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).