198 Shirazi and Vogel: TF in Fraxinus pennsylvanica var. subintegerrima indication that soil is warming up with a steady increase in soil temperature ≈10°C(≈50°F) to 15°C (59°F) during the spring. Peak of the soil temperatures are reached at ≈22°C (≈71.6°F) in mid-August and gradually decreases monthly to 20°C (68°F) in September, 17°C (62.6°F) in October, 12°C (53.6°F) in November, and 4°C (39.2°F) by December. The optimum temperature for root growth depends on species, genotype, stage of development, and availability of oxygen and water. The minimum temperature for root growth for temperate zone woody plants is between 0°C (32°F) and 5°C (41°F), whereas the optimum temperature is between 20°C (68°F) and 25°C (77°F). Lyr and Garbe (1995) reported the occurrence of lower root temperature optima in species of northern origin compared with species of southern origin. The impact of soil temperature on root growth is affected by soil moisture. Teskey and Hinckley (1981) demonstrated in white oak in Missouri that root elongation is affected by soil temperature below 17°C (62.6°F); however, above 17°C (62.6°F), it is soil moisture that is the dominant factor. Based on our results, September to November are the most appro- priate times for planting trees in northern latitudes because the temperature of the root zone makes an ideal situation for root growth. TF for January and mid-February are shown in Figure 5. On 15 February 2003, which was the coldest day recorded, the soil 30 cm (12 in) temperature reached approxi- mately −1°C (30.2°F), whereas sod and soil surface were ≈−2°C(≈28.4°F). Mulch and base temperature range was −1°C (30.2°F) to −5°C (23°F) and −2.5°C (27.5°F) to −7.5°C (18.5°F), respectively. Root core temperature was ≈−1°C (≈30.2°F), the trunk temperature was −2.5°C (27.5°F) to −3.5°C (25.7°F), whereas the canopy TF was −2.5°C (27.5°F) to −7.5°C (18.5°F). The south HOBO Pro tempera- ture sensor TF range was between −0.5°C (31.1°F) and −7.5°C (18.5°F) from midday to midnight. The fact that soil temperature does not fluctuate and that there is a lack of cold freezing temperatures in the root zone when compared with aboveground and canopy temperatures explains the lack of cold hardiness induction in roots. Overall, the underground organs are more sensitive to frost than the shoot organs. Root tip low temperature tolerance is between −1°C (30.2°F) and −3°C (26.6°F) (Sakai and Larcher 1987). Depending on the stage of development and species, the permanent roots freeze at −5°C (23°F) to −20°C(−4°F) (Sakai and Larcher 1987). In this study, the minimum root core temperature recorded was nearly −1°C (30.2°F). The location of the sensor in the root core was under mulch, making it less likely that the fine roots in the root zone would be affected by frost. Exposure of the roots to cold temperatures resulted in double the cold hardi- ness in both roots and shoots in Lonicera tatarica, Coto- neaster horizontalis, and Euonymus europaeus (Pellet 1971). However, this effect was not achieved in Ligustrum obtusi- folium. Root cold hardiness is species-specific as a result of different seasons of growth in root and shoot (Sakai and Larcher 1987). Repo et al. (2005), using simulated winter to summer conditions, reported that 2 weeks of delayed soil thawing caused death in the saplings of Pinus sylvestris L., whereas no delay or a short delay caused only minor damage and reversible recovery. There were significant temperature differences depending on time of day between the south and west sides compared with the north and east sides. These higher temperatures, during the winter months, have been associated with sun- Figure 5. Daily temperature fluctuation (TF) in Fraxinus pennsylvanica var. subintegerrima (red ash) tree and its surrounding environment for the 15th day of each month from January through mid-February 2003. The first graph for each month shows TF in soil surface, sod, mulch, base, and soil 30 cm. The second graph for each month shows TF in root, trunk, and canopy. The third graph for each month shows TF in north, south, east, west, and canopy. ©2007 International Society of Arboriculture Figure 6. Percent water content in stems and buds of Fraxinus pennsylvanica var. subintegerrima (red ash) tree. Arrow indicates date of budbreak. Curves were fit- ted (polynomial third order) through weekly observation from October 2002 to May 2003.
May 2007
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