Effect of Physiological Status and Growth of Ponderosa Pine (Pinus ponderosa) and Greenleaf Manzanita (Arctostaphylos patula) on Herbicide Selectivity

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Abstract
Correlations between herbicide damage and several physiological factors were examined in the field for ponderosa pine (Pinus ponderosaDougl. ex P&C Lawson) and greenleaf manzanita (Arctostaphylos patulaGreene). Pine injury caused by 2,4-D [(2,4-dichlorophenoxy) acetic acid], glyphosate [N-(phosphonomethyl) glycine], or triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid]} was compared to leader growth rate, needle growth rate, predawn xylem potential, daytime xylem potential, and photo synthetic rate occurring on the dates of herbicide application. Shrub injury for each of the three herbicides was compared to predawn xylem potential, daytime xylem potential, and photo synthetic rates. Both species exhibited less injury from herbicide applications made at the end of September than from any applications made from April through October. Comparison of factors highly correlated to herbicide damage indicates that highest herbicide selectivity occurs when pine has ceased growing, the xylem potential of the pine is relatively low (high water stress), and the xylem potential of the manzanita is relatively high.