Patterns of elevational distribution of alien plant species in the southern Sierra Nevada of California were used to test the hypothesis that alien plant species invading high elevations around the world are typically climate generalists capable of growing across a wide elevational range. The Sierra Nevada has been heavily impacted for more than a century and a half, first by heavy grazing up into high elevation meadows, followed by major logging, and finally, by impacts associated with recreational use. The comparative elevational patterns of distribution and growth form were compared for native and alien plant species in the four families (Asteraceae, Brassicaceae, Fabaceae, and Poaceae) that contribute the majority of naturalized aliens in the study area. The distribution of realized climatic niche breadth, as measured by elevational range of occurrence, was virtually identical for alien and native species, with both groups showing a roughly Gaussian distribution peaking with species whose range covers a span of 1500–1999 m. In contrast to alien species, which only rarely occurred at higher elevations, native species showed a distribution of upper elevation limits peaking at 3000–3499 m, an elevation that corresponds to the zone of upper montane and subalpine forests. Consistent with a hypothesis of abiotic limitations, only a few alien species have been ecologically successful invaders at subalpine and alpine elevations above 2500 m. The low diversity of aliens able to become established in these habitats is unlikely due to dispersal limitations, given the long history of heavy grazing pressure at high elevations across this region. Instead, this low diversity is hypothesized to be a function of life history traits and multiple abiotic stresses that include extremes of cold air and soil temperature, heavy snowfall, short growing seasons, and low resource availability. These findings have significant implications for resource managers.
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Vol. 36 • No. 3