Nelson, Neil DBuchman, DanielCai, MeijunMcMahon, Bernard GBerguson, William E2022-02-282022-02-282022-02https://hdl.handle.net/11299/226513The original version of this report (uploaded February 28, 2022) contained incorrect R sq values in a few places; these errors were corrected in a revised version (uploaded March 9, 2022). The revised version of this report (uploaded March 9, 2022) contained a numerical error in the abstract, where "13" should have been "14"; this error has been corrected in this new revised version (uploaded May 3, 2022).The effects of inter-tree competition on growth in family field trials (FFT), clone trials (CT) and yield blocks (YB) were studied in NRRI experimental field plots in Minnesota, USA. FFT and CT competition is inter-clonal , YB competition is intra-clonal. Two approaches were explored: (1) regression analysis of growth of individual trees versus growth of immediate neighboring trees in FFT and CT; (2) bole volume growth as measured by DBH2 in CT versus YB for the same clones on each site to determine whether inter- clonal competition in CT overestimates tree growth in YB. In CT on five sites planted the same year with the same population of clones (“simultaneous CT”), significant negative slopes, indicating the onset of inter-tree competition, occurred in the fifth and sixth years for the two fastest-growing CT. The top 50th growth percentile clone group in the fastest-growing of the simultaneous CT had a significant negative regression line slope; the lower 50th group did not. The three slower growing CT did not exhibit competition (significant negative slopes) from three through six years. A separate clone trial measured through 9 years showed little evidence of inter-tree competition. The regression slopes in FFT were almost all positive, indicating no inter-tree competition effects from three through ten years of stand age. All significant regression R2 values were low–a maximum of 24 % for CT, 22 % for FFT. Clonal genetic potential for growth likely predominates prior to significant inter-clonal competition, suggesting that randomization of single-tree replications of each clone within each block is effective in evaluating clone genetic growth potential within the initial six years selection window that we have used in our program. There was no significant difference between CT and YB for tree bole volume growth (yield) in a population of clones. There was wide variation in the YB/CT yield ratios between individual clones on a site. Some individual clones exhibited wide variation in YB/CT ratios between different sites, indicating a clone x site interaction for this trait. The commercial clone NM6, used as a check clone in most of our studies, had the widest variation of any clone in YB/CT ratios between sites, ranging from 53 % to 104 %. Of the 22 YB/CT yield ratios for specific clones on 14 sites, only four were above 100 %, indicating a clear trend for CT overestimating yields in YB. The average of the 22 YB/CT ratios was 86 %, again indicating overestimation of YB yield in the CT. The YB/CT ratio for NM6 averaged 79 %, while five elite (fast growing, disease resistant) clones averaged a YB/CT yield ratio of 89 % over the 14 sites. CT/YB yield ratios are too variable to use CT growth as an estimate of growth for specific clones under near commercial conditions (YB).enNatural Resources Research InstituteUniversity of Minnesota Duluthhybrid poplarclonespoplar breedingpoplar geneticspoplar field testingTechnical Report