R. Montes

Savanna vegetation-fire-climate relationships differ among continents.

Surveying Savannas Savannas are structurally similar across the three major continents where they occur, leading to the assumption that the factors controlling vegetation structure and function are broadly similar, too. Lehmann et al. (p. 548) report the results of an extensive analysis of ground-based tree abundance in savannas, sampled at more than 2000 sites in Africa, Australia, and South America. All savannas, independent of region, shared a common functional property in the way that moisture and fire regulated tree abundance. However, despite qualitative similarity in the moisture–fire–tree-biomass relationships among continents, key quantitative differences exist among the three regions, presumably as a result of unique evolutionary histories and climatic domains. Evolution cannot be overlooked when aiming to predict the potential global impacts on savanna dynamics in a warming world. Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.

Carbon stocks and fluxes in a temporal scaling from a savanna to a semi-deciduous forest

Abstract The strength of carbon sink and stock was assessed in a protected savanna of the Orinoco Llanos by the harvesting plant phytomass and using allometric relationships between the dry mass and the censuses of plant height. Thus, changes in the carbon stock and the proportion in the tree/grass proportion were evaluated throughout age states. Results indicate that the carbon stock in the vegetation increased from 207 to 9215 g C m −2 whereas in the soil, it varied 6680 to 12 196 g C m −2 . The carbon stock accumulation was mainly related to increases in the woody layer from 36 to 9215 g C m −2 (255-fold) and in the soil from 1341 to 12 196 g C m −2 (nine-fold), respectively. The estimated pool of carbon sequestered in the Orinoco Llanos by the restored forest in 51 years was 5.69 Pg C. The expansion and conservation of this carbon pool might remove CO 2 from the atmosphere to help compensate for CO 2 liberation associated with other land uses or industrial practices.

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

Summary 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.

Aluminium concentration in the biomass of native species of the Morichals (swamp palm community) at the Orinoco Llanos, Venezuela

Aluminium concentration was analyzed in roots, stems and leaves of species growing in morichals with extreme flooded acid soils, deficient in K, Ca, Mg and P. Mean aluminium saturation was 73%. Results seem to indicate that species depending on seasonal changes in the Al concentration of plant compartments, could be grouped into “perennial aluminium accumulators”, “late aluminium accumulators” and “threshold aluminium accumulators”.

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.

Chemical composition and nutrient loading by precipitation in the Trachypogon savannas of the Orinoco llanos, Venezuela

Samples of bulk precipitation were collected in the Trachypogon savanna, Calabozo, Venezuela, during three consecutive years. In the first year, rain samples were taken daily; in the following years the samples were grouped on a monthly basis. In addition, samples of dry deposition were collected during the dry seasons. All samples were analyzed for the following water soluble cations and anions: P04-P, S04-S, N03-N, NH4-N, Ca+2, Mg+2, K+, Na+ and H+. The mean annual input rate of chemical constituents (Kg ha-1 year-1) was: PO4-P (0.42); SO4-S (2.62); NO3-N (0.21); NH4-N (2.03); Ca+2 (3.50); Mg+2 (11.31); K+ (3.60); Na+ (5.93) and H+ (0.03). The total mean input of particulate material to the savanna during the dry season was 2.06 Kg ha-1 year-1, with a soluble fraction of 30%. Possible sources of nutrients input were analyzed.

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

Abstract 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.

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 areas 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.

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.

Diurnal patterns of carbon dioxide, water vapour, and energy fluxes in pineapple [Ananas comosus (L.) Merr. cv. Red Spanish] field using eddy covariance

We analyzed the eddy covariance measurements of momentum, mass, and energy taken daily throughout five consecutive seasonal courses (i.e. 840 d after planting) of a pineapple [Ananas comosus (L.) Merr. cv. Red Spanish] field growing in the Orinoco lowlands. This field provides an opportunity for micrometeorological studies because of the flat and windy site; the seasonal weather including ENSO effects and the Crassulacean Acid Metabolism (CAM) physiology of the crop were additional attributes. Soil CO2 flux was quantified and added to the net ecosystem exchange in order to obtain the canopy flux (FC). The canopy CO2 flux partially followed the four phases of CAM sensu Osmond (1978). The daily pattern of gaseous exchange in pineapple showed a continuum spectrum in which a major proportion of CO2 uptake occurring during the daytime was common and in which the CAM expression was related to day and nocturnal CO2 uptake. However, the benefits of CO2 uptake at low water cost were constrained by the limited nocturnal CO2 uptake. Seasonal and ontogenetic changes affected the energy exchange as well as the partitioning of available energy into sensible (QH) and latent (QLE) heat. When the hourly net radiation (QRn) reached its maximum value, latent heat flux (QLE) to available energy throughout the vegetative and reproductive stages was 0.65, 0.05, 0.30, 0.11, and 0.33 for the 1997 wet season, 1997/98 dry season, 1998 wet season, 1998/99 dry season, and 1999 wet season, respectively. Throughout the growth period, we found the pivotal role of surface conductance (gS) in both QLE and FC. Furthermore, the canopy responded to environmental changes. During the wet seasons the gS was strongly influenced by humidity mole fraction deficit and was usually lower than aerodynamic conductance, whereas during the dry seasons, soil water deficit limited evapotranspiration and production rates. For the fully canopy cover, the hourly trend of marginal water cost of pineapple carbon gain in the dry seasons indicated that gS became sufficiently efficient to reduce the amount of water transported per unit of carbon gain. In the wet season, the coupling of CO2 uptake and stomatal conductance was more effective in maintaining a higher proportionality between QLE and gS.