Eastern cottonwood clonal mixing study: intergenotypic competition effects

Intergenotypic competition of seven clones of eastern cottonwood (Populus deltoides) was evaluated in a replacement series experiment. A partial diallel competition design was used to choose pairs (binary sets) of clones for plot type treatments. Two separate treatments were established for each pair of clones, namely (1) 75% clone A: 25% clone B and (2) 25% clone A: 75% clone B. Twenty-one treatments were established in the study: seven pure clone treatments and 14 mixed treatments (seven pairs of clones each at two ratios). Two study sites (Vicksburg, Mississippi and Wickliffe, Kentucky) were used. Results are presented for stand ages two, three, and four years which corresponds to the lower to mid-length rotation for the species for a short rotation woody crop (either biomass for energy or fiber for pulp and paper). Average plot height at an age of 4 years was 13.23 m. Plot total yield was affected by intergenotypic competition. The type and level of response to mixing clones depended on the specific clones involved and the planting site. Usually, the most predictable opportunity for over- or underyielding when in binary mixture occurred for clones which differed substantially in pure plot growth and yield. The yield of mixtures of clones of more similar growth patterns sometimes differed significantly from that expected from an additive model, but this was less common. When significant differences did occur between pure clone yields and the yields of their binary mixtures, the plot yield was often a linear function of the proportion of the best clone. Overyielding of mixtures occurred, with additional yields of up to 27% of the mixture over the best clone's pure plot yield. (C) 1998 Elsevier Science B.V. All rights reserved.