Productivity of Closely-spaced Young Poplar on Agricultural Soils in Britain

Publisher Website
Google Scholar
Abstract
Recent ideas on ‘silage’ and ‘fuel’ forestry call for more information on the total harvestable woody dry matter produced by hardwoods grown at very close spacings in fertile soils and coppiced every few years. Yields of oven-dried stems and branches (S and B) are presented here for Populus trichocarpa Torr. and Gray, clone ‘Fritzi Pauley’. Plantings in Bedfordshire at 21 600 trees ha −1 had a mean annual increment (M.A.I. SB ) of 5.2 t ha −1 y −1 over five years, and plantings in the Cambridgeshire fens at 1480 trees ha −1 produced 4.8 t ha −1 y −1 over six years. Fan-shaped spacing experiments, established in Midlothian by inserting cuttings through black polythene into nursery soil with added fertilizers, gave 4.6 t ha −1 y −1 at the end of the first year and about 7 t ha −1 y −1 one year after coppicing, but only with over 250 000 stems ha −1 producing closed canopies with leaf area indices of about 4. Similar spacing experiments planted without fertilizer on farmland in Gloucestershire, Suffolk, Argyll and Midlothian gave average M.A.I. SB values of 6.5–7.0 t ha −1 y −1 after three years with over 25 000 trees ha −1 and similar values after five years with over 10 000 trees ha −1 . Peak current annual increments (C.A.I. SB ) averaged 10–12 t ha −1 y −1 . The maximum M.A.I. SB , attained in Gloucestershire, was 10.0 t ha −1 y −1 at age 5 with over 20 000 trees ha −1 , with maximum C.A.I. SB values of about 14 t ha −1 y −1 at age 4; M.A.I. SB values of about 11.5 t ha −1 y −1 were anticipated at this site by age 6–8. Equivalent stem volumes are given. As expected, trees subjected to competition accumulated greater proportions of their woody biomass in stems rather than branches. Biomass yields of fully-stocked young hardwood stands are independent of planting density. In Britain, M.A.I. SB values of 6–8 t ha −1 y −1 can be obtained over 1 or 25 years by planting 250 000 or 2000 trees ha −1 , using vigorous Populus spp, Salix spp or Nothofagus procera on good sites. Advantages and problems of ‘silage’ forestry are discussed, and it is considered that hardwood fuel coppices could not meet more than about 2% of national energy needs. The reciprocals of individual tree weights were linearly related to planting density.