Soil, climate and insularity variables
We collected 38 soil samples across 17 edaphic islands (i.e. 2-3 per
island depending on its area), close to the location of the climatic
data-logger stations (see below). We could not sample all the 20
outcrops due to logistic constraints (see Data analyses ). Each
sample included ~100 g of soil material. To determine
soil nutrient status, we measured eight parameters, namely pH, organic
matter, electrical conductivity, total nitrogen, ammonium, nitrate,
total phosphorus and exchangeable phosphorus (for details on analytical
procedures, refer to Supplemental Material 1). For soil textural
parameters, we measured clay, silt and sand content based on particle
size (clay: 0.1-2 µm; silt: 2-50 µm; sand: 50-2000 µm). We also measured
soil depth at each sampled plant.
We gathered information on 18 climate variables using 38 automatic
climatic stations (TOMST), recording every 15 minutes at 10 cm
aboveground and 10 cm belowground for one year (July 2019-July 2020).
Like for soil parameters, we located 2 to 3 stations on 17 outcrops. The
18 climate variables relate to temperature and moisture, namely: mean
annual value and coefficient of variation (CV) of air temperature, soil
temperature and moisture; minimum and maximum value of air temperature,
soil temperature and moisture during plant growing season (April to
October) and outside the growing season (November to March)
(Supplemental Material 1).
As a proxy of insularity, we used the target effect metric, which is
calculated as the natural logarithm of the ratio between the distance of
the target island to its putative species source (i.e. the largest and
specialist-richest island) and the square root of its area
(Méndez-Castro et al., 2021). Higher values of target effect imply that
islands are harder to colonize because of their smaller size or their
location far away from the species source (Conti et al., 2021;
Méndez-Castro et al., 2021). In the study system, the target effect
effectively captures different dimensions of insularity, namely
isolation, area, and connectivity (Supplemental Material 1). The target
effect (dimensionless) was calculated for each edaphic island.