Relationships between single tree canopy and grass net radiations

It is reported a simple approach to transform daily values of grass net (all-wave) radiation (Rn, MJ m−2 day−1), as measured over standard grass surface at meteorological stations, into whole tree canopy net radiation (A, MJ tree−1 day−1). The revolving Whirligig device [McNaughton, K.G., Green, S.R., Black, T.A., Tynam, B.R., Edwards, W.R.N., 1992. Direct measurement of net radiation and photosynthetically active radiation absorbed by a single tree. Agric. For. Meteorol. 62, 87–107] describing a sphere about the tree measured A in five trees of different species (walnut, dwarf apple, normal apple, olives and citrus), with leaf area LA varying from 8.65 to 40 m2. For each tree, A and Rn were linearly related (A = bRn), as previously reported elsewhere, but it was found that the slope of such regression was also a linear function of LA or, b = 0.303 (±0.032) LA. Consequently, the hypothesis that total daily tree canopy net radiation per unit leaf area is linearly related to grass net radiation could not be rejected after 86 days of measurements in five locations, and the empirical relationship is A = 0.303 (±0.032) RnLA (R2 = 0.9306).