Mechanical and Chemical Release in a 12-Year-Old Ponderosa Pine Plantation

Abstract

A 12-year-old ponderosa pine plantation on the Tahoe National Forest in northern California was mechanically treated with a Hydro-Ax in an attempt to increase the survival and growth of the planted seedlings. Other release methods were not feasible because the shrubs in the mixed-shrub community (greenleaf manzanita, mountain whitethorn, bittercherry, coffeeberry) were too large (3 to 5 feet tall) and well developed. Additional treatments were a chemical treatment, in which 2,4-D was applied to a portion of the study site that had been treated with the Hydro-Ax 1 year previously, and control. Eleven growing seasons after treatment (1993), average pine crown cover was statistically higher in the mechanical treatment (Hydro-Ax alone) than in the control. This was the only significant enhancement of pine growth by the Hydro-Ax alone. Mean pine diameter and height did not differ statistically from the control after 11 years. In contrast, the Hydro-Ax plus herbicide (chemical) treatment statistically increased pine crown cover, height, and diameter over the Hydro-Ax alone and the control. Mean crown cover was 104 percent greater in the treated trees than for pines in the control, height was 45 percent greater, and diameter was 47 percent greater. Relative costs were $225 per acre for the Hydro-Ax alone (mechanical) and $273 per acre for the Hydro-Ax + herbicide (chemical). Altogether, the most cost-effective treatment was Hydro-Ax + herbicide.