Interspecific Competition Interacts with the Spatial Distribution of a Palatable Grass to Reduce Its Recruitment

Abstract

Most arid and semiarid ecosystems around the world have been grazed by domestic herbivores. In many cases, grazing has degraded vegetation and soil. The possibility of restoring rangeland's good condition depends, partially, on the ability of remaining populations of desirable species to recover. In this work, we studied the exact spatial distribution of remaining palatable adult plants in fields with different grazing history (i.e., seed sources) and quantified the effect of interspecific competition with less palatable grasses on seedling emergence and survival (i.e., regeneration constraints). We worked in a Patagonian steppe composed of shrubs and perennial tussock grasses that has been grazed by sheep for .100 years. In order to evaluate the location of seed sources, we mapped the location of a palatable species (Bromus pictus Hook.) in paddocks with different long-term grazing intensity. In addition, we sowed seeds of B. pictus close to 2 dominant, less palatable grasses in 2 different years to evaluate the role of interspecific interactions on regeneration and the effects of climate variability. The proportion of B. pictus plants growing in protected places near less palatable species significantly increased with grazing intensity. Competition effects on emergence, survival, and growth depended on the year's moisture regime. During the dry year, competition with less palatable grasses reduced the emergence, survival, height, and number of leaves of palatable grass seedlings by 30%, 55%, 48%, and 40%, respectively. In the wet year, there were no effects of competition on emergence and height, and the effects on survival depended on the species of the less palatable neighbors. Our study supports the idea that management for recovering degraded rangelands in this ecosystem may benefit from considering the spatial distribution of remaining plants. It also indicates that the susceptibility of demographic processes to interspecific competition depends on the year and neighbor species. ResumenLa mayoría de los ecosistemas á ridos y semiá ridos del mundo han sido apacentados por herbívoros domésticos. En muchos casos, el apacentamiento ha deteriorado la vegetació n y el suelo. La posibilidad de restaurar la buena condició n de estos pastizales depende parcialmente de la habilidad de las poblaciones remanentes de especies deseables para recuperarse. En este trabajo, estudiamos la distribució n espacial exacta de las poblaciones remanentes de plantas adultas de un pasto palatable en pastizales con diferente historial de apacentamiento (fuentes de semillas) y cuantificamos el efecto de la competencia interespecífica con pastos menos palatables sobre la emergencia y supervivencia de plá ntulas (restricciones a la regeneració n). Trabajamos en una estepa semiárida Patagó nica co-dominada por arbustos y pastos perennes, que había sido apacentada por ovinos durante más de 100 añ os. Para evaluar la localizació n de las fuentes de semilla mapeamos la ubicació n de una especie palatable (Bromus pictus Hook.) en po...