142 Sax and Bassuk: Tree Growth and Gas Exchange Response of Ficus benjamina 'Evergreen' at field capacity. After soils had finished draining freely, each core was weighed to determine total start- ing weight at field capacity. Total weight included trees, soil, PVC container, mesh screen, clamp ring, plastic bag, rubber band, and twist tie. Plastic bags were tied around the PVC containers and the top of the soil profile to reduce water loss by soil evapora- tion. Volumetric water content was determined using a four prong Theta Probe (ThetaProbe ML2, Delta-T Devices, London, England). All PVC cores had two sets of three holes that were drilled into the side of the containers at 4 cm (1.5 in) and 8.5 cm (3.3 in) from the top, allowing access for the Theta Probe at depth. Soil cores were weighed every five days to determine water loss between days and accumulative water. Accumulative water loss was reported in kgs on a volumetric basis due to the fact that soil and container volume were known. Water Deficit Dry Down Period On February 2, 2014, the water deficit dry down experiment was initiated (Day 0). The first round of sampling occurred one day later on February 3, 2014. Sampling was repeated every five days and a total number of six times with the last measurement end- ing on Day 26. Measurements included gravimetric and volumetric water content of containers, leaf water potential (kPa), transpiration (mmol H2 0 m-2 m-2 S-1 S-1 S-1 ), vapor pressure deficit (kPa), photosynthetic rate (μmol CO2 H20 m-2 ). A LI-COR LI-6400 XT portable gas exchange system (Li-Cor Inc, Lincoln, Nebraska, U.S.A.) was used and was set to provide 450 PAR light, 400 ppm CO2 , 25°C (77°F) temperature and ambient humidity. Predawn leaf water potential was measured using a plant water status console pressure chamber (Soil Moisture Equipment Corporation, Santa Barbara, California, U.S.A.) (Scholander et al. 1965). For water potential testing, fully expanded leaves were removed from individual shoots with a razor blade and immediately placed into the pressure chamber. At the site of excision, naturally occurring latex was wiped away with cotton swab, so that leaf water could be observed unobstructed. A modified dissecting microscope was used to observe leaf water at the site of excision on the petiole. The final round of sampling occurred on Day 26, and the experiment concluded. Shoots were cut at the root flare and individual leaves removed. Fresh leaves and shoots were weighed. Fresh leaves were passed ©2019 International Society of Arboriculture ), and stomatal conductance (mol through a leaf area meter (Licor LI-3100C Lincoln, Nebraska, U.S.A.) and after measurement were placed in a paper bag for drying. Final Sampling and Experiment Conclusion Soils were collected from the sample cores and placed in airtight plastic bags. These bags were taken to the Cornell Soil Health Lab and assessed for water hold- ing capacity, texture, and organic matter. Soils were stored at 4°C (39.2°F) before assessment. All soils were oven-dried at 105°C (221°F) for 24 hours and then were passed through an 8 mm sieve to remove coarse organic matter. A sample splitter was used to randomize and homogenize soil samples before being tested. Texture was determined using a rapid suspen- sion method (Moebius-Clune et al. 2016). Organic matter was assessed by loss on ignition method (LOI). Percent loss on ignition was converted to percentage organic matter and multiplied by a correction factor. The equation utilized was: % Organic Matter = (% Loss on ignition × 0.7) − 0.23 (Burt 2014; Moebius- Clune et al. 2016). To determine water release characteristics, sam- ples were placed in pressure plate cell rings 7 cm (2.7 in) in diameter in a Pressure Plate Extractor (Soil Moisture Equipment Corporation, Santa Barbara, California, U.S.A.). Gravimetric water content was determined at five negative pressures (-10 kPa, -30 kPa, -100 kPa, -300 kPa, -1500 kPa) after samples had reached equilibrium. Water potentials at each step of the water release curve were converted from gravimetric to volumetric water content by multiply- ing the value by bulk density . Available water hold- ing capacity was determined by calculating the difference between water held at -10 kpa and -1500 kpa on both a gravimetric and volumetric basis. Soil weight was calculated by subtracting con- tainer weight, screen weight, ring weight, fresh roots weight, fresh shoots weight, and fresh leaves weight from the total weight. Bulk density was calculated by soil dry weight and container volume. Root systems were washed to remove excess soil, air dried for one hour, inspected to determine if water droplets were present, and then weighed to calculate fresh weight. Soil containers, metal screens, and clamp rings were weighed individually. Roots, shoots, and leaves were placed in paper bags in an oven at 70°C (158°F) for 24 hours and then measured to obtain dry weights.
July 2019
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