Tropical savannas

The research conducted in Lamto savannah under Maxime Lamotte direction, particularly in the International Biological Program (1968-1978), aimed at working out the energetic balance of savannah. It adopted a then pioneering systemic approach involving standard quantitative methods. The evaluation of primary production was central in such a task. The basis of the method was to follow up different kinds of phytomasses. Plants species and organs were sorted out according to their alive or dead condition and also to functional typologies of their phenological cycles. The collaboration with zoologists who were studying the consumers and decomposers of phytomass focused attention on the seasonal distribution of production. In the late 1960s and early 1970s, Lamto was one of the pioneering project where primary production estimates were founded not only on above-ground biomass changes, but also on estimates of below-ground production. Furthermore the methods of above grounds estimates were improved in order to take full account of organ losses and turnover of plant material. For instance additional studies of the clonal growth of grass species were developed. Such accurate estimates of the primary production set the ecological surveys of the vegetation at Lamto at the onset of the modern functional plant ecology. The Lamto project was one of the first able to demonstrate that the primary production of the savannah was higher than expected. Most of the production comes from grass species and especially from their underground parts. Analyses of vertical and horizontal structure of vegetation were also undertaken from the beginning to assess another type of variability. It led to explain how availability of plant material affects animal composition in the ecosystem. The results also contributed to explain how and why vegetation changes on the climatic gradient in West-Africa. Further work, mostly modelling, would go on with associating structure, function, and dynamics. Despite the fact that Maxime Lamotte was an early pioneer of quantitative work, his vision of ecosystem was however a deeply naturalist one, that also considered the long-term time and its evolutionary aspects. Therefore he encouraged various work, from inventories of fauna and flora to landscape's dynamics studies involving the “écocomplexe” concept. He also encouraged studies in physical and human geography. Studies on interaction between the different biocenosis of savannah-forest mosaic were thus performed. They particularly contributed to forge the perception, now well accepted, of fire as a “natural” disturbance and to elucidate the colonization dynamics of savannah by forest. Thus the imposed frame of quantifying the energy budget of savannah, which could be felt as limiting, did not in fact prevent the use of various methods and concept in plant ecology work in Lamto; it yielded a wealth of novel results.

The general aim of this project is to provide a new kind of approach for predictive modeling in GIS and potential palynological GIS applications. The specific objective of this study is to improve our understanding of the Colombian savanna, with the focus on the application of GIS which consists of: a) Database design b) Predictive modeling c) Palynological data implementation The Database design is the actual composition of a complete dataset in GIS of the selected area of interest. This includes data of the elevation, (political and natural) boundaries, land cover (vegetation), hydrology, climate and soil. The Predictive modeling-section aims at modeling - with GIS and a statistical technique called logistic regression - the occurrence of savanna. The Palynological data implementation in GIS is looked at from a data quality perspective, determining the suitability of palynological data use in geographical information systems.

La diversidad funcional permite analizar los papeles ecológicos de las especies y cómo su interacción influye sobre la estructura de los ecosistemas. Esta aproximación se constituye como un componente clave en el entendimiento de las dinámicas ecológicas de ecosistemas estratégicos para la conservación, como las sabanas inundables de la Orinoquia. Las sabanas han estado sometidas a cambios en su composición y estructura como consecuencia de la introducción de especies vegetales foráneas, de ganadería extensiva y de la quema indiscriminada. En este trabajo se realiza una aproximación a la diversidad funcional de los mamíferos de las sabanas inundables de Cravo Norte y Puerto Rondón. A partir de muestreos entre 2016 y 2018, se registraron 63 especies de mamíferos asociados a las sabanas inundables. Para estimar la diversidad funcional, se compilaron los atributos ecológicos de cada especie, incluyendo gremio trófico y categoría de amenaza nacional. En general, las sabanas están principalmente dominadas por procesos de regulación llevados a cabo por insectívoros, herbívoros y carnívoros, principalmente. Los gremios están compuestos de especies especialistas y altamente vulnerables a disturbios antrópicos. Estas especies, a través de sus papeles funcionales, están influyendo directa e indirectamente en la estructura vegetal de las sabanas inundables, moldeando así mismo la dinámica ecosistémica.

The dispersal of seeds by cattle on fallows was asscssed in a sudanian region of West Africa. AlI the feces were collected in four stands during two years from March 1994 to May 1996 and secds werc manually extracted. Cow dungs represent the main part of aIl the feces collected. They were found on fallows maînly during the dry season when the cultivated area is open to cattle grazing. CaUle disperse weeds and some woody species that are in majority leguminous. Specics diversity in dung is low and only sorne species such as Borreria stachydea. a weed, or Gardenia erubescens. a shrub, constitute the greatcr part of the seeds disperscd by caule. Seed content of dung varies along the year, it is high from November to March and low in October, April and May and during the wet season. Weeds are dispersed by cattle during the whole dry season but are especial1y abundant from December to February when woody species are mainly dispersed from January to March. Cattle acts principal1y as a disperser of speeies that colonize open habitat and play a role in the successions of vegetation that occur during the fal1ow-cu1tivation cycle.

Biomass of roots, the root : shoot ratio (ratio of below-to aboveground biomass) and carbon stocks belowground (to 100-cm depth) were estimated in different open savannah environments in the extreme north of the Brazilian Amazon. Sampling was conducted in permanent plots established in two open savannah areas in the state of Roraima. We identified four phytopedounits in the 27 plots sampled in two areas: four in dry grasslands on Argisol/Ultisol soils (DG-Arg), eight in dry grasslands on Latosol/Oxisol soils (DG-Lts), five in a mosaic of grasslands with savannah-parkland on Latosol/Oxisol soils (GP-Lts) and 10 in seasonally flooded (wet) grasslands on Hydromorphic/Entisol soils (WG-Hyd). Fine roots (<2 mm diameter) dominated the 0-100-cm vertical profile in the four phytopedounits (>92.5%). Biomass of the roots in WG-Hyd (29.52 AE 7.15 Mg ha -1 ) was significantly higher as compared with the other phytopedounits studied, although the carbon stocks did not differ among the phytopedounits (6.20-7.21 MgC ha -1 ). The largest concentration of roots was found in the upper three 10-cm sections of the soil profile, ranging from 56.3 to 82.9% in the four environments. The root : shoot ratio based only on living biomass of roots with diameter !2 mm (standard Intergovernmental Panel on Climate Change methodology) ranged from 0 for seasonally flooded grasslands to 0.07-0.20 for unflooded grasslands on clay soils. The results indicate that the root : shoot ratio (expansion factor) for belowground biomass in open savannah ecosystems in the northern Amazon are low and differ from the default values used in Brazil's reference report to the Climate Convention.

En Afrique de l'Ouest, plusieurs espèces ligneuses sont des ressource connues pour l'alimentation humaine ou animale; plusieurs études en ont souligné l'importance Chandrasekharan, 1993; F.A.O., 1995). Dans la région de Bondoukuy, certaine espèces fourragères sont ainsi fortement émondées et plusieurs espèces ligneuses spontanée aux fruits comestibles et recherchés sont pré ervées et fréquemment maintenues dans le champs. L'objectif de ce travail e tde mettre en évidence l'état de populations de ce espèce ain i que leurs préférences écologiques, en particulier vi -à-vis des sols, du couvert végétal et de l'utilisation des terres.

The presence of a savanna-forest vegetation mosaic in the Gran Sabana, Canaima National Park (CNP), has been more than an academic controversy since the 1980s through to the 2010s in Venezuela. Scientists, Park administrators and officials from institutions devoted to protect the Caroní river basin within the limits of the Park, argued that the presence of savannas under a tropical rainfall regime that could support humid forests was due to the fire practices used by the local Pemón Indigenous communities. This misconception justified applying fire suppression policies, aimed at putting out all types of fires in CNP, especially the “compulsive burning” (thus so called by some scientists) by Pemón people in forest areas. This paper describes the initiatives, pursued for more than 20 years, to consolidate intercultural and participatory fire management in the CNP, Gran Sabana, which evolved as result of several participatory action-research projects coordinated by academics, and suppo...

The presence of a savanna-forest vegetation mosaic in the Gran Sabana, Canaima National Park (CNP), has been more than an academic controversy since the 1980s through to the 2010s in Venezuela. Scientists, Park administrators and officials from institutions devoted to protect the Caroní river basin within the limits of the Park, argued that the presence of savannas under a tropical rainfall regime that could support humid forests was due to the fire practices used by the local Pemón Indigenous communities. This misconception justified applying fire suppression policies, aimed at putting out all types of fires in CNP, especially the “compulsive burning” (thus so called by some scientists) by Pemón people in forest areas. This paper describes the initiatives, pursued for more than 20 years, to consolidate intercultural and participatory fire management in the CNP, Gran Sabana, which evolved as result of several participatory action-research projects coordinated by academics, and suppo...

The West African savannas are subject to changes in fire regimes related to land use intensification, which may infer significant biological modifications. We investigated the efficiency of MODIS fire products to account for the variability of fire regimes in relation to changes in land cover and savanna vegetation. The specificity and complementarities of both MODIS active fire (MOD14A2 and MYD14A2) and burned area (MCD45A1) products were assessed in order to characterize fire regimes and to relate them with land cover. In addition the distribution of plant species between landscape units characterized by specific fire regimes was assessed. The calculation of the mutual information made it possible to set apart species more frequent in landscapes little or not at all subject to fires and species more frequent in burnt areas. The first group is the expression of the nature of the unburnt land-cover units, mainly constituted by the agricultural areas dominated by fields and fallows. It consists of more potential threats (weeds, encroachers, invasive species etc.) than the second group, which is more typical of sudanian savannas. It thus appears that unburnt landscape units are currently more sensitive and vulnerable to some biological threats than the burned savannas, where fire provides the long-term stability needed to preserve the ecosystem.

Termitaria are major sites of functional heterogeneity in tropical ecosystems, through their strong influence on soil characteristics, in particular soil physico-chemical properties and water status. These factors have important consequences on nutrient availability for plants, plant spatial distribution, and vegetation dynamics. However, comprehensive information about the influence of termite-rehandled soil on soil water regime is lacking. In a humid shrubby savanna, we characterized the spatial variations in soil texture, soil structure and maximum soil water content available for plants (AWC max) induced by a large termite mound, at three depths (0–0.10, 0.20– 0.30 and 0.50–0.60 m). In addition, during a three month period at the end of the rainy season, soil water potential was surveyed by matrix sensors located on the termite mound and in the surrounding soil at the same depths and for the 80–90 cm layer. Concurrently, the leaf shedding patterns of two coexisting deciduous shrub species exhibiting contrasted soil water uptake patterns were compared for individuals located on termite mounds and in undisturbed control areas. For all the soil layers studied, clay and silt contents were higher for the mound soil. Total soil clods porosity was higher on the mound than in control areas, particularly in the 0.20-0.60 m layer, and mound soil exhibited a high shrinking/swelling capacity. AWC max of the 0-0.60 m soil layer was substantially higher on the termite mound (112 mm) than in the surroundings (84 mm). Furthermore, during the beginning of the dry season, soil water potential measured in situ for the 0.20-0.90 m soil layer was higher on the mound than in the control soil. In contrast, soil water potential of the 0-0.10 m soil layer was similar on the mound and in the control soil. In the middle of the dry season, the leaf shedding pattern of Crossopteryx febrifuga shrubs (which have limited access to soil layers below 0.60 m) located on mounds was less pronounced than that of individuals located on control soil. In contrast, the leaf shedding pattern of the shrub Cussonia barteri (which has a good access to deep soil layers) was not influenced by the termite mound. We conclude that in this savanna ecosystem, termite mounds appear as peculiar sites which exhibit improved soil water availability for plants in upper soil layers, and significantly influence aspects of plant function. Implications of these results for understanding and modelling savanna function and dynamics, and particularly competitive interactions between plant species, are discussed.

Fine-scale biomass maps offer forest managers the prospect of more detailed and locally accurate information for measuring, reporting and verification activities in contexts, such as sustainable forest management, carbon stock assessments and ecological studies of forest growth and change. In this study, we apply a locally validated method for estimating aboveground woody biomass (AGWB) from Advanced Land Observing Satellite (ALOS) Phased Array-type L-band Synthetic Aperture Radar (PALSAR) data to produce an AGWB map for the lowland pine savannas of Belize at a spatial resolution of 100 m. Over 90% of these woodlands are predicted to have an AGWB below 60 tha −1 , with the average woody biomass of these savannas estimated at 23.5 tha −1 . By overlaying these spatial estimates upon previous thematic mapping of national land cover, we derive representative average biomass values of ~32 tha −1 and ~18 tha −1 for the previously qualitative classes of "denser" and "less dense" tree savannas. The predicted average biomass, from the mapping for savannas woodlands occurring within two of Belize's larger protected areas, agree closely with previous biomass estimates for these areas based on ground surveys and forest inventories (error ≤20%). However, biomass estimates derived for these protected areas from two biomass maps produced at coarser resolutions (500 m and 1000 m) from global datasets overestimated biomass (errors ≥275% in each dataset).

Unité de recherche 136 IRD, Institut de recherche pour le développement (IRD), Centre IRD d'Orléans, 5, rue du carbone, 45072 Orléans cedex 2, Unité mixte de recherche 5145 CNRS/Unité de recherche 136 IRD, Centre IRD d'Orléans, 5, rue du carbone, 45072 Orléans cedex 2,, Institut des sciences de la Terre d'Orléans, Rue de Saint-Amand, Bâtiment Géosciences, BP 6759, 45067 Orléans cedex 2.

Understanding movement patterns and home range of species is paramount in ecology; it is particularly important for threatened taxa as it can provide valuable information for conservation management. To address this knowledge gap for a range-restricted endangered bird, we estimated home range size, daily movement patterns and habitat use of a granivorous subspecies in northeast Australia, the black-throated finch (Poephila cincta cincta; BTF) using radio-tracking and re-sighting of colour banded birds. Little is known about basic aspects of its ecology including movement patterns and home range sizes. From 2011– 2014 we colour-banded 102 BTF and radio-tracked 15 birds. We generated home ranges (calculated using kernel and Minimum Convex Polygons techniques of the 15 tracked BTF). More than 50% of the re-sightings occurred within 200 m of the banding site (n = 51 out of 93 events) and within 100 days of capture. Mean home-range estimates with kernel (50%, 95% probability) and Minimum Convex Polygons were 10.59 ha, 50.79 ha and 46.27 ha, respectively. Home range size differed between two capture sites but no seasonal differences were observed. BTF home ranges overlapped four habitat types among eight available. Habitat selection was different from random at Site 1 (χ 2 = 373.41, df = 42, p<0.001) and Site 2 (χ 2 = 1896.1, df = 45, p<0.001); however, the preferred habitats differed between the two sites. BTF moved further than expected on the basis of current knowledge, with three individuals being resighted over 15 km from the banding location. However, BTF maintain small home ranges over short time-frames. Occasional long-distance movements may be related to resource bottleneck periods. Daily movement patterns differed between sites, which is likely linked to the fact that the sites differ in the spatial distribution of resources. The work provides information about home range sizes and local movement of BTF that will be valuable for targeting effective management and conservation strategies for this endangered granivore.

O destino de uma savana amazônica: plano de zoneamento do governo estadual ameaça o desenvolvimento sustentável no Amapá, o estado brasileiro mais protegido. Tradução de:
Hilário, R.R., J.J. de Toledo, K. Mustin, I.J. Castro, S.V. Costa-Neto, É.E. Kauano, V. Eilers, I.M. Vasconcelos, R.N. Mendes-Junior, C. Funi, P.M. Fearnside, J.M.C. Silva, A.M.C. Euler & W.D. de Carvalho. 2017. The fate of an Amazonian savanna: government land-use planning endangers sustainable development in Amapá, the most conserved Brazilian state. Tropical Conservation Science 10: 1-8. http://dx.doi.org/10.1177/1940082917735416. [open access]

Fine-scale biomass maps offer forest managers the prospect of more detailed and locally accurate information for measuring, reporting and verification activities in contexts, such as sustainable forest management, carbon stock assessments and ecological studies of forest growth and change. In this study, we apply a locally validated method for estimating aboveground woody biomass (AGWB) from Advanced Land Observing Satellite (ALOS) Phased Array-type L-band Synthetic Aperture Radar (PALSAR) data to produce an AGWB map for the lowland pine savannas of Belize at a spatial resolution of 100 m. Over 90% of these woodlands are predicted to have an AGWB below 60 tha−1, with the average woody biomass of these savannas estimated at 23.5 tha−1. By overlaying these spatial estimates upon previous thematic mapping of national land cover, we derive representative average biomass values of ~32 tha−1 and ~18 tha−1 for the previously qualitative classes of “denser” and “less dense” tree savannas. The predicted average biomass, from the mapping for savannas woodlands occurring within two of Belize’s larger protected areas, agree closely with previous biomass estimates for these areas based on ground surveys and forest inventories (error ≤20%). However, biomass estimates derived for these protected areas from two biomass maps produced at coarser resolutions (500 m and 1000 m) from global datasets overestimated biomass (errors ≥275% in each dataset). The finer scale biomass mapping of both protected and unprotected areas provides evidence to suggest that protection has a positive effect upon woody biomass, with the mean AGWB higher in areas protected and managed for biodiversity (protected and passively managed (PRPM), 29.5 tha−1) compared to unprotected areas (UPR, 23.29 tha−1). These findings suggest that where sufficient ground data exists to build a reliable local relationship to radar backscatter, the more detailed biomass mapping that can be produced from ALOS and similar satellite data at resolutions of ~100 m provides more accurate and spatially detailed information that is more appropriate for supporting the management of forested areas of ~10,000 ha than biomass maps that can be produced from lower resolution, but freely available global data sets.

The spatially explicit and mechanistic model TREEGRASS was used to test the effects of tree density, tree spatial distribution, and tree size distribution on radiation absorption, net primary production (NPP), and, water fluxes for a West African savanna . The model has previously been shown to adequately predict radiation absorption, production, and soil water balance in humid savannas . Using field measurements quantifying the spatial variations in grass NPP, the model was found to predict correctly the effect of tree abundance on grass NPP. Model simulations showed that annual photosynthetically active radiation (PAR), absorption efficiency, NPP, and transpiration of the tree layer increased with increasing tree density and decreased with increasing tree aggregation ; the grass layer showed the opposite trends . Total NPP (tree plus grass) remained stable for all tree densities and decreased slightly for aggregated tree distributions . Total transpiration increased with tree density and was not affected by tree aggregation. Changing the tree size distribution had no effect on radiation absorption or on carbon and water fluxes . This study showed that fine-scale vegetation structure can influence NPP and water fluxes. As such, it should be taken into account when assessing the functioning of tree- grass systems. However, the detailed fine-scale structure of the tree layer is rarely available for savanna ecosystems. Thus we assessed the reliability of two common descriptors of savanna vegetation, leaf area index (LAI) and tree canopy cover, to predict PAR absorption, NPP, and evapotranspiration. Neither of these descriptors could adequately predict all the simulated effects. However, tree canopy cover can be used to reasonably estimate radiation absorption and carbon fluxes, whereas LAI is a good predictor of water fluxes. Therefore, mapping and monitoring approaches incorporating both LAI and tree canopy cover appear to be suitable alternatives to quantifying the fine-scale vegetation spatial structure when assessing ecosystem function in savanna-like ecosystems .

It has been suggested that grazing could stimulate the aboveground dry mass and/or nitrogen yields available to grazers (i.e., the grazing optimization hypothesis, GOH), but the actual importance of this effect is still controversial. The GOH has only been tested for a few grasslands and one savanna ecosystem, but not for the moistest and warmest grasslands and savannas of the world. The objectives of this study were to test the GOH in the humid savanna of Lamto (West Africa) by analyzing the growth of grasses in response to a field trial with three levels of clipping two levels of fertilization. We quantified the effects of clipping and fertilization on the dry matter and nitrogen yields to grazers (i.e., mass or nitrogen amount in clipped-off tissues during the experiment) and on the remaining yield (i.e., mass or nitrogen amount in residual phytomass at the end of the experiment) over a three-month period. Total phytomass yield, the sum of yield to grazers, and the remaining yield was maintained under moderate clipping frequency and fertilization as compared to control conditions. Both clipping frequencies decreased the remaining phytomass yield as compared to control plots. Clipping frequency significantly increased nitrogen concentrations in the total yield, in the remaining yield, and in the yield to grazers. Total nitrogen yield and nitrogen yield to grazers were 65% and 91% higher on the plots experiencing moderate clipping frequency with fertilization as compared to control plots. The study shows that grazers in this humid savanna system could potentially modify ecosystem processes in such a way as to partly alleviate nutritional deficiencies, but only in the presence of increased nitrogen availability.

... 2002 ALPA. Todos los derechos reservados Arch. ... The experiments were carried out at the Carimagua Ex-periment Station located in the tropical savanna region of Colombia (4° 37&#x27; N, 70° 30&#x27; W; 150 masl; 2 100 mm rain-fall distributed between April and December). ...

The presence of a savanna-forest vegetation mosaic in the Gran Sabana, Canaima National Park (CNP), has been more than an academic controversy since the 1980s through to the 2010s in Venezuela. Scientists, Park administrators and officials from institutions devoted to protect the Caroní river basin within the limits of the Park, argued that the presence of savannas under a tropical rainfall regime that could support humid forests was due to the fire practices used by the local Pemón Indigenous communities. This misconception justified applying fire suppression policies, aimed at putting out all types of fires in CNP, especially the “compulsive burning” (thus so called by some scientists) by Pemón people in forest areas. This paper describes the initiatives, pursued for more than 20 years, to consolidate intercultural and participatory fire management in the CNP, Gran Sabana, which evolved as result of several participatory action-research projects coordinated by academics, and suppo...

Ecology Letters (2010) 13: 793–802Ecology Letters (2010) 13: 793–802AbstractWe test for two critical phenomena in Amazonian ecosystems: self-organized criticality (SOC) and critical transitions. SOC is often presented in the complex systems literature as a general explanation for scale invariance in nature. In particular, this mechanism is claimed to underlie the macroscopic structure and dynamics of terrestrial ecosystems. These would be inextricably linked to the action of fire, which is conceived as an endogenous ecological process. We show that Amazonian savanna fires display the scale-invariant features characteristic of SOC but do not display SOC. The same is true in Amazonian rainforests subject to moderate drought. These findings prove that there are other causes of scale invariance in ecosystems. In contrast, we do find evidence of a critical transition to a megafire regime under extreme drought in rainforests; this phenomenon is likely to determine the time scale of a possible loss of Amazonian rainforest caused by climate change.We test for two critical phenomena in Amazonian ecosystems: self-organized criticality (SOC) and critical transitions. SOC is often presented in the complex systems literature as a general explanation for scale invariance in nature. In particular, this mechanism is claimed to underlie the macroscopic structure and dynamics of terrestrial ecosystems. These would be inextricably linked to the action of fire, which is conceived as an endogenous ecological process. We show that Amazonian savanna fires display the scale-invariant features characteristic of SOC but do not display SOC. The same is true in Amazonian rainforests subject to moderate drought. These findings prove that there are other causes of scale invariance in ecosystems. In contrast, we do find evidence of a critical transition to a megafire regime under extreme drought in rainforests; this phenomenon is likely to determine the time scale of a possible loss of Amazonian rainforest caused by climate change.

1. The rate of CO2 emission by two major termite species (Ancistrotermes cavithorax and Odontotermes n. pauperans) was studied in a West African savanna (Lamto, Côte d’Ivoire). First, in three major savanna types (grassy, shrubby and woody savannas), CO2 emission from the soil surface was measured using a closed container system. Control soil without termite fungus-comb chambers, and soil of eroded termite mound with or without Odontotermes fungus-comb chambers was sampled in each savanna type. Second, the mass-specific respiration rate of the different components of termite fungus-comb chambers (i.e. workers, soldiers, fungus comb and chamber walls) was measured under laboratory conditions. CO2 emission by termites at the landscape-scale was computed from both field biomass data and laboratory measurements.
2. Whatever the savanna type, CO2 emission from the soil surface was not different between control soil and soil of eroded termite mound without termite fungus-comb chambers, but was significantly higher in areas with fungus-comb chambers than in areas without fungus-comb chambers (10 –19 μmol CO2 m−2 s−1 vs 5 –10 μmol CO2 m−2 s−1). 3. The mass-specific respiration rates were higher for individuals of O. pauperans than for individuals of A. cavithorax. Total respiration rate from an individual fungus-comb chamber was around 56 and 143 μmol CO2 h−1 for Ancistrotermes and Odontotermes, respectively.
4. Despite a low mass-specific respiration rate, fungus comb accounted for 51% of the total respiration flux from whole chambers in Odontotermes and for 82% in Ancistrotermes. The laboratory-derived respiration rate from individual Odontotermes chambers was consistent with the field estimates.
5. At the landscape-scale, the CO2 emission due to A. cavithorax and O. pauperans was 0·022 and 0·050 μmol CO2 m−2 s−1, respectively. This total (27·2 g C m−2 years–1) represented 4·9% of the total above-ground net primary production in this ecosystem and 11·3% of the carbon not mineralized by annual fires.

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties1–3. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover1,2,4,5, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than ,650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered ‘stable’ systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of ,650mm, savannas are ‘unstable’ systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation6 may considerably affect their distribution and dynamics.

The presence of a savanna-forest vegetation mosaic in the Gran Sabana, Canaima National Park (CNP), has been more than an academic controversy since the 1980s through to the 2010s in Venezuela. Scientists, Park administrators and officials from institutions devoted to protect the Caroní river basin within the limits of the Park, argued that the presence of savannas under a tropical rainfall regime that could support humid forests was due to the fire practices used by the local Pemón Indigenous communities. This misconception justified applying fire suppression policies, aimed at putting out all types of fires in CNP, especially the “compulsive burning” (thus so called by some scientists) by Pemón people in forest areas. This paper describes the initiatives, pursued for more than 20 years, to consolidate intercultural and participatory fire management in the CNP, Gran Sabana, which evolved as result of several participatory action-research projects coordinated by academics, and suppo...

Densidade da madeira (DM) é uma variável largamente utilizada para estimativas de estoques de carbono de árvores e arbustos em ecossistemas terrestres. Este tema é pouco investigado em áreas abertas da Amazônia, como as savanas e campinas. O objetivo deste estudo foi investigar a resposta da DM de árvores e arbustos típicos de áreas de savana do extremo norte amazônico em relação aos fatores edáficos e a altimetria em seis diferentes sítios de coleta. Coletamos cerca de 8 discos amostrais, em três diferentes classes de diâmetro (&lt;5cm, 5-10cm e &gt;10cm) de cada um dos 75 indivíduos observados ao longo de seis sítios amostrais em áreas de savana aberta de Roraima. Ao redor de cada indivíduo foram coletadas quatro amostras de solos, distantes 1m da base das plantas, nas direções N, S, L e O na profundidade de 20cm. As amostras de solo foram homogeneizadas e analisadas segundo a textura e a fertilidade. Verificou-se a variação da DM em função de: pH, bases trocáveis, textura e matér...

Understanding the main factors driving fire regimes in grasslands and savannas is critical to better manage their biodiversity and functions. Moreover, improving our knowledge on pyrogenic carbon (PyC) dynamics, including formation, transport and deposition, is fundamental to better understand a significant slow-cycling component of the global carbon cycle, particularly as these ecosystems account for a substantial proportion of the area globally burnt. However, a thorough assessment of past fire regimes in grass-dominated ecosystems is problematic due to challenges in interpreting the charcoal record of sediments. It is therefore critical to adopt appropriate sampling and analytical methods to allow the acquisition of reliable data and information on savanna fire dynamics. This study uses hydrogen pyrolysis (HyPy) to quantify PyC abundance and stable isotope composition (δ 13 C) in recent sediments across 38 micro-catchments covering a wide range of mixed C 3 /C 4 vegetation in north Queensland, Australia. We exploited the contrasting δ 13 C values of grasses (i.e., C 4 ; δ 13 C > −15‰) and woody vegetation (i.e., C 3 ; δ 13 C < −24‰) to assess the preferential production and transport of grass-derived PyC in savanna ecosystems. Analyses were conducted on bulk and size-fractionated samples to determine the fractions into which PyC preferentially accumulates. Our data show that the δ 13 C value of PyC in the sediments is decoupled from the δ 13 C value of total organic carbon, which suggests that a significant component of PyC may be derived from incomplete grass combustion, even when the proportion of C 4 grass biomass in the catchment was relatively small. Furthermore, we conducted 16 experimental burns that indicate that there is a comminution of PyC produced in-situ to smaller particles, which facilitates the transport of this material, potentially affecting its preservation potential. Savanna fires preferentially burn the grass understory rather than large trees, leading to a bias toward the finer C 4-derived PyC in the sedimentary record. This in turn, provides further evidence for the preferential production and transport of C 4-derived PyC in mixed ecosystems where grass and woody vegetation coexist. Moreover, our isotopic approach provides independent validation of findings derived from conventional charcoal counting techniques concerning the appropriateness of adopting a relatively small particle size threshold (i.e., ∼50 µm) to reconstruct savanna fire regimes