134 In 1996, Shaw and Pittenger (2004) evaluated the perfor- mance of 30 species of shrubs in Encinitas, CA. Following an establishment period of approximately 18-months, plants were irrigated for one year at 12%, 24%, and 36% ETo, while in 1997 and 1998 irrigation treatments were adjusted to 0%, 18%, and 36% of ETo “because initial treatments were not affecting plant quality.” Irrigation frequency was determined using a projected soil moisture deficit of 13 mm. The authors reported that “many species performed well at 36% and 18% ETo treatments, but suffered at 0 ETo.” Eight shrub species performed well with no irrigation, while 13 species performed well at 18% ETo. Three species did not become established in the study plot, and the remaining species required irrigation levels greater than 18% ETo. In an evaluation of herbaceous species, Reid and Oki (2008) conducted a one-year study in Davis, CA. After a one-year estab- lishment period in 2005, six species of California native plants were irrigated at four irrigation levels: 20%, 40%, 60%, 80% ETo. Plant performance was evaluated using a growth index. For all six species, no significant differences in summer growth or physical appearance were found for the four irrigation lev- els. Ongoing performance evaluations of the same species are being conducted in seven climate zones throughout California. Container and Lysimeter Studies Using both containers and lysimeters, Garcia-Navarro et al. (2004) measured the water use of four shrub species in Davis, CA. After a one-year establishment period, two parallel experi- ments (using containers and lysimeters) were conducted during the summer of 1998. The authors state that “the relative water use of the same species in 3.8-liter containers would be repre- sentative of the water use of the same species in the landscape.” Lysimeter plants were irrigated at 30% and 100% of ETo, while container plants received either daily irrigation or water was withheld until available water was depleted. Crop coefficients for the well-irrigated container plants (all four species) were found to range from 1.30 to 5.51, while three species showed a three-fold reduction in water use when water-stressed. At 30% ETo, water use was reduced by 52% to 55% for plants in lysimeters (three species). Growth of all species was affected by reduced irrigation, and visual appearance declined substantially for two species. DISCUSSION From the results of field studies, water needs assessments were found to vary according to vegetation types and species. For trees, seven species of arid-adapted trees were reported to per- form well under a 70% soil water deficit treatment, while two species performed better at a 50% depletion level (Schuch et al. 2010). Oaks receiving no irrigation performed as well as the same species receiving 25 and 50% ETo (Costello et al. 2005). Similarly, two palm species were found to perform optimally at 0% ETo, while three others were assessed as hav- ing acceptable quality without irrigation (Pittenger et al. 2009). Field studies of other vegetation types show a somewhat wider range of species water needs. An irrigation level of 12.5% ETo was found to be sufficient for eight hedgerow and groundcover species (Sachs 1991); while in another study (Staats and Klett 1995), two groundcover species performed well at 25% ETo, yet one species needed 75% ETo). For shrubs, eight species were reported to perform well at 0% ETo, while another 13 species ©2013 International Society of Arboriculture Costello: Urban Trees and Water required an irrigation level between 18% and 36% ETo (Shaw and Pittenger 2004). In a study evaluating six herbaceous spe- cies, all species performed well at 0% ETo (Reid and Oki 2008). In lysimeter studies where water supply was limited, no effect of reduced irrigation was found on trunk diameter growth for three species of trees (Devitt et al. 1994). In a follow-up study by Devitt et al. (1995), similar results were found for an additional three species of trees. In a container and lysim- eter study where water supply was limited for four shrub spe- cies, water use reductions resulted in growth reductions for all species and a decline in visual appearance for two species. In a lysimeter study where water supply was not lim- ited (Levitt et al. 1995), water-use coefficients were found to range from 0.5 to 1.0 for two tree species (based on a total leaf area) and from 1.4 to 1.6 (based on a projected canopy). For B&B stock planted into lysimeters, water-use coefficients ranged from 0.19 to 1.05 for five tree species. Collectively from field and lysimeter studies, many of the woody and herbaceous species evaluated were found to perform well at irrigation levels less than 25% ETo. Indeed, a number of species were found to perform well without irrigation (0 ETo). These findings are important for landscapes in climate zones where precipitation is limited during the year, such as in arid, semi-arid, and Mediterranean zones. The use of landscape species that require little or no irrigation once established will be of great value in creating and maintaining water-conserving landscapes. Certainly, a considerable amount of research still needs to be done. For instance, evaluations for only 17 tree species are reported in this review. Clearly, this number is not sufficient to provide useful guidance regarding the water needs of land- scape species. For trees in particular, much more research is needed to identify the needs of the hundreds of species used in urban forests in the western U.S. In addition and most importantly, a standardized protocol for conducting such re- search is critically needed. From this review, it is evident that a substantial level of variation exists in experimental design and methods. For the species evaluated, many differences can be found in treatment levels, methods for quantifying water supplied to individual plants, use of soil moisture measure- ments to schedule irrigations, length of establishment and treatment periods, stock types, irrigation systems used, and measures of plant performance. These are critical elements of an experimental design that affect the outcome of water- needs studies, making it difficult to compare study results. This leads to an important question: What is the best method of conducting water-needs studies? Unfortunately, as yet, there is no standardized method—but one needs to be established. A number of other questions regarding experimental meth- ods used for water-needs evaluations of trees can be listed: • What are reasonable levels of ETo for treatments? Certainly, no irrigation (0% ETo) should be included, but what amounts should be used for irrigation levels (10%, 30%, and 50% ETo)? • What is the best way to determine the amount of water that matches the desired level of reference evapotrans- piration? For example, how much water should be applied to a 2.5 cm caliper tree for 50% ETo? How much for a 5 cm caliper tree? What irrigation frequen- cy should be used?
May 2013
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