Arboriculture & Urban Forestry 38(1): January 2012 termite infestations (pers. obs.: R.C. Beeson), slow soil warming in spring (Litzow and Pellett 1983), retaining and/or repelling water (Gilman and Grabosky 2004), and girdling root formation (John- son and Hauer 2000). Lenticel size also increased when mulch rested against the trunk for 17 months after planting, perhaps to compensate for lowered oxygen levels (Greenly and Rakow 1995). There is little data describing mulch impacts on direct measure- ments of moisture relations within the root ball (Altland and Lan- thier 2007) after planting. The importance of water status within the root ball is obvious because it is more important to tree sur- vival in the weeks and months following planting than moisture in the surrounding landscape soil (Watson and Kupkowski 1991b). With questionable benefits of mulch to survival, growth, and health of newly planted landscape-sized trees under experimental conditions, combined with the potential disadvantages of mulch applied to the root ball surface, this study was designed to evaluate if application of mulch over the root ball surface impacts evapora- tion. The hypothesis was that mulch placed on the root ball surface had negligible impact on evaporative water loss. Aboveground ly- simeters were designed to eliminate perched water tables in order to simulate root balls planted into landscape soils. This allowed for gravimetric measurement of water loss from the root ball with and without pine bark mulch. No trees were in the lysimeters. MATERIALS AND METHODS Lysimeter Construction Lysimeters are non-porous containers made from plastic or other materials that can be positioned above or in the ground to simu- late plant water usage in the landscape (Pearson and Scheiber 2006). The authors of the current study used 360 L (105 cm top diameter × 49 cm tall) black plastic containers common in the nursery trade. Sides of containers were covered with alumi- num foil to reflect heat to better simulate a substrate or soil root ball planted in the ground. These lysimeters were placed above ground on platforms of industrial steel shelving measuring 109 cm × 117 cm resting on two 5 cm × 150 cm long, 6 mm thick, angle-irons. Four cm from the ends of the angle iron, about 45 cm of 7 mm thick chain (580 kg working load limit) was bolted to the angle iron forming a loop. These were used as lifting points to weigh the lysimeters and platforms together. Bricks were used as footings for angle irons to provide a level platform approxi- mately 10 cm above the soil surface. Platforms were stiff enough to prevent lysimeter deformation when lifting the platform. A system was devised to eliminate the perched water table that occurs in a container (Richards et al. 1986; Bilderback and Fon- teno 1987; Spooner 1974) by wicking water into the landscape soil beneath the lysimeter. This allowed for a better simulation of water drainage from a root ball planted into the landscape. A single layer of Aquamat (Soleno Textiles, Laval, Quebec, Can- ada) capillary mat fabric was cut to fit neatly across the bottom and 10 cm up the container sides to cover all drainage holes pre- viously drilled into the bottom and sides by the manufacturer. A slit was cut through fabric at the position of all nine drain- age holes on the bottom surface of the container. A piece of the same fabric (76 cm long × 2.5 cm wide) was pushed through the slit and out each drainage hole. Fifteen cm of the fabric wick rested on the fabric mat at the container bottom with the other end of each wick resting on bare landscape soil below the 19 lysimeter. When a lysimeter received water, it drained out the holes wicking through the fabric and into landscape soil below. Filling Lysimeters Each lysimeter for the container root ball test in Spring 2007 was loosely filled (no packing) with 120 kg of substrate (60 compos- ted pine bark:30 Florida peat:10 sand by volume, Florida Potting Soil, Inc., Orlando, Florida, U.S.) that was typical for a container nursery in southeastern United States. In autumn 2007, the sub- strate was replaced with 351 kg of native sand soil (Millhopper fine sand; loamy, siliceous, hyperthermic Grossarenic Paleudults) with less than 2% organic matter) for the field-grown root ball test. Either substrate or soil filled the container to 10 cm from the top. This system simulated planting a 360 L container or a field-grown root ball one meter in diameter into the landscape. Spring and autumn were chosen because weather in the state of Florida is similarly warm with infrequent rainfall. Periods of consecutive rain-free days were needed to conduct the study. Twenty-two lysimeters on platforms in full sun were placed 2 m apart in two rows. Treatments, mulched and non-mulched, were assigned at random to 18 uncovered lysimeters. After filling with substrate or soil, nine lysimeters received no mulch and nine received an 8-cm-thick layer (85.2 L) of large-particle (up to 8 cm long) non-composted pine bark as mulch. Pine bark was placed on a 25 mm sieve screen and gently shaken five times prior to application in an effort to standardize particle size distribu- tion in each mulched lysimeter. The particles that remained in the screen were applied to the surface. Sides of non-mulched lysimeters were shortened by 8 cm so the distance between the top surface of mulch or substrate to the top of lysim- eters was equal (5 cm). This eliminated differential shad- ing of the lysimeter surface. Four lysimeters were perma- nently covered with the plastic sheet described below except to facilitate irrigation when all 22 lysimeters received water. Measuring Evaporation In April 2007, substrate in four lysimeters was thoroughly wetted with five applications of 57 L of water spaced about 1.5 hours apart for two consecutive days. Water was applied uniformly us- ing a handheld nozzle attached to a hose. After the last application, each lysimeter was covered with 6 mm white polyvinyl sheets attached to the top sides of the lysimeter with 12 clips. Each ly- simeter was then weighed 30 minutes after the last irrigation that afternoon, and again at 10:00 am the following morning to evalu- ate the consistency of substrate saturation and drainage. This was repeated twice more within two weeks. Lysimeters, consisting of the steel platform and containers, were weighed together with one in-line 909 kg capacity load cell (SSM-AF-2000; Interface Inc., Scottsdale, Arizona, U.S.) suspended from a chain held up by a frontend loader mounted on a medium-size tractor. The same pro- tocol preceded the soil root ball test conducted in October 2007. On May 2 and 3, 2007, saturation procedure previously de- scribed was repeated on all 22 lysimeters, with an additional three applications on May 4 for a total of 738 L. Water drained out the bottom of lysimeters at every application on May 3 and 4. All 22 lysimeters then received 60 L water as described from 3:00 pm to 4:00 pm on all irrigation days beginning May 7, 2007, for the substrate test. Covered lysimeters were re-covered im- mediately after each application. Lysimeters were weighed 60 ©2012 International Society of Arboriculture
January 2012
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