AB - ABSTRACT: New AMS and palynological data are presented from the Colombian Andes to assess vegetational and climatic change during the Lateglacial-Holocene transition, with special emphasis on the Younger Dryas (YD) chronozone. The new evidence is compared and discussed with other Colombian cores and with data from other countries in the region. The Lateglacial climatic reversal in Colombia, known as the El Abra stadial, has an estimated lower boundary of 11,200 (conventional) and 10,900 (interpolated) (super 14) C yr BP, respectively. Although the El Abra stadial is assumed to be the equivalent of the European YD, the present data suggest that the El Abra stadial is a biostratigraphical signal representing both the YD chronozone and the earliest Holocene (11,000-9000 (super 14) C yr BP). On the basis of new AMS dates and a re-evaluation of the pollen zones, we divided the Colombian El Abra biozone into two phases. From ca. 11,000 to ca. 10,500 (super 14) C yr BP there is a sharp increase of subparamo and paramo pollen, reflecting a relatively cool phase during the YD chronozone (zone Y1). After ca. 10,500 (super 14) C yr BP, a slight increase of arboreal pollen and the presence of Cactaceae (zone Z1) point toward a relatively milder but drier phase extending to ca. 9000 (super 14) C yr BP in the earliest Holocene. Our conclusions add detail to the concept of astronomical forcing of contrasting rainfall changes in northern South America between 12,400 and 8800 (super 14) C yr BP. We propose an environmental drought during the El Abra biozone, with subsequent erosion of the sediments deposited during that period, as a major factor explaining the poor presence of YD evidence in northern South America. We conclude that at hydrologically sensitive sites without a clear lithological change around 11,000-9000 (super 14) C yr BP, a hiatus during the YD chronozone can only be detected if other cores are available with bracketed time control. We state that the start of the temperature decline in Central America and northern South America is related to the global YD cooling event. However, much work is still required to understand the duration and amplitude of the climatic reversal during the YD chronozone in this part of the globe.
AB - ABSTRACT: A common feature in the undisturbed forest ecosystems in the Middle Caqueta (Colombian Amazonia) is the presence of a thick litter layer with abundant fine roots over mineral soils which are highly weathered and very low in available nutrients. In these situations, the litter layer or the forest floor (FF) may play an important role in the forest water balance, controlling water fluxes and nutrient cycling. We investigated the forest floor water dynamics in four representative forest ecosystems in the Middle Caqueta, Colombian Amazonia (tertiary sedimentary plain, high terrace, low terrace and the rarely inundated flood plain). Meteorological conditions were measured on hourly basis, FF water storage capacity, TDR water content in the FF and litterflow or FF drainage were measured daily over a two year period. A dynamic model was developed to simulate FF water storage, root water uptake and drainage to the mineral soil. The four-parameter model was calibrated applying a step-wise procedure. Analysis of collected data showed that FF water content was generally lower in the sedimentary plain than in the other ecosystems, whereas FF water storage was the highest due to a high FF mass. The average storage capacity per unit FF thickness was 1.23 mm cm (super -1) . The sensitivity analysis and calibration of model parameters highlighted the relevance of storage capacity as the most sensitive parameter for the FF water dynamics. For the validation period, there is a good agreement between predicted and measured FF water storage and especially between predicted total drainage from the forest floor and measured litterflow. Model predictions indicate that water uptake from the FF"s during the validation period (190 days) differed between ecosystems, ranging from 15 to about 28% of the reference transpiration. This seems to be related to the fraction of fine roots in the FF in each ecosystem and to the water availability. On the other hand, total drainage to the mineral soil was very similar among ecosystems, except for the sedimentary plain, where total drainage was the lowest with about 87% of incoming throughfall.
AB - ABSTRACT: The partitioning of gross rainfall into throughfall, stemflow and evaporation of intercepted rainfall was studied in four forest ecosystems in the Middle Caqueta, Colombian Amazonia. Data on climate was collected automatically on an hourly basis during a five-year period. Weekly measurements of rainfall, throughfall and stemflow were carried out during a period of two years, while daily measurements, on an event basis, were carried out during two subsequent years. Throughfall, stemflow and evaporation in each forest were checked for correlations with gross rainfall characteristics, canopy gap fraction, tree crown area and bark texture. Canopy gap fraction differed between forests, ranging from 9% on the flood plain to 17% on the Tertiary sedimentary plain. Rainfall was rather evenly distributed over the year, with one dry period from December to February. 92% of the rain fell in single showers of less than 30mm and most of the storms (56%) fell in less than one hour, during the afternoon or early night. Throughfall ranged from 82 to 87% of gross rainfall in the forests studied and varied with gross rainfall in all forests. It depended on the amounts and characteristics of rainfall, but differences in throughfall among forests, when comparing similar rainfall events, clearly indicated that throughfall also depends on forest structure. Stemflow contributed little to net precipitation (on average 1.1% of gross rainfall in all forests) and showed a power relation with gross rainfall. Correlations between stemflow per tree, projected crown area and bark texture were very poor as indicated by the low coefficients of determination. Evaporation during rainfall events exhibited a linear relation with rainfall duration and the ratio of evaporation over gross rainfall increased with forest cover (1-gap fraction) in the forests studied. The structure of the forests seemed to vary considerably and given its influence on rainfall partitioning it may explain both differences and similarities between results from this study and those from most other studies within Amazonia.
AB - ABSTRACT: The pollen record of a 65 cm long core Laguna Carimagua-Bosque (4 degrees 04'N, 70 degrees 13'W) shows the late Holocene environmental history from a lake located within the gallery forest of the savannas of the Llanos Orientales of Colombia. Nine AMS radiocarbon dates of the organic deposits show that the core represents the period from ca. 1300 (super 14) C yr BP to the present. The lake evolved from an active drainage system. During the period from ca. 1300 to 875 (super 14) C yr BP (zone CMB-Ia), Mauritia-dominated swamp and gallery forest was present, dominated by Cecropia, and later also Acalypha and Alchornea. From 875 to 700 (super 14) C yr BP (zone CMB-Ib), the lake was completely surrounded by gallery forest. Mauritiella and Cecropia occurred around the lake. Cecropia pioneer forest reached its greatest abundance and became gradually replaced by a more species-rich gallery forest, including Acalypha, Alchornea, Euterpe/Geonoma, Moraceae/Urticaceae, Piperaceae, and Virola. From 700 to 125 (super 14) C yr BP (zone CMB-II), Cecropia lost its dominant role, and Mauritiella palms became more frequent. The main vegetation categories were swamp forest, gallery forest, understory elements, savanna shrubs and trees, and grass savanna. From 125 (super 14) C yr BP to recent (zone CMB-III), the plant diversity in the gallery forest became highest, Mauritiella became very abundant, and among the savanna elements, woody Didymopanax increased. Comparison of four pollen records from savanna sites shows that pollen of savanna vegetation is markedly underrepresented in lake sediments when the lake lies within the gallery forest. As most of the drainage system of a savanna is hidden by gallery forest, we also expect a significant underrepresentation of the savanna ecosystem in river-transported pollen assemblages.
AF - AUTHOR AFFILIATION: Florida Museum of Natural History, Paleobotany Laboratory, Gainesville, FL, United States
SO - SOURCE: Geology (Boulder). 28; 9, Pages 815-818. 2000.
PB - PUBLISHER: Geological Society of America (GSA). Boulder, CO, United States. 2000.
CP - COUNTRY OF PUBLICATION: United-States
PY - PUBLICATION YEAR: 2000
LA - LANGUAGE: English
AB - ABSTRACT: The late Paleocene-early Eocene interval was characterized by a long period of global warming that culminated with the highest temperatures of the Tertiary. This interval was also associated with plant extinctions and a subsequent increase in plant diversity in temperate latitudes. However, tropical regions remain largely unknown. We compare the microfloral diversity of the late Paleocene with the late early to middle Eocene in flood plain, coastal plain, and estuarine facies of a section in the Colombian eastern Andes. Several techniques such as range-through method, rarefaction, bootstrap, detrended correspondence analysis, and Simpson index were used to assess the significance of the diversity pattern observed throughout the section. The microfloral record indicates a distinct, diverse Paleocene flora declining toward the end of the Paleocene, being replaced by a different and much more diverse Eocene flora. It is uncertain, however, how these floral changes correlate with the latest Paleocene thermal maximum and Eocene thermal maximum events.