Table 3--Grass and forb control after one application of herbicides in southwestern Oregon

1/Pounds active ingredient per acre for atrazine and terbacil; pounds

acid equivalent for 2,4-D. All chemicals were applied in water carriers.

DISCUSSION

Excellent survival of Douglas-fir seedlings planted in untreated grass and forb communities in and near the fog zone indicates that grass and forb control may not be necessary on typical coastal sites. Summer drought conditions are probably less severe than on inland sites. Adequate soil moisture storage, a high percentage of overcast days, and periodic fog evidently interact to reduce moisture stress in freshly planted tree seedlings. Stone (1957) showed that artificial dew (spray) increased the length of time ponderosa pine (Pinus ponderosa Laws.), incense-cedar (Libocedrus decurrens Torr.), and white fir (Abies concolor (Gord. and Glend.) Lindl.) survived after soil moisture had been depleted to the ultimate wilting point of sunflower. Cool coastal fog may have a similar effect and reduce moisture stress in Douglas-fir seedlings.

On the three dry sites, survival of Douglas-fir seedlings sprayed with 2,4-D or with a combination of 2,4-D and atrazine was less than survival of trees sprayed with atrazine alone. This strongly suggests that broadcast sprays of 2,4-D have an adverse effect on newly planted Douglas-fir seedlings experiencing transplant shock on summer-dry sites. On such sites, young Douglas-firs must also endure severe drought stress as well. The herbicidal effect of 2,4-D is evidently sufficient to tip the balance from survival to death for many trees on dry sites.

Better established trees in older plantations evidently can withstand any adverse effects of 2,4-D. In

an earlier study (Gratkowski 1976), established Douglas-firs survived broadcast sprays of 3 lb active ingredient of atrazine plus 1/2 lb acid equivalent of 2,4-D per acre (3.4 kg atrazine plus 0.6 kg 2,4-D per ha).

In contrast to effects on established seedlings, the data from dry sites indicate that terbacil may damage freshly planted Douglas-fir seedlings. Although terbacil provided appreciable grass and forb control for two summers, survival of Douglas-fir seedlings was less than that obtained with atrazine. Herbicidal activity of terbacil has also been reported to vary with differences in soil type (Kratky and Warren 1973, Stewart and Beebe 1974). This variation further complicates silvicultural use of terbacil and favors use of atrazine in broadcast sprays for grass control in new plantations of Douglasfir.

CONCLUSIONS

An important result of this study is the strong indication that grass control is not usually needed to establish Douglas-fir plantations in grass-forb communities at low elevations on the western slope of the Coast Ranges in southwest Oregon. Under normal climatic conditions, environmental stress--especially summer drought--is evidently ameliorated by fog on forest sites in and near the fog belt along the Oregon coast. On drier inland areas, where summer drought is more severe, and even on droughty sites in the Coast Ranges, grass control is needed to insure survival of new Douglas-fir plantations. Almost

all forest areas east of the crest of the Coast Ranges can be considered dry sites.

On dry sites, herbicidal sprays broadcast over the trees, grasses, and forbs in new Douglas-fir plantations effectively controlled grasses and increased survival of the tree seedlings. A more advisable procedure, however, would be to use the herbicide, for site preparation before planting. Damage to the small, susceptible conifers would be minimized if spray solutions were not allowed to fall on their foliage.

Atrazine applied in a water carrier at a rate of 4 lb active ingredient per acre (4.5 kg/ha) was the most effective treatment after planting on the three drier areas in this study. Atrazine provided excellent grass control and resulted in best survival of Douglas-fir seedlings at the end of the second summer after planting. Survival on atrazine-sprayed plots was more than double that of trees in untreated control plots (48 percent vs. 23 percent).

Results of this experiment strongly suggest that low volatile esters of 2,4-D should not be added to atrazine in broadcast sprays to control forbs in new plantations of Douglas-fir on forest sites east of the crest of the

Coast Ranges. This combination of herbicides (probably the 2,4-D component) may have damaged or killed weaker trees that were suffering transplant shock and environmental stress during the dry summer season. Early spring application of atrazine alone at a rate of 4 pounds active ingredient per acre is a far better

treatment.

1970. Herbicides in forestry. In Oregon Weed Control Handbook, Coop. Ext. Serv., Oreg. State Univ., Corvallis, Oreg., p. 232. Stewart, R. E., and T. Beebe. 1974. Survival of ponderosa pine seedlings following control of competing grasses. In Proc. West. Soc. Weed Sci. 27:55-58. Stone, Edward C.

1957. Dew as an ecological factor. I-II. The effect of artificial dew on the survival of Pinus ponderosa and associated species. Ecology 38(3):407-422.