Radiation Interception and Productivity of Willow

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Abstract
(1) Stands of Salix viminalis L. (clone Bowles hybrid), growing near Edinburgh [Scotland], were studied in two successive years. In 1984, the stands grew from coppiced stumps at 0.5 m spacing in the field. In 1985, the stands grew from cuttings in containers of compost, at 0.3 m spacing, supplied wtih trickle irrigation and nutrients. (2) Interception of total solar radiation by the canopies was measured using tube solarimeters. Regular harvests were made to determine above-ground dry matter in 1984 and both above-ground and root dry matter in 1985. Leaf area index (L) was also determined, and total stem surface area was estimated in 1985. (3) Accumulated dry matter was linearly related to accumulated intercepted radiation unit heavy leaf fall. The slopes of the relationships for above-ground dry matter were 0.99 g MJ-1 in 1984 and 1.38 g MJ-1 in 1985. The difference in light-use efficiency (rather than a difference in amounts of light intercepted) was primarily responsible for a difference between years in above-ground dry matter production (11 t ha-1y-1 in 1984 and 14 t ha-1y-1 in 1985). Greater efficiency in 1985 was attributed to better water and nutrient supplies and lower levels of incoming radiation. (4) L reached 2.4 in 1984 and 4.5 in 1985. The relationship between L and fractional interceptance (f) was: f = 0.95-1.07 exp(-0.65 L). Extinction coefficients increased from about 0.4 in June to 0.6-0.7 in September. (5) Stems alone intercepted 60% of total solar radiation after leaf fall in 1984 and 47% in 1985 when the ''stem area index'' reached 1.5. (6) Leaf-area development in spring was a function of thermal time, with an earlier starting time for the coppiced stand in 1984, resulting in about 20% greater potential light interception (1240 MJ m-2y-1). (7) A simple model, which simulated canopy development and the interception of 1240 MJ m-1y-1 of total radiation by coppiced willow growing near Edinburgh, suggested that 2014 MJ m-1y-1 might be intercepted at Long Ashton near Bristol. Light interception, and hence dry matter production, was relatively insensitive to an increase in L above 4.5, or to delayed leaf fall, but was increased greatly by earlier canopy development.

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