Venezuelan Savanna Research Articles

EFECTOS DE LAS AGUAS RESIDUALES DE POZOS PETROLEROS SOBRE LOS SUELOS DE SABANAS UBICADAS CERCA DE EL FURRIAL, ESTADO MONAGAS, VENEZUELA

In terrestrial environments affected by oil operations, oil drilling and sewage fluids (SF) represent an environmental and aesthetic problem of importance. Before watered to the environment, SF rich in suspended solids and with potential contaminants, are transported to a tank treated with a flocculant agent and corrected for pH. The resulting waters are then disposed to the local savannas. It is important to know the effects that those treated SF may have on the surrounding ecosystems. This contribution aims to analyze the levels of soluble salts and heavy metals in the soils adjacent to oil wells located in the eastern part of the Venezuelan savannas after irrigation with treated SF. The treated SF watered to the environment have a liming effect on the natural acidity of soils. In general, no high levels of heavy metals (Fe, Mn, Cu and Zn) were found nor soluble aluminum in treated soils, which is due to the rise in pH after treatment. The soils treated with effluents generally had a higher sodium concentration. However, there is no potential environmental risk in the amounts of soluble sodium released to the sandy ultisols since the good internal drainage of these and the proper precipitation of the area allow the soon eviction of salts without danger of salinization. Nevertheless, strict monitoring is recommended, in particular of the pH levels of wastewater before being irrigated to the surrounding savannas.

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.

Phosphorous cycling and biogeochemistry in a Venezuelan-flooded savanna

In 1970, a programme of land recuperation started in Venezuelan savannas, strongly affected by the seasonality of precipitation; therefore, a network of dykes has been built to alleviate the floods and retain water throughout the dry period. Under the dyked system, the environment has been altered, allowing a change in the herbaceous vegetation towards aquatic species and an increase in primary production. It is assumed that a considerable quantity of nutrients is lost from the ecosystem through the floodgates, a situation that could be worsened with the climate change. This contribution describes the atmospheric input and total output in stream run-off of phosphorous (P) in a flooded savanna. Internal pools of the biogeochemical cycle of P associated with terrestrial compartments are described. In the flooded savanna, a large amount of P is immobilised (29.6 kg ha−1) in their above ground biomass by grasses, and in soil microbial biomass. The P budget was nearly balanced, as measured losses were cancelled out by the inputs in rainfall. Soils act as a sink, retaining P coming either from precipitation or from desorption/mineralisation processes. That interruption can be maximised, and losses of P and other nutrients can be minimised with an adequate management of the floodgate.

Impact of land protection in soil quality properties and in earthworm biomass in Venezuelan savannas

We studied soil parameters and the earthworm biomass and density in a protected savanna (PS) from fire and cattle raising for more than forty years located at Estacion Biologica de los Llanos, Venezuela (EBLL), and an adjacent non-protected savanna (NS) under frequent burning and agricultural activities, located less than 1 km apart. Earthworm sampling was carried out at the end of the dry season (April), and at the peak of the wet season (July-August) using a hand sorting extraction method. In each plot, five to seven sampling units were selected corresponding to soil monoliths of 25x25x30 cm. The main physical, chemical, microbial (biomass) and enzymatic activities of soils in both systems were estimated. The presence of an abundant litter layer under the tree canopies in the PS have increased the soil water retention capacity, the organic-C, C-microbial biomass, dehydrogenase activity and fertility levels respect NS. This may allow for an increase in the density and biomass of earthworms in the PS compared with the adjacent NS. The systems were characterized by abundance in juvenile individuals belonging to the Glossoscolecidae family; two different morphotypes of the genus Aicodrilus were registered. Earthworms found were located in an endo-anecic ecological category. Results suggest that, agricultural management in savannas by modification of physical and biochemical soil properties can decrease an important fraction of pedofauna, particularly their earthworm communities. However, earthworm populations and soil quality indicators can be restored after long-term protection.

Leaf trait variation of a dominant neotropical savanna tree across rainfall and fertility gradients

The evergreen tree Curatella americana dominates the woody stratum in Venezuelan savannas, along substantial rainfall and soil fertility gradients. This work determines intra-specific variation of leaf traits in Curatella which may account for its extended ecological range. Six Curatella populations, from sites spanning a 1800 mm rainfall and a 5-fold soil fertility range, were selected. Specific leaf area (SLA), N and P content, photosynthesis rate ( A), and the 13δC signature as an indication of long term water use efficiency (WUE lt) were measured at the peak of the rainy season of two years. The sclerophyllous and long-lived Curatella leaves had relatively low SLA, leaf N and P and A which differed among populations but were in the range of other evergreen savanna trees. SLA and A mass were positively related with rainfall whereas 13δC was negatively related. The responses to soil fertility were less clearly defined. There was a marked P limitation in most populations and nutrient resorption accounted for >50% of the leaf N and P budget. The variation in leaf traits of Curatella populations is comparable to inter-specific variation seen in other studies. Multivariate analysis established that the ecophysiological distance among populations of Curatella was mostly accounted by SLA and leaf N differences. The leaf trait response of Curatella to rainfall and fertility position it at the slow end of the global leaf economics spectrum.

Phosphorus fertilisation management modifies the biodiversity of AM fungi in a tropical savanna forage system

In the present study we investigated how the community of arbuscular mycorrhizal fungi (AMF) in roots of Centrosema macrocarpum responded to different doses and sources of phosphorus (40 kg ha −1 of P as rock phosphate, 150 kg ha −1 of P as rock phosphate and 75 kg ha −1 of P as diammonium phosphate together with 75 kg ha −1 of P as rock phosphate) in a Venezuelan savanna ecosystem. We also related AMF diversity to soil parameters (total N, total P, available P, extractable K, total Ca, total Mg, total Fe, total Cu, total Zn, total Mn, glomalin-related soil protein, microbial biomass C, dehydrogenase, urease and acid phosphatase activities, water-soluble carbon and carbohydrates and aggregate stability) at different doses of P. The AM fungal small-subunit (SSU) rRNA genes were subjected to PCR, cloning, SSCP, sequencing and phylogenetic analyses. Nine fungal types were identified: six belonged to the genus Glomus and three to Acaulospora. The majority of fungal types showed high similarity to sequences of known glomalean isolates: Aca 1 to Acaulospora mellea, Aca 2 to Acaulospora rugosa, Aca 3 to Acaulospora spinosa, Glo 1 to Glomus intraradices and Glo 3 to Glomus fasciculatum. The control treatment was dominated by species belonging to the genus Acaulospora. However, when the soil was fertilised with low doses of P, the colonisation of roots increased and there was a change in the AMF diversity, the genus Glomus dominating. The AM development and the abundance of AM fungal types in roots were decreased dramatically by the fertilisation with high doses of P, without differences between the sources of P used. The available P in soil was negatively correlated with the AMF diversity. In conclusion, the application of low doses of P as rock phosphate stimulated mycorrhization and enhanced the soil quality parameters except water-soluble carbohydrates, helping to offset a loss of fertility in P-poor tropical savanna soils.

Effect of conversion of natural grassland to cropland on nitric oxide emissions from Venezuelan savanna soils. A four-year monitoring study

Conversion of “natural” savanna to agricultural soil is proceeding at a very fast rate. In this work, nitric oxide (NO) emissions were measured immediately after conversion of a “natural” grassland to fertilized agricultural fields (corn, sorghum and cultivated pasture), and also during the second and fourth year after conversion. Large fluxes were observed after plowing grassland soil, and almost no difference was observed between ammonium-fertilized and unfertilized plowed grassland soils. Soil water content and pH related negatively to NO emissions. NO emissions were positively correlated with soil nitrate concentrations and carbon content (corn only) whereas little or no correlation was found with ammonium concentrations. This suggests that in these savanna soils, the NO emitted from soil is mainly denitrification-derived. The disturbed soils emitted on average ∼7 times more NO than the original grassland. During the second and fourth year of cultivation the emissions from corn and sorghum fields (plowed every year) were around 10 times higher than from the control grassland soils. The fertilizer-induced emission (FEI) values for NO emission estimated in this work show that similar agricultural practices could lead to dramatically different FEI values depending on the water content of the soils. These FEI results corroborates that modeling approaches to determine the global agriculture-derived NO emissions should take into consideration the main factors that regulate the NO emission at the scale of functional units with similar climate, soil and management conditions instead of expressing it as a percentage of the fertilizer applied. In our sandy loam savanna soils these factors are inorganic nitrogen content, pH and WFPS.

Growth of tropical savanna grass plants in competition: A shoot population model

The Venezuelan savannas, and in general Neotropical savannas, are composed of herbaceous co-dominant species and less abundant trees and shrubs, conditioned in great measure by the occurrence of annual fire events. We studied the growth dynamics of three grass species representative of the major phenological and architectural types: a precocious, bunch grass, Elyonurus adustus, an early, medium tall grass Leptocoryphium lanatum and a late, tall species Andropogon semiberbis. The experimental design included individuals of all these species growing alone, interacting in species pairs and all three species in competitive interaction. Close inspection of the data showed that the species differ widely in the spatial and temporal patterns of occupation of aboveground space, showing important levels of neighborhood interference among them. Shoot emergence during the time period between fire events (usually 1 year) is not a continuous process but occurs as a set of discrete cohorts. We model plant growth as a population of shoots governed by ordinary differential equations that accounts for the existence of different number of cohorts as a consequence of shoot competition. A comparison of simulated patterns to those observed in the field was assessed by the root mean square (RMS) error between the transient behaviors of both patterns. L. lanatum resulted both the most impaired and the least impairing of the three species and the opposite is valid for A. semiberbis. For the former species, the competition effect when growing together with the other two is stronger that when growing in pairs and close to an additive process. For E. adustus and A. semiberbis the addition of the third species does not substantially modified the effect obtained when competing in pairs. This model can be applied to the analysis of the herbaceous plant growth process on savannas subject to fire. With modifications, the model can be applied to other grass plants and other grassland ecosystems. We suggest that a possible mechanism involved in this process is the growth inhibition of the basal shoots by the older shoots of the same plant.

Ecology of pollination in a tropical Venezuelan savanna

Pollination modes ecology of a total of 164 plant species was evaluated according to habitats and plant life forms in the Venezuelan Central Plain. Frequency distribution of nine pollination modes showed that, at the community level bee pollination (38.6%) was dominant. Butterfly (13.9%), fly (12.7%), and wasp (10.8%) pollination were the second most frequent. Moth (6.2%) and wind (10.4%) pollination occurred with similar frequency, and the least common were bird (3.1%), beetle (2.3%) and bat (1.9%) pollination. There was a significant interaction effect indicating that pollination mode was affected by the type of habitat. Bee pollination was the most common pollination mode in all habitats with butterfly, fly and wasp pollination being secondary for forest and forest-savanna transition; and butterfly, wasp, wind and fly pollination being secondary for savanna. Wind, butterfly and fly pollination were found in disturbed areas as secondary pollination modes. Pollination modes were significantly associated and affected by life forms. Bee pollination was dominant in all life forms with wasp, butterfly and fly pollination being the secondary for trees, shrubs, and lianas; and butterfly and wind pollination being the secondary for herbaceous species. The number of pollination modes (richness) among life forms ranged between four and nine for epiphytes and perennial herbs respectively. The highest values of diversity indexes among life forms were found in trees and shrubs. The richness and diversity indices of pollination modes were statistically higher for more structured habitats, forest and forest-savanna transition, than herbaceous habitats, savanna and disturbed areas, which is associated with the highest values of diversity indexes in trees and shrubs. Equitability was higher for forest and disturbed areas than forest-savanna transition and savanna. The results of comparative richness, equitability, diversity indices, and the frequency distribution of pollination modes of 19 samples from tropical and temperate communities indicated that richness of pollination modes may be different between tropical and temperate communities. The proportion of each pollination mode suggests four grouping: (1) rain forests and their strata, (2) grassland savanna, and associated disturbed areas, (3) temperate communities, and (4) the most heterogeneous group, contained mostly neotropical communities, including the four habitats of the Venezuelan Central Plain. The frequency of pollination modes, richness, diversity and equitability of communities, habitats, successional stages, and vegetation strata varies with respect to geography, vegetation structure, and plant species richness.

Quantitative trait, genetic, environmental, and geographical distances among populations of the C4 grass Trachypogon plumosus in Neotropical savannas

ABSTRACTVenezuelan savannas are exposed to land‐use changes and biological invasions which compromise their persistence and function. The native C4 grass Trachypogon plumosus is the most important component of the savannas under diverse combinations of climate and soils, suggesting substantial interpopulation variation. We examined quantitative traits and isozyme variation of nine populations of this grass and related these estimates to geographical and environmental features of sampled locations. Isozyme diversity estimates were based on 10 polymorphic enzyme systems whereas 21 quantitative traits, from field and controlled growth conditions, were evaluated. Distance matrices for quantitative traits, isozyme, geographical and environmental data were subjected to clustering analysis. Correspondence between quantitative trait distance and genetic distance, and their association to geographical and environmental distances were analysed with Mantel tests. All quantitative traits differed significantly among populations. The average QST calculated for eight quantitative traits measured in the greenhouse was 0.157. Isozyme diversity differed significantly among populations. About 28% of total isozyme variation occurred among populations. Significant positive associations were detected between environmental, quantitative field traits, and geographical distance as well as between the later and genetic distances. Genetic distances did not correspond significantly with quantitative traits nor did environmental distances. Ecologically meaningful associations were detected between field quantitative traits, environmental, and geographical data using cluster analysis. Our results support the hypothesis that processes of the neutral type are mainly responsible for the variation patterns observed in T. plumosus populations in Venezuelan savannas. Variation observed for quantitative traits among populations seems to be due to the effect of environmental conditions on phenotypically plastic traits, and not the result of directional selection favouring different phenotypes in different populations.

Growth dynamics of tropical savanna grass species using projection matrices

Savannas are very important tropical ecosystems characterized by co-dominance of herbaceous vegetation and less abundant trees and shrubs. Models of the dynamics of the spatial distribution of aboveground components of grass plants can be a useful tool to understand the dynamics of savanna ecosystems. We developed a model of grass plant growth as a collection of the individual dynamic behavior of shoots inspired in data for plants of three species of common grasses in the Venezuelan savannas. The species are: Elyonurus adustus (Trin.) E. Ekman., Leptocoryphium lanatum (Kunth) Nees and Andropogon semiberbis (Nees) Kunth. These species represent various types of architecture and regeneration response to fire. The individual-shoot model is based on shoot emergence, mortality, and elongation given by Richards’ equation, plus a few simple geometric considerations. Model output is shoot density in each cell of a square grid at several vertical levels. Differences in patterns of shoot density among species are explained by changing a set of parameter values related to growth form and phenology. Vertical distribution of shoot density was calculated from the simulation results and the field data with the purpose of deriving a simpler lumped shoot-population model. This simpler demographic model is based on a projection matrix that predicts the essential dynamics of growth in the vertical dimension. The transient and final behavior of vertical distribution of shoot density are calculated with the matrix model and compared to field data yielding good fit as assessed by the root mean square (RMS) error of difference between the field data and the model results. This error is of the same magnitude as the RMS difference among the three field data replicates. Sensitivity analysis of the matrix model for the three species was used to identify those vertical levels for which the damping ratio is more sensitive to a perturbation. For early and precocius species ( E. adustus and L. lanatum), the top level is more sensitive whereas the late blooming species ( A. semiberbis) is more sensitive at lower levels. Although developed for these three species, the matrix model is generic and therefore can be applied to plants with the basic growth form of perennial grasses of seasonal savanna ecosystems. Applications include scaling-up the individual-shoot model to larger areas and analysis of savanna dynamics subject to fire.

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.

Effects of sociality on short‐distance, female‐biased dispersal in tropical wrens

Summary Dispersal from the natal area, or between breeding sites, marks a critical transition for individual animals, and a fundamental demographic and genetic process. Limitations on dispersal in natural populations are important in predicting population dynamics and structure. Tropical habitats that can be naturally patchy are also being fragmented rapidly, and these habitats harbour many social species. We studied dispersal in cooperatively breeding stripe‐backed wrens (Campylorhynchus nuchalis) in the Venezuelan savanna, using 20 years of data from marked populations over 40 km2. We test the hypotheses that dispersal is limited by competition for social position and that such limitation is sex‐specific. Dispersal is strongly female‐biased, and results mainly in attainment of breeding status in neighbouring groups. Reproductive success is determined by the structure of the breeding group, and is concentrated in a small minority of individuals. Natal dispersal is delayed, and represents a dramatic shift from indirect contribution to kin production (helping) to the direct approach of breeding. Monogamy limits breeding opportunities, compared to other social species, and both natal and breeding dispersal are constrained to short distances, particularly when compared to non‐cooperative breeders. Males are more philopatric compared to females, for which more frequent and distant dispersal leads to greater reproductive success. Competition for breeding status in an established social order impedes dispersal, in part because experience and previous familiarity are important, particularly in males that rarely disperse to productive large groups. Diverse patterns of sex bias in dispersal among cooperatively breeding birds can be explained by the comparative diversity of reproductive alternatives available to males and females. A ‘breeding diversity’ hypothesis is illustrated by comparison with male‐biased dispersal in brown jays. Social resistance to movement establishes a brake on gene flow and demographic interaction among populations that has important implications for population viability.

Growth dynamics of three tropical savanna grass species: an individual-module model

We model the dynamics of grass plant growth as a collection of the individual dynamic behavior of shoots and leaves. The model is inspired in data for plants of three species ( Elyonurus adustus, Leptocoryphium lanatum and Andropogon semiberbis) of common grasses in the Venezuelan savannas that were sampled monthly for 1 year. These species represent different architecture and regeneration response to fire. Modules (shoots and leaves) were counted in each cell of a square grid in each one of several vertical levels. Module density per cell provides the horizontal distribution within a level and is aggregated by level to obtain vertical distribution. Both distributions are simulated by a dynamical model based on shoot emergence and mortality, elongation of shoots and leaves given by Richards’ equation, plus a few simple geometric considerations. For quantitative comparisons of model results to data, the transient and final values for vertical distribution plus two metrics of horizontal distribution at each level, were calculated for the simulation results and the field data. Proportion of occupied cells and maximum distance to the center of growth were the two metrics selected to capture the dispersion and range of the horizontal distribution. The model results indicate predictable final vertical profiles (of proportional density plus the two metrics) similar to the profiles of measured distributions for each species. A reasonable prediction of the transient behavior was also obtained but with larger deviations as evaluated by the root mean square error between model and data. Differences in vertical and horizontal patterns of module density among species are explained by changing a set of parameter values related to growth form and phenology. Thus, the model could be applied to generate plant functional types for analysis of savanna dynamics subject to fire. With modifications, the model is potentially applicable to other grass plants and other grassland ecosystems.

Dinámica Estacional del Banco de Semillas en una Sabana en los Llanos Centro-Orientales de Venezuela1

Little is known about seed bank dynamics in Neotropical savannas. We studied temporal variation in seed density, species richness, species composition, seed mass, and seed number in the topsoil seed bank of a savanna on “La Iguana” Experimental Station, Guárico State, Venezuela. Total soil surface sampled was 57,600 cm2, in 400 cm2 units, at four times of the year. Seeds from 124 plants from 20 families were encountered. Species composition varied seasonally, but grass seeds dominated almost all the year, both numerically and in species number. Seed density and species richness were highest in the dry season and declined to a minimum in early wet season. Mean density was 1702 seeds/m2, with a range of temporal variation from 235 to 4624 seeds/m2. Average seed mass across species by date ranged from 0.97 to 2.53 mg, but because tiny seeds were most numerous throughout the year, the abundance-weighted averages only varied from 0.8 to 1.19 mg. We found a high spatial variability in the seed distribution in the soil throughout the year. There were also substantial changes in the topsoil seed bank among seasons, as a consequence of herbaceous plant phenology and germination strategies, which are tightly coupled with hydroperiodism in Venezuelan savannas. Based on the species-area curves, we concluded that the minimal sampling effort needed to adequately estimate the species richness in the topsoil of this savanna is 11 fold higher than that used in most seed bank studies in tropical environments.

Dietary relationships among four granivorous doves in Venezuelan savannas

A total of 537 crops from four coexisting granivorous doves (Columbina minuta, C. passerina, C. talpacoti and Scardafella squammata), collected on seven sampling dates in a Venezuelan savanna, were examined to assess food partitioning. These closely related doves are resident birds, and overlap broadly both in daily activity patterns and habitat use; in circumstances like this, segregation in the use of food resources often occurs. It was found that: (a) differences in body dimensions did not account for variations in seed size taken by doves; they all ate seeds in the same size range and they showed high similarity in seed size preferred; (b) the importance of different seed sizes in the diets changed seasonally with similar tendencies for all four species, (c) there were differences up to 4-fold in mean seed size consumed by individuals of the same species on the same date; (d) seeds of 63 plant species were consumed by the four dove species and constituted from 98 to 99.5% of the diets; (e) high dietary overlap both in prey size and prey type was found throughout the year. The dietary relationships found in this study support the idea that these birds forage opportunistically, and that some differences found among their diets may be the result of random seed sampling from the heterogeneous seed pool available.

Social Structure of Feral Horses in the Llanos of Venezuela

Feral horses (Equus caballus) in the Venezuelan savannas (llanos) formed bands of 3-35 individuals; mean group size varied between 15 and 21. Some bands had up to three adult males, and there was a positive correlation between the number of males and the number of females in bands. Adult males also were seen solitary or formed bachelor groups. One association of three females was observed for 3 months. We noted a correlation between number of females in bands and number of foals per female born during our study (Jan­ uary-July 1992). The advantages of being in larger groups were unclear, because natural predators were rare. Perhaps females in larger groups have more time available for foraging due to reduced individual vigilance or that larger groups live in better-quality home ranges.

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.

Extra-pair paternity uncommon in the cooperatively breeding bicolored wren

We investigated parentage using multilocus DNA fingerprinting for 222 juveniles produced during 99 group-years in the bicolored wren Campylorhynchus griseus, a cooperatively breeding bird of the Venezuelan savanna. Young adult bicolored wrens (auxiliaries) remain in their natal territories and substantially enhance the production of young there. We have previously used behavioral indicators of dominance by a single male/female pair (principals) to infer breeding status, resulting in the commonly applied model of helping in which current fitness accrues to auxiliaries only indirectly, in proportion to their relatedness to the principals and the effect of their assistance on breeding success. Our parentage analysis has demonstrated that 8.6% of the juveniles found on territories were not produced by the principal pair. Parentage of 4.1% of the juveniles was completely outside the social group; these appear to result from early dispersal of juveniles rather than from brood parasitism, most likely resulting from breakup of nearby groups. Principal females mated outside of their group (2.3%), or with an auxiliary male (2.3%), in the remaining cases of parentage outside the principal pair. No matings were detected between close relatives (e.g. mother-son); matings detected between the principal female and an auxiliary male followed a typical replacement of the principal female by an unrelated immigrant female. Our finger-printing results indicate that: (1) current fitness benefits accruing to most auxiliaries do not exceed their likely reproductive success had they dispersed successfully to a breeding position; (2) nearly all wren mating is monogamous and (3) behavioral dominants (especially females) can monopolize breeding.

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.