Growing Degree-Hours at Selected Oregon Locations: A Complementary View of Temperature

R. D. RuDD and D. C. Newland Oregon State University Growing Degree-Hours at Selected Oregon Locations A Complementary View of Temperature Men have wrestled with the quantitative expression of heat requirements of growing plants at least since the early 1700's when Rene de Reamur proposed his thermal constant of phenology. A wholly satisfactory achievement of such an expression continues to elude its pursuers because of the changing relationships between temperature and the maturing plant and of the interrelationships between plant growth and other environmental parameters.1 As a result , such expressions as have been devised (heat units, growing degree-days, etc.) enjoy only qualified acceptance, although practical considerations encourage their continued use.2 The employment of the heat unit as an expression of the temperature element of climate—as an indicator of heat experience—affords an intriguing alternate use of this concept. To be sure, annual curves of average growing degree-days or degree-hours would seem to offer little more than do curves of average temperatures; however, there are advantages to discrete quantitative units that may be summed for variable periods. Assuming arbitrary threshold values, locations may be contrasted in terms of cumulative annual or seasonal totals, cumulative heat experience by selected dates, rates of accumulation, and the like. In addition, despite the problems at1 For an excellent summary of the problem, see Jen Yu Wang, "A Critique of die Heat Unit Approach to Plant Response Studies." Ecology (41): 785-790. (October 1960.) 2 Bernard E. Detiiier and Morril T. Vittum. "Growing Degree-Days." New York State Agricultural Experiment Station Bulletin 801, Geneva, 1963. 169 170ASSOCIATION OF PACIFIC COAST GEOGRAPHERS tending the definition of threshold, growing degree-hour figures imply more about the potential for plant growth in an area than standard temperature data. The purposes of this paper are to employ the growing degree-hour concept in an effort to express a complementary view of temperature at several locations in Oregon and, thereby, to make more clear the extent and character of climatic variation within the state. Scope and Method Four stations were selected to illustrate temperature contrasts in Oregon in this fashion. They are Astoria on the coast, Salem in the Willamette Valley, Medford in the southwestern valleys, and Pendleton in the northeast. Daily temperature normals published by the Weather Bureau are available for each.3 The method here employed for determining growing degree-hours, or heat units, was devised by Lindsey and Newman.4 It differs chiefly from the conventional procedure in employing the daily maximum and minimum temperatures rather than the daily mean. According to Lindsey and Newman, use of the mean yields true degree-hours only when the threshold does not exceed the minimum. When the daily mean and the threshold temperature are the same, subtraction of the threshold from the mean in the calculation of heat units yields a value of zero; yet the temperature is above the threshold for a substantial part of the day. Even in the instance in which the threshold lies between the mean and the minimum temperature, calculation employing the daily mean is believed to yield values too small. The method here used for obtaining heat unit values is summarized by the following two formulae: ( 1 ) 24 ( min — threshold ) + 24 ( max — min ) 2 used when the threshold is below or equal to the minimum for the day, and (2) 12 (max -threshold)2 max - mm 3 Data in this paper are for the 1921-1950 period and are from U. S. Weather Bureau, "Daily temperature, degree day and precipitation normals: Selected Stations in Oregon." Washington, D. C, United States Government Printing Office, 1953. 4 Alton A. Lindsey and James E. Newman. "Use of official weaüier data in spring time-temperature analysis of an Indiana phenological record." Ecology (37): 812-823. (October 1956.) VOLUME 28 YEARBOOK 1966 171 used when the threshold exceeds the minimum. Although formula 2 will yield a positive value when the threshold exceeds the maximum temperature, the definition of threshold should rule out such interpretation . Daily values thus derived are the basis for the graphs in this paper. Two temperatures were arbitrarily selected as thresholds, 40° F and 50° F. These two...