Intra-annual growth and mortality of four Populus clones in pure and mixed plantings

Intra-annual growth rates were assessed during 3 years for four Populus clones grown in pure and mixed clonal stands at square spacings of 0.5, 1.0 and 1.5 m in western Washington, USA. Height growth rate peaked in August, except at the 0.5-m spacing where it peaked in July and June in years 2 and 3, respectively. Diameter growth rate generally peaked in June. Seasonal growth patterns in mixed clonal stands, relative to those in pure clonal stands, varied by clone. Following early season moisture stress in year 2, growth of a P. balsamifera ssp. trichocarpa clone declined significantly relative to three P. balsamifera ssp. trichocarpa x P. deltoides clones; this trend was most evident at the 0.5-m spacing. During this period of moisture stress, P. balsamifera ssp. trichocarpa had 11 and 36% mortality in pure and mixed stands, respectively, at the 0.5-m spacing; this clone had no mortality at wider spacings during the same period. Although height rankings among clones shifted during the first growing season, rankings at the end of year 1 were very similar to those at the end of year 3.