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.