Sampling design
Sampling in the PASTUS database was designed according to a stratified
random scheme, where samples were selected at random within strata. This
process was done using the software ArcMap 10 (ESRI, Redlands, CA, USA).
The basis for randomization was the map of habitats of Catalonia 1:50000
(Carreras, J. & Diego, 2006) for the Eastern and Central sectors, the
map of habitats of Madres-Coronat 1:10000 (Penin, 1997) for the
North-Eastern sector and the land use map of Navarra 1:25000 (Gobierno
de Navarra, 2003) for the Western sectors. Four variables were initially
considered for sampling stratification within each sector: altitude
(< 1800 m; 1800-2300 m; > 2300 m), slope (0-20º;
20-30º; > 30º), macrotopography (mountain
top/southern-facing slope; valley bottom/northern-facing slope) and
grazing management (sheep grazing; cattle grazing; mixed grazing).
Accordingly, we determined a set of homogeneous grassland patches by
crossing the stratification variable layers. Grassland patches were then
listed by type and arranged within each list randomly to determine
sampling priority. At least one to two replicates of each patch type
were sampled.
In each sampled grassland patch, a 10 x 10 m2 plot was systematically
placed in the middle of each homogeneous grassland patch, including a
particular plant community. Soils and vegetation were sampled inside
this 100 m2 plot, and environmental variables assessed (see Rodríguez et
al., (2018) for additional details about sampling design).