Effects of Land Use Change and Seasonality of Precipitation on Soil Nitrogen in a Dry Tropical Forest Area in the Western Llanos of Venezuela

We evaluated changes of different soil nitrogen forms (total N, available ammonium and nitrate, total N in microbial biomass, and soil N mineralization) after conversion of semideciduous dry tropical forest in 5- and 18-year-old pastures (YP and OP, resp.) in the western Llanos of Venezuela. This evaluation was made at early rainy season, at end rainy season, and during dry season. With few exceptions, no significant differences were detected in the total N in the three study sites. Compared to forest soils, YP showed ammonium losses from 4.2 to 62.9% and nitrate losses from 20.0 to 77.8%, depending on the season of the year. In OP, the ammonium content increased from 50.0 to 69.0% at the end of the rainy season and decreased during the dry season between 25.0 and 55.5%, whereas the nitrate content increased significantly at early rainy season. The net mineralization and the potentially mineralizable N were significantly higher P<0.05 in OP than in forest and YP, which would indicate a better quality of the substrate in OP for mineralization. The mineralization rate constant was higher in YP than in forest and OP. This could be associated with a reduced capacity of these soils to preserve the available nitrogen

Basin-wide variations in Amazon forest structure and function are mediated by both soils and climate

Forest structure and dynamics vary across the Amazon Basin in an east-west gradient coincident with variations in soil fertility and geology. This has resulted in the hypothesis that soil fertility may play an important role in explaining Basin-wide variations in forest biomass, growth and stem turnover rates. Soil samples were collected in a total of 59 different forest plots across the Amazon Basin and analysed for exchangeable cations, carbon, nitrogen and pH, with several phosphorus fractions of likely different plant availability also quantified. Physical properties were additionally examined and an index of soil physical quality developed. Bivariate relationships of soil and climatic properties with above-ground wood productivity, stand-level tree turnover rates, above-ground wood biomass and wood density were first examined with multivariate regression models then applied. Both forms of analysis were undertaken with and without considerations regarding the underlying spatial structure of the dataset. Despite the presence of autocorrelated spatial structures complicating many analyses, forest structure and dynamics were found to be strongly and quantitatively related to edaphic as well as climatic conditions. Basin-wide differences in stand-level turnover rates are mostly influenced by soil physical properties with variations in rates of coarse wood production mostly related to soil phosphorus status. Total soil P was a better predictor of wood production rates than any of the fractionated organic- or inorganic-P pools. This suggests that it is not only the immediately available P forms, but probably the entire soil phosphorus pool that is interacting with forest growth on longer timescales. A role for soil potassium in modulating Amazon forest dynamics through its effects on stand-level wood density was also detected. Taking this into account, otherwise enigmatic variations in stand-level biomass across the Basin were then accounted for through the interacting effects of soil physical and chemical properties with climate. A hypothesis of self-maintaining forest dynamic feedback mechanisms initiated by edaphic conditions is proposed. It is further suggested that this is a major factor determining endogenous disturbance levels, species composition, and forest productivity across the Amazon Basin. © 2012 Author(s). CC Attribution 3.0 License

Byrsonima crassifolia

Angiosperm

Growth ring periodicity in Pinus caribaea var. hondurensis from a Mérida state plantation, Venezuela

CONTENIDO Editorial.La educación agroforestal en la facultad de ciencias forestales y ambientales.Agroforestry education at the forestry and environmental sciences faculty. Petit Aldana, Judith Artículo Mejoramiento en la eficiencia de fertilizantes químicos con biofertilizantes para producir maíz en suelo Alfisol (Chac-lu’um). Chemical fertilizer efficiency improvement with biofertilizer to produce maize in Alfisol soil (Chac-lu’um). Uribe V., Gabriel; Petit, Judith y Roberto, Dzib E. Relaciones fenéticas en especies de Sterculia L. (Sterculiaceae) en Venezuela. Sterculia L. species (Sterculiaceae) phenetic relationships in Venezuela. Mondragón, Alcides; Máyida, Elsouki y Castillo, Aníbal Principales causas de la deforestación en la vertiente sur del Parque Nacional Sierra Nevada, Venezuela. Deforestation main causes in the south slope of Sierra Nevada National Park, Venezuela. Gómez T., Hugo A. y Molina, Misael Dinámica sucesional del componente arbóreo, luego de un estudio destructivo de biomasa, en el bosque universitario San Eusebio, Mérida-Venezuela. Arboreal component sucesional dynamics, after a destructive study of biomass, in San Eusebio university forest, Mérida, Venezuela. Ramos, María Cristina y Plonczak Ratschiller, Miguel Andrés Establecimiento de lianas sobre los árboles de un bosque del chaco húmedo argentino. Tree liana establishment of a forest from the argentinean humid chaco. Lorea, Luciano y Brassiolo, Miguel Periodicidad de los anillos de crecimiento en Pinus caribaea var. hondurensis proveniente de una plantación delestado Mérida, Venezuela. Growth ring periodicity in Pinus caribaea var. hondurensis from a Mérida State plantation, Venezuela. Melandri, José Luis; Dezzeo, Nelda y Espinoza de Pernía, Narcisana Planificación de la explotación de impacto reducido como base para un manejo forestal sustentable en un sector de la Guayana venezolana. Reduced impact exploitation planning as a base for a sustainable forest management in an area of the Venezuelan Guyana. Noguera L., Oscar E.; Pacheco A., Carlos E.; Plonczak Ratschiller, Miguel Andrés; Jeréz R., Mauricio; Moret, Ana Yajaira; Quevedo R., Ana Mercedes y Carrero G., Omar E. Potencial papelero de clones de Hevea brasiliensis procedentes del estado Amazonas, Venezuela. Paper potential of Hevea brasiliensis clones from Amazonas State, Venezuela. Zerpa, Juan; Mogollón, Gladys M.; Gutiérrez G., Ilvania y Aguilera, Antonio V. Ecuaciones de volumen para árboles de Samán (Samanea saman (Jacq.) Merr.), provenientes de potreros en el municipio Machiques de Perijá, Estado Zulia, Venezuela. Volume equations for Saman (Samanea saman (Jacq.) Merr.) trees growing in pasture lands of Zulia state, Venezuela. Moret, Ana Yajaira; Ortiz, Alba Guadalupe; Pérez, Yorfreddy; Quijada, Marcelino y Jeréz R., Mauricio Variabilidad de los vasos en Gmelina arborea Roxb. (Verbenaceae) proveniente de plantaciones del estado Portuguesa (Venezuela). Variability in vessels of Gmelina arborea Roxb. (Verbenaceae) from plantations in Portuguesa State (Venezuela). León Hernández, Williams J. y Quintero M., Maria [email protected]@ula.veNivel analíticosemestra

Changes in structure and composition of evergreen forests on an altitudinal gradient in the Venezuelan Guayana Shield

There have been several ecological studies in forests of the Guayana Shield, but so far none had examined the changes in structure and composition of evergreen forests with altitude. This study describes and analyzes the structure, species composition and soil characteristics of forest stands at different altitudinal zones in Southeastern Venezuelan Guayana, in order to explain the patterns and the main factors that determine the structure and composition of evergreen forests along the altitudinal gradient. Inventories of 3 948 big (>10cm DBH) and 1 328 small (5-10cm DBH) woody stems were carried out in eleven plots, ranging from 0.1 to 1.0ha, along a 188km long transect with elevations between 290 and 1 395masl. It has been found that 1) hemiepihytes become more dominant and lianas reduce their dominance with increasing altitude and 2) the forest structure in the study area is size-dependent. Five families and 12 genera represented only 9% of the total number of families and genera, respectively, recorded troughout the gradient, but the two groups of taxa comprised more than 50% of the Importance Value (the sum of the relative density and the relative dominance) of all measured stems. Moreover, the results suggest that low species richness seems to be associated with the dominance of one or few species. Stand-level wood density (WD) of trees decreased significantly with increasing elevation. WD is an indicator of trees’life history strategy. Its decline suggests a change in the functional composition of the forest with increasing altitude. The Canonical Correspondence Analysis (CCA) indicated a distinction of the studied forests on the basis of their altitudinal levels and geographic location, and revealed different ecological responses by the forests, to environmental variables along the altitudinal gradient. The variation in species composition, in terms of basal area among stands, was controlled primarily by elevation and secondarily by rainfall and soil conditions. There are other interacting factors not considered in this study like disturbance regime, biological interactions, productivity, and dispersal history, which could affect the structure and composition of the forests in the altitudinal gradient. In conclusion, it appears that the structural and floristic variability observed in the studied transect is produced by a combination of different climates and randomly expressed local processes interacting across a complex physical landscap

Spatial and temporal variability of soil N₂O and CH₄ fluxes along a degradation gradient in a palm swamp peat forest in the Peruvian Amazon

Mauritia flexuosa palm swamp, the prevailing Peruvian Amazon peatland ecosystem, is extensively threatened by degradation. The unsustainable practice of cutting whole palms for fruit extraction modifies forest's structure and composition and eventually alters peat-derived greenhouse gas (GHG) emissions. We evaluated the spatiotemporal variability of soil N2O and CH4 fluxes and environmental controls along a palm swamp degradation gradient formed by one undegraded site (Intact), one moderately degraded site (mDeg) and one heavily degraded site (hDeg). Microscale variability differentiated hummocks supporting live or cut palms from surrounding hollows. Macroscale analysis considered structural changes in vegetation and soil microtopography as impacted by degradation. Variables were monitored monthly over 3 years to evaluate intra- and inter-annual variability. Degradation induced microscale changes in N2O and CH4 emission trends and controls. Site-scale average annual CH4 emissions were similar along the degradation gradient (225.6 ± 50.7, 160.5 ± 65.9 and 169.4 ± 20.7 kg C ha−1 year−1 at the Intact, mDeg and hDeg sites, respectively). Site-scale average annual N2O emissions (kg N ha−1 year−1) were lower at the mDeg site (0.5 ± 0.1) than at the Intact (1.3 ± 0.6) and hDeg sites (1.1 ± 0.4), but the difference seemed linked to heterogeneous fluctuations in soil water-filled pore space (WFPS) along the forest complex rather than to degradation. Monthly and annual emissions were mainly controlled by variations in WFPS, water table level (WT) and net nitrification for N2O; WT, air temperature and net nitrification for CH4. Site-scale N2O emissions remained steady over years, whereas CH4 emissions rose exponentially with increased precipitation. While the minor impact of degradation on palm swamp peatland N2O and CH4 fluxes should be tested elsewhere, the evidenced large and variable CH4 emissions and significant N2O emissions call for improved modeling of GHG dynamics in tropical peatlands to test their response to climate changes.</p