84 Costello et al.: Irrigation Effects on Growth of Newly Planted Oaks limiting bulk density for loam is approximately 1.6 g/cc (Daddow and Warrington 1983). Acorn-propagated container stock (19 L [5 gal]) of three species of California native oak were selected for uniformity of size, health, and vigor from nurseries in the San Francisco Bay area. Species included Quercus lobata (valley oak), Q. agrifolia (coast live oak), and Q. douglasii (blue oak). At planting (March 1997), soil was loosened by hand from the root ball, peripheral roots were spread in the planting hole, and the top of the root ball was set approximately 2.5 cm (1 in.) above the soil line. After backfilling with plot soil, berms were formed to create watering basins, and all plants were hand-watered twice a week (approximately 11.4 L [3 gal] per tree per irrigation) for 3 months after planting. Neither fertilizer nor mulch was applied. Weeds were controlled by mowing and applications of glyphosate (as needed). Average annual rainfall at BAREC is 40 cm (16 in.). Trees were spaced on 3.3 m (10 ft) centers. Randomized complete blocks were established with three irrigation treatments for each species per block. Eight replicates for each treatment and species gave a total of 72 trees in the study plot. Irrigation treatments were established as fractions of reference evapotranspiration (ET0 (no irrigation). ET0 May through August is 16.5 cm (6.5 in.), with a range of 14.5 to 18 cm (5.7 to 7.1 in.). After the 3-month period of hand watering, all plants received 1 ET0 Management Information System (CIMIS) station located adjacent to the study plot. Average monthly ET0 was measured from a California Irrigation for the period ): 0.25 ET0 , 0.5 ET0 , applied twice weekly using a drip irrigation system. This irrigation program during the first season ensured the survival of all trees and allowed for a root establishment period. Irrigation treatments were initiated in the second year (1998) and continued for 3 years. The specific amount of water delivered to 0.25 and 0.5 ET0 treatment trees was determined from a measure of average crown projection (ground shading) by individual plants (Lindsey and Bassuk 1991; Costello and Jones 2000). Using a 50 cm (20 in.) diameter for crown projection, plants initially shaded an area of 2 m2 ). Being small plants, leaf area index (LAI) was set at 1. For this area, 5.1 L (1.35 gal) of water was needed to supply 2.5 cm (1 in.) of water (1 ft2 (21.5 ft2 -inch volume of water = 0.62 gal). On average, 0.25 ET0 and 0.5 ET0 treatments received 13.3 L (3.5 gal) per week treatments received 26.6 L (7 gal) per week. Irrigation water was supplied using a Netafim Techline pressure-compensating drip system (1.27 cm [0.5 in.] polyethylene tubing). In-line emitters were placed in a circle within the canopy perimeter (drip line) of each tree, with four emitters per tree. In 1999, an additional line of emitters was added to the outside of the original line to provide water to the enlarging root system. Each emitter supplied 3.8 L (1 gal) per hour. To monitor total water supplied to all plants in each irrigation treatment, flow meters (Neptune® 1.6 cm [0.63 in.]) were installed in each main line. This provided assurance ©2005 International Society of Arboriculture , and 0 that 0.5 ET0 amount of water of 0.25 ET0 treatments were actually receiving two times the treatments. Water supply was week, respectively. Watermark® soil moisture sensors (Irrometer Corp., Riverside, CA) were installed 15.3 and 30.5 cm (6 and 12 in.) deep within the irrigated zone of plants (approximately one- half the distance between in-line emitters and plants) at seven locations across the plot. Sensors were used to monitor water application uniformity and assess moisture depletion between irrigations. Periodic measurements of water applied by emitters to each tree and the use of flow meters to monitor total water applied to treatments substantiated that water supply and distribution were in agreement with scheduled amounts. Trunk diameter was measured annually for all trees at 15.3 cm (6 in.) above ground line. Although plant height was recorded, it was not used to assess treatment effects due to variability in tree form. In summer 2001, root systems of coast live oak were excavated to quantify root mass and distribution (Figure 1). Three trees from each irrigation treatment were measured. Trenches were dug 2 m (6 ft) deep and 1.6 m (5 ft) from the trunk on two or three sides of excavation trees using a backhoe. Trenches were used as a receptacle for soil removed from roots. An area 2.6 × 2.6 m (8 × 8 ft) around each tree was excavated to a 1.6 m (5 ft) depth using a pneumatic tool (Air-Spade®). In some cases, individual roots were excavated beyond this volume to their length and depth (to 2 mm [0.08 in.] diameter). Based on a visual assessment after excavations, it was estimated that less than 5% of roots remained unexcavated. Following excavation, intact root systems were removed from the field, suspended from a greenhouse roof beam, and controlled with a six-station irrigation timer (Hardie Rain Dial®), set to irrigate 0.25 and 0.5 ET0 treatments once and twice per Figure 1. After 4 years, root systems of coast live oak were fully excavated using a pneumatic excavation tool (Air-Spade®), backhoe, and jackhammer.
March 2005
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