Arboriculture & Urban Forestry 47(3): May 2021 MATERIALS AND METHODS Experimental Set-Up Four-year-old, container-cultivated pine trees (Pinus sylvestris L.) were planted in containers and placed in a greenhouse to provide protection from rainfall and to allow measurements within the whole range between saturated and dry soil. Trees were 40 cm to 60 cm tall, with shoots measuring about 15 cm to 20 cm, and terminal shoots of 20 cm to 25 cm. They had been cultivated in 1.2-L containers filled with a mixture of wood fibers, compost, peat, sand, and lime- stone. Evergreen conifers were chosen to avoid effects of leaf senescence when continuing the experiment until autumn. The first treatment, hereafter referred to as the “loam treatment,” was run from 22 August 2018 to 28 September 2018. Sixteen trees were planted separately into 7.5-L containers using sandy loam as substrate. 111 Two soil water potential sensors were installed in each container in contact with and at the same height as the root ball (Figure 1). The sensors were placed outside the small root balls (approximately 10 cm in diameter) to prevent damages during installation. Between 02 April 2019 and 15 May 2019, the experiment was repeated using Vulkatree 0/32 mm (VulkaTec GmbH, Kretz, Germany) as substrate, a lava-based, load-bearing, compaction-resistant soil which conforms to the German FLL-guidelines (FLL 2010). One sensor was installed 2 cm apart from the root ball, while the second sensor was placed in con- tact with the root balls as in the first treatment. The intention of the different placements was to detect possible relationships between sensor location and variability of sensor readings. To prevent the sensor placed further away from being too close to the con- tainer wall, 13-L containers were used. This second treatment is referred to as the “lava treatment.” Measurement of Soil Water Status Soil water potential (ψS ) was measured with granular Figure 1. Experimental set-up. In the loam treatment, both sen- sors were placed like sensor 1 in the diagram, i.e., in contact with the root ball. In the lava treatment, sensor 1 was installed as in the loam treatment, and sensor 2 was placed 2 cm away from the root ball. matrix sensors (Watermark Model 200SS, Irrometer, Riverside, CA, USA). They consist of an encapsulated granular matrix with 2 embedded electrodes. The enclosing porous membrane allows the moisture of the surrounding soil to equilibrate with the moisture of the granular matrix inside the sensor. The electric conductivity of the granular matrix correlates with its moisture content. By applying a source of electricity to the sensor’s electrodes, the electric conductivity can be measured. In combination with the separately measured soil temperature and empirically developed equations, the soil water potential can be calculated (Shock et al. 1998; Irmak et al. 2016). The sensors had been conditioned (3 cycles of watering overnight after completely drying) and were installed in a fully wet state. A hole with a smaller diameter than that of the sensor was placed into the soil, and the sensor was pushed into it. According to Irmak et al. (2016), no slurry was used for sensor installation to prevent the sensors from losing contact with the surrounding soil because of shrinking slurry. Soil water potential sen- sors were read with a handheld Soil Moisture Meter (Irrometer, Riverside, CA, USA, range of measure- ment 0 kPa to −199 kPa) at predawn right after taking the needle samples for measuring the predawn water potential. ©2021 International Society of Arboriculture
May 2021
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