3 research outputs found
Encroachment of sandplain heathland (kwongan) by Allocasuarina huegeliana in the Western Australian wheatbelt: the role of herbivores, fire and other factors
No full textKwongan, also known as sandplain heathland, occurs in remnant vegetation throughout the fragmented landscape of the Western Australian wheatbelt. This vegetation community has high levels of species richness and endemism, and is of high conservation value. In many vegetation remnants in the wheatbelt the native tree species Allocasuarina huegeliana (rock sheoak) is expanding out from its normal range and encroaching into kwongan. A. huegeliana may ultimately dominate the kwongan, causing a decline in floristic diversity. Altered disturbance regimes, particularly the absence of fire and reduced or absent browsing mammal herbivores, are likely to be responsible for causing A. huegeliana encroachment.
This study used experimental and observational data from patches of kwongan in three Nature Reserves in the central and southern wheatbelt to investigate the role of fire, native mammal activities and interactions between these two factors in shaping A. huegeliana woodland kwongan community boundaries. Investigations were carried out into the characteristics of encroaching A. huegeliana populations; the environmental factors affecting the extent of encroachment, naturally recruited juveniles, and seedling emergence and establishment; historical and current abundances of native mammals; and the effects of mammal herbivores on seedling establishment during inter-fire and post-fire periods.
Results from this study confirm that A. huegeliana has encroached into kwongan throughout the wheatbelt region and recruitment appears likely to continue in most areas. Few of the environmental factors measured in this study affected the extent of encroachment, the locations of naturally recruited A. huegeliana juveniles, and seedling germination and establishment. Western grey kangaroos (Macropus fuliginosus) browsed extensively on seedlings, which largely prevented them from establishing in open areas of kwongan. However, numerous A. huegeliana seedlings escaped browsing herbivores by establishing in perennial shrubs, where they appeared to be tolerant of increased levels of inter-specific competition.
There was no native mammal common to all three Reserves that declined around the time that A. huegeliana encroachment most likely began in the 1970s. In addition, tammar wallabies (Macropus eugenii) had little effect even where their densities were high. It is therefore unlikely that the decline of an individual mammal species initiated encroachment. A. huegeliana encroachment appears to be driven by increased propagule pressure, which is in turn caused by increased inter-fire intervals. Long periods of time without fire have enabled fire-sensitive A. huegeliana trees to produce increasing quantities of seed that are continuously released into kwongan. A range of other factors may interact synergistically with this process to affect encroachment and these are also discussed. This study considered the implications of these findings for management of remnant vegetation in fragmented landscapes, particularly kwongan in the Western Australian wheatbelt, and areas for further research are suggested
Influencia de la señal climática global del Oligoceno en el Caribe colombiano: Calibración astrobiocronológica de una sucesión del Oligoceno temprano y sus implicaciones tectonoestratigráficas
No full textIlustraciones, mapas, fotosspa:El Caribe colombiano es un margen activo caracterizado por una constante e intensa actividad tectónica desde el Cretácico al Reciente. No obstante, estudios recientes han reportado un período de relativa estabilidad durante el Oligoceno temprano para algunas cuencas del Caribe como la Cuenca Sinú-San Jacinto (CSSJ). Se presenta el estudio del registro bioestratigráfico y registro de pozo de una sucesión sedimentaria de 143 metros del pozo ANH-San Jacinto-1 en la CSSJ. Este es el primer modelo de edad astrobiocronológico construido para el Oligoceno temprano (32-28 Ma) basado en el uso integrado de la bioestratigrafía de foraminíferos planctónicos y el registro gamma ray (GR). Las alternancias rítmicas del registro GR, asociadas con cambios en la concentración de arcilla/materia orgánica, exhiben un patrón regular y están correlacionadas en amplitud e intensidad con los ciclos orbitales de excentricidad de la Tierra. La correlación con la curva astronómica de la excentricidad permitió establecer un modelo de edad con una resolución de 400 kyr para el Oligoceno temprano, que será fundamental para definir futuras correlaciones en las cuencas del Caribe colombiano. Los patrones del registro GR indican que durante los periodos de excentricidad máxima existieron condiciones climáticas húmedas fomentando mayores acumulaciones de arcilla y/o materia orgánica en el área del Caribe. En tanto que, condiciones de reducida precipitación persistieron durante los periodos de excentricidad mínima. La amplitud del registro GR también sugiere que, en momentos de menor insolación ecuatorial (en precesión máxima), la oblicuidad ejerció un mayor control sobre la entrada de sedimentos ricos en arcilla y/o materia orgánica, probablemente a través de la modulación del desplazamiento latitudinal de la Zona de Convergencia Intertropical.engThe Colombian Caribbean is an active margin characterized by constant and intense tectonic activity from the Cretaceous to the present. However, recent studies have shown that some Caribbean basins, such as the Sinú-San Jacinto Basin (CSSJ), have experienced a period of relative stability during the early Oligocene. This study analyzed the biostratigraphic record and well-log of a 143-meter sedimentary succession from the ANH-San Jacinto-1 well in the CSSJ. This is the first astrobiochronological age model for the early Oligocene (32-28 million years ago) by integrating planktonic foraminiferal biostratigraphy and the gamma- ray (GR) log. The rhythmic alternations in the GR record, which are associated with changes in clay and/or organic matter concentration, show a regular pattern correlated with Earth's orbital eccentricity cycles. This correlation allowed to create an age model with a resolution of 400,000 years for the early Oligocene, which will be necessary for defining future correlations in Colombian Caribbean basins. The patterns in the GR record suggest that during periods of maximum eccentricity, humid climatic conditions promoted the accumulation of clay and/or organic matter in the Caribbean region. On the other hand, conditions of reduced precipitation occurred during periods of minimum eccentricity. The amplitude of the GR record also suggests that, during times of lower equatorial insolation (at maximum precession), obliquity had a greater influence on the input of clay- and/or organic matter-rich sediments, possibly through the modulation of the latitudinal displacement of the Intertropical Convergence Zone.1. Introducción / 1.1 Objetivos / 1.1.1 Objetivo general / 1.1.2 Objetivos específicos / 2. Escenario geológico / 2.1 Geología regional / 2.2 Escenario global del Paleógeno tardío / 3. Ciclos de Milankovitch y Cicloestratigrafía / 3.1 Excentricidad (400-100 kyr) / 3.2 Oblicuidad (41 kyr) / 3.3 Precesión (21 kyr) / 3.4 Origen de la ciclicidad en sucesiones sedimentarias / 3.5 Cicloestratigrafía / 4. Los Foraminíferos / 4.1 Biología de los foraminíferos planctónicos / 4.1 Modo de vida / 4.2 Conceptos filogenéticos / 4.3 Características taxonómicas / 4.3.1 Textura de la pared / 4.4.2 Disposición de las cámaras / 4.4.3 Otras características / 5. Metodología / 5.1 Obtención de muestras / 5.2 Descripción litológica / 5.3 Preparación de muestras / 5.4 Análisis de las muestras / 5.4.1 Selección y clasificación / 5.4.2 Parámetros de preservación y abundancia de los foraminíferos planctónicos / 5.5 Análisis de foraminíferos / 5.5.1 Determinación bioestratigráfica / 5.6 Datos de registro de pozo: modelo de edad y análisis de series de tiempo / 5.7 Microscopio Electrónico de Barrido (MEB) / 5.8 Software para gráficos de datos micropaleontológicos / 6. Resultados / 6.1 Litoestratigrafía / 6.2 Parámetros de abundancia y preservación / 6.3 Análisis micropaleontológicos / 6.4 Datos de registros de pozos: Caracterización de patrones visuales y análisis Redfit / 6.5 Modelo de edad y calibración orbital / 6.6 Determinación del modelo de edad / 6.7 Evaluación del modelo de edad / 7. Discusión / 7.1 Registro micropaleontológico del Oligoceno temprano en el Caribe colombiano / 7.2 Origen y conservación de los ciclos en el Oligoceno temprano / 8. Conclusiones / 9. ReferenciasMaestríaMagister en Ciencias de la TierraCicloestratigrafía, Micropaleontología, Bioestratigrafía, Gamma ra
