Responses of Douglas-Fir to Elevated Atmospheric Fluorides

The response of Pseudotsuga menziesii (Mirb.) Franco to fluorides was studied near a phosphate reduction plant around which some 200 acres of timber had been killed. Study trees selected at increasing distances from the operation were placed into groups according to the average fluoridecontent of the leaves. Radial growth, needle length, and dry weight of trees in the various groups were compared before and during operations. Radial growth of trees with fluoride concentration under 50 ppm (the control) was greatest during the period the phosphate plant was operating. Whereconcentrations were higher radial growth was least during operations and significantly less than in the control even in the absence of leaf necrosis. Normal growth resumed following closure of the phosphate plant. Needle growth followed an opposite pattern. Needle length was positively correlatedwith fluoride levels and negatively correlated with radial growth. Stimulation of needle elongation appeared to be the earliest response of needles to fluorides. Growth suppression was evident at higher concentrations. Needle burning and mortality developed when fluoride concentration of theleaves exceeded several hundred ppm.