## 4 Discussion  
Our results demonstrate that the substrate contained different levels of volumetric water content (VCW) according to species, suggesting that the plants' seasonal development and biomass structures were the primary cause of these differences [21; 33]. We also found that biomass alone is not sufficient to explain the behavior of various species, although it is important to understand the biomass distribution. The water content in substrate of \(D\). _margaretae_ was higher than other species in both season due to the compact nature of its roots and leaves, limiting also the evaporation process of the substrate. This species has a compact low-growing structure, developing leaves and roots on the substrate, and it is  
Figure 5: Annual vegetation cover for each species and irrigation treatments (0–20–40% ET\({}_{0}\)). Capital letters indicate significant differences throughout the trial period for the same treatment. Lowercase letters indicate significant differences between irrigation treatments in the same month. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Plants Species** & **Irrigation** & **Above Ground (g)** & **Roots (g)** & **Total (g)** \\ \hline \multirow{4}{*}{_D. margaretae_} & 0\% ET\({}_{0}\) & / & / & / \\  & 20\% ET\({}_{0}\) & 17.1 & 53.63 & 70.63 \\  & 40\% ET\({}_{0}\) & 2.83 & 13.13 & 15.96 \\  & Mean & 19.52 & 77.13 & 96.64 \\ \hline \multirow{4}{*}{_F. laevis_} & 0\% ET\({}_{0}\) & 30.15 & 3.15 & 33.30 \\  & 20\% ET\({}_{0}\) & 415.73 & 44.21 & 459.94 \\ \cline{1-1}  & 40\% ET\({}_{0}\) & 558.93 & 40.50 & 599.43 \\ \cline{1-1}  & Mean & 334.94 & 29.29 & 364.22 \\ \hline \multirow{4}{*}{_I. Intescens_} & 0\% ET\({}_{0}\) & 78.91 & 56.27 & 135.18 \\  & 20\% ET\({}_{0}\) & 136.83 & 105.38 & 242.21 \\ \cline{1-1}  & 40\% ET\({}_{0}\) & 193.22 & 103.24 & 296.46 \\ \cline{1-1}  & Mean & 136.32 & 88.30 & 224.62 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Means of dry weight (g) \(\times\) m\({}^{2}\) of the tested species in the three irrigation protocols. the only species whose root biomass was greater than its upper parts. _F. laevis_, a semi-shrub species, has very thin roots occupying all available substrate. Its more complex rooting stem structure allowed the substrate to store less water than _D. margaretae_, suggesting that it consumed a larger amount of water. Therefore, the biomass of _I. lutescens_ was mainly concentrated in its large rhizomes, developing principally in the spring and summer. The substrate of this species stored the least water amount. We suggest that root biomass, improving the soil structure, helps in the rainwater conservation and using different plant combinations makes it possible to achieve specific results according to the context in which a green roof is built. In a Mediterranean climate, _D. margaretae_'s ability to keep humidity in the substrate above minimum levels could be a positive sign with regard to the survival of vegetation on green roofs. Substrate water content variations before and immediately after rainfall, could also be described as the soil's recharge capacity. Our study confirms that, this recharge capacity varied seasonally between summer and winter rainfall events, and in relation to the species. Moreover, in our work, two rainfall events occurring in the hottest period, although different in intensity, resulted in similar substrate humidity variations, confirming that the soil's storage capacity changed according to the season [50]. In accordance with [PERSON] [7], our study found that average water content was higher in the winter, leaving the substrate with a reduced capacity for storage during the rainiest period of the year. Humidity variations in the substrate 60 h after a rainfall event can supply us with information about what happens to the water stored in the soil. Comparing the data, _F. laevis_ (semi-shrub) > _D. margaretae_ (groundcover) > _I. lutescens_ (Rhizomatous-herbaceous). The highest variations occurred in _F.