## 2 Materials and Methods
### Measurements' Design  
For each species we recorded the following parameters: volumetric water content (VWC; \(\theta\) m\({}^{3}\)/m\({}^{-3}\)), as the maximum amount of water absorbable by the substrate; relative extractable water (REW), as the amount of substrate water available to the plants; rainwater retention capacity; maximum-minimum average substrate temperatures; green cover; final root and aerial biomasses.  
To evaluate the maximum water holding capacity of the substrate we subtract the field capacity (VWC field capacity) by the lowest water content registered during the whole trial period (minimum  
Figure 1: The experimental design: 9 plots of 1 m\({}^{2}\), each one with 9 plant of each species and for each irrigation treatment.  
VWC). Later, we used this result to obtain the amount of REW according to the equation defined by [PERSON] [44] and [PERSON] et al. [45]:  
\[\text{REW}=(\text{actual VWC}-\text{minimum VWC})/(\text{maximum VWC}-\text{minimum VWC})\times 100\]  
In calculating the rainwater holding capacity of the substrate we looked at five different types of rainfall as classified by AEMET. We selected as _'very heavy'_ the rainfall event of 14 September 2009; as _'heavy'_ the rainfall events of 9 August 2009 and 20 September 2009; as _'moderate'_ the rainfall events of 8 February 2010 and 7 March 2010. For each of these rainfall types we looked at the VWC variation over a period of 24 h before the rainfall and immediately afterwards, when the substrate was saturated. We also calculated water content variation in the saturated substrate over a period of 60 h following rainfall. Holding capacity values were expressed as L/m\({}^{2}\) by multiplying the water volume (cm\({}^{3}\)/cm\({}^{-3}\)) by the substrate depth of 110 mm.  
Ech2O probes estimate the substrate water content by indirectly measuring the dielectric substrate permittivity. A relationship between substrate water content and dielectric permittivity was estimated in the laboratory following [PERSON] et al. [46]. To obtain a range of water contents from dry to near saturation, different volumes of deionized water were added to the substrate and mixed thoroughly to obtain uniformity. The weight of the container with substrate and different water contents was taken and a sensor was inserted, and dielectric permittivity was measured according to [PERSON] [47]. Finally, the daily changes of substrate water content were calculated from data recorded each 10 min.  
To calculate the insulation capacity of the substrate and plants in high temperature conditions, we looked at the experimental hottest period, since July to September, and compared average maximum substrate temperatures for each species and each irrigation treatment plot (0%, 20%, 40%) with average maximum air temperatures. Later, to evaluate insulation capacity in conditions of low temperatures we compared daily minimum substrate and air temperatures during the coldest period, from November to March. Finally, we compared the average minimum temperature values for each species in all irrigation treatment plots with air temperatures to evaluate the differences.  
We measured the plant cover photographically with a Nikon EOS 500 also. Later, we processed the images with Greenpix digital analysis software according to [PERSON] et al. [48,49]. Finally, to calculate final biomass, we removed all plants from the soil and we separated the upper sections (leaves and stems) from the roots, before washing them carefully. In this way, we located all resulting material in an oven at 65 \({}^{\circ}\)C until it was at a constant weight.