Arboriculture & Urban Forestry 33(3): May 2007 199 scald, which often results in low temperature-induced cracks on the southwest side of the trunk (Sakai 1966). A higher south side temperature was observed in December and Feb- ruary, but not in January, because we had a very mild January in 2002. This higher peak on the south side also was observed in September and October as well as in December. When approaching budbreak, bud water content was sig- nificantly higher than stem water content (Figure 6). The stem water content and budbreak have a direct relationship; however, root zone temperature and soil moisture also con- tribute to earlier budbreak, with the exception of genetically controlled species. These TF observations and TFF determi- nations may be useful for a better understanding of how a tree can sense its surrounding environment and how temperatures fluctuate in and around a tree. This data are also useful for freeze protection as well as horticultural and arboricultural practices in urban environments. More research is needed for understanding the interaction of temperature and other envi- ronmental factors such as soil moisture, soil structure, and availability of oxygen in the root zone. Regarding mulch depth, more research is warranted for better landscape man- agement and healthier trees in urban environments. Having higher TF than expected in mulch prompted the reinvestigation of TF with regard to mulch depth. The TF of mulch at different depths is shown in Figure 7. TF varies depending on time of year. In October, 15 cm (6 in) mulch was several degrees warmer (P < 0.05) than ground, 7.5 cm (3 in) mulch, and 30 cm (12 in) mulch. However, in Decem- ber and February, 30 cm (12 in) mulch was significantly warmer (P < 0.05) (Figure 7). The microbial activity in the mulch may contribute to the higher temperature. Borges and Chaney (1989) indicated that the development and efficiency of mycorrhizal fungi is higher when soil temperature is be- tween 8°C (46.4°F) and 27°C (80.6°F). Our results indicated that the higher the mulch depth, the warmer the temperature. However, overmulching may cause problems in urban land- scapes such as adventitious root formation at the base of the trees, which may result in root girdling. LITERATURE CITED Borges, R.G., and W.R. Chaney. 1989. Root temperature af- fects mycorrhizal efficacy in Fraxinus pennsylvanica Marsh. The New Phytologist 112:411–417. Fitter, A.H., and R.K.M. Hay. 1987. The temperature fluc- tuations of plants, p. 187. In Environmental Physiology of Plants. 2nd Edition. Academic Press, New York, NY. Figure 7. Temperature fluctuation in different mulch depths in four Acer saccharum (sugar maple) during Oc- tober and December 2004 and February 2005. Mean separation by Duncan’s multiple range test (P< 0.05). Data are means of ≈3,000 observations. Huttunen, L., and J. Soveri. 1993. The regional and temporal variation of frost in natural conditions in Finland. Public. Nat. Board of Watersand the Env. Series A 139:1–74. In Finnish with English summary. Kozlowski, T.T., and S.G. Pallardy. 1997. Environmental regulation of vegetative growth, pp. 195–322. In Growth Contol in Woody Plants. Academic Press, San Diego, CA. ©2007 International Society of Arboriculture
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