Herb I Study
Twelfth-Year Response of Douglas-fir Seedlings to Area of Weed Control and Herbaceous versus Woody Weed Control
Introduction
Last fall (2004), at the end of the 12th growing season since planting, we remeasured the Summit and Marcola Herb I sites. This cornerstone VMRC study was featured at the Fifth International Conference on Forest Vegetation Management-both in the field tour as well as a presentation on the first day of the conference; we will be submitting a manuscript to be included in the proceedings. These sites were most recently remeasured in 2000 (year 8), and those results were reported in the 2000-2001 annual report and will be published later this year in the Annals of Forest Science.
Methods
The study is being followed at Summit, OR, a hardwood conversion site in the central region of the Oregon Coast Range (site index = 41 m [135 ft]) and Marcola, OR, located in the western Cascade Mountain foothills east of Springfield, where the prior stand consisted of 65-yr-old Douglas-fir (site index = 37 m [121 ft]). At both sites slash was removed and the ground was scarified with ripper blades and subsoiled with a winged blade to a depth of approximately 60 cm. Douglas-fir 1+1 seedlings were planted in winter 1993. The perimeters of both sites were fenced to prevent deer browse.
Eight area-of-control (spot) or selected control treatments (Table 1) are replicated three times per site for a total of 24 plots per site.
Each plot consists of 49 seedlings planted in a 3.1 x 3.1 m (10 x 10 ft) grid surrounded by a buffer strip of two tree rows. For spot treatments, herbaceous weeds were controlled within the spots and all woody competition was controlledin the entire plot-allowing only herbaceous competitors outside the treated areas. Woody vegetation was controlled in these treatments to prevent its rapid invasion into the spot treatments. This invasion was expected to be intense due to the fencing of plots to prevent deer browse.
Herbaceous treatments-applied within spot treatment areas and broadcast across entire plots in both TVC and herbaceous-only treatments-were designed to control all herbaceous species present, requiring the use of several chemicals. Hexazinone was applied at a rate of 1.68 kg/ha (ai) in year one, and both hexazinone (1.12 kg/ha) (ai) and sulfometuron (0.07 kg/ha) (ai) were applied in year two. The herbicides were applied from a backpack with a gas-powered boom sprayer with nozzles adjusted for treatment sizes. Applications were made before budbreak in early spring in years one and two only. Treated areas were maintained throughout the first two growing seasons by directed applications of glyphosate in a 1% aqueous solution. On all plots except the check and herbaceous-only control, woody vegetation was controlled over the entire plot by a directed basal application of 3% triclopyr in diesel applied prior to bud break in spring. Woody control treatments were applied for the first three years after planting. Vegetation covers through the third year of the study are shown in Figure 1.
Figure 1. Mean total vegetation cover (sum of individual species cover) for Summit and Marcola sites measured in July for each of the first three years post planting.
Results
Herbicide treatments significantly influenced the amount of hardwood basal area present at both Summit (p=0.0237) and Marcola (p=0.0278). Basal area was much greater at Summit, ranging from 1.6 m2/ha (TVC) to 23.8 m2/ha (check), than at Marcola, ranging from 0.2 m2/ha (5.95 m2) to 2.0 m2/ha (herbaceous-only) (Table 2). At Summit all treatments that involved woody control have significantly less hardwood basal area than the check treatment, and herbaceous-only control reduced basal area to levels not significantly different from all other treatments except TVC. At Marcola the two treatments without any woody control (check and herbaceous-only) had significantly more basal area than all but the smallest area-of-control treatment (0.38 m2). The large difference in basal area between sites is due to differences in species composition rather than differences in the number of hardwood stems (Tables 2, 3).
Figure 2. Cumulative mortality (since year 1) and height: diameter ratios (years 5 through 12) by site for all treatments.Mortality in the check treatment at Summit has risen sharply since year 5, reaching 23% in year 12. However, there were no significant differences in cumulative mortality at year 12 (Figure 2) due to high plot-to-plot variability. At Marcola mortality in the 0.38 m2 area-of-control treatment (20.1%) was significantly greater than mortality in all other treatments (7.2 to 10.5%) (Figure 2). However, mortality in this treatment was nearly 16% the first year and has only risen 4% since, suggesting this response may be related to other factors in addition to area of weed control.
Height, DBH and stem volume have responded differently between sites (Figure 3). At both sites height, DBH and volume tended to increase with increasing area of herbaceous weed control, but treatment differences were less at Marcola than at Summit. At Summit total vegetation control increased twelfth-year individual-tree volume by 336%, but at Marcola total vegetation control improved individual-tree volume by just 64%. Another difference between the sites is that volume has increased nearly linearly with increasing area of weed control at Summit, but at Marcola, greater than 3.35 m2 of weed control failed to bring about a positive response (Figure 4). TVC improved volume by 38% at Summit and by just over 3% at Marcola relative to a 3.35 m2 area of control. At Summit woody-only control improved growth in all parameters relative to the check treatment, and all non-check treatment means appear to be separating from the check treatment. At Marcola, on the other hand, no parameter means in year 12 were greater in the woody-only control treatment than in the check treatment.
Figure 3. Treatment means by site for years 5, 8 and 12 for height (A), DBH (B), Individual-tree volume (C) and volume per hectare (D).
Figure 4. Individual tree volume response to area of herbaceous weed control at the Summit and Marcola Herb I sites. Summit: volume = 90.14 + 64.06 (1 - 0.87)x, r2 = 0.83, p<0.0001. Marcola: volume = 46.68 + 21.52 (1 - 0.51)x, r2 = 0.60, p = 0.0011.
Discussion
Herbaceous Control
Two years of complete herbaceous control plus three years of woody control (TVC) increased 12th year volume by 101% at Summit and 53% at Marcola relative to no herbaceous control plus three years of woody control (woody-only control treatment). This differential response to area-of-control treatments between sites is likely due to differences in weed control effectiveness between the sites. Reinvasion of treated plots by bracken fern at Marcola made it difficult to achieve the same level of control at Marcola as was achieved at Summit (Figure 1).
Woody Control
Woody-only control increased volume relative to the check by 117% at Summit and 11% (ns) at Marcola. The differential response to woody-only control is explained by site differences in species composition. Bitter cherry, a species capable of competing aggressively with Douglas-fir, was widespread at Summit. No highly competitive hardwood present in significant numbers at Marcola. The impact of fencing to prevent deer browse played a large part in the response at Summit. Bitter cherry is preferred deer forage and fencing enabled cherry to reinvade the site much more aggressively than is typical in the surrounding stands. The Summit site provides an excellent data set for understanding the relationship between vegetation control and woody plant response that can be applied to any location where competitive woody species occur.
