Arboriculture & Urban Forestry 32(5): September 2006 203 pollinated outcrossing species with half-sibs likely derived from a large number of pollen parents (Schwarzmann and Gerhold 1991). In the four families studied, there was at least a threefold difference in height growth. Also, within the Quercus subgenera, many hybrids have been described or listed (Palmer 1948; Jensen and Eshbaugh 1976; Jensen et al. 1984; Manos and Fairbrothers, 1987; Tomlinson et al. 2000; Aldrich et al. 2003). Palmer (1948) lists six red oak hybrids (Q. rubra with palustris, ilicifolia, velutina, phellos, shumar- dii, and imbricaria). Soil moisture stress and oak water relations have been studied extensively (Abrams 1990), but only two studies de- scribe whole plant water use (Bourdeau 1954; Drunasky and Struve 2005). Under greenhouse conditions, unstressed bur (Quercus macrocarpa) and chestnut oak (Quercus prinus) seedlings used similar amounts of water, although bur oak seedlings were shorter and had less leaf surface area than chestnut oak. Stressed seedlings of both species used less water compared with unstressed seedlings. Quercus macro- carpa seedlings had greater root surface area and root:leaf area ratio than Q. prinus seedlings, but root systems of Q. prinus seedlings absorbed more water per unit root area per day than Q. macrocarpa. Unstressed Q. prinus seedlings ab- sorbed 2.2 times more water, whereas stressed seedling ab- sorbed 2.4 times more water than unstressed and stressed Q. macrocarpa seedlings. Thus, two drought-resistant species had significantly different growth habits and water use char- acteristics. Bourdeau (1954) found twofold difference in wa- ter use cm−2 leaf surface area among the five oak species studied. Xeric-site adapted species had higher rates of water use relative to mesic species. He also found great variation in water use among individual seedlings within the species studied. This study describes a series of four experiments con- ducted to determine the relationship between seedling growth habit and water use characteristics of six oak species. We hypothesize that seedling morphology and water use charac- ters may be used as a screening method to identify drought resistant individuals better able to survive stressful urban planting sites. In the first experiment, Q. shumardii, Q. rubra, and Q. velutina, which represent a continuum of wet-mesic to xeric-site adapted species, was studied. In the second experi- ment, a deep-rooted species (Q. macrocarpa) and two shal- low-rooted species (Q. palustris and Q. prinus) were studied. In the third experiment, two drought-resistant species were studied (Q. macrocarpa and Q. prinus). The fourth experi- ment was conducted to study within family variation in three species: Q. macrocarpa, Q. palustris, and Q. rubra. MATERIALS AND METHODS Four water use experiments were conducted over 10 years under different experimental conditions; however, similar data were collected. The first experiment used acorns col- lected from one red (Quercus rubra) and one black oak (Q. velutina) tree and acorns from two Shumard oak (Q. shumar- dii) trees. The second used bulked acorns from at least three bur (Q. macrocarpa), pin (Q. palustris), and chestnut (Q. prinus) oaks from each species. The third used acorns col- lected from one bur and one chestnut oak tree. The fourth used acorns collected from individual bur, pin, and red oak trees and maintained mother tree identity. In the fourth ex- periment, all acorns were collected from mother trees native to Franklin County, Ohio (40.08°N, 83.07°E), except for the chestnut oak acorns, which were from trees native to central Indiana (approximately 39.46°N, 86.10°E). Acorn collection, handling, and germination procedures were similar in all the experiments. Acorns were picked from trees in early to mid- September ≈1 week after the first acorns dropped. Typically, the fallen acorns were heavily infested with weevils; those remaining on the tree were relatively weevil-free. Within 4 hr of collection, the acorns were placed in unsealed plastic bags in a 7°C (44.6°F) refrigerator until sown. In March, acorns were sown in flats (Kandon Corp., Dayton, OH) in Metro Mix 360 (SunGro Horticulture Canada, Ltd., Vancouver, BC) substrate and germinated in a double polycarbonate-glazed greenhouse (25/18°C [77.0/ 64.4oF], day/night temperature) under natural photoperiods. As soon as shoots emerged, seedlings were removed from the flats, the tap roots pruned to 5 cm (2 in) length, and trans- planted to plastic containers using the same substrate. The containers’ interior surfaces were coated with SpinoutTM (Griffin Chemical, Co., Valdosta, GA) to encourage root de- velopment throughout the growing medium (Arnold and Struve 1993). Seedlings were initially placed container-to- container on greenhouse benches and grown under the con- ditions used for germination. At canopy closure, seedlings were spaced at twice the containers’ top dimension where they remained until the experiment was terminated. Seedlings were hand watered as needed to prevent moisture stress. At the beginning of Quercus Morphological Index (QMI) Lag-I (Hanson et al. 1986), the seedlings were fertilized once per week with 100 mg L−1 N from a water-soluble fertilizer (20N-8.6P-16.4K, 20 to 20 to 20 Peters; Scotts Miracle Gro, Marysville, OH). An Integrated Pest Management program was used to monitor insect populations, which were con- trolled with chemical pesticides. Water use in experiments 2 through 4 was determined by watering seedlings to saturation, allowing the substrate to drain for 1 hr and then weighing the seedlings with an elec- tronic balance (TR-12001; Denver Instrument Company, Denver, CO). Twenty-four (for experiment 2) or 48 (experi- ments 3 and 4) hr later, the seedlings were reweighed. The difference in weight was divided by the length of the water use period and used as an estimate of daily evapotranspiration referred to as water use for the remainder of the paper. Pre- ©2006 International Society of Arboriculture
September 2006
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