Relationships between single tree canopy and grass net radiations
Introduction
Orchard is an assemblage of fruit trees whose leaf area does not evenly cover the ground surface. The discontinuous spatial distribution of trees with inter-plant spacing of about the height of the plants increases the difficulty in characterizing the net radiation regime of orchards and its use in energy balance studies. Measurements of tree canopy net radiation do not follow a standard experimental setup and they can be grouped into three classes of design: (i) above; (ii) above-and-below; and (iii) around the tree or hedgerow canopy.
The above canopy measurements group includes experiments with one, two or more sensors. Single sensor experiments has it positioned either well above the canopy row (about two times the plant height), to include the tree crown and the inter-row space in the view angle (Ben-Asher and Sammis, 1978, Trambouze et al., 1998), or immediately above the tree (Thorpe, 1978, Judd et al., 1986, Caspari et al., 1993, Azevedo et al., 2003, Green et al., 2003a, Green et al., 2003b, Pereira et al., 2003, Williams et al., 2004). Experiments with two (Testi et al., 2004) or four (Rana et al., 2005) sensors have them evenly positioned above the trees row and the inter-row.
The above-and-below measurements group can have two sensors above and two below sensing the row and the inter-row space, positions of maximum and minimum ground cover (Heilman et al., 1996, Villalobos et al., 2000), or only one sensor well above the row and four evenly distributed below the canopy (Daamen et al., 1999). Yunusa et al. (2004) combined measurements of all-wave radiation well above a vineyard row with shortwave radiation above and a series of three linear solarimeters below a vine.
The around-the-canopy group uses an array of eight net radiometers arranged at equally spaced fixed positions around an imaginary volume enclosing the tree foliage. Static linear sensors (0.9 m long) were used around apple trees in hedgerows (Thorpe, 1978), and windbreak trees (Smith et al., 1997). A dynamic system (named Whirligig) of eight regular sensors rotating in a circle around the tree was used for isolated trees (McNaughton et al., 1992, Green, 1993, Green and McNaughton, 1997, Pereira et al., 2001, Green et al., 2002, Green et al., 2003a, Green et al., 2003b).
Besides the operational difficulties with any measurement of tree net radiation the results represent only the sampled trees and rarely can be taken as representative of others with different canopy sizes (leaf areas) in the same orchard. Thorpe (1978) found that the around-the-canopy net radiation per unit leaf area represented about 50% of the net radiation of the single sensor well above the tree row. McNaughton et al. (1992) reported that daily integrated net radiation (A) absorbed by the 10.6 m2 of leaf area (LA) of a false acacia tree (Robinia pseudoacacia L.) was nearly equal to the net radiation absorbed by 8 m2 of grass (Rn). Thus, tree net radiation per unit leaf area was 0.75 (=8/10.6) Rn. Pereira et al. (2001) found that a citrus tree (Citrus latifolia Tan.) with LA = 40 m2 absorbed the equivalent of 12.66 m2 of grass surface, or A = 0.32 (±0.02) Rn on a leaf area basis. Inverting the Penman–Monteith equation, using the sap flow as inputs, Zhang et al. (1997) found that the net radiation per unit leaf area of poplar trees (Populus spp.) was equal to 0.484 (±0.046) of Rn measured 7 km away.
This paper reports the results of a study of around-the-tree canopy measurements of net radiation compared with net radiation of standard weather station to determine an empirical transfer function including the tree leaf area. The hypothesis that tree net radiation per unit leaf area is a unique function of grass net radiation was tested and accepted.
Section snippets
Material and methods
Four data sets are from experiments in New Zealand. One set is from a 3.5 m tall and isolated walnut tree (Jugans spp.) at Massey University Research Orchard near Palmerston North (40.2°S, 175.4°E), during 9 days of January 1992 (see Green, 1993). Tree leaf area (LA) was estimated as 26.4 m2 by measuring 10% of the total number of leaves.
Another data set is from a 4-year-old orchard olive tree (Olea europaea L. cv. Barnea) at the Ponder Estate (41.5°S, 173.9°E) in Marlborough between January and
Results and discussion
The results reported in this paper are based on 86 daily integrations of Whirligig measurements of tree net radiations (A, MJ tree−1 day−1) around five trees of four different species and canopy sizes, and the corresponding grass net radiations (Rn, MJ m−2 day−1), in five different locations. As indicated by McNaughton et al. (1992) it was possible to develop the simple linear relationship A = bRn. The small spread of the points around the regression line indicates the goodness of each fit (Fig. 1);
Conclusions
Considering the paucity of detailed measurements of tree net radiation there is a need for a transfer function to use the more readily available grass net radiation of standard weather stations. Results for five trees of four different species with canopy leaf area up to 40 m2, in five locations support the hypothesis that daily integrated tree net radiation per unit leaf area is a unique linear function of grass net radiation. This empirical approach compensates for not having to use any
Acknowledgement
A.R.P. and N.A.V.N. were partially supported by the Brazilian National Research Council.
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Comment on "Relationships between single tree canopy and grass net radiations"
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