Site selection and sampling design
The present study was conducted in the Kashmir Himalayan region which is recognized as a biogeographical province situated along the north-western side of the Himalaya (Dar & Khuroo 2020). The region is surrounded by Zanskar Range of the Greater Himalaya in the north and northeast and PirPanjal Range of the Lesser Himalaya in the south and south-west. The study region lies between 32° 20′ to 34° 54′ N latitude and 73°55′ to 75°35′ E longitude and covers an area of about 15,948 km2 with 64% of the region being mountainous (Husain 2002; Khuroo et al., 2007). Due to relatively dry and hot summers followed by wet and cold winters, temperature usually fluctuates between 15°C - 31°C during summer and −4°C - 4°C in winter. The average annual precipitation received by the region, most of which comes in the form of snow during winter, is ca. 1055 mm. Coniferous evergreen forests and the alpine meadows are the characteristic natural vegetation types of the region (Dar & Khuroo 2013).
Field surveys were conducted across the study region for selection of appropriate sampling sites. In the region, the model invasive plant species (i.e., Leucanthemum vulgare ) occurs along an altitudinal gradient ranging from c. 1700 to 2900 metres above sea level (m.a.s.l) (Ahmad et al., 2019a, b). Therefore, for the present study, we adopted the space-for-time substitution approach (Diekmann et al., 2016; Kiełtyk & Delimat 2019) by following coordinated distribution experiment methodology developed by the Global Invader Impact Network (Barney et al., 2015). This approach adopts the within-site comparison, wherein invaded and uninvaded plots that are located as close as possible are compared (Ahmad et al., 2019 a, b). Being the most preferred methodological design adopted globally for standardized evaluation of ecological impacts of invasive species, this method allows direct attribution of any ecological impacts to a specific invasive species (Barney et al., 2015; Tekiela & Barney 2017; Ahmad et al., 2021; Brooks et al., 2021). A total of four sampling sites namely Kunzar (KZ), Tangmarg (TM), Kashmir University–Gulmarg Research Station (KU) and Kongdoori (KD) were selected along the altitudinal gradient, where Kunzar is the low-altitude and Kongdoori the high-altitude site (Fig. 1). For soil microbiome and nutrient analysis, at each sampling site, a suite of two different types of plots (i) invaded (with L. vulgare present and well colonized i.e., > 50% cover), and spatially separated (ii) uninvaded (L. vulgare absent) plots were selected. At each site, the two types of plots were selected in such a manner that they were environmentally similar (i.e., almost similar slope, aspect, vegetation, land-use history).