Estimaciones Contables Críticas

The saturated SMCs of Stachys and Salvia were greater than for Heuchera, Sedum and control in all trials, possibly due to differences in rooting densities between species affecting the maximum retention capacity of the substrate (Nagase and Dunnett, 2012; Berretta et al., 2014). However, SMC of all species/control declined over the course of the trials, with Stachys and Salvia SMC decreasing rapidly due to their high ET rates so that after 96 hours their SMCs were similar to

Heuchera and significantly lower than Sedum SMC in all treatments except ‘dry’, in which Sedum

was similar.

For ET to continue at its maximum potential rate, water must be continually supplied to the evaporating surfaces (i.e. the substrate or leaf surfaces; Verhoef and Egea, 2013). As SMC declines over time after wetting, the water supply to the evaporating surfaces becomes increasingly restricted, causing ET rates to fall (Stovin et al., 2013). This was clearly observed in all T/RH trials in the presented experiment, with daily ET declining concurrently with SMC, depending on species/control, and there was a strong relationship between antecedent SMC at the

111 beginning of each day and ET in the subsequent 24 hours identified for species with the highest ET rates (Stachys and Salvia: adjusted R2 _{= 0.76 and 0.44 respectively). Indeed, daily ET with Stachys}

declined rapidly over time in all trials, particularly in the first 3 days after saturation (e.g. from 15.17 day-1 _{on Day 1 to 4.22 mm day}-1 _{on Day 3 in the ‘dry’ treatment), probably as a result of}

reduced availability of substrate moisture (SMC declined from 0.495 to 0.109 m3 _m-3 _{over the}

same period). Consequently, by Day 3 Stachys ET was lower than Heuchera and Salvia ET (significantly so in the ‘hot’, ‘dry’ and ‘warm’ trials) and similar to Sedum ET; by the end of all trials, the daily ET rate of Stachys was approaching that of the control. These results imply that the majority of a green roof’s substrate retention capacity would be restored very quickly (in the first 2 – 3 days following rainfall) with species with high transpiration rates, thus enabling the green roof to store a large volume of water in subsequent rainfall events that occur relatively close together.

Since the daily ET of these species declined whilst those with lower ET rates did not, the available substrate retention capacity would become more similar with all species (and control) after this time, indicating that the advantage of species with high ET rates (e.g. Stachys) over species with lower ET rates (e.g. Sedum) would be greatest in the first 2 – 3 days following rainfall. A similar decline in daily ET rates and a convergence of the cumulative ETs of different species were observed with succulent species in other studies (Berghage et al., 2007; Voyde et al., 2010b; Poë

et al., 2015), although typically over a much longer period of time (28 days), which may account

for why this was not observed for all species/control in the short duration of these trials.

In all T/RH treatments, declines in SMC and daily ET with Stachys, and to a slightly lesser extent

Salvia, were accompanied by wilting (Figure 4.6), presumably indicating low leaf water status with

these species in response to drought conditions. Cameron et al. (2006) observed similar behaviour with Forsythia under severe regulated deficit irrigation, and suggested that vigorous species may continue to photosynthesise at maximum potential (based on available light), thus maintaining high stomatal conductance even when water deficit is high and plants become stressed. Similarly, (Voyde et al., 2010b) noted that Disphyma australe did not adapt to drought stress during drying, instead depleting plant water content in order to maintain higher transpiration rates, and thus resulting in plant wilting. It is likely, therefore, that Stachys and

Salvia may continue to photosynthesise, and thus transpire, at their full potential whilst light is

available, quickly depleting substrate moisture and resulting in a rapid decline in subsequent ET and also leaf water status. Indeed, the gs of these species, an indicator of ET, has previously been found to only be significantly reduced when SMC falls below 0.150 – 0.200 m3 _m-3 _{(Blanuša et al.,}

112 2013; Vaz Monteiro et al, 2016a). Additionally, once Stachys became wilted the plants formed a very dense cover over the substrate (Figure 4.6 B), probably restricting evaporation from the substrate surface and further reducing overall ET.

Wilting of Stachys and Salvia plants was, however, temporary and they recovered quickly when irrigated at the end of each trial, with no permanent damage evident. It is therefore likely that these species could provide good rainfall retention on a green roof as long as supplementary irrigation is provided in periods of drought to ensure plant survival and continuation of high ET rates. Although the substrate retention capacity available for water storage during the next rainfall event can only be restored by a finite amount, maintaining high ET rates is advantageous for the provision of additional ecosystem services (ESs), particularly cooling (Vaz Monteiro et al, 2016a). Results of this experiment suggest that irrigation would be required after an average of 3 - 4 dry days, depending on T and RH conditions, in order to maintain plant health and high ET rates with Stachys.

Figure 4.6: Example of a Stachys plant under well-watered conditions (A) and the same plant after 4 days with no irrigation in the ‘dry’ treatment (B), showing extreme turgor loss.

In contrast, with Sedum and the control, the relationship between antecedent SMC and daily ET in all trials was extremely weak, with adjusted R2 _{values of 0.01 and 0.02 respectively. Moreover, in}

contrast to other studies (e.g. Voyde et al., 2010b), ET rates did not decline over the course of the trials, probably because of the short duration of the trials and the consistently low daily ET rates limiting the depletion of substrate moisture. A rapid decline in stomatal conductance (and thus transpiration) of Sedum species when they switch to CAM photosynthesis in response to low SMC has previously been observed (D'Arco et al., 2017). Additionally, Sedum spurium is considered to have relatively low leaf succulence compared to other Sedum and succulent species, likely reducing its ability to survive in severe drought conditions (Farrell et al., 2012). However,

113 substrate moisture did not appear to become restricted with Sedum in the presented experiment due to the short duration of the trials and its low ET rate, and so these effects were not observed.