Relationship Between Xerophyllum tenax and Canopy Density in the Southern Cascades of Washington Stewart Higgins, Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164-6420; Keith Blatner, Department of Natural Resource Sciences, Washington State University, Pullman, WA 99164-6410; Becky K. Kerns, United States Department of Agriculture Forest Service, Pacific Northwest Research Station, Corvallis, OR 97331; and Alexis Worthington, Department of Biology, Western Washington University, Bellingham, WA 98225-9160.
ABSTRACT: Large-scale commercial harvest of beargrass (Xerophyllum tenax) has been taking place in the Cascades of Washington and Oregon for the past 15 to 20 years. The long, slender leaves are either used fresh or dried and dyed for use in the floral industries in the United States and Europe. Our objectives were to develop a better understanding of beargrass production under different tree canopy (overstory) densities in the Pacific silver fir/big huckleberry/beargrass and the mountain hemlock/big huckleberry/beargrass plant associations in and around the Cispus Adaptive Management Area. We examined differences in beargrass production for different overstory canopy conditions on 10 sites in each association. Results indicated that beargrass quality is not of commercial grade under open canopies (⬍60% overstory density). For medium and high densities, the interaction between plant association and overstory density was significant for all response variables except harvestable dry mass. Harvestable dry mass of beargrass did not differ between the two associations, but was greater under medium- compared with high-density conditions. For the Pacific silver fir association, the high-overstory-density class had greater basal area of beargrass per site, and plants were larger with longer leaves compared to medium-canopy-density sites. We did not find this relationship for the mountain hemlock association, except for the longest leaf variable. It is unclear why basal area and size of beargrass were more closely related to overstory conditions for the Pacific silver fir association. Evaluation of the sustainability of beargrass as a nontimber forest product will require long-term study of the relationships among environmental variables, beargrass productivity, and beargrass population dynamics. West. J. Appl. For. 19(2):82– 87. Key Words: Beargrass, nontimber forest products, NTFP, overstory/understory relationships.
Beargrass (Xerophyllum tenax) is an understory species in mixed-coniferous forests and meadows throughout the Northwest. Inhabiting a wide range of forest types, beargrass is usually found as an understory dominant in cool western spruce-fir forests. Large-scale commercial harvest of this species has been taking place in the southern Cascades of Washington and Oregon for the past 15 to 20 years. The long, slender leaves are either used fresh, or dried and dyed for use in the floral industries of the United States and Europe (Schlosser et al. 1992a). To meet current industry NOTE:
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Keith Blatner can be reached at Department of Natural Resource Sciences, Washington State University;
[email protected]. We thank D. Massie for help in the field, and L. Archer for help with library research. Copyright © 2004 by the Society of American Foresters. WJAF 19(2) 2004
standards, leaves must be dark green in color, free of blemishes and brown tips, and up to 76 cm long (Schlosser et al. 1992b). As beargrass harvest activities continue to expand, managers have expressed concern regarding sustainable harvest levels. Relatively little is known about the productivity of beargrass or the long-term impact that increasing harvest levels may have on the resource. Despite its ubiquitousness and the growing interest in beargrass as a nontimber forest product, we know little about the species’ commercial characteristics in relation to different plant associations and overstory conditions. Floral greenery species such as beargrass require partial shade provided by overstory forests to meet industry standards (Schlosser et al. 1992a), but reduced light environments may limit productivity and regeneration. It is generally
assumed that productivity and other characteristics of understory plants are tightly coupled with overstory tree canopy development and successional patterns. In the Pacific Northwest, researchers have examined understory-overstory relationships, often identifying strong predictable patterns during early succession, stand closure, and following silvicultural thinning (Alaback 1982, Bailey et al. 1998, Halpern and Franklin 1990, Klinka et al. 1996, Thomas et al. 1999). Empirical data have also been used to model relationships among understory abundance, overstory characteristics, and environmental factors (Klinka et al. 1996, McKenzie and Halpern 1999, McKenzie et al. 2000). Our objectives were to develop a better understanding of beargrass production under different canopy-density conditions in the Pacific silver fir/big huckleberry/beargrass (Abies amabilis/Vaccinium membranaceum /Xerophyllum tenax) (Brockway et al. 1983) and the mountain hemlock/big huckleberry/beargrass (Tsuga mertensiana/V. membranaceum/ X. tenax) (Diaz et al. 1997) plant associations in and around the Cispus Adaptive Management Area (AMA). We examined differences in beargrass total basal area, mean basal area per plant, fresh leaf length, and mass (fresh and dry) of harvestable leaves for different overstory canopy densities in each of the two plant associations studied. We also visually evaluated beargrass leaf color.
Background Beargrass occurs in mixed-coniferous forests and meadows in the mountains of northern California and the Pacific Northwest, ranging north into the southeast corner of British Columbia and into the northern Rocky Mountains of Idaho and Montana. In the Rocky Mountains, it can be found between 2,000 and 7,000 ft. It infrequently occurs along the coast ranges at sea level, and just below the summits of the coastal ranges between west-central California and northwestern Washington (Maule 1959). In the Sierras and Cascades, beargrass occurs between 2,000 and 7,000 ft. In the Cascades, it can be a dominant understory species under mixed-evergreen subalpine forests of subalpine fir (Abies lasiocarpa), mountain hemlock (Tsuga mertensiana), western white pine (Pinus monticola), Pacific silver fir (A. amabilis), Douglas fir (Pseudotsuga menziesii), and lodgepole pine (Pinus contorta) (Brockaway et al. 1983). It is often found on steep sites in which the soils are saturated in the spring and well drained in later months (Maule 1959). It does well in shallow, rocky soils near ridge tops, but also occurs in forested areas with seasonally saturated soils. Beargrass is extremely frost tolerant and can be an indicator of frost-prone sites (Brockaway et al. 1983). In the Mount Rainier province, beargrass most often occurs on South-facing slopes as steep as 55°, and is found as an understory dominant in the more xeric associations of the upper Pacific silver fir and lower mountain hemlock zones (Franklin et al. 1988, Maule 1959). Beargrass is often associated with huckleberry species (Vaccinium alaskaense, V. parvifolium, and especially V. membranaceum), prince’s pine (Chimaphila umbellata), sidebells pyrola (Pyrola secunda), and twinflower (Linnaea borealis) (Brockaway et al. 1983).
A member of the lily family, beargrass is one of only two members of its genus, along with Xerophyllum asphodeloides, which occurs in the Appalachian Mountains. It is an evergreen herbaceous perennial with wiry, scabrous, linear leaves (5–10 dm long; 3– 6 mm wide) that sprout from a short woody rhizome to form dense clumps or tussocks. When a plant blooms, it produces a leafy flowering stalk (3–15 dm high) crowned by a raceme of small cream-colored flowers (Hitchcock and Cronquist 1973). Beargrass flowers between May and Aug., depending on the location (Maule 1959). Beargrass is harvested by individuals who obtain permits to gather on USDA Forest Service and other public lands, or who obtain a contract or permit or otherwise make arrangements to harvest on private forest lands. A contractor also may hire several people to harvest beargrass on land for which he/she has obtained a lease. Commercial harvesters only remove the inner whorl of leaves (the new growth). Two different harvesting methods are used. Some harvesters grasp the inner-whorl leaves and then twist and pull the leaves to free them from the rhizome. Others prefer to cut the leaves off at ground level with a knife. Once the leaves have been removed, a harvester holds the bunch by the tips of the longest leaves and shakes gently to allow the shorter leaves to fall to the ground. Then, holding the leaves by their base, the bunch is hit against the harvester’s leg or other solid object several times to remove any brown tips. The leaves are then gathered and tied with a rubber band in small bundles, each weighing about two pounds (the bundles are referred to as “hands” by the nontimber forest products industry). The leaves are sold to contractors or buyers on a fresh weight basis. The hands are typically rebundled at the buying station to meet desired fresh weight requirements, as it is difficult to produce hands of a uniform weight under field conditions.
Materials and Methods The study took take place in and around the Cispus Adaptive Management Area on the Gifford Pinchot National Forest, on the west side of the southern Cascades of Washington. The Cispus AMA is approximately 120 km northeast of Portland, OR. The study sites were located in the Pacific silver fir/big huckleberry/beargrass plant association and in the mountain hemlock/big huckleberry/beargrass plant association. For brevity, we will refer to these two plant associations as the Pacific silver fir and the mountain hemlock types. Ten sample sites were selected in each plant association for a total of 20 sites. Sites were selected to span a range of canopy densities from low to medium and high. Categories were defined as follows: low density was 30 – 60%, medium density was ⬎60% but ⬍90%, and high density was ⬎90%. Determination of the boundary between the low- and medium-density categories (60%) was based on the observation that no beargrass of commercial quality was found under overstory canopies with a density of less than 60%. In these stands, leaves of beargrass had a chlorotic appearance and were often plagued with necrotic spots. For the high-density WJAF 19(2) 2004
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category (⬎90%), we wanted to include only the darkest understory conditions, so that we would be assessing beargrass response to the most reduced light conditions; this category had to have a wide enough range to ensure we could find enough stands to sample. A boundary of 90% fulfilled these requirements. Within each sample site, four 1 ⫻ 10-m belt transects were placed perpendicular to each other and were oriented around a central point. Overstory canopy density was determined using a spherical densiometer (Forest Densiometers, Bartlesville, OK) (Lemmon 1957). Densiometer readings were taken at five locations within each plot: at the ends of each of the perpendicular transects, and at the center of the plot where the transects intersected. Overstory density of the plot was determined from the average of these five readings taken during July and August and was expressed as a percentage. For example, a canopy density of 80% indicated that the sky was obscured by overstory vegetation in 80% of the densiometer’s designated sample points. In all four belt transacts, individual beargrass plants were assessed in terms of their quality (marketability) and basal area. Basal area of beargrass was determined using an elliptical model. The major and minor axes were measured to the nearest centimeter with a tape at the base of each individual tussock. Basal area was calculated as Area ⫽ ab, where a ⫽ 1⁄2 the length of the major axis and b ⫽ 1⁄2 the length of the minor axis. Two belt transacts were randomly selected for harvest. The other two were reserved as controls for possible future remeasurement. The inner whorl (new growth) of each beargrass tussock that was rooted within the belt transect was harvested and weighed following normal commercial harvest practices with the leaves severed from the plant using a knife (see Background) in early to mid September. The harvest was timed to allow for maximum plant growth for the year, while avoiding the heavy fall rains and light snows that commonly occur in the study area in late September and early October. The fresh weight, dry weight (60°C, 72 h), and length of the longest leaf were recorded for each inner whorl. Leaf color was determined by comparison of the leaves with Munsell Color Charts for Plant Tissues (Killmorgen Instruments Corporation, 405 Little Britain Road, New Windsor, NY). Diameter at breast height (dbh) was measured on all trees within a 20 ⫻ 20-m macroplot superimposed on the four
10-m belt transects. Coverage of shrub species was determined by line interception from two 20-m transects within the macroplot. Basal area (m2)/ha was calculated for each tree species. Coverage of herbaceous species other than beargrass was estimated using coverage classes (Daubenmire and Daubenmire, 1968) within 20 2 ⫻ 5-dm microplots located at 1-m intervals adjacent to two of the four belt transects.
Analysis Because only those plots having greater than 60% tree canopy density produced commercially harvestable quality beargrass during the late summer harvest, further statistical analyses were based on the medium and high-canopy-density plots. This left a total of eight sample sites within the Pacific silver fir type and nine sites within the mountain hemlock type. The response variables analyzed were total basal area of beargrass per site, mean basal area of beargrass per plant, length of the longest leaf per plant, and the fresh and dry mass of harvestable leaves per plant. The effects of canopy density (medium versus high) and plant association (Pacific silver fir versus mountain hemlock) were assessed using a two-way analysis of variance (ANOVA) model with subsampling (Steel and Torrie 1960). Both canopy density and plant association were considered to be fixed factors. The analysis was performed using PROC GLM with mean separation determined on the basis of the PDIFF option in the LSMEANS statement (SAS, Inc. 1988). The analysis of the response of beargrass leaf color to overstory density was done by inspection rather than by formal statistical analysis.
Results For mean harvestable dry mass of beargrass per plant, the only independent variable that had a significant effect was overstory canopy density (medium versus high, P ⬍ 0.01). For the medium-density category, mean harvestable dry mass of beargrass was 3.6 ⫾ 0.07 g, and for the high-density category, 3.2 ⫾ 0.06 g). There was a significant interaction between plant association and canopy density for total basal area of beargrass (P ⫽ 0.03), mean basal area per plant (P ⬍ 0.01), fresh leaf length (P ⬍ 0.01), and fresh mass of harvestable leaves per plant (P ⫽ 0.02). In the Pacific silver fir association, total basal area of beargrass, basal area per plant, and length of
Table 1. The effect of plant association and overstory canopy density on plant characteristics of beargrass. Data are means followed by one standard error of the mean in parentheses.
Plant association
Canopy density class*
Sample size (n)
Total basal area of beargrass per site (dm2/m2)
Basal area per plant (dm2)
Length of longest leaf (cm)
Fresh mass of harvestable leaves (g)
Pacific silver fir Pacific silver fir Mountain hemlock Mountain hemlock
High Medium High Medium
5 3 5 4
18.4 (2.0) a† 6.3 (2.6) b 12.4 (2.0) ab 11.0 (2.3) b
3.3 (0.1) a 1.0 (0.1) c 1.5 (0.1) b 1.3 (0.1) b
62.6 (0.5) b 56.2 (0.7) c 67.3 (0.5) a 63.3 (0.5) b
8.5 (0.4) b 11.0 (0.5) a 10.3 (0.4) a 10.5 (0.4) a
* 60% ⱕ medium density ⬍ 90%; high density ⱖ90%. † Values within a column and followed by the same letter are not significantly different (P ⱕ 0.05). For all tabled response variables, the interaction between plant association and canopy density class was significant. All plots were located in either the Pacific silver fir/big huckleberry/beargrass plant association or in the mountain hemlock/big huckleberry/beargrass plant association in the southern Cascades of Washington.
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Table 2. Leaf colors of beargrass collected from plots in which beargrass leaves were of commercially acceptable quality. All plots were located in either the Pacific silver fir/big huckleberry/beargrass plant association or in the mountain hemlock/big huckleberry/beargrass plant association in the southern Cascades of Washington. Hue*
Value*
Chroma*
5 GY 5 GY 5 GY 5 GY 5 GY 7.5 GY 7.5 GY 7.5 GY 7.5 GY 7.5 GY 7.5 GY
3 4 4 4 5 3 4 4 5 5 6
4 4 6 8 6 4 4 6 4 6 4
In contrast, for the mountain hemlock plant association, few differences were detected between canopy-density categories (Table 1). Total basal area of beargrass per site, mean basal area per plant, and fresh mass of harvestable leaves per plant were similar regardless of overstory density. Only the length of the longest leaf was greater under the high-overstory-density class. The leaf colors identified in beargrass leaf samples from plots with commercially useful leaves is presented in Table 2. In general, leaves with a color value of ⱖ6 were too light to be of use, the exception being when chroma was ⱕ4 (See the footnote on Table 2 for definitions.). Big huckleberry and beargrass dominated the shrub and herb layers, respectively (Table 3). These were the only two understory species that showed consistent, high coverage in the stands. Most of the studied stands were late successional stands as indicated by Douglas fir (Table 3). These Douglas fir trees were generally large, few in number, and the species had poor reproduction on the respective site. For example, the 22.3 m2 /ha contributed by Douglas fir in the high-canopy density (92.4%), mountain hemlock plot (leftmost association column in Table 3) was the total of three large trees. Only one Douglas fir seedling was noted in the plot (data not shown). The 66.6 m2 /ha of mountain hemlock in
* See Munsell Color Chart for Plant Tissues. Hue is the general color, e.g., red, yellow, red-yellow, etc.; value is the degree of lightness or darkness relative to a gray scale with 0 as black and 10 as white; chroma is the strength or saturation of a color, with a value of 0 being neutral gray and progressively higher values being increasing degrees of saturation.
longest leaf were significantly greater under the high-density class (Table 1). The fresh mass of harvestable leaves, however, was significantly greater under the medium-density class (Table 1).
Table 3. Vegetative characteristics of selected plots in the mountain hemlock/big huckleberry/beargrass (mountain hemlock) and Pacific silver fir/big huckleberry/beargrass (Pacific silver fir) associations of the southern Cascades of Washington. Plant association of plots Characteristics
Species
Canopy density (%) Trees basal area (m2)/ha Abies amabilis A. lasiocarpa A. procera Picea engelmannii Pinus monticola Pseudotsuga menziesii Tsuga heterophylla T. mertensiana Shrub coverage (%) C. umbellata G. ovatifolia M. ferruginea P. myrsinites R. gynmocarpa S. aucuparia V. membranaceum
Mountain hemlock
Mountain hemlock
Pacific silver fir
Pacific silver fir
92.4
85.2
78.1
92.3
⬍0.1 22.0 — 0.6 — 22.3 — 17.5
0.1 5.3 0.7 — — 19.2 0.4 10.3
8.0 2.4 — — 1.0 4.9 — 0.8
19.2 — — — — 16.6 0.5 66.6
⬍1 — — ⫹* — — 24
— — — ⬍1 ⬍1 — 26
6 — — — — ⬍1 21
2 1 ⬍1 — — — 35
— — — ⫹ — — — — ⫹ ⬍1 ⬍1 11.2†
— — — — 3 — 5 ⫹ ⫹ 2 ⬍1 12.9
3 — 1 — 3 — 7 — — 10 ⬍1 5.9
— ⫹ — ⫹ — 1 — — — — — 28.8
Herbs coverage (%) A. margaritacea C. uniflora E. spp. G. oblongifolia H. albiflourm L. borealis L. polyphyllus P. recemosa P. secunda R. lasiococcus V. orbiculata X. tenax
* “⫹” indicates the species was present in the stand, but did not fall within the sample plots. † Basal area (dm2)/m2.
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the high-canopy density Pacific silver fir plot was the result of only four trees. Reproduction of mountain hemlock in the plot was lacking, whereas Pacific silver fir trees showed a size distribution characteristic of a reproducing population (data not shown).
Discussion Several important conclusions regarding commercial beargrass characteristics and overstory canopy and plant association can be made from our study. It is clear that beargrass quality is not of a commercial grade under the open canopies of recent burns or clear-cuts. The 60% density threshold could be used by managers to predict sites that beargrass harvesters are likely to avoid, regardless of plant association. After management or natural disturbance destroys the overstory, commercial quality beargrass could be obtained most rapidly if the overstory is quickly reestablished. Hence, thinning and other forms of partial cutting should be favored if harvestable quantities of beargrass are considered an important management objective. Managers could use the 60% density threshold in areas where beargrass is being managed for commercial harvest. Leaves with color that was commercially acceptable fell within a fairly wide range of color. Although 5GY 3/4 and 7GY 3/4 are both very dark, 5GY 5/6 is a comparatively light color and was still acceptable. Leaves with color values ⱖ6 tended to be too chlorotic for commercial harvest. Leaves with this high a value occurred almost exclusively in the plots with ⱕ60% overstory density. In these more open plots, even the numerous leaves that had darker, otherwise acceptable color had necroses that rendered the leaves unacceptable. The mean harvestable dry mass of beargrass per plant did not differ between the Pacific silver fir and mountain hemlock plant associations. Harvestable dry mass was greater, however, under medium- compared to high-canopy-density conditions for both plant associations. Therefore, for either of these plant associations, harvesters will likely have greater commercial yields of dry plant matter if they harvest under overstory canopies with 60 –90% density, rather than areas that exceed 90%. For the Pacific silver fir association, the high-overstorydensity class had significantly greater total basal area of beargrass, and individual beargrass plants were significantly larger with longer leaves than in sites of medium-canopy density. We did not find this relationship for the mountain hemlock association, except for the longest leaf variable. Thus total basal area of beargrass and plant size appears to be more closely related to overstory conditions in the Pacific silver fir plant association compared to the mountain hemlock association. In both plant associations, the length of the longest leaf was greatest under the high-overstorydensity class. Because the plants are larger with longer leaves, harvesters may tend to concentrate their activities in these more shaded stands, particularly in the Pacific silver fir association even though both dry and fresh harvestable mass might be higher under the medium-density canopy. 86
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It is unclear why the total basal area and size of beargrass appears to be more closely related to overstory density conditions in the Pacific silver fir association. A number of biotic and abiotic processes could be responsible for this result. In selecting our plots for study, we were unable to control for site history, including overstory disturbance history, stand age, and beargrass harvest on the site. Sites on which beargrass had been recently harvested were avoided, but it is unknown what influence may have been exerted by harvest 4 to 5 or more years prior to site selection. It is frequently assumed that beargrass plants fully recover in terms of appearance within 4 to 5 years, but no studies have been published to confirm this assumption. The best way to avoid the potential confounding influences of past harvest would be to use plots in logistically difficult and often politically untenable wilderness areas or parks. Alternatively, beargrass characteristics may be more closely related to such things as competition, environmental factors, or soil fertility. Evaluation of the sustainability of beargrass as a non timber forest product will require long-term study of the relationships among environmental variables, beargrass productivity, and beargrass population dynamics.
Literature Cited ALABACK, P.A. 1982. Dynamics of understory biomass in Sitka spruce–western hemlock forests of southeast Alaska. Ecology 63:1932–1948. BAILEY, J.D., C. MAYRSOHN, P.S. DOESCHER, E. ST. PIERRE, AND J.C. TAPPEINER. 1998. Understory vegetation in old and young Douglas-fir forests of western Oregon. For. Ecol. Manage. 112: 289 –302. BROCKWAY, D.G., C. TOPIK, M.A. HEMSTROM, AND W.H. EMMINGHAM. 1983. Plant association and management guide for the Pacific silver fir Zone, Gifford Pinchot National Forest. PNW R6-ECOL-13OA-1983, Portland, OR. 122 p. DAUBENMIRE, R., AND J.B. DAUBENMIRE. 1968. Forest vegetation of eastern Washington and northern Idaho. Agric. Res. Ctr., XT 0060 Coll. of Agriculture and Home Economics, Washington State Univ., Pullman, WA. 104 p. DIAZ, N.M., C.T. HIGH, T.K. MELLEN, D.E. SMITH, AND C. TOPIK. 1997. Plant association and management guide for the mountain hemlock zone, Gifford Pinchot and Mt. Hood National Forests. USDA For. Ser., PNW Region, R6-MTH-GP-TP-08-95, Portland, OR. 101 p. FRANKLIN, J.F., W.H. MOIR, M.A. HEMSTROM, S.E. GREENE, AND B.G. SMITH. 1988. The forest communities of Mount Rainier National Park. Scientific monograph series no. 19, USDI National Park Service, Washington, DC. 194 p. HALPERN, C.B., AND J.F. FRANKLIN. 1990. Physiognomic development of Pseudotsuga forests in relation to initial structure and disturbance intensity. J. Veg. Sci. 1:475– 482. HITCHCOCK, C.L., AND A. CRONQUIST. 1973. Flora of the Pacific Northwest. Univ. of Washington Press, Seattle, WA. 730 p. KLINKA, K., H.Y.H. CHEN, Q. WANG, AND L. DE MONTIGNY. 1996. Forest canopies and their influence on understory vegetation in early seral stands on West Vancouver Island. NW Sci. 70:193–200. LEMMON, P.E. 1957. A new instrument for measuring forest overstory density. J. For. 55:667– 668. MAULE, S.M. 1959. Xerophyllum tenax, Squawgrass, its geographic distribution and its behavior on Mount Rainier, Washington. Madrono. 15:39 – 48. MCKENZIE, D., AND C.B. HALPERN. 1999. Modeling the distributions of shrub species in Pacific Northwest forests. For. Ecol. Manage. 114:293–307. MCKENZIE, D., C.B. HALPERN, AND C.R. NELSON. 2000. Overstory influences on herb and shrub communities in mature forests of western Washington, U.S.A. Can. J. For. Res. 30:1655–1666. SAS INSTITUTE, INC. 1988. SAS/STAT® User’s Guide, release 6.03 Ed. SAS Institute, Inc., Cary, NC. 1,028 p.
SCHLOSSER, W., K.A. BLATNER, AND B. ZAMORA. 1992A. Pacific Northwest forest lands potential for floral greenery production. NW Sci. 66(1):44 –55. SCHLOSSER, W.E., C. TALBOTT ROCHE´ , K.A. BLATNER, AND D.M. BAUMGARTNER. 1992B. A guide to special forest products of the Northwest. Washington State Univ. Coop. Ext. Bull EB1659, Pullman, WA. 11 p.
STEEL, R.G.D., AND J.H. TORRIE. 1960. Principles and procedures of statistics. McGraw-Hill Book Company, Inc., New York, NY. 481 p. THOMAS, S.C., C. HALPERN, D. A. FALK, D. A. LIGUORI, AND K.A. AUSTIN. 1999. Plant diversity in managed forests: Understory responses to thinning and fertilization. Ecol. Applications 9:864 – 879.
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