Arboriculture & Urban Forestry 41(6): November 2015 There is a great need to evaluate tree species to determine species that are adapted to the limited soil moisture and harsh climate oſten found in urban landscapes (Costello 2013). ‘Autumn Blaze’ maple is one of the many Freeman maple cultivars (Acer × freemanii) developed from crosses between silver maple (A. saccharinum) and red maple (A. rubrum) (Santamour 1993). Research indicates Freeman maples are more resistant to stress when compared to red maples (Bachtell 1989), and Zwack et al. (1999) suggests Freeman maples are more ornamental when compared to silver maples. Shan- tung maple (A. truncatum) is native to China (Pair 1987), and has been found to be a drought- and heat-tolerant species (Liu 1989; Pair 1994; Wang et al. 2006). Even though each of these species are considered tolerant of high temperatures and adapted to xeric conditions, research investigat- ing response of Freeman and shantung maples to limited soil moisture while growing in a semi-arid (high VPD) climate has yet to be conducted. There- fore, in a semi-arid, high-VPD site (used to simulate an urban environment), this research investigated water relations, gas exchange, and growth of two newly transplanted, field-grown maple varieties subjected to three ETo-based irrigation regimes. MATERIALS AND METHODS Research was conducted in a field nursery located in Lubbock, Texas, U.S. Prior to year one of the study, nine container-grown (11.4 L) trees of A. × freemanii ‘Autumn Blaze’ and A. truncatum were planted 2.5 m apart in east-west rows with 3.0 m between each row. Soil consisted of an Amarillo fine sandy loam (fine-loamy, mixed, superactive thermic Aridic Paleustalfs) with a pH of 8.5, organic matter con- tent of 0.8%, and CEC 13.5 meq 100 g-1 . Irrigation regimes were based upon estimated tree root area (cm2 ) and local ETo (mm). During the first growing season of the study (April–October), tree root area was estimated using the radius of the plant’s con- tainer (23.0 cm) plus an additional 15.2 cm. Follow- ing the first growing season, radius means of each variety were estimated to equal 122 cm and 183 cm, for the second and third growing seasons, respec- tively (Fox et al. 2014). Weather data were collected from an onsite weather station (Campbell Scientific, Inc., Logan, Utah, U.S.). Weather data were used to calculate daily total ETo. Reference evapotranspira- 337 tion was calculated using ETo calculation soſtware (Allen 2000). Based on total weekly ETo and esti- mated root surface area, irrigation was applied once each week at one of three PF values [100%, 60%, and 30% of ETo (high, intermediate, and low, respec- tively)]. Irrigation volume was calculated as follows: [2] V = [((ETo) – (P)) * (A)] / (1000)] * (PF) where V is irrigation applied each week (L), P is weekly precipitation (cm), A is mean soil surface area above each tree’s roots (cm2 ), and PF is frac- tion of ETo (1.0, 0.66, or 0.33). Once each week trees were irrigated through a drip irrigation system. To achieve the desired irrigation volume, each tree had one, two, or three emitters (3.8 L hr-1 ) placed at the base of the tree. Trees were not fertilized or pruned during the experiment, and weed control was per- formed by hand (each tree was surrounded by bare soil from trunk to drip line). To aid establishment, during the growing season prior to the study all trees were irrigated at 100% ETo. Irrigation treat- ments began during the second growing season, and continued for two additional growing seasons. Daily maximum air temperature, and VPD at maximum daily air temperature during each grow- ing season were plotted against day of the year (01 May through 31 August). From May through Sep- tember, monthly pre-dawn ψl , and midday gs were (Central Daylight Savings Time) on two randomly selected, mature leaves from each tree. Leaves were excised before dawn, immediately sealed in a plas- tic bag, and placed in a portable cooler. Pre-dawn ψl was measured within a half-hour of excision 1400) gs was measured with a steady-state porom- eter (model LI-1600; LI-COR®, Lincoln, Nebraska, U.S.). For gs data collection purposes, single trees of each treatment combination (irrigation regime × species) were grouped, and one measurement cycle (time required to measure gs on each tree in group) with a pressure chamber (model 3005; Soilmois- ture Corp., Santa Barbara, California, U.S.). Each day pre-dawn ψl was measured, midday (1200 to ©2015 International Society of Arboriculture measured six days aſter an irrigation event. Water relations and gas exchange data were measured the following dates for year one (16 June, 30 June), year two (27 May, 03 June, 17 June, 01 July, 21 July, and 05 August), and year three (09 June, 30 June, and 16 September). Pre-dawn ψl was measured at 0600
November 2015
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