Trachypogon Savannas Research Articles

The Presence of Beneficial Organisms Associated to N and P Economy in the Rhizosphere of Native Vegetation in an Oligotrophic Savanna of Guárico State, Venezuela

Background:In natural conditions, tropical plants are adapted to different ecological niches that can be associated to soil microorganisms which play a key role in nutrient cycling likeArbuscular mycorrhiza(AM), phosphate solubilizing bacteria (PSB) and/or nitrogen fixing rhizobia.Methods:We report a survey of the presence in a Trachypogon savanna located at Estación Experimental La Iguana (EELI) in Central Venezuela, of some beneficial plant-microorganism associations. In this savanna, plants present a high AM symbiosis affinity. The high mycorrhization and the presence of potential PSB suggest a synergic effect in plant P-uptake.Results:After screening the rhizospheres of 25 plant species from the zone, we could isolate a high proportion of potential PSB in relation to the total bacteria number from the rhizospheres ofCentrosema venosumandGalactia jussiaeana.Conclusion:Therefore, the presence of potential PSB in the rhizosphere of those species constitutes an important finding to discover novel biofertilizers for crop plants.

Arbuscular mycorrhizal fungal assemblages in biological crusts from a Neotropical savanna are not related to the dominant perennial Trachypogon

Knowledge of the arbuscular mycorrhizal fungal assemblages in the Trachypogon savanna ecosystems is very important to a better understanding of the ecological processes mediated by this soil microbial group that affects multiple ecosystem functions. Considering the hypothesis that the biocrusts can be linked to vegetation through the arbuscular fungi mycelial network, the objectives proposed in this study were to determine (i) whether there are arbuscular mycorrhizal fungi (AMF) in the biocrusts (ii) whether arbuscular mycorrhizal fungal assemblages are linked to the Trachypogon patches, and (iii) whether the composition of the assemblages is related to soil properties affected by microbiological activity. The community structure of the AMF was investigated in three habitats: rhizospheric soil and roots of Trachypogon vestitus, biological soil crusts, and bare soil. The canonical correspondence analysis showed that two soil properties related to enzymatic activity (protease and β-glucosidase) significantly affected the community composition of the AMF. The biocrusts in the Venezuelan savanna are colonized by an AM fungal community linked to that of the bare soil and significantly different from that hosted by the roots of the surrounding T. vestitus, suggesting that assemblages of AMF in biocrusts might be related more closely to those of annual plant species appearing in favorable conditions.

N biogeochemistry and cycling in two well-drained savannas: a comparison between the Orinoco Basin (Llanos, Venezuela) and Ivory Coast (western Africa)

When a savanna burns, a decline in the input of organic matter and nutrients to the soil occurs. However, the existence of recurrent fires is a natural condition and N depletion by fire is not incompatible with the existence of savannas per se. Consequently, savanna vegetations have evolved under fires, implying a near to steady-state N budget. In some Australian and African ecosystems, N fixation appears to be insufficient to replace losses inducing soil-N depletion, whereas in neotropical Llanos and western African savannas, the N-fixation and precipitation seem to be enough to maintain production despite fires. This review presents information about well-drained savannas, namely Trachypogon savannas in Orinoco Llanos and Andropogonae savannas in Ivory Coast. The sites present similarities in climate and fire regime differing in soil parent material. A report on N budgets is presented. The budget was positive, since losses seem to be balanced by inputs in rainfall and biological fixation. Uncertainties in flux measures indicate that more work on those aspects are required. Results indicate that the estimated amounts of dinitrogen fixation and deposition seem to be enough to maintain plant production. Data support the hypothesis that the vegetation in the natural savanna has evolved under fire constraint.

Earthworm Populations in Savannas of the Orinoco Basin. A Review of Studies in Long-Term Agricultural-Managed and Protected Ecosystems

Earthworm biomass and production in savannas are limited by seasonal precipitation and the lack of organic and nutrient resources; I hypothesize that after a long-term protection of savanna from fire and agricultural activities drastic changes in the physical and chemical characteristics of the soil occur with a concomitant increase in earthworm abundance and activities. Similar changes might occur after a long-term fertilization of savannas with manure. This review article considers the earthworm communities and other soil quality indices in Trachypogon savannas of the Orinoco Basin in an organic agricultural forestal savanna (OAFS) amended with compost over forty years in Puerto Ayacucho, Venezuela, and in an Experimental Station long-term protected (PS) from fire and cattle raising from more than four decades in Central Llanos, Venezuela, comparison is made with results from similar savannas. Long-term additions of organic manure or a long protection have induced significant changes in the soil physical and chemical properties of the natural savanna (NS) soils that induce a significant increase in the density and biomass of earthworm populations. On the other hand, the protection of the savanna promotes an improvement in the physical and chemical properties of the soil, which favors an increase in the density and biomass of earthworms in the PS compared with the NS subjected to recurrent burning and grazing. The results emphasize the importance of appropriate organic matter management and the relevance of earthworms in such agroecosystems.

Granivory, seed dynamics and suitability of the seed-dish technique for granivory estimations in a neotropical savanna

Spatial and seasonal patterns of seed removal by rodents, ants and birds were evaluated using seed-dish experiments in a Trachypogon savanna in Venezuela. Four-day experiments were conducted on three dates and at four different sites. Seeds from 10 native plant species representing a broad mass range were offered at densities equivalent to 3145 seeds m−2. We found that total seed removal and relative importance of granivorous groups varied widely among sites and dates. Rodents were the major seed predators in all the habitats, except for the savanna/forest border where birds dominated. Ants removed more seeds than birds, but since they preferred the smallest seeds, ants ranked last in mass removed. We found a temporally variable relationship between seed removal rates and seed rain or seed bank. We inferred monthly seed predation from seed-dish experiments as well as through the decline of the seed bank in the environment during the dry season, and compared both estimations to evaluate the suitability of the seed-dish technique for estimating granivory. We initially hypothesized that seed-dish experiments that emulate seed availability more closely may yield a better estimate of seed predation, but in fact, they may still overestimate granivory intensity.

Contribution of nitrogen-fixing organisms to the N budget in Trachypogon savannas

Trachypogon savannas in Venezuela are mainly used for extensive cattle raising. These savannas are currently affected with man-made or natural fires. During fires, 24% of the nitrogen (N) necessary for primary production is lost through volatilisation. More is lost by leaching and/or erosion. Since those losses are not compensated for by N input through precipitation, N balance in these savannas depends on biological mechanisms. In this study we explore the possible forms of biological N fixation, in particular the cyanobacterial activity from soil microbial crusts, and the contribution of grass rhizosphere microorganisms. Determinations were made by using, in situ, the method of acetylene reduction as an estimate of nitrogenase activity (NA). N 2 fixation due to NA in the soil–plant system is 13.7 and 7.8 kg ha –1 year –1 for the burned and protected plots, respectively. Even considering the lowest fixation values by microbial crusts, they could provide 6% of the N needed for annual production of the vegetation of the savanna under fire, and 9% in the protected savanna. These amounts of N 2 sustained the productivity of the vegetation experiencing periodical fires.

Vegetation–environment relationships and classification of the seasonal savannas in Venezuela

The seasonal savannas dominated by the C 4 grasses of the genus Trachypogon, are widespread in northern South America. In Venezuela, they extend from the central lowland Llanos to intermediate elevations in the Coastal and Andean Mountains and to the Guiana Plateau (Gran Sabana) in the south. This study aims to classify these savannas and to understand the environmental factors that determine their composition and structure. Thirty-seven sites were sampled, plant cover and density were measured in 376 one square meter quadrats and the importance value index (IVI) was calculated. Climate data were obtained from climatological stations and soil properties were analyzed. The results were classified by clustering and TWINSPAN. Floristic and environmental data were ordered with the canonical correspondence analysis (CCA). Trachypogon savannas are heterogeneous, both floristically and environmentally. An altitudinal gradient of ∼2000 m separates the intermediate elevation Coastal Mountains (Type I) and the Guiana savannas (Type II) from the lowland Llanos. In the lowlands, sparsely covered and treeless communities on extremely oligotrophic and sandy soils (Type IV) differ structurally and floristically from woody savannas (Type III). The floristics and structure of the savannas respond directly or indirectly to elevation and water and nutrient availability. These responses were evidenced by: (a) floristic composition, as Neotropical lowland savanna species merged with those of the Andean sub-páramo in Type I savannas or with those from the Guianan flora in Type II savannas. (b) Species richness, which ranged from an average of 13.8 per site in sandy Type IV savannas to 38.9 per site in Type I intermediate elevation savannas. (c) Community physiognomy, which changes from an homogeneous herbaceous matrix with dispersed trees in Type III savannas to a shrubby Type I savannas to “treeless” Types II and IV savannas. (d) Plant cover which varied from 93.8% in the fertile Type I savannas to around 80% in the sandy oligotrophic savannas of Types II and IV. Each savanna type is related to one of the major geomorphological regions of Venezuela.

Constraints to colonization and growth of the African grass, Melinis minutiflora, in a Venezuelan savanna

Melinis minutiflora Beauv. (Poaceae) is an African grass that is invading mid-elevation Trachypogon savannas in Venezuela. The objective of this study was to investigate the influence of soil fertility, competition and soil disturbance in facilitating Melinis' invasion and growth in these savanna sites. We manipulated soil fertility by adding nitrogen (+N), phosphorus and potassium (+PK), or nitrogen, phosphorus, and potassium (+NPK). We simultaneously manipulated the competitive environment by clipping background vegetation. In a separate experiment, we mechanically disrupted the soil to simulate disturbance. We hypothesized that germination and growth were bottlenecks to early establishment in undisturbed savanna, but that disturbance would alleviate those bottlenecks. We measured Melinis seed germination and subsequent establishment by adding seeds to all plots. We examined Melinis growth by measuring biomass of Melinis seedling transplants, 11 months after they were placed into treatment plots. Germination and establishment of Melinis from seed was extremely low. Of the 80,000 seeds applied in the experiment, only 28 plants survived the first growing season. Mortality of Melinis seedling transplants was lowest in PK fertilized plots, but in the absence of PK mortality increased with N additions and clipping. By contrast, fertilization of the savanna with NPK greatly increased Melinis seedling biomass and this effect was greatly enhanced when competition was reduced (e.g. clipping). Melinis transplant growth responded strongly to soil disturbance- a response not fully explained by removal of competitors (clipping) or changes in soil nutrients and moisture. We suspect that disruption of the soil structure allowed for greater root proliferation and subsequent plant growth. We believe that native savanna is relatively resistant to Melinis invasion, since Melinis seedlings persisted in intact savanna but exhibited little or no growth during the first year. The significant enhancement of Melinis seedling growth with clipping and nutrient additions suggests that low soil nutrients and the presence of native savanna species are important factors in the ability of native savanna to resist Melinis establishment. However, the potential for Melinis growth increases enormously with soil disturbance.

Allocation of phosphorus in a tropical savanna

The allocation of phosphorus (P) was studied in the different components of a Trachypogon savanna ecosystem, particularly in those related to biological productivity processes. The research was based on the sequential fractionation of soil P and on the determination of P content in different plant components. Soil is the principal reservoir of P (more than 99%), which is distributed mostly in forms of low availability. Microbial soil P represents about 6% of total P in this ecosystem, whereas cattle and vegetation accumulate about 1% P. The relatively high P content in microorganisms suggests the important role of this pool as a source of labile P and as a catalyzer of P transformations. Although available P forms are scarce, microorganisms and plants can satisfy their P requirements from the organic labile and moderately labile fractions, which account for 24% of total P in the soil.

Nitrogen mineralization in soils under grasses and under trees in a protected Venezuelan savanna

Nitrogen mineralization was evaluated in soils beneath the most common woody species growing isolated within the grass matrix of a Venezuelan Trachypogon savanna, which has been protected from fire and cattle grazing since 1961. Adult trees of three evergreen species, Byrsonima crassifolia (L) H. B. K., Curatella americana L., and Bowdichia virgilioides H. B. K; and two deciduous, Godmania macrocarpa Hemsley and Cochlospermun vitifolium (Wild) Spreng were selected. The amount of N mineralized (NH 4 +-N+NO 3 −-N) during 15 weeks of laboratory incubation of soils collected from beneath trees, was significantly higher ( p<0.01) than those from under grasses. Values of N mineralized on soil from under trees were from 21.28 to 82.65% greater than for soil from under grasses. A highly significant ( p<0.01) positive correlation, for all soils, was found between Nm and SOC, and between Nm and Nt. The higher N mineralization rates under trees would reflect a higher soil biological activity, due to higher SOC and Nt, of the soils under the tree canopies than those under grasses. The N availability values obtained under all species reveal the importance these trees have for creating enriched areas on generally oligotrophic soils. Nitrogen mineralized in the soil from beneath evergreen trees was significantly ( p<0.01) higher than from under deciduous trees, being 25.87% higher on average. Similarly to the relation found for all soils, a highly significant ( p<0.01) positive correlation between Nm and SOC and between Nm and Nt was also obtained for soils beneath all trees, indicating the importance of SOC and Nt for nitrogen mineralization processes in this savanna. The higher SOC and Nt contents found under evergreen trees are probably due to the longer time they have been established on the site as compared to the deciduous ones. The chemical quality of fresh fallen leaves (as measured by their lignin/nitrogen ratio) did not seem to influence the quality of the SOM (as measured by C/N ratio), since the C/N ratio was not statistically different for the soils under the two groups of trees. Apparently, whatever the chemical quality of the fresh fallen leaves (higher lignin/nitrogen ratio in evergreen trees), the humification processes in this savanna soil environment seems to homogenize the SOM beneath both types of trees.

Competition between Perennial Grasses in a Neotropical Savanna: The Effects of Fire and of Hydric-Nutritional Stress

1 Pulse perturbation experiments, in which a percentage of above-ground biomass of a given species in a plot was removed on a single occasion, were performed in plots of a Trachypogon savanna in Northern Venezuela. These showed interspecific competition between the dominant C4 perennial grasses Trachypogon vestitus and Axonopus canescens in protected (i.e. unburned and ungrazed) savanna plots, regardless of the stress status. 2 Press perturbation experiments, in which a given species in a plot was continuously removed, showed that A. canescens had a higher maximum yield (defined as the density of green biomass in the permanent absence of its potential competitor), and a more deleterious influence on T. vestitus only in the protected plots. This suggests that the interaction between the two grasses is asymmetric, in agreement with the findings of the pulse experiments. 3 Press perturbation experiments revealed that the demographic importance (measured as the relative growth rate in green biomass) of interspecific interactions appears to be low, regardless of fire and stress degree. This is consistent with the low fertility of most Trachypogon savanna soils, and together with the asymmetric interaction between the two species may contribute to explain the slow replacement of T. vestitus by A. canescens in a long-term protected plot of a Trachypogon savanna in Central Venezuela. 4 Fire and stress status interacted to modify the occurrence and symmetry of the competitive interactions between the dominant grasses in Trachypogon savannas. The structure of the herbaceous community resulted from the complex interplay between biotic interactions, fire and stress status.

Vegetation and soil analysis of topo-sequences in the Orinoco Llanos

Two contiguous topo-sequences in the Orinoco Llanos were analyzed. The results indicated that the relief functioned differentially in the two environmental situations involving ancient alluvium from the Guarico River covered by the Trachypogon savannas and a swamp palm community, and recent alluvium from the Orituco River covered by a seasonal gallery forest, a seasonally flooded palm savanna, and a seasonally flooded herbaceous savanna. In the soil catena (first topo-sequence), the dissected relief (via the water table) determined a lack of a dry season for the swamp palm community. On the other hand, relief in the second topo-sequence (via precipitation and run-off) prolonged the effects of the wet season on the seasonally flooded palm and the herbaceous savanna. The described vegetation were found to occupy distinct soil types. However, all the studied soils were highly infertile, the swamp palm community being the extreme case. The collection of vascular plants in the communities included 515 species, distributed in 93 families. The seasonal gallery forest presented the highest species richness with 243 species, whereas the lowest value was found in the swamp palm community (79). Comparisons of the life form spectra indicated that the Teroculmi and Teroherbae were common in all communities and both possessed a tolerant-dormant stage which allowed for their survival under stressful conditions. Chorological-ecological analysis of the data indicates that the flora of the Orinoco savannas seems to have different species sources with high affinity to the nearby systems.

Emission of nitric oxide from soils and termite nests in a Trachypogon savanna of the Orinoco basin

Nitric oxide fluxes from soils in the Trachypogon savanna of the Orinoco basin were determined during the dry season using the static chamber method. The emission from dry soils fluctuated from 0.4 to 3 ng N m−2 s−1 and increased up to 25 ng N m−2 s−1 after moderate watering or light rain-falls (1 to 5 mm). The mean emission values are up to 6 times lower than one observed earlier at the Chaguaramas site, but up to 10 times higher than one recorded at the Guri site, indicating an important spatial variability in NO fluxes of the Venezuelan savanna region. The changes observed after the addition of nitrogen to the soil, in the form of ammonium and/or nitrate, indicate a high denitrification potential in this acidic soil. Burning of the surface vegetation produced an increase by a factor of 10 in the emission rate of NO, but the effect was relatively short in time, about 5 days. It was estimated for the savanna region that burning increases the total NO soil emission during the dry season by 15% compared to the unburnt case. Soils with termite nests emit 10 times more NO than soil without nests, but the contribution from this source is less than 2% of the total savanna soil flux.

Changes in bulk precipitation reactivity throughout the vegetation/soil continuum in a Trachpogon savanna (Venezuela)

Abstract The reactivity of bulk precipitation moving into the soil of a bush island Trachypogon savanna throughout a forest grove and the herbaceous layer was analyzed during four consecutive years. In the forest grove, there was a significant decrease of annual hydrogen (H+) content (meq/ha) of 44–75% as rainfall leached through the canopy, whereas the bulk throughfall pH in the grass layer was similar to the bulk precipitation pH. As throughfall passed throughout the soil profile of the grove, the H+ content decreased 87–70%. Results evidenced a buffering capacity of the system when bulk precipitation was acid during the last measured year. The savanna system retained 92–93% of H+ input from the bulk precipitation indicating that a redistribution of H+ ocurred instead of a net input or net loss. Canopy neutralization of bulk precipitation appears to take place in the grove canopy mainly by organic and bicarbonate salts.

Rainfall partitioning by a semideciduous forest grove in the savannas of the Orinoco Llanos, Venezuela

A study of rainfall partitioning into throughfall, stem flow and interception loss was conducted during five annual cycles in a patch of semideciduous forest within the Trachypogon savanna in the Orinoco Llanos. Annual seasonal rainfall (April-November) was 1238 mm, 1228 mm, 1531 mm, 1345 mm and 1398 mm for 1981, 1982, 1983, 1984 and 1985, respectively. During the years analyzed (1981–1984), the throughfall measured using a roving gauge technique was 75.7% (±1.2%), 81.8% (±1.2%), 74.1% (±1.1%) and 77.4% (±0.2%), respectively, of the gross precipitation. The rain required to initiate average throughfall in the grove ranged from 0.6 to 1.0 mm. Of the rainfall reaching the grove canopy, 1.0% (±0.2%), 1.1% (±0.2%) and 0.4% (±0.00%) flowed down the trunks as stem flow in 1983, 1984 and 1985, respectively. Consequently, interception loss accounted for 19% and 12% of gross rainfall for 1983 and 1984, respectively. Regression analysis indicated that throughfall, stem flow and interception loss as a function of gross rainfall followed similar linear patterns. Volume of stem flow was related to tree diameter but it decreased at the highest diameter class.

Carbon Dioxide and Ammonia Exchange in the Trachypogon Savannas of the Orinoco Llanos

Atmospheric carbon dioxide (CO 2 ) and ammonia (NH 3 ) exchanges were determined in the Trachypogon savannas of the Orinoco Llanos uring the energy balance approach. Total dry mass and separate dry mass values of plant parts were used for a growth analysis of the community and for measurements of nitrogen content. During the growth period, the net assimilation (P w ) ranged from 0.102 to 0.127 MJ m −2 d −1 (6.6-7.9 g dry mass m −2 d -1 ). These figures were similar to mean crop growth rate measured using the mass balance approach (2.8-6.9 g dry mass m −2 d −1 ) (...)

Regional interpretation of environmental gradients which influence Trachypogon savannas in the Orinoco Llanos

Multivariate structural data describing Trachypogon savanna relationships were used to characterize the sources of variation among savannas as a function of the environmental characteristics of the Orinoco Llanos. Twenty-six savannas including major physiognomic types of the Orinoco Llanos ranging from herbaceous to bush island savannas were ordinated on a regional scale. Results of Detrented Canonical Correspondence Analysis (DCCA) using floristics variables indicated that savannas were ordinated along two complex gradients. A soil physical and chemical gradient as expressed by changes in bulk density and magnesium concentration were evident after the analysis of the first DCCA axis. The second axis was a climatic gradient of decreasing annual precipitation and incresing monthly precipitation during the dry season. The impact of human disturbance on the savanna composition was also an explanatory variable of the second DCCA axis. Savanna sites overlapped considerably in composition and most species were widely distributed, with aboveground phytomass abundance depending upon the site.

CO2 Exchange in soil algal crusts occurring in the trachypogon savannas of the Orinoco Llanos, Venezuela

CO2 exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities to savanna woodlands. A maximum CO2 flux of 0.207 mg m-2 s-1 was measured in the crusts of the Guanipa savannas, while in the other examined crusts (0.035–0.105 mg m-2 s-1) the flux was similar to values reported for terrestrial algae. The CO2 flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic efficiencies of the crusts were compared. The activation energy calculated for the CO2 fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12 kcal mole-1), whereas they were evident in the other crust studied. An optimum CO2 incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the basis of the results and characteristics of the habitats.

Spatial Patterns of Trees and Structuring Factors in a Trachypogon Savanna of the Orinoco Llanos

Tree dispersion was evaluated in a Trachypogon savanna by measuring the distance from individual trees to their nearest conspecific neighbor. The vegetation is a closed bush island savanna with coexistent isolated trees and small patches of semideciduous forest. The isolated trees were in clumps similar to trees growing in savannas and tropical forests. In our study, adults were less clumped compared to juveniles. Environmental and biotic differences related to edaphic heterogeneity, life history, and reproductive strategy could account for the clumped pattern. Different spatial patterns emerged for stems in groves. Thus, uniform spatial distribution was evident in the individuals of the fire resistant species (Curatella americana, Byrsonima crassifolia and Bowdichia virgilioides) and a gap filling species (Casearia decandra). Changes in the demographic components between different age groups provide evidence for density-dependent mortality. A hypothesis mainly based on two scales of disturbance (human impact and gap formation) acting synergetically with a large scale factor (substrate patchiness) is put forward to explain the dynamics of tree invasion and the maintenance of the Trachypogon savanna.

Environmental Factors Related to the Occurrence of Mound-Building Nasute Termites in Trachypogon Savannas of the Orinoco Llanos

The incidence of termite mounds was determined in twenty-six sites in the Trachypogon savannas at the Orinoco Llanos, including a wide range of physionomic types. A multiple discriminate analysis was utilized to analyze ecological variables as the best indicators of a given area's suitability for support of terrestrial Velocitermes and/or Nasutitermes species. The variables included physical attributes (texture, ineffective soil mass, water holding capacity, and infiltration rate), chemical attributes (cation exchange capacity, base saturation, and organic matter content), soil characteristics, and the above-ground phytomass accumulated at each stand. Results indicated that soil physical variables determining water balance during critical dry periods were of paramount value for the successful establishment of mound-building nasute termites in Trachypogon savannas.