Yield improvements through modification of planting density and harvest frequency in short rotation coppice Salix spp. - 1. Yield response in two morphologically diverse varieties

An experiment comparing the dry matter yield of intensively managed short rotation coppice (SRC) under factorial combinations of two plant varieties (S. viminalis cv. Jorunn and S. x dasyclados), five planting densities (10,000-111,000 plants ha(-1)) and two harvesting frequencies is reported. Data are presented from the first biennial and triennial harvest cycles at two sites (East Anglia and Warwickshire, UK) planted in spring 1996 and flailed in autumn 1996. Higher annual yields were attainable by more intensive packing of plants combined with more frequent harvesting. With S. viminalis cv. Jorunn, yield increased by 34% between: the lowest and highest planting densities. Biennial harvesting increased yield compared with triennial harvests. S. viminalis, on average, yielded 2.7 t ha(-1) yr(-1) more than S. x dasyclados, with peak yields of 11.4 t ha(-1) yr(-1). Significant differences in yield between sites were noted, with an average of 1 t ha(-1) yr(-1) benefit on a mineral soil compared with a peaty loam. There were no statistically significant interactions in the data. Biennial plots reached peak heights of 254-297 cm (S. viminalis) and 239-273 cm (S. x dasyclados) in 1998. Peak heights of triennial plots (achieved in 1999) were 401-515 cm. (S. viminalis) and 316-420 cm (S. x dasyclados). There was a gradual increase in height through the three years and a non-significant trend towards decreased height with increasing planting density at the ends of the growing seasons. Both varieties demonstrated phenotypic plasticity, with individual plant weight and stem number decreasing as a function of increasing density. A range of parameters including stem width reduction, stem mortality and plant mortality were seen to vary. The population of primary stems was found to increase up to the time of canopy closure, on a per plant basis and per:unit ground area basis. After this time, competition for light resulted in self-thinning of stems through until final harvest for S. dasyclados but not S. viminalis. Stem numbers per plant, recorded after leaf fall, showed different responses between both harvesting frequency and variety. With S. viminalis, stem numbers remained constant after cut back for up to 2 yr, but increased rapidly,as a result of a harvest after 2 yr. With S. x dasyclados, a similar regrowth of stems was observed after a 2-yr harvest. Prior to the harvest, stem numbers per plant decreased steadily for 2 of the 3 yr depending on harvest cycle. After 3 yr stem populations with S. x dasyclados were also decreasing. The degree to which non-destructive measurements could be used to (a) determine annual increment and (b) The used to predict eventual yield was examined. Cylindrical volume was the only growth measurement that enabled a reasonable fit between crop morphology and final yield to be made. The relationship was found to be a good fit and the fit improved with a longer harvest interval. Regression equations were not significantly different between sample sites but were significantly different between both variety (P < 0.001) and harvest interval (P < 0.05). In summary, we deduce that modem varieties of S. viminalis such as Jorunn, are more suited to higher planting density and intensive harvesting, due to more erect growth reducing intra-specific competition at high planting densities. (C) 2002 Elsevier Science Ltd. All rights reserved.