Paul van Gardingen
p.vangardingen at ed.ac.uk
Fri Dec 1 14:16:26 EST 1995
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.
Wang, JY (1960) A critique of the heat unit approach to
plant response studies. Ecology 40, 785-790.
Hope this helps,
Dr G. Russell
University of Edinburgh
Institute of Ecology & Resource Management (Agriculture Building)
West Mains Road
Edinburgh EH9 3JG
International phone +44 131 535 4063 Fax +44 131 667 2601
UK phone 031 535 4063 Fax 031 667 2601
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