The risk of spread and establishment of invasive species to interior habitat within urban parks is of great concern to park managers and ecologists. Informal trails as a vector for this transmission are not well understood. To characterize effects of informal trails on understory plant communities, I conducted a study of the informal trail network in Forest Park, Portland, Oregon. The system of 382 informal trails was mapped and evaluated qualitatively, and from this population a systematic sample was selected for analysis. To identify hotspots of informal trail activity, showing the relationship of informal trails to formal trails, other park features, and trail use level, I evaluated all mapped trails using line density spatial analysis tools. To characterize understory communities, thirty transects were placed along informal trails, with paired transects along nearby formal trails for comparison. I measured percent cover by species for non-graminoid understory plants, and percent total plant cover at different structural layers, for quadrats at regular intervals from the trail edge. I calculated richness and Shannon-Weaver diversity for non-graminoid understory plants. For community analysis, species were grouped by dispersal strategy, native status, and growth form. Observations from system mapping suggest that "hidden" behaviors drive many informal trails: bathroom stops, party spots, waste dumping, and camps make up 28% of all informal trails. Trails to private property are few but represent over 29% of total trail length. Informal trail density is highest along Balch Creek. Hotspots of informal trail presence are associated with trailheads, trail intersections, and water access. Quadrats located within one meter of informal trails showed higher richness and diversity due to increased number of introduced and ruderal species. Formal trails exhibit these same patterns to a stronger degree and over a greater distance (two meters) from the trail edge. Distance from trail edge explained variation in plant communities when grouped by dispersal type, but not by growth form. This study shows that although informal trails are widely distributed throughout the park, they are concentrated in high use areas. The presence of informal trails leads to significant changes in Forest Park plant communities that favor invasive and ruderal species, but these effects appear limited to two meters from the trail edge.