<p>Land use and land cover changes significantly affect C storage in terrestrial ecosystems. Programs intended to compensate land owners for the maintenance or enhancement to C stocks are promising, but require detailed and spatially explicit C distribution estimates to monitor the effectiveness of management interventions. Savanna ecosystems are significant components of the global C cycle, however, they have not received much attention for the development of C monitoring approaches. In this dissertation I have investigated three of the aspects related to woody plant cover dynamics in semiarid savannas of central Argentina: spatio-temporal dynamics, precise field surveying and scaling from field to region with the use of freely available remotely sense data. </p><p>To examine the long term changes in woody plant cover, I first carefully extracted information from historical maps of the Caldenal savannas of central Argentina (190,000 km2) in the 1880s to generate a woody cover map that was compared to a 2000s dataset. Over the last ~120 years, woody cover increased across ~12,200 km2 (14.2 % of the area). During the same period, ~5,000 km2 of the original woody area was converted to croplands and ~7,000 km2 to pastures, about the same total land area as was affected by woody plant encroachment. A smaller area, fine scale analysis between the 1960s and the 2000s revealed that tree cover increased overall by 27%, shifting from open savannas to a mosaic of dense woodlands along with additional agricultural clearings. Statistical models indicate that woody cover dynamics in this region were affected by a combination of environmental and human factors.</p><p>To assess the consequences of woody cover dynamics on C, we also measured ecosystem C stocks along a gradient of woody plant density. I characterized changes in C stocks in live biomass (woody and herbaceous, above- and belowground), litter, and soil organic carbon (to 1.5 m depth) pools along a woody plant cover gradient (0 to 94 %). I found a significant increase in ecosystem C stocks with increasing woody cover, with mean values of 4.5, 8.4, 12.4, and 16.5 kg C m-2 for grasslands, shrublands, open and closed forests, respectively. Woody plant cover and soil silt content were the two primary factors accounting for the variability of ecosystem C. I developed simple regression models that reliably predict soil, tree and ecosystem C stocks from basic field measurements of woody plant cover and soil silt content. These models are valuable tools for broad scale estimation if linked to regional soil maps and remotely sensed data, allowing for precise and spatially explicit estimation of C stocks and change at regional scales.</p><p>Finally, I used the field survey data and high resolution panchromatic images (2.5 m resolution) to identify tree canopies and train a regional tree percent cover model using the Random Forests (RF) algorithm. I found that a model with summer and winter tasseled cap spectral indices, climate and topography performed best. Sample spatial distribution highly affected the performance of the RF models. The regression model built to predict tree C stocks from percent tree cover explained 83 % of the variability, and the spatially explicit tree C model prediction presented an root mean squared error (RMSE) of 8.2 tC/ha which represented ~30% of the mean C stock for areas with tree cover. Our analysis indicates that regionally over the last ~120 years, increases in woody plant cover have stored significant amounts of C (95.9 TgC), but not enough to compensate for in C generated by the conversions of woodlands and natural grasslands to croplands and pastures (166.7 TgC), generating a regional net loss of 70.9 TgC. C losses could be even larger in the future if, as predicted, energy crops would trigger a new land cover change phase in this region.</p> / Dissertation

La interacción entre especies es un fenómeno ecológico habitual en la naturaleza. En sistemas donde las plantas leñosas forman parte de la comunidad vegetal, como sucede en el sur del Distritito Fitogeográfico del Caldén (Caldenal), su presencia altera el medio ambiente de forma tal que influyen sobre los organismos vegetales bajo su área de influencia. Las relaciones leñosas/gramíneas incluyen efectos positivos (facilitación) y negativos (competencia) que ocurren simultáneamente. Así, el efecto neto de una planta sobre otra estará determinado por el balance de diferentes mecanismos en determinadas condiciones físicas y bióticas. El objetivo general del presente trabajo fue estudiar el efecto de las especies leñosas Prosopis caldenia (caducifolia) y Larrea divaricata (perennifolia) sobre el crecimiento aéreo de las macollas, la producción de biomasa aérea y la calidad forrajera de Piptochaetium napostaense y Nassella clarazii, dos gramíneas perennes forrajeras nativas del Caldenal. Además, se evaluaron las modificaciones de las condiciones microambientales y edáficas debajo y fuera de la canopia de ambas especies leñosas. La altura, la longitud del tallo más vainas, la longitud de láminas verdes y la longitud total verde de las macollas de las plantas de P. napostaense y N. clarazii que crecieron debajo de la canopia de P. caldenia y L. divaricata fueron en general mayores que las registradas en las macollas de aquellas plantas que crecieron en los sitios abiertos entre individuos de esas especies leñosas. Asimismo, las macollas de las plantas de P. napostaense y N. clarazii que crecieron debajo de la canopia de P. caldenia tendieron a mostrar una mayor longitud total verde que las macollas de aquellas plantas que crecieron debajo de la canopia de L. divaricata. La biomasa aérea mostró un comportamiento diferencial entre P. napostaense y N. clarazii en función de la especie leñosa considerada. La presencia de L. divaricata habría favorecido la producción de biomasa aérea de P. napostaense; mientras que, la presencia de P. caldenia habría favorecido la producción de biomasa aérea de N. clarazii. El contenido de proteína bruta, al final del período de crecimiento vegetativo, en las plantas de P. napostaense que crecieron debajo de la canopia de ambas especies leñosas estudiadas fue menor que el de aquellas plantas que crecieron en los sitios abiertos. Sin embargo, el contenido de proteína bruta de las plantas de N. clarazii que crecieron debajo de la canopia de P. caldenia fue mayor que el de aquellas plantas que crecieron debajo de la canopia de L. divaricata. Al final del período de crecimiento vegetativo, el mayor porcentaje de fibra detergente neutra y ácida en las gramíneas estudiadas se registró en las plantas que crecieron debajo de la canopia de L. divaricata. Al final del período de crecimiento reproductivo, el porcentaje de fibra detergente neutra en las plantas de N. clarazii que crecieron debajo de las canopias de P. caldenia fue mayor que el de las plantas que crecieron debajo de la canopia de L. divaricata. El porcentaje de lignina detergente ácida de la plantas de P. napostaense y N. clarazii, al final de ambos períodos de crecimiento, que crecieron debajo de la canopia de P. caldenia y en los sitios abiertos entre individuos de esta leñosas fue mayor que el de aquellas plantas que crecieron debajo de la canopia de L. divaricata y en los sitios abiertos entre individuos de la misma. En ambos períodos de crecimiento evaluados, la menor digestibilidad in vitro de P. napostaense y N. clarazii se registró en las plantas que crecieron debajo de la canopia de P. caldenia y en los sitios abiertos de esta leñosa. En general, los resultados obtenidos en este estudio sugerirían que P. caldenia y L. divaricata al mejorar en su área de influencia el contenido de nitrógeno, materia orgánica y densidad aparente del suelo tendrían un cierto nivel de facilitación sobre P. napostaense y N. clarazii; sin embargo, esta mejora no favoreció significativamente el crecimiento aéreo de sus macollas, la producción de biomasa aérea y/o algunos parámetros asociados con su calidad forrajera. En conjunto los resultados obtenidos, bajo las condiciones en que se desarrolló el presente estudio, sugieren que P. caldenia y L. divaricata tienen un efecto neutro o negativo sobre el crecimiento, la producción de biomasa aérea y la calidad de P. napostaense y N. clarazii en los pastizales naturales semiáridos del sur del Caldenal / The interaction between woody and grass species is a common feature of semi-arid rangelands, as is southern Caldenal in central Argentina. Woody species can exert positive and/or negative influence on the growth of neighbouring grass species through imposing changes in the abiotic conditions in which the latter species grow, leading to a balance in which either facilitative or competitive interactions prevail. The objective of this work was to assess the impact of woody species on grass growth over a 2-y period. The study comprised two morpho-physiologically different woody species and two cool-season perennial grass species highly preferred by domestic livestock. The species chosen for the study were Prosopis caldenia and Larrea divaricata, a warm-season deciduous and an evergreen woody species, respectively; and Nassella clarazii and Piptochaetium napostaense as representatives of perennial grasses highly preferred by livestock. The study involved the assessment of tiller growth parameters, biomass production and forage quality in grasses growing beneath the canopy of woody species and in open rangeland areas. Tiller growth was evaluated through measurements of height, pseudostem length, green blade length and total green length as the sum the latter two parameters. In addition, soil temperature, pH and water, nitrogen, phosphorus and organic matter content were estimated at the same study sites. Tiller growth parameters of grasses growing beneath the canopy of woody species were generally higher than those of grasses growing in open areas. Also, total green legth of grass tillers tended to be higher when growing beneath the canopy of P. caldenia than L. divaricata. Biomass production differed between grasses and woody species under consideration for L. divaricata might have favoured biomass production of P. napostaense while P. caldenia might have favoured that of N. clarazii. At the end of vegetative growth period, crude protein content of P. napostaense was lower beneath the canopy of woody species than in open areas. Whereas, crude protein content of N. clarazii was higher beneath the canopy of P. caldenia than L. divaricata. Fiber content, both acid and neutral detergent, at the end of vegetative growth period was highest in grasses growing beneath the canopy of L. divaricata. At the end of reproductive growth period, neutral detergent fiber content in N. clarazii was higher beneath the canopy of P. caldenia than L. divaricata. Acid detergent fiber content at the end of both growth periods was higher beneath the canopy of P. caldenia and in open areas between individuals of this species than beneath the canopy and open areas between individuals of L. divaricata for both grass species. The lowest records of in vitro digestibility were registered beneath the canopy and open areas between individuals of P. caldenia for both grass species and for both growth periods. In general, the results obtained in this study suggest that P. caldenia and L. divaricata, when improving in their area of influence the nitrogen content, organic matter and apparent density of the soil will have a certain level of facilitation on P. napostaense and N. clarazii. However, this improvement did not favor substantially the aerial growth of their tillers, the production of aerial biomass and / or some parameters associated with their forage quality. The results obtained, under the conditions under which the present study was carried out, suggest that P. caldenia and L. divaricata have a neutral or negative effect on growth, aerial biomass production and quality of P. napostaense and N. clarazii in the semi-arid natural grasslands of southern Caldenal.