Arboriculture & Urban Forestry 39(3): May 2013 Basically, tree growth and visual appearance for trees receiving no irrigation were not different from those receiving 25% or 50% of reference ETo. Average annual rainfall at the study site is 40.6 cm. To assess the effect of irrigation level on the performance of palm species, Pittenger et al. (2009) conducted a study in Irvine, CA. Five landscape species (Archontophoenix cunninghamiana, Chamaerops humilis, Syagrus romanzoffi- ana, Trachycarpus fortunei, and Washingtonia filifera) were irrigated at three levels of reference ETo: 0%, 25%, and 50%. All species maintained at least minimally acceptable visu- al quality at the 0% ETo treatment (no irrigation), and two species were found to have near optimum performance with no irrigation, while two species produced more leaves with additional irrigation. The authors note that the water needs of landscape palms are considerably less than that those of commercial palms, such as date, oil, and coconut palms. Container or Lysimeter Studies To measure tree water use and/or determine species water needs, a number of studies have been conducted using plants in con- tainers or lysimeters. Here, studies are sorted into two groups depending on whether plant water supply was limited or not. Water supply limited In these studies, experimental design included treatments where the supply of available water was limited to some extent. In a two-year study, Devitt et al (1994) measured water loss from three landscape species in Las Vegas, NV. Three container stock sizes (3.8, 18.9, and 56.8 L) of Prosopis alba, Chilopsis linearis, and Quercus virginiana were planted into 190-liter lysimeters. After a three-month establishment period, three irrigation treatments were imposed as leaching fractions of +0.25, 0, and –0.25 (drainage volume/irrigation volume). Although water loss from species (ETa) was affected by tree size and leaching fraction treatments, there was little or no effect of irrigation level on trunk diameter growth. In a similar study, Devitt el al. (1995) measured water loss from three tree species (Washingtonia robusta, Pinus eldarica, and Cercidium floridum) planted as container stock (#5 and #15) into 190-liter lysimeters. Treatments were similar to Devitt et al. (1994) with irrigation levels expressed as leaching frac- tions of +0.25, 0, and -0.25. After a three-month establishment period, treatments were imposed for a six-month period. Although significant differences in water use were found for species resulting from planting size and leaching fraction (irrigation level), no significant effect on canopy volumes or basal cano- py areas were found, and few significant differences in trunk diameter were found across irrigation treatments for all species. Water supply not limited In these studies, experimental design did not include treat- ments where the supply of available water was limited to some extent (i.e., water was continuously available to the plant). In a three-month study, Levitt et al. (1995) measured water loss from Prosopis alba (Argentine mesquite) and Quercus virginiana (southern live oak) growing in containers in Tucson, AZ. A gravimetric method was used to determine actual plant water loss, and water-use coefficients were cal- 133 culated as the ratio of water loss to reference evapotrans- piration for the study site. Water-use coefficients of 0.5 for southern live oak and 1.0 for mesquite were reported using water loss values for the total leaf area, and 1.4 (oak) and 1.6 (mesquite) for water loss on a projected canopy basis. To determine water needs of balled and burlapped (B&B) stock during the first year after planting in a semi-arid climate, Montague et al. (2004) conducted a one-year study in Logan, UT. The performance of five species (Platanus × acerifolia, Salix matsudana, Tilia cordata, Acer platanoides, and Fraxinus penn- sylvanica) was evaluated using locally grown field stock planted into lysimeters. Water loss was measured from trees under non- limiting conditions, and a water loss coefficient (Kc) was calcu- lated as the ratio of actual water loss (based on total leaf area) to total daily ETo. Tree water loss varied with species, and water loss coefficients ranged from 0.19 for A. platanoides to 1.05 for S. alba. To quantify the influence of shading on water loss, Costello et al. (1996) conducted a study using container plants in Palo Alto, CA. Three tree species (Sequoia sempervirens, Mag- nolia grandiflora, and Liquidambar styraciflua) and one shrub species (Pittosporum tobira) were placed in full sun (86,000 lux) or a shaded environment (820 lux) and water loss was measured gravimetrically over a two-week period. Plants in the shaded environment were found to lose on aver- age 58% less water than those in the full sun environment. OTHER PLANT TYPES (GROUND COVERS, SHRUBS, AND HERBACEOUS PLANTS) Field Studies In a relatively early report, Sachs (1991) conducted a two-year study at two sites in California (San Jose and Irvine), evaluat- ing the performance of hedgerow and ground cover plantings irrigated at 0%, 12.5%, and 100% ETo. Species included were Ligustrum lucidum, Pittosporum tobira, Nerium oleander, Coprosma baueri, Xylosma congestum, Eugenia uniflora, Hedera canariensis, and Carpobrotus sp. Plantings were established in 1965 and treatments were initiated six years later (1971). At both study sites, an irrigation level of approximately 12.5% ETo was sufficient to maintain all plants in a healthy condition with good appearance. Higher amounts of water caused increasing amounts of growth, which also increased their pruning requirements. A two-year study to assess the performance of three ground- cover species irrigated at 0%, 25%, 50%, 75%, and 100% of ETo was initiated in 1991 by Staats and Klett (1995). A principal goal of the study was to identify water-conserving species that could serve as alternatives to Kentucky bluegrass (Poa praten- sis). They reported that an optimum irrigation level for Ceras- tium tomentosum and Sedum acre was 25% ETo (after becoming established), while Potentilla tabernaemontii required 75% ETo. In 1990, Pittenger et al. (2001) initiated an evaluation of the performance of six groundcover species at four irriga- tion levels: 20%, 30%, 40%, and 50% ETo. All species had been established for a one-year period in 1989 at 25%, 50%, 75%, and 100% ETo (Pittenger et al. 1990). After a 17-month treatment period, they reported that Drosanthemum hispidum, Baccharis pilularis, and Hedera helix performed well at 20% ETo, while Vinca major required 30%. Both Potentilla tabernae- montii and Gazania sp. were found to need greater than 50% ETo. ©2013 International Society of Arboriculture
May 2013
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