Beyond the trail: understanding non-native plant invasions in mountain ecosystems

In an era of rapid climate change and increased human pressure on mountains, the introduction and spread of non-native plant species have become a critical focus of research. Road construction and motorized vehicle traffic are often cited as major sources of disturbance and dispersal vectors facilitating the encroachment of non-native plants into these ecosystems. In contrast, the impacts of outdoor activities, such as hiking, remain less studied, despite hiking trails being a key component of tourist movements in mountains and providing access to higher elevations that are often free of plant invasions. Our aim was to conduct a multi-regional assessment of the abiotic, biotic, and anthropogenic drivers of non-native plant species distribution along hiking trails. To achieve this, we implemented a standardized sampling design across nine mountain regions on six continents. By establishing T-shaped sample sites parallel to trails and leading perpendicular into adjacent vegetation, we assessed the main drivers contributing to non-native species presence, richness, and cover. We found that at the global scale abiotic (climatic) variables explained most of the variation of non-native species presence and richness, while for non-native species cover, biotic factors were most important. Anthropogenic factors, including distance to the trail and livestock grazing, played a relatively minor role. While the total number of non-native species differed across regions, the patterns explaining plant invasions were consistent. Our results show that trails were important conduits for non-native plants into mountain areas, even if anthropogenic drivers had a lower impact on non-native species distribution along trails than previously observed along roads. Importantly, non-native species were not constrained to trails, suggesting that wandering off-trail by hikers and domestic animals plays an important role in the spread of non-native species away from trail edges. This highlights the importance of limiting off-trail visitor use in areas of high conservation value. 

Methods
The dataset has been collected using the MIREN T trail protocol, as described in Liedtke et al. (2020) [Liedtke, R., Barros, A., Essl, F., Lembrechts, J.J., Wedegärtner, R.E.M., Pauchard, A. & Dullinger, S. (2020) Hiking trails as conduits for the spread of non-native species in mountain areas. Biological Invasions, 22, 1121–1134.]. The dataset   includes nine mountain regions on six continents, including countries in North America (USA), South America (Argentina and Chile), Europe (Sweden, Norway, Czech Republic), Africa (South Africa), Asia (China) and Oceania (Australia) (Table 1). It covers 55 mountain trails, 680 sample sites and more than 2000 plots.

Each regional contributor/s submitted the dataset to the lead authors (Barros, A., Fuentes Lillo, E., Lembrechts, J.) using the template format developed through the MIREN T trail protocol. The dataset includes information on: environmental information (site code, transect, plot, elevation, geographic coordinates, distance to trail head, distance to trail edge, trail use intensity, exotic herbivory), bioclimatic variables (Bio 1, Bio 5, Bio 6, Bio 12), and vegetation variables (native and non-native species richness, native and non-native cover, non-native occurrence, shrub cover, tree cover). In addition, the occurrence of each non-native species and its frequency was provided by each regional contributor. This information is included Supplementary Information in the manuscript. 

The information was curated in Excel and R and all statistical analyses were performed using R software. 

We include in this repository the Excel file and the scripts used to undertake the statistical analyses.