Herbicide Tolerance in Relation to Growth and Stress in Conifers

Herbicide injury to five coniferous species was determined for the butoxyethanol ester of 2,4-D [(2,4-dichlorophenoxy)acetic acid], the isopropylamine salt of glyphosate [N-(phosphonomethyl)glycine], and the triethylamine salt of triclopyr {[(3,5,6-trichloro-2-pyridinyl) oxy] acetic acid} throughout the growing season of 1981. The relationship of herbicide tolerance to growth rate, water stress, and photosynthesis was determined for Jeffrey pine (Pinus jeffreyiiGrev. & Balif), sugar pine (P. lambertianaDougl.), red fir (Abies magnificaA. Murr.), white fir [A. concolor(Gord. & Glend) Lindl.], and Douglas fir [Pseudotsuga menziesii(Mirb.) Franco]. Although most of the species showed a high correlation of injury to leader or needle growth rate and xylem pressure potential, the seasonal trend in the degree of injury and the relationship of herbicide tolerance to various physiological factors were unique for each species. Criteria for determining herbicide application periods depended on species and herbicide.