Identifying Plant Communities Resistant to Conifer Establishment Along Utility Rights-Of-Way In Washington And Oregon, U.S.

METHODS

In 1999, three separate ROWs, approximately 5 ha each, were selected in the western foothills of the Cascade Mountains in Washington and Oregon (Figure 1). They represent only a subsample of the conditions in the region. Each ROW passes through a mountainous landscape covered by a dense, coniferous forest. The Willamette foothills site (WF) in Oregon had been used by the power utility since 1965. The Columbia River Gorge (CRG) and the Tacoma City Watershed (TCW) sites in Washington state had been used since 1938 and 1972, respectively. The vegetation along these rights-of-way had not been treated for at least 3 years before the beginning of the project. The CRG site was the lowest, at 100 m above sea level; TCW, the highest at 450 m; and WF, intermediate, at 320 m. The ROW was 80 to 100 m wide at WF and CRG and 160 m wide at TCW.

• Download figure
• Open in new tab
• Download powerpoint

Figure 1.

Schematic of the Pacific Northwest region; Xs indicate location of rights-of-way included in our survey.

Vegetation sampling was done along a 2 m wide transect across the ROW from forest edge to forest edge. The first transect was placed at an arbitrary location, and subsequent transects were placed every 20 to 25 m. Sampling plots (2 × 2 m) were placed at the beginning of the transect and at 4 m intervals. In mid-spring, after budbreak but before full leaf expansion, we visually estimated the percentage of cover and measured the maximum height of each species of tree, shrub, and fern intercepting the plot. Total grass cover and total forb cover were recorded by group rather than by individual species. Conifer tree seedlings were counted if they were rooted within the sampling plot. Between 470 and 511 plots covering 0.2 ha, or 3% to 5% of the study area, were sampled at each site.

Classifying sample plots based on their growth habit (physiognomy) and percentage of cover estimate involved three steps. First, species were assigned to one of several growth form types: forb, grass, fern, hardwood tree, tall shrub, and scotch broom (Cystisus scoparius, a non-native shrub), etc. Secondly, we used cluster analysis (McCune and Mefford 1999) to assign each plot to a community type based on the percentage of cover of each growth form (Table 1). The program grouped plots based on whether the plants were mainly herbaceous, native or exotic shrubs, trees, or tall shrubs. The analysis quickly and efficiently divided several hundred sample plots into six groups. A review of the original species cover estimates provided a check of the reliability of the cluster analysis groupings.

If all patch types were equally suitable for Douglas-fir, each patch type would have an equal density of Douglas-fir. The expected number of Douglas-fir seedlings within a community was proportional to the area occupied by that type (expected seedling number = percentage of area × number of seedlings in all areas). We compared the observed and expected frequency of Douglas-fir occupying each community type using a goodness-of-fit test (G-test, Sokal and Rohlf 1995). If the number of seedlings observed was significantly less than expected, the patch type was deemed resistant to invasion by Douglas-fir. Because many of the Douglas-fir seedlings had survived previous cuttings, we ran the test using total seedling counts and again using only counts of previously uncut, and therefore the youngest, seedlings. The results were similar under both scenarios; hence, we report statistics only for the test using all seedling counts.

RESULTS AND DISCUSSION

The cluster analysis defined six different plant communities within our study sites, representing a gradient of woody plant cover and height (Table 1). Four of the community types were common to all sites. The relative amount of right-of-way covered by each community type was between 2% and 43% (Table 2). No single plant species or community dominated large contiguous areas of the ROW; rather, their distribution was very patchy. Much of this patchiness appeared to be related to environmental factors, such as slope position (ridge versus swale), aspect, and past disturbance, particularly road construction. For example, abandoned access roads were dominated by scotch broom at WF and CRG and red alder at TCW.

Douglas-fir seedlings were not randomly distributed among the various plant communities (Table 2). At TCW, three of the five communities present had fewer Douglas-fir than expected by chance. At CRG and WF, only two of the five communities had fewer Douglas-fir than predicted (Table 2). Given the density of mature Douglas-fir trees bordering all three ROWs, the seed rain onto the various community types is likely frequent and abundant. Since the proportion of Douglas-fir seedlings was not evenly distributed among the different patch types, it is reasonable to conclude that some patch types were more resistant to seedling establishment than others. Consistently across the three ROWs, the tree/tall shrub community contained fewer Douglas-fir seedlings than expected and was considered the most resistant to seedling establishment. For at least two of the three sites, the shrub and the herbaceous communities appeared to be resistant to Douglas-fir as well (Table 2).

Resistance to seedling invasion is conferred, in part, by the growth habit of the individual species making up the community. Fifty woody plant species were recorded for the three sites combined. At least 17 of these were common to all sites, and an additional 15 were common to at least two sites. Four of the 17 species common to all sites—salal (Gaultheria shallon), thimbleberry (Rubus parviflorus), bracken fern (Pteridium aquilinum), and sword fern (Polystichum munitum)—were likely candidates for low-growing plants capable of inhibiting tree regeneration (Haeussler et al. 1990). Three more species—snowberry (Symphoricarpos albus), Oregon-grape (Berberis nervosa), and salmonberry (Rubus spectabilis)— were abundant on at least one site and exhibit a low-growth form capable of forming dense cover under favorable conditions.

Although forbs and grasses were common throughout the ROWs, under a dominant cover of woody plants, herbaceous cover was typically less than 1%. Grasses reached higher cover values (>20%) along access roads and within a meadow at the base of a steep slope at TCW and in patches at CRG and WF. Forb-dominated patches were not frequent enough to determine if they would effectively impede tree establishment.

An obvious similarity between many of the common plants within the ROWs is the vegetative growth capability. Several shrub species occupying our sites can form dense layers of roots [e.g., vine maple (Acer circinatum), beaked hazelnut (Corylus cornuta var. californica), salal, and salmonberry]. Their belowground shoots also have numerous buds. Where disturbance, including ROW maintenance, removes the aboveground material, these species can respond with a rapid flush of growth (Haeussler et al. 1990). These characteristics make them suitable candidates for low-growing plant communities capable of filling in the gaps left after the removal of unwanted vegetation. Some grass-dominated patches also appeared resistant to tree seedling establishment. Future work will include identifying which grass species are present and which might be most resistant to tree invasion.

Douglas-fir seedlings were 0.5 to 3 m tall (average 1.4 m). The density of Douglas-fir seedlings ranged from several hundred to more than 1,500 stems/ha (Table 3). Many seedlings had already been cut down with a chain saw and regrew from an uncut lateral branch (Table 3). Cutting occurred during the last maintenance cycle, 3 or more years before our survey. More careful removal of the treetop and all lateral branches would achieve a lower density of conifer seedlings and perhaps a longer interval between successive treatments. Certainly, less effort would be required each maintenance cycle to remove Douglas-fir trees.

SUMMARY AND CONCLUSIONS

Several patch types in our study areas were composed of low-growing plants. We observed examples of both woody and nonwoody communities, which in some settings were resistant to Douglas-fir establishment but in other settings appeared to permit it. Communities occupied by tall trees and shrubs were most resistant to Douglas-fir establishment, but obviously this type does not conform to our low-growing plant criteria.

We calculated the density of Douglas-fir seedlings occupying rights-of-way several years after the most recent vegetation control and maintenance cycle. Thus, the data from one survey represent the combined results of several growing seasons and an overall measure of the resistance of each community type to Douglas-fir. Of particular interest regarding stable, low-growing plant communities were the number of turfforming grasses and clonally reproducing shrubs. Moderate to high cover of these species was correlated with low levels of conifer seedling establishment. Further investigation of the character of these communities, how they are encouraged, and how they affect tree seedling establishment and growth is warranted. In general, management practices that maintain or expand their presence on the ROWs have a great potential to reduce maintenance (Hill et al. 1995). The ROW vegetation described in this study and its relationship to seedling establishment will be used to develop further questions and to inform future research, including experimental manipulations.