Relationships of exotic plant communities with native vegetation, environmental factors, disturbance, and landscape ecosystems of Pinus ponderosa forests, USA

Invasions by exotic plant species can threaten forest ecosystems in numerous ways. Theories on relationships of exotic species invasions with native vegetation, resource availability, and disturbance could assist in managing exotic plants by identifying parts of the landscape and ecological conditions most susceptible to invasion. However, generality of these theories is complicated by considerable variation among landscapes and ecology of individual species. We assessed hypotheses on relationships of exotic plant communities with native vegetation, environmental (soil, topographic, and climate), and disturbance (roads and grazing) variables using data from 66 plots within a 110,000-ha Pinus ponderosa forest landscape in northern Arizona, USA. We further assessed exotic plant relationships with ecosystem classification, which has a long history of use in forest ecology and management but has been underutilized for understanding exotic species distributions. Plots contained a total of 251 native and 20 exotic plant species. As hypothesized for distributional studies, native and exotic species were positively correlated, but the correlation varied with scale for species richness (m2, r = 0.65; 500 m2, r = 0.41) and was weaker for cover (r = 0.32). Multiple regression, including native vegetation and environmental variables, accounted for similar amounts of variation in exotic species richness (50% of variation) and cover (51%). Disturbance variables only entered regression models when native vegetation was excluded from models. Exotic species richness m−2, cover, and community composition were strongly related to the ecosystem classification. For example, mean exotic cover ranged from <0.1% in ecosystems with volcanic cinder soils to 7% in ecosystems containing Populus tremuloides. As hypothesized, moist ecosystems were generally most invaded. However, also considering factors such as past management of the ecosystems and literature on the introduction histories (e.g., intentional seeding for range improvement) of the species, we hypothesize that the ecosystem classification integrated co-varying environmental, disturbance, and management variables related to exotic plant communities. Ecosystem classification was a useful framework for understanding distributions of exotic plant communities across the landscape.

► Exotic plant invasions often threaten indigenous forest ecosystems.
► We examined exotic plant communities on a southwestern USA forest landscape.
► Native vegetation and environment portrayed 50% of exotic richness variance.
► Exotic richness and cover corresponded to landscape-scale ecosystem distribution.
► We hypothesize that ecosystem classification integrated factors affecting invasions.