine how overall herbaceous production, functional group production and diversity respond in the short‐term to interactions between forecasted environmental conditions and prescribed fires. Location Western Edwards Plateau, Texas (latitude 31°N, longitude 100°W). Methods We randomly assigned full‐factorial treatment combinations of rainout shelters, simulated nitrogen deposition and prescribed fires to field plots in an herbaceous‐dominated area of a semi‐arid savanna. Aboveground net primary productivity (assessed via destructive harvest) and diversity (using Shannon’s index) were assessed as indicators of ecosystem functioning. Results Total aboveground net primary production was reduced by fire in the short‐term (4 months), and reduced by drought at 8 months, but drought reductions were later overridden by lagged responses to large precipitation events (12 months). Forb production increased in response to nitrogen addition (8 months) and drought (12 months), but decreased as a result of fire (12 months). Live grass production was consistently reduced by drought. Plant species diversity was positively responsive to nitrogen addition, particularly in the absence of drought. Conclusions Our findings suggest that the concentrated precipitation events that are forecasted to follow extended droughts may aid rapid recovery of drought‐induced production decreases. In addition, the small‐scale diversity of this semi‐arid savanna may be driven more by resource availability than light‐competition in the short‐term. Managers and ecologists can use these results to help disentangle the ecosystem functioning that may be observed in the presence of future droughts, nitrogen deposition and prescribed fire. Understanding these processes will be key to protecting the integrity of savannas.
Article
Climate change is expected to lead to more frequent, intense and longer droughts in the future, with major implications for ecosystem processes and human livelihoods. The impacts of such droughts are already evident, with vegetation dieback reported from a range of ecosystems, including savannas, in recent years. Most of our insights into the mechanisms governing vegetation drought responses have come from forests and temperate grasslands, while responses of savannas have received less attention. Because the two life forms that dominate savannas—C3 trees and C4 grasses—respond differently to the same environmental controls, savanna responses to droughts can differ from those of forests and grasslands. Drought‐driven mortality of savanna vegetation is not readily predicted by just plant drought‐tolerance traits alone, but is the net outcome of multiple factors, including drought‐avoidance strategies, landscape and neighborhood context, and impacts of past and current stressors including fire, herbivory and inter‐life form competition. Many savannas currently appear to have the capacity to recover from moderate to severe short‐term droughts, although recovery times can be substantial. Factors facilitating recovery include the resprouting ability of vegetation, enhanced flowering and seeding and post‐drought amelioration of herbivory and fire. Future increases in drought severity, length and frequency can interrupt recovery trajectories and lead to compositional shifts, and thus pose substantial threats, particularly to arid and semi‐arid savannas. Synthesis. Our understanding of, and ability to predict, savanna drought responses is currently limited by availability of relevant data, and there is an urgent need for campaigns quantifying drought‐survival traits across diverse savannas. Importantly, these campaigns must move beyond reliance on a limited set of plant functional traits to identifying suites of physiological, morphological, anatomical and structural traits or “syndromes” that encapsulate both avoidance and tolerance strategies. There is also a critical need for a global network of long‐term savanna monitoring sites as these can provide key insights into factors influencing both resistance and resilience of different savannas to droughts. Such efforts, coupled with site‐specific rainfall manipulation experiments that characterize plant trait–drought response relationships, and modelling efforts, will enable a more comprehensive understanding of savanna drought responses. Drought‐driven mortality of savanna vegetation is the net outcome of multiple factors, including drought tolerance and avoidance strategies, landscape and neighbourhood context, and past and current stressors including fire, herbivory and competition. Our ability to predict savanna drought responses is currently limited by availability of relevant data, and there is an urgent need for campaigns quantifying drought‐survival traits across diverse savannas. Importantly, these campaigns must move beyond reliance on a limited set of plant functional traits to identifying suites of physiological, morphological, anatomical and structural traits or “syndromes” that encapsulate both avoidance and tolerance strategies.
Chapter
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Resumo O caso das savanas tropicais da América do Sul ilustra as complexas inter-relações entre as características ecológicas dos ecossistemas terrestres naturais e os seus usos. As savanas tropicais estão determinadas por um conjunto de limita-ções ambientais, i.e., marcada sazonalidade das precipitações, períodos de ex-cesso e déficit hídrico no solo, altas temperaturas e altas taxas de evapotranspiração potencial, conduzindo ao desenvolvimento de solos pobres em nutrientes e a queimadas recorrentes. Porém, as savanas tropicais são ecossistemas muito ricos em espécies, o que leva a considerar esta diversidade entre as mais importantes do mundo. Além da riqueza em espécies, a sua diversidade funcional é também elevada, devido a que as savanas se caracterizam por um expressivo mosaico de diferentes ecossistemas quando consideradas na escala espacial da paisagem. O funcionamento das savanas influencia claramente as alternativas de uso da terra nas regiões que estas ocupam. Nas maiores regiões de savana tropical da América do Sul, os Llanos Colombo-Venezolanos, o Cerrado Brasileiro e os Llanos de Beni na Bolívia, um sistema produtivo baseado na criação extensiva de gado chegou a ser dominante desde 1700 até 1950. Os sistemas de fazendas, i.e. hatos na América hispana, foram ecologicamente sustentáveis porque exploraram enor-mes extensões de savana com cargas animais muito baixas; no entanto, foram também economicamente bem-sucedidos porque, uma vez assegurada a proprie
Article
Savanna vegetation maintains its openness and its diverse plant composition because of frequent fire events; however, when these are suppressed, encroachment is caused by increases in the tree density. In the neotropical savanna (cerrado of Brazil), typical forest trees are invading savanna areas, altering abiotic conditions and affecting the persistence of their exclusive species. Here we studied changes in abiotic conditions, species richness and diversity of a non-arboreal community (herbs, vines, grasses, subshrubs and shrubs) in a gradient of encroachment at a site where fire has been suppressed for more than 50 years in south-eastern Brazil. Encroached communities were more shaded and possessed a wetter and richer soil (higher contents of organic matter and P) compared with the typical savanna. These abiotic changes were related to a less rich and less diverse plant community in encroached savanna compared with typical savanna. The most important abiotic variables driving such changes were photosynthetically active radiation (PAR) reaching the understorey and soil P content: communities with lower species richness and diversity had lower PAR incidence and higher soil P content. Our results suggest that non-arboreal savanna species may be under serious threat of extinction given the expected ecological changes caused by the widespread expansion of forest on the savannas in the absence of fire.