Energy budgets in pine forest

Abstract Hourly energy budgets measured in Thetford Forest, Norfolk, are analysed for seven fine days in months from May to September. Values found for the Bowen ratio β ranging from near 1 to 4 or more, are used to show that the bulk physiological resistance r ST of the forest exhibits a consistent diurnal trend, from near 1·2 s cm −1 in the forenoon (once the trees are dry) to as much as 4 s cm −1 by late afternoon ‐ consistent with independent biological measurements (Robins, personal communication). In contrast, the forest's bulk aerodynamic resistance r generally lies between 0·05 and 0·10 s cm −1 . The ratio r ST/r alone, of order 20:1, is shown to imply that transpiration from the forest must occur at rates much less dependent, primarily, on net radiation, R n , than on ambient vapour pressure deficit (v.p.d.), (provided that the latter is not too small, i.e. ≮ 1 mb per 100 W m −2 of R n , say); and also that the evaporation of intercepted rainfall from the trees must occur at about 5 times the corresponding transpiration rate under the same meteorological conditions. In addition, because lighter winds during fine weather tend to favour larger v.p.d's the observed decrease in transpiration rate with increasing wind speed is much larger than that expected from the accompanying decrease in r alone. Other conclusions are (i) that the storage associated with a 1° C change in air temperature per hour, within the canopy, is 18 W m −2 , which is not always negligible, as it is for short vegetation; and (ii) that values derived for the dimensionless excess resistance parameter, B −1 , although remarkably small, 2 to 3 for u * ∼ 0·75 m s −1 , are commensurate with other bulk aerodynamic resistances in the system: accordingly, the easily‐derived surface resistance parameter, rs , (Monteith 1965; Thom 1972) provides an estimate of rST for the forest within 10 per cent for all β > 0.