Use of degree-days in multiple-temperature experiments

Abstract

This research compared results from germination and growth when the experiment duration was chronologically set or based on degree-days. Seeds of smooth brome (Bromus inermis Leyss.), plains rough fescue (Festuca altaica Trin. subsp. hallii (Vasey) Harms), prairie coneflower (Ratibida columnifera (Nutt.) Woot. and Standl.), and silver sagebrush (Artemisia cana Pursh.) were germinated at 5, 10, 15, 20, and 25 degrees C for 28 days or 400 degree-days (Base temperature = 0 degrees C). Root and shoot weights of seedlings of these species were compared at 5, 10, 15, 20, 25 and 30 degrees C after growing them 20 days or 200 degree-days. With the exception of prairie coneflower, optimal temperatures for germination were 2 to 4 degrees C lower when incubated 400 degree-days compared to 28 days. Total germination for prairie coneflower was not significantly different (P = 0.454) at 28 days or 400 degree-days. Interacting effects of the duration of experiments and temperature significantly (P less than or equal to 0.001) influenced root and shoot weight of all species. Except for shoot weight of smooth brome, predicted optimum temperatures for root and shoot growth were 7 to 21 degrees C lower at 200 degree-days than 20 days. These experiments illustrate that results from germination and growth studies can vary substantially depending on whether chronological time or degree-days are used as the end point. Thus, ecological interpretations or management recommendations can be quite different. Degree-days may be more meaningful than chronological units for germination and growth studies because they integrate time and temperature. The use of degree-days as an end point for experiments rather than chronological time deserves further consideration by researchers.