An Ecological Assessment of Dorena Prairie

Transcription

1 An Ecological Assessment of Dorena Prairie Prepared by Mark V. Wilson Submitted to Bureau of Land Management Eugene District December 23, 1996

2 Summary Field observations allowed the ecological assessment of Dorena Prairie, an upland prairie site managed by the BLM and nominated as an Area of Critical Environmental Concern. Species-poor patches of woody and other pest plants constituted much of the site (about 40%). Over the remainder of the site, native species contributed 25% of the total plant cover (45%, if Festuca rubra is considered a native species). Although I judged the vegetation of the site to be of medium quality, the rarity of native upland prairies in the Willamette Valley qualifies Dorena Prairie as an Area of Critical Environmental Concern. The value of Dorena Prairie to BLM s conservation effort would be greatly enhanced by active habitat restoration. Introduction Dorena Prairie is a remnant upland prairie managed by the Eugene District of the U.S. Bureau of Land Management. Based on information from 1987 (The Nature Conservancy, 1987), this small tract (2.3 ha) was nominated by BLM as an Area of Critical Environmental Concern (ACEC). The purpose of this report is to update the ecological assessment of the site, to evaluate its status as a potential ACEC, and to recommend management to improve prairie conditions. My activities in support of this report included several visits to Dorena Prairie, twice with Nancy Wogen and other BLM personnel, and vegetation sampling and analysis. I also conducted a survey of other upland prairies to help understand Dorena Prairie s value in a Valley-wide context. Survey results are in a separate report (Wilson, 1996). Methods I divided the site into six bands centered on lines extending across the prairie (Figure 1). Sampling occurred throughout the prairie, except 10m buffers next to woods and fences on the east, south, and west sides of the site. Preliminary surveys showed that much of the site contained patches dominated by woody plants, tall oatgrass (Arrhenatherum elatius 1 ) or bracken fern (Pteridium aquilinum). I measured the cover of these patches using the line intercept method, with a total of 234 m measured (Figure 1). Patches were defined as having at least 50% cover of the dominant. For the remaining portions of the site, I sampled the vegetation using 1-m 2 square quadrats. I divided the site into 18 cells, 3 cells along each of the six lines. Quadrat location within each cell was determined using corrected random polar coordinates. Because two cells were almost 1 Plant nomenclature is after Hitchcock and Cronquist (1973). Dorena Prairie Page 1 Wilson

3 entirely dominated by woody plants or bracken fern, the total number of quadrats sampled was 16 (Figure 1). Within each quadrat, I first listed all species present, noting the two most abundant species. I then recorded cover by eight species groups: native perennial grasses, native annual grasses, native perennial forbs, native annual forbs, exotic perennial grasses, exotic annual grasses, exotic perennial forbs, and exotic annual forbs. Festuca rubra was recorded separately, because of its questionable taxonomic status. Cover was estimated visually, using a calibrated template. Results Patches of woody plants, tall oatgrass, and bracken fern Tall oatgrass was the single most abundant patch type, covering 12% of the site (Table 1). Patches of all types of woody plants covered about 20% of Dorena Prairie. Bracken fern was also common, occurring in patches in 9% of the prairie. Because patches of woody plants, tall oatgrass, and bracken fern contain few native species, these patches were excluded from the rest of the sampling. Thus, the remainder of results pertain only to the more grassland-like sections of Dorena Prairie (about 60% of the site). Species frequency and cover I encountered 42 species in the 16 sample quadrats. Anthoxanthum odoratum, Chysanthemum leucanthemum, Festuca rubra, Fragaria virginiana, Plantago lanceolata, and Sanguisorba minor were all more-or-less ubiquitous on the site, each found in at least 75% of the quadrats (Table 2). Frequency of dominance was somewhat different. Festuca rubra was one of the two dominant plants in 53% of the quadrats. Other frequent dominants were Arrhenatherum elatius (dominant in 27% of the quadrats), Chysanthemum leucanthemum (27%), Fragaria virginiana (20%), and Lupinus rivularis. Dorena Prairie was dominated by exotic perennial grasses (26%) (Table 3), although native perennial forbs (16%) and exotic perennial forbs (15%) were also abundant. Festuca rubra, which was recorded separately, had 15% cover. Vegetation quality Native and exotic species. There is no accepted measure of vegetation quality. A simple measure that reflects many individuals subjective sense of quality is the proportion of the vegetation in native plants. In nearly all cases, the native status of a plant species is clear. I choose to assign very widespread species like Achillea millefolium and Pteridium aquilinum as cosmopolitan, rather than native. The situation for Festuca rubra is more difficult. The taxon at Dorena Prairie is var. commutata. Barbara Wilson (pers. comm.), based on enzyme, seed Dorena Prairie Page 2 Wilson

4 protein, and DNA analyses, thinks that the Festuca rubra var. commutata at Dorena Prairie is derived from European stocks, probably brought in as forage. The native bunchgrass fescue in the Willamette Valley is probably Festuca idahoensis Elmer var. roemeri Pavlick. F. rubra var. commutata, even if an exotic species, is probably ecologically similar to F. idahoensis var. roemeri. Evidence for this conclusion includes the years of botanists mistaking one taxon for the other. Rather than try to resolve this issue, I have chosen to present the analysis in two forms, one treating the fescue as a native species, one as an exotic species. Richness. Of the 42 species I encountered in the vegetation sampling, 43% were native species (40% not including Festuca rubra). Because exotic species were a bit more common than native species, values for quadrats are lower than for the entire site (Table 4). On average, 40% of plant species in the 1-m 2 quadrats were native species (32% not including F. rubra). Cover. Native plants contributed 45% of the vegetative cover of Dorena Prairie (25% not including Festuca rubra) (Table 4). The large difference in these two numbers reflects the abundance of F. rubra at the site. Additions to the species list I noted five species (Table 5) not listed on the Dorena Prairie Plant List dated 9/15/90. Two of these species, Bromus carinatus and Elymus glaucus, are important additions because they are native grasses of remnant upland prairie in the Willamette Valley. No BLM special status plant species (BLM, 1995) appear to be present at Dorena Prairie. Discussion Vegetation and site quality The vegetation of Dorena Prairie is typical of many native upland prairies in the Willamette Valley. Dorena Prairie has a significant component of native species. The site includes key grasses like Bromus carinatus, Danthonia californica, and Elymus glaucus, although it lacks Festuca idahoensis var. roemeri and Stipa lemmonii. Showy native herbs include Calochortus tolmei, Lupinus rivularis, and Sidalcea virgata. No species on the BLM lists of special-status plants is present. The Prairie has been degraded by past land use, however, to the point that exotic species are now more abundant natives. The Prairie also faces severe current threats from woody plant encroachment and spread of the aggressive herbaceous weeds Arrhenatherum elatius and Pteridium aquilinum. These pest plants are vigorous at the site and appear to be expanding. If patches of pest plants expanded at an average rate of 20 cm per year, about 80% of the site would be covered by pest patches in 10 years, compared with 40% today. The urgency for protecting Dorena Prairie depends in part on how it compares with other remnant Willamette Valley upland prairies. In a separate report (Wilson, 1996), I have compiled Dorena Prairie Page 3 Wilson

5 information on all known native upland prairies in the Willamette Valley. Each site was ranked for vegetative quality based on subjective assessments by local experts. For sites with vegetation data, including Dorena Prairie, site quality was also assessed numerically. In both approaches, Dorena Prairie is scored as having medium quality (Table 6). The taxonomic problem of whether Festuca rubra is native to the site has an important but limited effect on Dorena Prairie s standing compared with other prairies, moving it from mid-category to the top of the group of medium quality sites. Evaluation as a candidate Area of Critical Environmental Concern (ACEC) The two main criteria for inclusion of a site as an ACEC are relevance and importance. Dorena Prairie is relevant for inclusion as an ACEC because it is one of the few remaining representatives of upland prairies, an extremely rare community in the Willamette Valley (Wilson, 1996; Wilson, in preparation). Based on size and plant species composition, including key and rare prairie species, Dorena Prairie probably ranks as the 14th to 20th best example of upland prairie remaining in the Valley. Surveys of insects, lichens, and mammals are unavailable for Dorena Prairie and would increase our understanding of the non-plant components of the community. Dorena Prairie has general, rather than simply local, importance because of the distinctiveness and rarity of upland prairies throughout the region. Dorena Prairie is also important according to BLM criteria because it is threatened by pest plant encroachment. Active management would be required to maintain or restore the quality of the site. The results presented in this report show that Dorena Prairie meets the broad BLM requirements of an Area of Critical Environmental Concern. The value of listing the site as an ACEC is less clear. Without vegetation management, Dorena Prairie would quickly lose its merit as an ACEC; with an active restoration program to improve vegetation quality, Dorena Prairie would be an important representative of the once widespread upland prairie ecosystem. Recommendations for managing Dorena Prairie The most urgent vegetation management issue at Dorena Prairie is reducing woody plant cover. Native herbaceous plant diversity is extremely low in woody patches. Continued increase in woody cover would decrease site quality over the next five or ten years to the point that the site would no longer qualify as an Area of Critical Environmental Concern. Although fire reduces woody plant cover (Pendergrass, 1995; Wilson and Clark, 1995; Clark and Wilson, 1996), its effect on native herbaceous plants can sometimes be unclear or even detrimental (Wilson et al., 1993; M. Maret and M. Wilson, unpublished data). A botanically safer approach than prescribed fire would be to remove woody plants manually. Dorena Prairie is small enough that a small crew could clear most of the patches in a day or two. Topical herbicides should be applied to stumps of cut woody plants, since all woody species at the site (except very old Cytisus scoparius) are capable of resprouting. Within a year after cutting, the Dorena Prairie Page 4 Wilson

6 exposed ground should be prepared and sown with native species. Current research (e.g., C. Schultz, unpublished data) should provide guidelines for best establishment techniques. Failure to establish native plants would lead to increases in exotic species. The aggressive herbaceous pest plants Arrhenatherum elatius and Pteridium aquilinum also form species-poor patches and require control within the next five years. These species would be best controlled by spot mowing with a hand-help rotary mower. Recent research (Wilson and Clark, 1996 for Arrhenatherum elatius; P. Hammond, 1987 for Pteridium aquilinum) provides guidelines for most effective mowing times and heights. Repeated annual mowing would be necessary for adequate control. After pest-plant control, Dorena Prairie would be suitable as a site for re-establishing BLM special-status species (e.g., Aster curtus, Erigeron decumbens var. decumbens) and other species of concern (e.g. Lupinus sulphureus ssp. kincaidii and Icaricia icarioides fenderi). Expanding the distribution of these upland prairie species would reduce their vulnerability because of threats at other sites. References Bureau of Land Management Record of Decision and Resource Management Plan. Eugene District Office. Clark, D. L., and M. V. Wilson Effects of fire, mowing and hand-removal on woody species in a native wetland prairie. Unpublished report to the Bureau of Land Management, Eugene District. Hammond, P Ecological investigation of Viola adunca. USDA Forest Service, Pacific Northwest Region, Siuslaw National Forest. Hitchcock, C. L., and A. Cronquist Flora of the Pacific Northwest: An Illustrated Manual. University of Washington Press, Seattle. Pendergrass, K. L Vegetation composition and response to fire of native Willamette Valley wetland prairies. Master's thesis, Oregon State University. The Nature Conservancy Threatened and endangered plant species habitats, Cottage Grove Lanke and Dorena Lake Projects. Unpublished report to the Oregon Division of State Lands and the U. S. Army Corps of Engineers. Wilson, M. V A survey of the native upland prairies of the Willamette Valley. Unpublished report to the Bureau of Land Management, Eugene District. Dorena Prairie Page 5 Wilson

7 Wilson, M. V., in preparation. Upland prairie. In: Willamette Valley Recovery Plan, U.S. Fish and Wildlife Service, Portland, Oregon. Wilson, M. V., and D. L. Clark Effects of fire and fire fighting techniques on Fender s blue butterfly and Kincaid s lupine. Unpublished report to the U.S. Fish and Wildlife Service, Portland, Oregon. Wilson, M. V., and D. L. Clark Controlling tall oatgrass in Fender s blue butterfly habitats. Unpublished report to the Oregon Natural Heritage Program. Wilson, M. V., K. P. Connelly, and L. E. Lantz Plant species, habitat, and site information for Fern Ridge Reservoir. Report submitted to Waterways Experiment Station Army Corps of Engineers. Table 1. Cover of patches dominated by woody plants, tall oatgrass, or ferns. Data are from 234 m of line intercept sampling. Species Cover (%) Arrhenatherum elatius 12.1 Cytisus scoparius 6.5 Pteridium aquilinum 8.8 Quercus garryana 2.2 Rhus diversiloba 2.6 Rosa spp. 3.7 Symphoricarpus albus 4.7 Total 40.6 Dorena Prairie Page 6 Wilson

8 Table 2. Frequency of species encountered in 16 quadrats and the rank order of frequencies. Group codes: N = native species, E = exotic species, C = cosmopolitan species. Species Group Frequency Rank Achillea millefolium C 38% 9 Aira caryophyllea E 13% 22 Anthoxathum odoratum E 100% 1 Arrhenatherum elatius E 56% 7 Bromus carinatus N 25% 16 Bromus mollis E 6% 28 Calochortus tolmei N 6% 29 Camassia quamash N 13% 23 Centarium umbellatum E 6% 30 Chrysanthemum leucanthemum E 75% 6 Cirsium arvense E 6% 31 Cirsium vulgare E 13% 24 Convolvulus nyctagineus N 19% 19 Crategeus sp. N 6% 32 Cytisus scoparius E 19% 20 Dactylis glomerata E 25% 17 Danthonia californica N 31% 13 Daucus carota E 38% 10 Elymus glaucus N 19% 21 Equisetum sp. N 6% 33 Eriophyllum lanatum N 6% 34 Festuca arundinacea E 38% 11 Festuca rubra N 94% 2 Fragaria virginiana N 94% 3 Galium aparine E 6% 35 Galium parisiense E 6% 36 Geranium bicknellii N 6% 37 Geranium carolinianum E 6% 38 Hypochaeris radicata E 6% 39 Lupinus rivularis N 31% 14 Plantago lanceolata E 81% 4 Prunella vulgaris N 6% 40 Pseudotsuga menziesii N 6% 41 Pteridium aquilinum C 13% 25 Rosa spp. 6% 42 Rubus discolor E 13% 26 Rubus ursinus N 44% 8 Rumex acetosella E 31% 15 Sanguisorba minor E 81% 5 Sidalcea virgata N 38% 12 Symphoricarpus albus N 13% 27 Vicia hirsuta E 25% 18 Dorena Prairie Page 7 Wilson

9 Table 3. Plant cover, by species groups. Results from quadrat sampling. Festuca rubra is listed separately. Group Average cover (%) Native perennial grasses 2.6 Native annual grasses 0.0 Native perennial forbs 16.2 Native annual forbs 0.1 Exotic perennial grasses 26.0 Exotic annual grasses 0.2 Exotic perennial forbs 14.9 Exotic annual forbs 0.1 Festuca rubra 15.0 Sum of herbaceous cover 75.1 Shrubs 5.4 Trees 0.5 Ferns 2.8 Dorena Prairie Page 8 Wilson

10 Table 4. Average species richness (number of species per 1-m 2 quadrat) and cover for native and exotic plant species. Calculations are presented two ways, corresponding to treating Festuca rubra as a native species and treating it as an exotic species. See Table 2 for the species in each group. Species group Richness Proportion Cover (%) Proportion Festuca rubra as native species Native species % % Exotic species % % Other species 0.6 5% Festuca rubra as exotic species Native species % % Exotic species % % Other species 0.6 5% Table 5. Species at Dorena Prairie not included in previous species lists. Species Family Status Apocynum androsaemifolium Aponcynaceae Native Bromus carinatus Poaceae Native Elymus glaucus Poaceae Native Galium parisiense Rubiaceae Exotic Geranium bicknellii Geraniaceae Native Dorena Prairie Page 9 Wilson

11 Table 6. Native upland prairie sites in the Willamette Valley, grouped by quality (from Wilson, 1996). A site s assignment was based on its highest quality portions, which often are small in extent. Sites that probably include relatively large areas (generally $ 2 ha) of high or very high quality are marked in bold. For the five sites or portions of sites with vegetation data, ratios of native cover to total cover are shown in brackets. Bald Hill (south, west [61%]) Baskett Butte (south [48%], NE, west) Butterfly Meadows (NW [73%], SE and SW) Carson Prairie [54%] High or very high quality Coburg Ridge (north, saddle, south, south of Baldy?) Forest Peak Horse Rock Ridge Kingston Meadows Noble Pasture? Open Space Park Philomath Prairie? Sublimity Grassland Preserve Wren? Camassia Natural Area Dallas Dorena Prairie [25%, 45% if Festuca is native] Edison Road Grassland? Fern Ridge Fire Knob Medium quality Hidden Oaks McKenzie Drive Mill Creek Mt. Pisgah Oak Ridge? Pigeon Butte Rattlesnake Butte Row Point Sanford Drive Shoulder-to-Shoulder Farm Spencer Butte Summit West Hills Road Willow Creek Baldy (Finley NWR) Dolph Corner Hills Fir Butte Grand Ronde Strip Hilaire Rd. Low quality Jackson Prairie NE of Estacada Peterson Butte Riches Road Tower Ridge Twin Buttes Unnamed Butte Wisner Cemetery Dorena Prairie Page 10 Wilson

12 Figure 1. Dorena Prairie. Dashed lines are the borders of the study area and approximate the boundary of the Prairie. Solid lines are sampling transects. Boxes denote sampling quadrats. Dorena Prairie Page 11 Wilson