(C) PLOS One [1]. This unaltered content originally appeared in journals.plosone.org.
Licensed under Creative Commons Attribution (CC BY) license.
url:https://journals.plos.org/plosone/s/licenses-and-copyright

------------



Influence of water storage and plant crop factor on green roof retention and plant drought stress

['Lubaina Soni', 'School Of Ecosystem', 'Forest Sciences', 'Faculty Of Science', 'The University Of Melbourne', 'Richmond', 'Victoria', 'Christopher Szota', 'Tim D. Fletcher', 'Claire Farrell']

Date: 2022-06

Effect of increasing plant density and substrate depth on evapotranspiration

We hypothesized that doubling plant density would double ET due to a proportional increase in K c [30, 45]. However, doubling plant density from one to two plants per pot increased ET by only 12–20%. When plant density was quadrupled, there was only a 30–40% increase in ET relative to that of a single plant. The observed range in daily ET in our study (0.8–7.5 mm d-1) is very high as compared with other green roof studies. However, these studies mainly used shallower substrates planted with succulent species, which have generally lower ET rates due to their conservative water use. For example, Voyde et al., (2010) [46] observed daily rates of 2.19–2.21 mm d-1 and Sherrard et al., (2012) [47] found average daily ET rates of 0.52–1.24 mm d-1. Since, the species used in our study had very high rates of ET, up to 7.5 mm d-1, which is similar to ET recorded for the same species in biofiltration studies [48, 49]. This shows that the high ET rates in our study are very much due to the selection of a higher water-using species in deeper substrates and under well-watered conditions. However, as increasing plant density did not proportionally increase ET in our glasshouse experiment, this suggests limitations in the gains in overall water use for densely planted green roofs.

Plant biomass and relative growth rates increased by approximately 70% when planting density increased from one to two plants, regardless of substrate depth. This is consistent with Schmid et al., (2008) [50], who found that greater biomass and productivity would retain more water and show higher ET. Although the increase in ET for our plants was related to increases in biomass, there were proportionally lower gains in ET with increasing plant density, which may suggest that ET at higher plant densities was limited by plants shading themselves, neighboring plants, and/or substrate surfaces. Although plants were well-watered in our glasshouse study when crop factors were derived, and therefore experienced no drought stress. On green roofs, shading has also been shown to reduce plant drought stress [51] and ET [52–54]. This is further supported by ET per unit biomass, which significantly decreased with increasing plant density, indicating that a single plant was more efficient at using water.

In planted pots, we observed a 28–38% increase in ET from the 150 mm to 300 mm deep substrates. This increase in ET was proportionally less than the increase in substrate depth and is consistent with other green roof studies [40, 55, 56]. For example, Buccola et al., (2011) [55] found only a 36% increase in hydrological performance when almost tripling substrate depth from 50 to 140 mm. Further, as Soulis et al., (2017) [56] showed that doubling substrate depth (80 to 160 mm) only increased plant water use when plants were able to dry out substrates and significantly replenish green roof storage between rainfall events. Hence, doubling substrate storage is unlikely to double ET where water is not limiting [57, 58], which was the case in our experiment as plants were well-watered each day to determine their K c . The effect of well-watered conditions on reducing ET is also reflected in the minimal increase in ET for bare unplanted pots, where increasing substrate depth from 150 mm to 300 mm only increased cumulative ET by 9 mm during our study period. As plant biomass was the same for both substrate depths when planted with the same number of plants, this suggests that plants in deeper substrates were unable to access more water at depth. The scoria-based substrate had a water holding capacity of ~50%, and therefore in 150 mm deep substrate, nearly 75 mm of water was available for daily ET. However, the maximum daily ET observed in 150 mm deep substrate was 5.5 mm d-1 at the highest density (4 plants per pot), suggesting that ~70 mm of water remained unused. Since our plants were fully grown with full canopy cover at the start of the glasshouse experiment, they should have had maximum ability to extract water from the pots [27, 30, 54, 59]. Plants in deeper substrates may have had lower allocation to roots and this may also have reduced their water use. However, we were unable to partition root and shoot biomass or measure root depth. Plants were not stressed during this experiment as they were kept well-watered. Under water-deficit conditions, we expect that plants in deeper substrates would show greater ET, due to greater available storage, than plants in shallower substrate. However, under well-watered conditions and regardless of the mechanism, additional storage achieved through increasing substrate depth only marginally increased ET.

In our study unplanted substrates had 78–81% lower ET than planted substrates. These results are lower than the results found in literature [12, 37, 60, 61], and are likely due to our experimental conditions. For example, a previous study in green roof modules showed that unplanted modules had 93% of the ET of modules planted with similar high water-using species [12]. However, plant coverage in our study was far greater (i.e., leaves extended well outside the pot area) than this previous module study, and this is likely to have caused greater ET in planted versus bare substrate. It is plausible that the lower ET of bare substrates was reduced by the pot sidewalls causing shading or reducing air flow across the surface. However, the greater ET in our planted substrates is more likely due higher canopy cover [62] and high water-using species [63]. Hence, coverage with high water-using plants will considerably increase ET rates as compared to unplanted green roofs, suggesting that green roofs should be planted to maximize plant coverage to ensure greater plant water use and in turn greater replenishment of substrate water storage between rainfall events.

[END]

[1] Url: https://journals.plos.org/water/article?id=10.1371/journal.pwat.0000009

(C) Plos One. "Accelerating the publication of peer-reviewed science."
Licensed under Creative Commons Attribution (CC BY 4.0)
URL: https://creativecommons.org/licenses/by/4.0/


via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/plosone/