Thermal Time

[Paul van Gardingen]

These comments have been forwarded in reply to the request by Carl-Otto 
Ottosen for information about thermal time:

================================


Hello.  Your message was passed on to me.  I think that
the term 'thermal time' is probably due to John Monteith
although the concept of accumulated temperature or heat
units goes back to Reaumur in about 1730.

Thermal time is the integral over time of temperature above 
a threshold (the base temperature).  In practice this is 
approximated with various degrees of error by accumulating 
either the mean 24h temperature measured in a 
meteorological screen (above the base temperature) or by 
taking account of the diurnal cycle.  It is perfectly 
possible for the mean daily temperature to be below the 
threshold but for part of the day to be above.

Thermal time should probably be thought of as an index of 
development since what is normally assessed is not the 
temperature of the meristematic regions.  

Base temperature is either assumed, on the basis of previous 
studies or is estimated for individual experiments by 
regressing rate of development against temperature.  
However, the data are often not unbiased.  I feel that it 
is better to use a constant base temperature for a species 
as it allows easier comparison between sites and years even 
though the confidence limits for individual dates of 
achievement of stages of development are widened.

Photothermal time takes some account of photoperiod by the 
statistical trick of adjusting the apparent base temperature.

Reference 
Wang, JY (1960) A critique of the heat unit approach to 
plant response studies.  Ecology 40, 785-790.

Hope this helps,
Graham Russell.

Dr G. Russell
University of Edinburgh
Institute of Ecology & Resource Management (Agriculture Building)
West Mains Road
Edinburgh EH9 3JG
SCOTLAND
International phone +44 131 535 4063  Fax +44 131 667 2601
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