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Determinants of Tropical Savannas
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.
... Together with herbivory, fire and precipitation are major factors driving species distribution and the cyclicity between dry and wet seasons in savannas (Solbrig et al., 1996). By the end of the dry season, the herbaceous layer (mostly grasses) dries out, increasing the flammability of the system (Zanzarini et al., 2022). ...
Question
How does seasonality affect the below‐ground bud bank dynamics in open savannas from the Cerrado and the density of below‐ground bud‐bearing organs? As the Cerrado is subject to marked seasonality, many herbaceous plants may lose their above‐ground biomass during the dry season, and regenerate it later in the rainy period through resprouting from below‐ground buds. Herein, we tested the hypothesis that seasonality may directly affect bud bank dynamics and below‐ground bud‐bearing organs, with a larger bud bank size and higher densities of below‐ground organs in the rainy season.
Location
Open savannas of the Cerrado, southeastern Brazil.
Methods
The bud bank was sampled in three open areas of the Cerrado. In each area, 10 plots were established, and, within each plot, the biomass was sampled to 10 cm below‐ground level. In the lab, buds and below‐ground organs were counted. We fitted generalized linear mixed models (GLMM) to test the differences in the bud bank and the below‐ground organ densities between the dry and rainy seasons.
Results
The bud bank size was smaller in the dry season. Buds found in the dry season suggest they were developed in the rainy season and remained dormant, not forming new shoots. Non‐woody rhizomes of caespitose grasses were more abundant during the rainy season. Xylopodia and woody rhizome densities were not significantly affected by seasonality.
Conclusions
Bud bank dynamics in the Cerrado is affected by seasonality. There is a decrease in bud bank density in the dry season, suggesting that buds were developed into new branches in the rainy season, but not all of them develop into new shoots in the same season. Thus, the bud bank evaluated in the dry season probably is formed by an accumulation of dormant preformed buds, highlighting the occurrence of a resprouting dynamic of native species of the Cerrado in the rainy season.
... 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. ...
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). ...
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). ...
The frequency of extreme drought events has been rising worldwide, but due to its unpredictability, how plants will respond remains poorly understood. Here, we aimed to characterize how the hydraulics and photosynthesis of savanna plants respond to extreme drought, and tested whether they can subsequently recover photosynthesis after drought. There was an extreme drought in 2019 in Southwest (SW) China. We investigated photosynthetic gas exchange, leaf-, stem-, and whole-shoot hydraulic conductance of 18 plant species with diverse leaf habits (deciduous, semi-deciduous and evergreen) and growth forms (tree and shrub) from a dry-hot valley savanna in SW China for three rainy seasons from 2019 to 2021. We also compared photosynthetic gas exchange to those of a regular year (2014). We found that leaf stomatal and hydraulic conductance and maximum photosynthetic rate were significantly lower during the drought in 2019 than in the wetter years. In 2019, all studied plants maintained stomatal conductance at their minimum level observed, which could be related to high vapor pressure deficits (VPD, >2 kPa). However, no significant difference in stem and shoot hydraulic conductance was detected across years. The reductions in leaf hydraulic conductance and stomatal regulation under extreme drought might help keep the stem hydraulic function. Stomatal conductance and photosynthesis after drought (2020 and 2021) showed comparable or even higher values compared to that of 2014, suggesting high recovery of photosynthetic gas exchange. In addition, the response of hydraulic and photosynthetic traits to extreme drought was convergent across leaf habits and growth forms. Our results will help better understand the physiological mechanism underlying the response of savanna ecosystems to climate change.
Spiders are extremely abundant throughout the country, but our knowledge of the Indian spiders is very fragmentary. Nonetheless only a few studies have been carried out on spiders of Andaman and Nicobar Islands. The primary objective was to provide an exhaustive checklist comprising the spider fauna of Great Nicobar Biosphere Reserve. The second objective of this study was to study the effects of habitat dynamics of agricultural fields and forest structure on orb-web weaving spiders (Araneae: Araneidae) and ground hunters. Agrobionts are species which reach high dominance in agroecosystems. The aim was to obtain a better understanding of how the life history of agrobiont or weavers enables them to persist in highly disturbed and ephemeral agricultural habitats. The survey was carried out in ten different locations, and sampling was done during December 2017 to November 2018. We collected 1507 spider samples from which we identified 55 species. Salticidae, Corinnidae, Sparassidae, and Hersiliidae were among the taxonomically problematic families. The spider species diversity, evenness, and species richness from studied sample show mixed data for each location. To a certain degree, increased abundance in the spider community paralleled to relative vegetation cover, but not for all months, and these results also cope with previous studies. Preservation of the diversity of biosphere reserves and also supplementing more monitoring data can conserve spiders in such a large landscape.KeywordsSpidersDistributionDiversityAbundanceGreat Nicobar
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.
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.
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—C 3 trees and C 4 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.
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
The aim of this paper is to describe spatial variation in the woody plant biomass in Burkea africana savannas, and some other related southern African savannas. Specific components of biomass considered include plant organ type, vertical distance from ground level and lateral extent from the plant base.