Evaluation of non-native and native plant species distributions along an elevation gradient in the Wallowa Mountain Range, Oregon

Background/Question/Methods

Mountain environments are currently among the least invaded ecosystems, however, these biodiversity hotspots are increasingly under threat of exotic plant invasion. The resistance of mountain ecosystems to exotic species invasions is likely due to a combination of low anthropogenic disturbances, low propagule supply, and extreme environmental conditions. The importance of any one of these factors is highly debated and likely ecosystem dependent. The objectives of this study were to evaluate the main drivers of plant invasions in the Wallowa Mountain Range of northeastern Oregon and explore to what extent patterns of non-native species distributions and their causes differ from native species along an elevation gradient. We conducted a complete understory vascular plant inventory in summer 2012 along three roads located in the Wallowa Mountain Range of northeastern Oregon.  Transects (n=20) were evenly stratified by elevation (60m) along each road. Transects were subdivided into three (50m x 2m) plots with one plot parallel to the road edge and the other two plots perpendicular to the road plot, together forming a “T” and extending 100m from the road. All vascular plants were identified to the species level and species abundance, herbivory, and environmental parameters were measured. 

Results/Conclusions

Non-native species richness decreased linearly with increasing elevation.  In contrast, native species richness displayed a unimodal distribution with maximum richness occurring at the mid–elevations. As shown by non-metric multidimensional scaling, species composition along Fish Lake road was strongly related to elevation and disturbance gradients. Non-natives were particularly concentrated in sites subjected to human disturbance. Despite a high level of disturbance at high elevation sites due to road traffic and a recent fire, non-native species were primarily concentrated at the low elevation sites with moderate to high disturbance levels. Non- parametric multiplicative regression modeling indicated that precipitation and disturbance were the strongest predictors of non-native species proportional canopy cover and elevation and disturbance were the strongest predictors of non-native species richness. Unlike native species, non-native species composition appeared nested in relation to elevation indicating that non-native species found at high altitudes were a subset of the population at lower altitudes. These data suggest that elevation, human disturbance, and precipitation are the main drivers structuring non-native plant distributions along elevation gradients in the Wallowa Mountains.  Additionally, the nested structure of non-natives with respect to elevation supports findings that non-natives occurring at high elevations tend to be generalist species.