Growing degree-day

Growing degree-day (GDD) is a heat unit, expressed in units of °C-day. GDD aims to describe the heat energy received by a given plant over a given time period. It is often argued that the calculation of GDD in plant phenology and development science has greatly improved the description and prediction of phenological events compared to approaches such as number of days or time of year, for instance (Richardson et al.2006). GDD is usually calculated by the following equation (McMaster and Wilhelm 1997):

  is the daily maximum air temperature (°C)
 is the daily minimum air temperature (°C)
 is the temperature below which the process of interest does not progress (°C).
In some cases, for simplification, is set equal to daily average temperature. T base varies among plant species and it is also likely to be dependent upon the growth stage being considered. Similarly the heat accumulation can also be measured at hourly frequency to generate growing degree hours (GDH). GDD and GDH can also be summed cumulatively over time as a measure of accumulated heat received by plants (Chuine 2000; Richardson et al. 2006).

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