Eastern cottonwood clonal mixing study: intergenotypic competition effects

Intergenotypic competition of seven clones of eastern cottonwood (Populus d eltoides) was evaluated in a replacement series experiment. A partial diall el competition design was used to choose pairs (binary sets) of clones for plot type treatments. Two separate treatments were established for each pai r of clones, namely (1) 75% clone A: 25% clone B and (2) 25% clone A: 75% c lone B. Twenty-one treatments were established in the study: seven pure clo ne treatments and 14 mixed treatments (seven pairs of clones each at two ra tios). Two study sites (Vicksburg, Mississippi and Wickliffe, Kentucky) wer e used. Results are presented for stand ages two, three, and four years whi ch corresponds to the lower to mid-length rotation for the species for a sh ort rotation woody crop (either biomass for energy or fiber for pulp and pa per). Average plot height at an age of 4 years was 13.23 m. Plot total yiel d was affected by intergenotypic competition. The type and level of respons e to mixing clones depended on the specific clones involved and the plantin g site. Usually, the most predictable opportunity for over- or underyieldin g when in binary mixture occurred for clones which differed substantially i n pure plot growth and yield. The yield of mixtures of clones of more simil ar 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 2 7% of the mixture over the best clone's pure plot yield. (C) 1998 Elsevier Science B.V. All rights reserved.