Abstract:In extensive green roof settings, plant communities can be more robust than monocultures. In addition, native plants might be hardier and more ecologically sound choices than non-native plants in green roof systems. The objectives of this research were to (1) compare the performance of plant communities with that of monocultures and (2) compare the growth of natives to non-natives in a simulated green roof setting. We conducted a two-year experiment at an outdoor site in a desert environment using four plant morphological types (groundcover, forb, succulent and grass). Native plants selected were Chrysactinia mexicana, Melampodium leucanthum, Euphorbia antisyphilitica, and Nassella tenuissima, and non-natives were Delosperma nubigenum, Stachys byzantina, Sedum kamtschiaticum and Festuca glauca. Plants were assigned randomly to either monoculture or community and grown in 1 m × 1 m custom-built trays filled with 15 cm of a proprietary blend of 50/20/30 lightweight aggregate/sand/compost (by volume). Native forb, Melampodium, in community had greater coverage for four of the five measurements in the first year over native forb in monoculture and non-native forb regardless of setting. Native forb coverage was also greater than non-native forb for three of the four measurements in year 2, regardless of setting. Coverage of native grass was significantly greater than non-native grasses throughout the experiment. Coverage was also greater for eight of nine measurements for native succulent over non-natives succulent. However, non-native groundcover coverage was significantly greater than native groundcover for seven of nine measurements. On 1 November 2016, relative water content (RWC) for succulents (p = 0.0424) was greatest for native Euphorbia in monoculture at 88%. Native Euphorbia also had greater RWC than non-native Sedum on 4 April 2017 (78%) and 4 July 2017 (80%). However, non-native Sedum had greater root length (6548 cm), root dry weight (12.1 g), and root-to-shoot dry weight ratio (0.45) than native Euphorbia. At the end of year 2, the relative growth rate (RGR) of native Euphorbia of 0.15 g·g −1 ·d −1 was greater than that of Sedum. While the native succulent had a smaller root biomass, its greater RWC and RGR would indicate it had better plant water status and grew faster than the non-native. The lack of differences in plant performance regardless of assignment to monoculture or community would imply that communities and monocultures are equally suitable for arid region green roofs.

With increased interest in water conservation, traditional residential landscapes, which often include turfgrass and other high-water use plants, are often eliminated in favor of low water use desert landscape plants. But even with a desert landscape, home owners often overwater, thereby reducing any water conservation possibility. This experiment was designed to demonstrate that plants can retain physiological health even when on reduced irrigation. Three 26.8-m 2 plots each of two contrasting landscapes, designated as either traditional or desert, were installed in a desert environment using a pot-in-pot in-ground system. Plots were irrigated at 100% or 50% of evapotranspiration (ET) with either sprinklers (turf) or drip emitters (trees and shrubs) using a modified crossover design. Midday stem water potentials (Ψ md) for Arizona ash, Indian hawthorn and Cleveland sage exhibited seasonal differences. In Chinese pistache, Ψ md remained stable when irrigation treatments were lowered from late spring/early summer (−0.26 MPa) to late summer (−0.35 MPa). Chinese pistache maintained less negative osmotic potential when irrigation increased from 50% ET (−5.13 MPa) to 100% ET (−3.68 MPa) in early fall. The ability of Chinese pistache to maintain Ψ md and osmotic potential may indicate this species' resilience to drought. Surprisingly, Arizona ash sustained greater relative water content in late fall when irrigated at 50% ET (92% elative water content (RWC)) than when irrigated at 100% ET (76% RWC) and this might be due to osmotic adjustment. The plants used in this study recovered after two weeks of full (100% ET) irrigation suggesting that landscape managers could irrigate at 50% ET for a limited period (approx. four weeks) as a way to conserve water.

The contiguous geographic range of bigtooth maple (Acer grandidentatum Nutt.) covers Utah, Idaho, Wyoming, Arizona, New Mexico, and Texas and suggests that this deciduous tree is a potential landscape plant for many regions. Using bigtooth maples selected from provenances in New Mexico (NM), Utah (UT) and Texas (TX), we evaluated physiological and growth traits of plants subjected to root zone salinity treatments at concentrations 0 (control), 2.5, 5.0 or 10.0 dS·m−1 (0, 1,600, 3,200, or 6,400 ppm). At harvest, foliar Kjeldahl nitrogen, potassium, magnesium, phosphorus, and calcium concentrations of salinity-treated plants were not different from control plants. Plants from the TX provenance had the highest leaf dry weight (DW) (15.7 g [0.55 oz]), larger stem diameter (11.4 mm [0.45 in]), less foliar injury, and less negative midday stem water potentials while accumulating three and two times more foliar sodium than plants from the UT and NM provenance plants, respectively. Total DW (95.9 g [3.4 oz]) of TX plants was triple that of the other two provenances. While bigtooth maples from the three provenances tolerated salinity, those from the TX provenance show enhanced resiliency to root zone salinity.

Extreme evaporative demand makes substrate depth a critical design factor in arid-climate green roofs. The objective of this study was to determine whether a shallow irrigated substrate could support the growth of hens and chicks (Sempervivum calcareum L.) and iceplant [Delosperma nubigenum (Hook.f.) L.Bolus] in an arid environment. First, an experiment was conducted in the greenhouse that established that plants survived in 10 cm (3.9 in), 15 cm (5.9 in), and 20 cm (7.9 in) substrate depths, which then lead to a second experiment in an outdoor environment. The substrate was heat-expanded clay:sand:worm castings (6:3:1, by volume) in a greenhouse experiment and heat-expanded clay:zeolite:worm castings (6:3:1, by volume) in an outdoor experiment. In the greenhouse experiment, deep root length density (RLD) was significantly greater in the 10 cm-deep (3.9 in) substrate, while outdoors, deep RLD was highest for plants grown in the 15 cm-deep (5.9 in) substrate. Outdoors, iceplant had significantly greater mean coverage and shoot dry weight than hens and chicks. Lack of significant differences in quality and coverage due to substrate depth, coupled with higher RLD in the 10 cm (3.9 in) and 15 cm (5.9 in) depths in both experiments provides evidence that shallow irrigated substrates support the growth of both taxa. Index words: iceplant, hens and chicks, plant coverage, root length density, quality, zeolite, heat expanded clay. Species used in this study: hens and chicks (Sempervivum calcareum L.); iceplant [Delosperma nubigenum (Hook.f.) L. Bolus].