Step description
1. Systematic Grid Survey：This method aimed to obtain spatially balanced and ecologically representative samples independent of the road network.
Grid Design and Site Selection: The study area was evenly divided into a survey grid of 20 km × 20 km cells. Within each cell, one primary survey point was selected. Site selection was not random, but strategically targeted towards areas presumed to have higher invasion susceptibility, such as locations with complex vegetation structure, distinct habitat edges or a history of anthropogenic disturbance.
Fixed-Area Reconnaissance: A 300 m × 300 m square area was established, centred on the geographic coordinates of the sampling point. This predefined area was thoroughly and systematically searched to locate all patches of invasive plants.
Targeted Plot Establishment and Quantitative Recording: When a discrete patch or notable concentration of invasive plants was identified during reconnaissance, a sampling plot was immediately established at that precise location. Plot size was determined by the life form of the species, as follows. Herbaceous plants: A 1 m × 1 m quadrat was used. Shrub plants: A 5 m × 5 m quadrat was used. Within each plot, all invasive plant species were identified to the species level and their total percentage cover was estimated. These records constitute the core quantitative presence-abundance data in our dataset. (Note: As the invasive flora in this region is overwhelmingly herbaceous and the only shrub species encountered (Amorpha fruticosa L.) was primarily cultivated with only one wild individual found, the 5 m × 5 m protocol yielded only a single record.)
Opportunistic Presence Recording (Around Grids): To maximise spatial coverage beyond the systematic plots, any invasive plants observed during travel to/from or in the vicinity of the grid point, but outside the formal 300 m × 300 m boundary, were also recorded. For these opportunistic observations, we collected presence-only data, documenting the species identity and precise GPS coordinates without establishing a quantitative plot.
2. Roadside Transect Survey: This method was specifically designed to sample linear dispersal corridors and anthropogenically disturbed habitats along roads, which are key pathways for invasion.
Transect Establishment: The road sections we selected met two criteria: (a) a minimum continuous length of 5 km to ensure spatial representativeness and (b) an absence of major recent disturbances, such as ongoing construction. To avoid bias, one side (left or right) of each selected road section was randomly chosen for survey.
Systematic Point Sampling: Along the surveyed side of the road, a sampling point was marked at 1 km intervals. A two-phase protocol was followed at each point: Quantitative Sampling: A visual survey was conducted within the "road-effect zone" of the sampling point (a band extending approximately 20-50 m from the road edge). At locations where invasive plants were found to be concentrated, 3 - 5 quadrats were established. All invasive species within each quadrat were recorded and their percentage cover was estimated.
Opportunistic Presence Recording: For invasive plants encountered along the road, but not within the immediate area of the sampling points, the species name and GPS coordinates were recorded.