Neotropical Savanna Research Articles

Earthworm communities in native savannas and man-made pastures of the Eastern Plains of Colombia

The structure and seasonal changes of earthworm communities were evaluated in a natural savanna and in a improved grass-legume pasture in a Colombian oxisol over a period of 18 months. One plot of 90×90 m was isolated in each of the systems and each month five samples of 1 m2×0.5 m and ten of 20×20×20 cm were randomly selected from a stratified block design. Species richness was similar in the two evaluated plots (seven species), whereas diversity measured by the index, H (Shannon and Weaver 1949) was clearly different, i.e. H=2.89 in natural savanna and H=1.29 in pasture. This is explained by differences in earthworm community structure. The average annual density in the savanna was 49.8, ranging from 10.8 to 135.8 individuals (ind) m–2, and biomass was 3.3 g m–2 (hand-sorting method), ranging from 0.9 to 11.5 g m–2. In the man-made pasture, density was 80.1 ind m–2 on average, ranging from 24 to 215.8 ind m–2 and biomass was more than tenfold higher, ranging from 29.2 to 110.4 g m–2. This was especially due to the presence of a large glossoscolecid anecic species, Martiodrilus carimaguensis Jiménez and Moreno, which has been greatly favoured by conversion of savanna to pasture. Endogeic species were dominant in the natural savanna whereas the anecic species accounted for 88% of total earthworm biomass in the pasture. Total earthworm density and biomass were significantly different in the two systems studied (t-test). The results indicate a clearly positive response of earthworm communities to improved pastures, a type of land use that is being increasingly adopted in moist neotropical savannas.

The Dilemma of Sampling Streams for Macroinvertebrate Richness

The Dilemma of Sampling Streams for Macroinvertebrate Richness

The savannization of moist forests in the Sierra Nevada de Santa Marta, Colombia

In the Rio Ranchería watershed of the Sierra Nevada de Santa Marta, between 500 and 1500 m, savanna vegetation is interspersed with moist forests. The savannas are composed of native savanna grasses like Aristida adscensionis L., Arundinella sp., Panicum olyroides Kunth, and Schyzachyrium microstachyum (Desv.) Roseng., Arrill & Izag and the African Melinis minutiflora P. Beauv. There is also Curatella americana L. and Byrsonima crassifolia (L.) H.B.K., two typical tree species of the neotropical savannas. Although moist forest patches occur more often on lower slopes and narrow valley bottoms, they can also be found on mid‐ and upper‐slopes and less often on ridges. Thus, these forest patches are not gallery forests as are found throughout the neotropics, but the result of deforestation and fractionation of a continuous forest. A comparison of soil profiles between the savannas and remnant forest patches on the same slope, showed the disappearance of the A and B horizons (approx. 50 cm) under savanna vegetation. The sharp difference between the savanna and forest soils at the Rio Ranchería does not appear to be due to a change in soil water status along a toposequence or differences in the underlying bedrock. We hypothesize that the savannas of the Rio Ranchería watershed, are the result of deforestation and land practices on infertile soils derived from granite. The savannization process was likely initiated by Amerindians by means of the frequent use of fire or clearing lands for the cultivation of maize. The introduction of cattle by Spaniards (c. 1530) and the frequent use of fire to maintain grazing fields, contributed to further degradation of the habitat. While some tropical landscapes recovered their forest cover when human pressure was removed approximately 500 years ago, areas such as the Rio Ranchería watershed have suffered permanent damage. The savannas of this region are likely to remain unless fire is suppressed and soil restoration practices implemented.

On the Use of Path Analysis and Related Procedures for the Investigation of Ecological Problems

C) and, based on that assumption, partitioned relationships among different pathways. In this original form, path analysis was not a simultaneous method (in the Smith et al. (1997) recently drew attention to some pit- sense that all parameters were estimated simultaneously) falls that can arise when trying to predict the responses nor did it include overall tests of model fit to data. In the of experimental manipulations from nonexperimental early 1970s, more general procedures were developed for

Post‐burn reproduction of woody plants in a neotropical savanna: the relative importance of sexual and vegetative reproduction

1. Burning typically occurs at intervals of 1–3‐years in the Brazilian cerrado, a rate that exceeds the precolonization fire regime. To determine if woody plants of the cerrado successfully reproduce within the short span of time between burns, experimental burns were used to quantify the effects of fire on sexual and vegetative reproduction of six species of resprouting trees and shrubs.2.Four of the six species reproduce vegetatively by producing root suckers. For three of these species,Rourea induta,Myrsine guianensisandRoupala montana, sucker production was seven to 15 times greater in burned plots than in unburned controls.3.Fire had a negative impact on sexual reproduction. Fire caused an immediate reduction in sexual reproductive success by destroying developing reproductive structures and seeds. Additionally, five of the six study species exhibited overall reductions in seed production in the years following fire. Fire had this effect by reducing the individual size of all species and, for three species, by reducing size‐specific reproductive output. Only the treePiptocarpha rotundifoliaexhibited increased seed production following burning.4.Fire caused substantial mortality to both seedlings and suckers. Suckers were larger than seedlings and experienced lower mortality rates for two of three species. Fire‐induced mortality of seedlings varied greatly among species, ranging from 33% to 100%.5.The results indicate that vegetative reproduction is much more successful than sexual reproduction under the high fire frequency typical of current fire regimes. It is concluded that current fire regimes must be causing a shift in species composition, favouring species capable of vegetative reproduction.

Comparison of Factors Affecting Water Transfer in a Cultivated Paleotropical Grass (Brachiaria decumbensStapf) Field and a Neotropical Savanna during the Dry Season of the Orinoco Lowlands

Abstract Partitioning of available energy into sensible (H) and latent (LE) heat exchange was simultaneously measured in a closed Brachiaria field and an open neotropical savanna during the dry season of the Orinoco lowlands. These communities, differing in canopy coverage and aerodynamic forcing conditions, had major effects on the radiation and energy budgets. Thus, in the Brachiaria canopy, seasonally averaged daily LE was 55% (6.635 ± 0.92 MJ m−2 day−1) of the seasonally averaged daily available energy. In the savanna, it was 30% (2.841 ± 0.65 MJ m−2 day−1). The high degree of coupling between the Brachiaria and savanna surfaces to the prevailing weather of the dry season, as expressed by the seasonally averaged daily omega factor (Ω = 0.056 ± 0.005 and 0.042 ± 0.003, respectively), indicates that LE depended on the atmospheric driving potential and the canopy conductance (gc) to vapor diffusion. Consequently, daily LE was proportional to the imposed evapotranspiration rate. The hourly LE measured/LE ...

Comparison of water transfer as a component of the energy balance in a cultivated grass ( Brachiaria decumbens Stapf.) field and a savanna during the wet season of the Orinoco Llanos

Flux densities of sensible ( H) and latent heat (LE) were comparatively measured over a closed canopy of a cultivated paleotropical grass ( Brachiaria decumbens) field and an open canopy of a native neotropical savanna with upright-tiller grasses. The measurements were conducted during non-rainy days of the wet season (May–November). Results indicated that these two canopies, differing in closure and forcing conditions, were able to affect the energy budget. Consequently, seasonally averaged daily net radiation ( R n) was, on average, 1.1 fold higher over the Brachiaria canopy. However, contrast in canopy closure had a major effect on the partitioning of seasonally averaged daily R n into LE, H and soil heat flux ( G) as seasonally averaged daily LE over the Brachiaria canopy dissipated 85% of the available energy (7.5±0.6 MJ m −2 day −1). Over the savanna, the value was 64% (4.9±0.3 MJ m −2 day −1). Seasonally averaged daily measurements of H and G in the closed canopy were 53 and 66% lower than those in the open canopy. In the Brachiaria canopy, physical factors related to the hourly available energy were mainly controlling the energy partitioning and hourly LE was proportionally related to the equilibrium evaporation rate (LE eq). In the savanna canopy, the canopy cover and relatively higher soil exposure to solar radiation ( R s) were mainly controlling the stand evapotranspiration as the differential effect of R s and the air water saturation deficit on the vegetation and soil evaporating surfaces. The evapotranspirative adaptation of Brachiaria in relation to the dynamic between water-transpirating and water absorbing surfaces as well as surface conductance sensitivity to soil transfer and yielding capacity are probably responsible for the widespread nature of this forage in the Orinoco Llanos. However, the difference in production between the Brachiaria and savanna stands is economically determined under the limiting environmental conditions of the Orinoco Llanos, where yield increases, which may be gained throughout the agricultural inputs must justify the required addition of resources.

Dioecy in the Cerrado vegetation of Central Brazil

Summary Occurrence of dioecy in Cerrado, Neotropical savanna region in Central Brazil, is analyzed here. Frequency of dioecious species and their phenological and morphological characteristics were studied for a cerrado woody community near Brasilia, and a broader comparative survey was done using phytosociological data for different cerrado areas and vegetation physiognomies. Nine dioecious species were found in the studied community (15% of the species in the area), most with simple open flowers visited by small insects. Flowering phenology differed from species to species including long, sparse flowering and comucopian strategies. Distance between flowering individuals varied from 13 to 70 m. Gender related differences in spatial distribution and in height or circumference were tested for three of the species and significant departure from random was found only for Rapanea guianensis . The regional survey showed frequencies of dioecy higher in denser woodlands and forest than in open cerrado areas. Most species presented small promiscuous flowers, probably associated to pollination by small insects, and fleshy fruits. Among the species occurring in the cerrado areas one could separate a group of dioecious shrubs and treelets really adapted to cerrado environment and another group of evergreen forest species which occur only in denser woodlands, the cerradao. Differences in the frequency of dioecy seems to be related to the occurrence of this latter group and the appearance of cerradao.

The Effects of Fire and Cover on Seedling Establishment in a Neotropical Savanna

1 It was hypothesized that if facilitation is important for seedling establishment in savanna, then fire should reduce seedling establishment. 2 This was tested in the cerrado savanna of Brazil with a factorial experiment designed to evaluate the effects of cover and prescribed burning on seedling establishment. 3 Seeds of 12 species of trees and shrubs were sown in plots located in sites providing three densities of woody cover and four times since last burning. 4 Seedling establishment generally was greater under the crowns of trees than in open grassland, but individual species responded differently to cover. Eight of the 12 species responded favourably to cover, but a single species showed reduced establishment with increasing cover. 5 Burning had an overall negative effect on seedling establishment in the first year following burning. By the second year following burning, establishment returned to control levels. 6 An irrigation experiment with two species demonstrated that rain-free periods within the wet season had negative effects on seedling establishment. 7 A litter-removal experiment with Miconia albicans revealed that, at least for this species, litter has a negative effect upon seedling establishment in densely wooded sites and positive effects in open grassland.

Resistance to Leafcutter Ants (Hymenoptera: Formicidae) and Inhibition of Their Fungal Symbiont by Tropical Forage Grasses

Four measures of resistance to the leafcutting ant Acromyrmex landolti (Forel) in 5 forage grasses were evaluated in a neotropical savanna: damage caused by cutting of seedlings during pasture establishment, colonization by A. landolti of established swards, fresh weight of fungus gardens of excavated A. landolti colonies, and ability of the leafcutters’ symbiotic fungus to grow on artificial medium containing aqueous extracts of susceptible and resistant grass cultivars. Susceptible forages ( Andropogon gayanus ‘Carimagua 1’ and Brachiaria dictyoneura ‘Llanero’) had more seedlings cut during establishment and greater colonization by A. landolti in established swards compared with resistant cultivars ( B. brizantha ‘Marandu’, B. decumbens ‘Basilisk’, and B. humidicola ‘Pasto Humidicola’). Fresh weights of the fungus gardens of excavated colonies that had been restricted to cutting resistant cultivars (Pasto Humidicola, Marandu, or Basilisk) were reduced compared with colonies restricted to Carimagua 1, Llanero, or native savanna (control). Growth of the symbiotic fungus isolated from fungus gardens of A. landolti and Atta laevigata (F. Smith) was reduced when grown on agar with aqueous extracts of resistant cultivars compared with growth of the fungus on agar alone. When cultured on agar with aqueous extract of Carimagua 1, fungus growth was equivalent to the control. Growth of the fungus with homogenate of Llanero was intermediate to control and resistant cultivars. Agreement between these measures of resistance to leafcutting ants and their symbiont indicates that resistance in Brachiaria is conferred by plant factors inhibitory to the symbiotic fungus.

Dynamics of energy and nutrient concentration and construction cost in a native and two alien C4 grasses from two neotropical savannas

In Venezuela, the alien grasses Melinis minutiflora Beauv. and Hyparrhenia rufa (Nees.) Stapf tend to displace the native savanna plant community dominated by Trachypogon plumosus (Humb. and Bonpl.) Nees. This occurs in either relatively wetter and fertile highland savannas or in drier and less fertile lowland savannas. Although the native and aliens are perennial C4 grasses, higher net assimilation leaf biomass per plant and germination rate of the latter are some causes for their higher growth rates and for their competitive success. The objective of this study is to compare seasonal tissue energy, N, P and K concentrations and the calculated construction costs (CC) between the native grass and either one of the alien grasses from lowland and highland savannas. We predict that, in order to out-compete native plants, alien grasses should be more efficient in resource use as evidenced by lower tissue energy and nutrient concentrations and CC.

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.

Water flux through a semi-deciduous forest grove of the Orinoco savannas.

Water relations were analysed in a semi-deciduous forest grove occurring in the oxisols of the Orinoco savannas. This grove has a shallow unconsolidated ironstone cuirass, which is overlaid by a sandy loam layer (0.0-0.5 m) that contains more than 90% of the grove forest root phytomass. Evapotranspiration and through drainage were calculated by using data from the soil profile as related to physical characteristics of the site root zone, hydraulic conductivity, volumetric water content and potential hydraulic gradient. Mean annual evapotranspiration was 783 mm year-1 and annual through drainage below the root zone was 14% (162 mm year-1) of the gross rainfall. This drainage recharged the 42% of the annual saturation deficit of the water table. Similar mean annual evapotranspiration (770 mm year-1) was also calculated by using the water balance components. The mean daily coupling omega factor (Ω) between the grove canopy and the surrounding atmosphere indicated that a high degree of coupling (Ω=0.14±0.16) occurs in the grove and evapotranspiration was mainly controlled by surface conductance. As the dry season proceeded, the soil saturation deficit (δθ) increased rapidly resulting in a threshold surface conductance (0.030-0.005 m s-1) for δθ ranging from 0.05 to 0.10. Hypotheses to explain the omnipresence of perennial species in the wide range of physical conditions in neotropical savannas are discussed.

Effect of seasonal drought and cactus X virus infection on the crassulacean acid metabolism ofAgave sisalanaplants growing in a neotropical savanna

Healthy and cactus X virus-infected Agave sisalana plants were collected during the dry and rainy seasons in two contrasting savanna sites. At site 1, the soil water content showed no seasonal variations, but at site 2 it was much lower during the dry season than in the rainy season. When compared to healthy plants (HP), virus infection reduced the total leaf area, the Δ of titratable acidity and the water content of the plants, regardless of the season. In virus-infected plants at site 1 (VIP-1) the magnitude of the effect elicited by the virus did not differ between seasons, whereas, in virus-infected plants at site 2 (VIP-2) the deleterious effect of the virus was more pronounced during the dry season. In the dry season, VIP-1 and VIP-2 accumulated more fructans along the diurnal cycle and had higher net diurnal carbon gain than HP, with higher values in VIP-2. With the onset of rain the total leaf area of VIP-2 increased and the Δ of titratable acidity and water content reached the values seen in VIP-1. During the rainy season there was less accumulation of fructans and the net diurnal carbon gain was lower in both VIP-1 and VIP-2 compared with HP, with VIP-2 showing the lower values. In VIP-1 and VIP-2 there was also a marked reduction in the diurnal accumulation of α-amino-N at both seasons. The data obtained in young developing leaves allowed us to discuss the present data in terms of alterations in the plants sink/source ratio caused by drought and cactus X virus infection

Maintenance of Rain Forest Diversity in Riparian Forests of Tropical Savannas: Implications for Species Conservation During Pleistocene Drought

Structural and floristic attributes of riparian forest patches in a Neotropical savanna were studied in order to evaluate their potential to maintain rain forest plant diversity during periods of drought. Tree density was significantly greater, and basal area significantly lower, than pantropical means for these variables, thus these riparian forests were characterized as low biomass, many-stemmed communities. A total of 292 species ? 0.5 m high were found in an aggregate vegetation sample of 1.6 ha. Most species were typical rain forest taxa, indicating that this flora is not specialized to the riparian environment. Isolation of these patches was found to have some negative effects on species composition, since dioecious and mammal dispersed species were more poorly represented in riparian than in continuous forests. While this riparian system is not floristically depauperate, species densi- ties in individual plots indicate that its richness is comparable to that of continuous forests in Central America. These results suggest that tropical riparian forest fragments have the poten- tial, albeit limited, to maintain large numbers of rain forest species and, thus, that analogous forest fragments may have played an important role as safe sites for tropical rain forest floras during periods of drought in the Pleistocene.

Water relations of native and introduced C4 grasses in a neotropical savanna.

Introduced African grasses are invading Neotropical savannas and displacing the native herbaceous community. This work, which is part of a program to understand the success of the African grasses, specifically investigates whether introduced and native grasses differ in their water relations. The water relations of the native Trachypogon plumosus and the successful invader Hyparrhenia rufa were studied in the field during two consecutive years in the seasonal savannas of Venezuela. The two C4 grasses differed clearly in their responses to water stress. H. rufa consistently had higher stomatal conductance, transpiration rate, leaf water and osmotic potential and osmotic adjustment than the native T. plumosus. Also, leaf senescence occurred much earlier during the dry season in H. rufa. Both grasses showed a combination of water stress evasion and tolerance mechanisms such as stomatal sensitivity to atmospheric or soil water stress, decreased transpiring area and osmotic adjustment. Evasion mechanisms are more conspicuous in H. rufa whereas T. plumosus is more drought tolerant and uses water more "conservatively". The evasion mechanisms and oportunistic use of water by H. rufa, characteristic of invading species, contribute to, but only partially explain, the success of this grass in the Neotropical savannas where it displaces native plants from sites with better water and nutrient status. Conversely, the higher water stress tolerance of t. plumosus is consistent with its capacity to resist invasion by alien grasses on shallow soils and sites with poorer nutrient and water status.

Contrasting breeding systems in twoEriotheca (Bombacaceae) species of the Brazilian cerrados

The pollination biology and breeding systems ofEriotheca pubescens andE. gracilipes have been studied. These two species occur as trees in cerrado vegetation, the neotropical savannas of Central Brazil, with partially sympatric distributions. They have similar phenology and floral structure, although the flowers ofE. pubescens are larger. Both species have nectar flowers pollinated by largeAnthophoridae bees but the main pollinators of each species differ in size. The species have markedly different breeding systems: late-acting self-incompatibility inE. gracilipes and apomixis stimulated by pollination inE. pubescens.

Responses to simulated herbivory and water stress in two tropical C4 grasses.

The African grass Hyparrhenia rufa has established itself successfully in South American savannas (Llanos) and displaced dominant native grasses such as Trachypogon plumosus from the wetter and more fertile habitats. Several ecophysiological traits have been related to the higher competitive capacity of H. rufa. To further analyze the behavior of both species, their growth, biomass allocation, physiological and architectural responses to defoliation and water stress were compared under controlled conditions. Although total, aerial and underground biomass decreased under defoliation in both grasses, increases in clipped-leaf biomass and area compensated for defoliation in H. rufa but not in T. plumosus. This difference was due mainly to a higher proportion of assimilates being directed to leaf and tiller production and a higher leaf growth rate in the African grass as compared to T. plumosus, which showed incrased senescence under frequent defoliation. In both species, water stress ameliorated the effects of defoliation. The ability to compensate for defoliated biomass in H. rufa is possibly related to its long coevolution with large herbivores in its original African habitat and is apparently one of the causes of its success in Neotropical savannas.

Population Responses to Fire in a Tropical Savanna Grass, Andropogon Semiberbis: A Matrix Model Approach

SUMMARY (1) To study the population-level effects of fire on the savanna grass Andropogon semiberbis, size-classified matrix population models were constructed for an annually burnt population and for a population protected from fire. These models were used to examine the effects of fire on population growth rate, stable size distributions and reproductive value, and to simulate different fire regimes. (2) The burnt population is capable of increasing rapidly (kX = 1 2524, r = 0-2251), whereas the unburnt population is unable to persist (kX = 0 2762, r = -1.2886). Most of this difference is due to effects on the growth, survival and reproduction of the smallest two size classes, which are shown by elasticity analysis to be the most important to population growth in both populations. The stable size distributions and reproductive values are similar in the two populations. (3) Both deterministic and stochastic analyses reveal a critical frequency of fire (= 0.85) below which this species is unable to maintain itself. This apparent reliance on fire frequency suggests that the spread and evolution of this species has been closely related to human occupation of neotropical savannas.

Parasite loads in parthenogenetic and sexual lizards (Heteronotia binoei) : support for the Red Queen hypothesis

One version of the Red Queen hypothesis suggests that sexual reproduction is maintained in populations because of the need to continually create genotypes that confer resistance against rapidly evolving pathogens and parasites. Here, we report that parthenogenetic individuals of the Heteronotia binoei species complex are much more prone to infection by mites than are their sexual relatives. This accords with a central prediction of the Red Queen hypothesis. The greater susceptibility of the parthenogens is consistent across localities with different combinations of parthenogenetic genotypes and sexual chromosome races and occurs despite the unusually high genetic diversity of the parthenogenetic form. These observations support the contention that clonal reproduction increases the susceptibility of hosts to infection by parasites.