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Seasonal Effects of Phenoxy Herbicides on Ponderosa Pine and Associated Brush Species

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Ponderosa pines at two locations in southwestern Oregon were sprayed on 10 dates between late June and mid-September to determine changes in susceptibility to low volatile esters of 2,4-D and 2,4,5-T. Water and oil-in-water emulsions were tested as carriers. Brush species selected for comparison were snowbrush ceanothus, varnishleaf ceanothus, and Pacific madrone. As in earlier studies, ponderosa pines were damaged less by 2,4,5-T than by 2,4-D. Late August-early September aerial application of 2 lb acid equivalent of 2,4,5-T per acre (2.24 kg per ha) in a water carrier with a spray volume of 10 gal per acre (37.85 1 per ha) is recommended for releasing ponderosa pines from shrub competition in western Oregon. Ponderosa pine's resistance to phenoxy herbicides increased rapidly after cessation of height growth in early July, and no damage was caused by sprays applied in late August and mid-September. In contrast, snowbrush ceanothus was still highly susceptible to herbicides during that time. Varnishleaf ceanothus and Pacific madrone, although not as susceptible as snowbrush ceanothus, were adequately controlled to release intermingled pines. Late summer is a suitable time for application of aerial sprays to release pines from brush competition in southwestern Oregon without damaging the trees. Several hypotheses are offered as possible explanations for late summer resistance of pines and delayed response of shrub species to late summer-early autumn applications of phenoxy herbicides. Forest Science 23:2-12.

Keywords: Pinus ponderosa; Silviculture; brush control; pine release

Document Type: Journal Article

Affiliations: Research Forester, Forestry Sciences Laboratory, maintained at Corvallis, Oregon, by the Pacific Northwest Forest and Range Experiment Station, USDA Forest Service

Publication date: March 1, 1977

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  • Forest Science is a peer-reviewed journal publishing fundamental and applied research that explores all aspects of natural and social sciences as they apply to the function and management of the forested ecosystems of the world. Topics include silviculture, forest management, biometrics, economics, entomology & pathology, fire & fuels management, forest ecology, genetics & tree improvement, geospatial technologies, harvesting & utilization, landscape ecology, operations research, forest policy, physiology, recreation, social sciences, soils & hydrology, and wildlife management.
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