Factors affecting exotic annual plant cover and richness along roadsides in the eastern Mojave Desert, USA

Roads are often considered a critical point of entry for invasive exotic plant species. We evaluated the cover and richness of exotic plants relative to road type (gravel or paved) and distance from roads in the eastern Mojave Desert where exotic annual species are increasing the flammability of the desert. Exotic and native annual plant cover and richness were sampled from five to 45 m from roads in three prevalent microsite types for this system: Larrea tridentata microsites, Ambrosia dumosa microsites, and interspaces between shrubs. Six exotic and 58 native annual species were detected during sampling. There was no distinction between the behavior of exotics and natives relative to road type or distance from the roads. A. dumosa and L. tridentata microsites had more exotic cover than interspaces. Likewise, exotic richness was significantly higher in the shrub microsites. A taxonomically controlled analysis involving three plant families revealed no significant differences in cover of native or exotic species relative to their distance from the road. Our results suggest that exotic plant species in the Mojave Desert are not necessarily more prevalent near roadsides than in adjacent undisturbed desert. Therefore, roadside surveys alone may not be adequate to detect exotic species presence.