194 Shirazi and Vogel: TF in Fraxinus pennsylvanica var. subintegerrima Huttunen and Soveri (1993) reported that soil freezes to a mean annual depth of 15 (6 in) to 150 cm (60 in) depending on the location, soil moisture, soil texture, air temperature, and snow depth (Sakai 1970; Solantie 2000). The same fac- tors also determine the time of soil thaw (Solantie 2003). Root zone temperature is dependent on soil depth and prop- erties such as color, bulk density, moisture, texture, and type of vegetative cover. Extensive study on TF in wintering trees was first reported by Sakai in 1966. He found that the bark temperature on the south side of a tree increased sharply at noon to ≈21°C(≈69.8°F), whereas the temperature on the north side stayed nearly constant ≈−1°C(≈30.2°F) to −2°C (28.4°F) in Kalopanaz septemlobus koidz in Sapporo, Japan. He also reported that TF in the center of a stem is dependent on stem diameter. In the center of a tree (Ulmus davidiana planch var. japonica nakai) with a diameter of 86 cm (34.4 in), the temperature remained nearly constant throughout the day, from −0.5°C (31.1°F) to −2°C (28.4°F) in the winter. With advanced computerized sensors, we monitored TF at 14 locations in an 18-year-old Fraxinus pennsylvanica var. sub- integerrima located at The Morton Arboretum, Lisle, Illinois, U.S. The objective of this study was to understand more about how trees sense their surrounding environment and how temperatures fluctuate within a tree. This information gives a greater understanding of the role temperature plays in tree physiology as well as improving horticultural and arbo- riculture practices in urban environments. MATERIALS AND METHODS HOBO Pro (Onset Corp., Bourne, MA) temperature sensors were placed at 14 different locations: east, west, north, south (1.37 m [4.5 ft] above the soil), trunk (10 cm [4 in] inside the trunk of the tree, 1.41 m [4.7 ft] above the soil layer), root (1.25 cm [0.5 in] inside the root, 30 cm [12 in] under the soil, 20 cm [8 in] from the tree), canopy (7.5 m [24.8 ft] inside the tree canopy), soil surface (under the mulch, approximately 15 cm [6 in]), mulch (7.5 cm [3 in] inside the mulch layer), base (approximately 15 cm [6 in] above mulch layer on the west side), soil 30 cm (12 in) (30 cm [12 in] into the soil under mulch), ground (placed 45 cm [18 in] aboveground, 1.8 m [5.9 ft] from the trunk), sod (placed just under the sod 1.8 m [5.9 ft] from the trunk), and snow (2.5 cm [1 in] above the mulch at trunk flair on the south side). All locations were positioned by using a handheld GPS receiver (eTrex Summit; Garmin, Olathe, KS) (Figure 1; Table 1). The temperature was recorded every 15 min from December 2001 through February 2003. Data were downloaded with an Onset HOBO Shuttle at the site and transferred to a computer in the labo- ratory (The Morton Arboretum, Lisle, Illinois, U.S.). The water content of the stem and buds was measured by weighing the fresh tissues (FW) and placing them in a con- ventional oven at 75°C (167°F) for 48 hr. Samples were then weighed again for dry weight (DW). Percent water content ©2007 International Society of Arboriculture Figure 1. Location of the HOBO Pro temperature sensors on the Fraxinus pennsylvanica var. subintegerrima (red ash) tree and its surrounding environment. was calculated by subtracting the DW from FW, dividing by FW, and multiplying by 100 (percent water content((FW- DW)/FW) × 100). Three samples were taken weekly from October 2002 through May 2003. For the mulch study, mixed wood mulch was placed around four Acer saccharum (sugar maple) 60 cm (24 in) from the trunk and HOBO Pro temperature sensors were placed approximately 20 cm (8 in) from the trunk and 10 cm (4 in) below the surface of the mulch layer. Mulch depths were 7.5, 15, and 30 cm (3, 6, and 12 in) and a bare ground area served as the control. A stake secured the HOBO Pro temperature sensors for all locations. Statistical analysis was performed using SAS software 2003 (SAS Institute Inc, Cary, NC). Analysis of variance for data were performed using time interval as the replication. RESULTS AND DISCUSSION There was a significant difference (P < 0.05) in TF among locations and years. Total mean temperature observation for
May 2007
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