We examined the roles of dispersal mechanism, a biological barrier; light availability, an environmental barrier; and level of disturbance, a physical barrier, in explaining the spatial patterns of exotic plant species along road and stream segments in a forest landscape in the western Cascade Range of Oregon (U.S.A). The presence or absence of 21 selected exotic plant species and light levels were observed along 0.3-to 1.0-km transects within four habitat types. Each habitat represented a different level of disturbance: high-use roads, low-use roads, abandoned roads, and streams in the H. J. Andrews Experimental Forest. Nearly 300 50 ϫ 2-m sampling units were surveyed along five transects in each habitat type. We used ordination (nonmetric multidimensional scaling) and logistic regression to analyze data. All of the nearly 200 sampling units along roads with high and low levels of vehicle traffic contained at least one exotic plant species, and some contained as many as 14. Streams that were most recently disturbed by floods 20-30 years ago and abandoned spur roads with no traffic for 20-40 years also had numerous exotic species. Roads and streams apparently serve multiple functions that enhance exotic species invasion in this landscape: they act as corridors or agents for dispersal, provide suitable habitat, and contain reservoirs of propagules for future episodes of invasion. Species-specific dispersal mechanisms, habitat characteristics, and disturbance history each explain some, but not all, of the patterns of exotic species invasion observed in this study.Disponibilidad de Luz y Mechanismos de Dispersión e Invasión de Plantas Exóticas a lo Largo de Caminos y Arroyos en el Bosque Experimental H. J. Andrews, Oregon Resumen: Para explicarnos los patrones espaciales de plantas exóticas a lo largo de segmentos de caminos y de arroyos en un paisaje boscoso del Cascade Range, Oregon, examinamos el papel de los mecanismos de dispersión (una barrera biológica), de la luz (una barrera ambiental) y del nivel de perturbación (una barrera física). Se observaron la presencia o ausencia de 21 especies de plantas exóticas y los niveles lumínicos a lo largo de transectos de 0.3 a 1.0 km en cuatro tipos de hábitat. Cada hábitat presentaba diferentes niveles de perturbación: caminos de uso frecuente, caminos de uso no frecuente, caminos abandonados y arroys en el Bosque Experimental H. J. Andrews. Se trabajó en cerca de 300 unidades de muestreo de 50 ϫ 2 m, a lo largo de cinco transectos en cada tipo de hábitat. Para el análisis de datos utilizamos ordenamiento (escala multidimensional no-numérica) y regresión logística. Las cerca de 200 unidades de muestreo a lo largo de caminos con uso frecuente y no frecuente presentaron por lo menos una especie de planta exótica y algunas presentaron hasta 14. Los arroyas recientemente perturbados por inundaciones (20-30 años) y los caminos abandonados hace 20 -40 años también presentaron numerosas especies exóticas. Aparentemente, los caminos y arroyos realizan varias funciones que acrec...
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