Great Basin Naturalist
Volume 55 Number 1 Article 4
1-16-1995
Historic expansion of Juniperus occidentalis (western juniper) in southeastern Oregon
Richard F. Miller East Oregon Agricultural Research Center, Burns, Oregon
Jeffery A. Rose East Oregon Agricultural Research Center, Burns, Oregon
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Recommended Citation Miller, Richard F. and Rose, Jeffery A. (1995) "Historic expansion of Juniperus occidentalis (western juniper) in southeastern Oregon," Great Basin Naturalist: Vol. 55 : No. 1 , Article 4. Available at: https://scholarsarchive.byu.edu/gbn/vol55/iss1/4
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. G,eal Basin Naturnlist 55(1), i:> 1995, pp. 37-45
HISTORIC EXPANSION OFJUNIPERUS OCClDENTAUS (WESTERN JUNIPER) IN SOUTHEASTERN OREGON
Richard F. MilI«1 and Jeffel)' A. Rosel
ABsTRACT.-The chronology ofJuniperus occidentalis (western juniper) expansion in eastern Oregon, the effect of plant canopy and interspace on J. occidentoIis seedling establishment and growth rates, and the age ofJ. occidentalis maximum reproductive potential were determined. Measurements were recorded in twenty-two O.4-ha plots estab lished in sagebrush-grassland communities and six O.l-ha plots in Populus tremuloides (quaking aspen) couununities.]. ve.cidentaUs began increasing dUring the 18805 in stands containing trees > 130 yr old. Relatively steady establishment ensued into the J950s and then began to progress at a geometric rate in the 19605. J. occidentalis encroachment into aspen stands began between 1910 and 1920. The largest proportion of juvenile trees established beneath ArtemWa species in sagebrush.grassland conununities. J. occidentalis trees appeared to reach fun reproductive potential at >50 yt ofage. The ratio ofmaleJemaIe trees increased &om 1.7 in scattered]. occidmtalis stands to 3.8 in closed stands. The initiation ofJ. occidentalis encroachment during: the late 1800s coincides with optimal climatic conditions for Juniperus berry production and establishment, reduced 6re-remm intervals, and heavy livestock grazing. The accelerated increase in J. occiJk,ltalis expansion since 1960 may be due 10 the continued absence of fire, abundant ~-oody planl cover, and the large increase in J. occidentalis seed production.
Key worcU: western juniper, Juniperus occidentalis, expansion, Great Basin, intermountain shmh steppe, aspen. Populus tremuloides, succession.
One of the most pronounced plant commu nah-like (Nichol 1937, West 1988), or confined nity changes in the 20th century has occurred to rocky surfaces or ridges (Cottam and Stewart in the juniper and pinyon-juniper woodlands, 1940, Barney and Frishknecht 1974, Hopkins a major vegetation type characterizing the 1979, Johnson and Simon 1987}.]. occidentalis Intermountain Region. These woodlands, began increasing in both density and distribu sometimes described as pygmy forests, cur tion in the late 1800s (Burkhardt and Tisdale rently occupy 17 million ha throughout this 1976, Young and Evans 1981, Eddleman 1987), region (West 1988). Juniperus occidentalis ssp. invading Artemisia !ridentata subsp. vaseyana occidentalis Hook. (western juniper) is consid (mountain big sagebrush), Artemisia arbuscula ered the Northwest representative of the pin (low sagebrush), Populus tremuloides (quaking yon-juniper zone in the Intermountain Region aspen), and riparian communities. Although ]. (Franklin and Dyrness 1973) and occupies occidentalis is long lived (Vasek 1966, Lanner over 1 million ha (Dealy et al. 1978) in eastern 1984), less than 3% ofthe woodlands in Oregon Oregon, southwestern Idaho, and northeastern are characterized by trees > 100 years old California (Cronquist et al. 1972). This sub (USDI-BLM 1990). In 1825, Ogden obsel'Ved species of]. occidentalis is found primarily only occasional]. occidentalis (reported as north of the polar front gradient (Neilson cedars) growing on hillsides while t,aveling 1987; parallel to the O lEastem Oregon Agrieultunl Resew:ch Center, He 714.51 H,"Y 205, Burns. OR 97720. The ENtem ~D Agricultu.nll Resean:h Center. including the Bunu and Union JlalioD$, is jointl)' operated by the Oregon AgricultuDl .E.x-periment Station of Oregon State University and USDA AgricultW1l1 Re~eareh Service. 37 38 GREAT BASIN NATURALIST [Volume 55 occidentalis densities on this site range expanding on the majority of sites measured, between 125 and 250 ha-I . In Silver Lake, Early observations on Steens Mountain indi Oregon,]. oeeidentalis density increased from cate the landscape contained only scattered 62 ha-I in 1890 to over 400 ha-I by 1970 stands of]. oeeidentalis (Griffiths 1902). Since (Adams 1975). On another site in central 1900 the abundance of]. oeeidentalis pollen in Oregon where trees were absent prior to the Steens Mountain area has increased five 1880,]. oeeidentalis increased to 1018 ha-I by fold (Mehringer and Wigand 1990). 1980 (Eddleman 1987). Recent expansion is Plant communities characteristic ofJ. occi similar to increases in other Juniperus species dentalis woodlands are Artemisia tridentata throughout western United States (Ellis and ssp. vaseyana!Festuea idahoensis (Idaho fes Schuster 1968, Tausch et al. 1981, West 1984, cue), Artemisia arbW3eulnlF idahoeWJis, and E Tausch and West 1988). tremuloides. P. tremuloides communities on The objectives of our study were to (1) Steens Mountain range in elevation from 1760 describe the chronology of J. oeeidentalis to 2400 m. At lower elevations, in the]. oeei expansion during the past several centuries in dentalis woodland belt, E tremuloides stands southeastern Oregon, (2) determine the effect form long, narrow communities along north of plant canopy and interspace on]. oecUkn aspects, which capture windblown snow and talis seedling establishment and growth rates, runoff. and (3) determine the age when]. oeeidentalis reaches maximum reproductive potential. Plot Layout Plot locations were selected in an attempt METHODS to reflect sagebrush-grassland communities in different stages of]. oeeidentalis invasion on Study Area the west slope of Steens Mountain. Old stands The study area is located on Steens Moun on the rocky outcrops, which make up only a tain in southeastern Oregon, approximately 80 small percentage of present-day woodlands, km south of Burns. This isolated volcanic were not measured. Sites selected support, or fault-block, which lies in the extreme north have the potential to support, sagebrush-grass west Basin and Range Province (Fenneman land communities. Currently these sites are 1931), is about 80 km long and oriented in a occupied by varying numbers and sizes of]. northeast direction (Baldwin 1981). The eleva occidentalis dominance, creating a woodland tion of Steens Mountain ranges from 1268 to structure of dispersed, intermediate, and 2949 m, with a steep east-facing escarpment closed tree stands (Table 1). Twenty-two O.4-ha and a gentle west-facing slope. Climate is cool plots were located within the]. oecidentalis and semiarid, characteristic of the northern belt of Steens Mountain; they ranged from 1500 Great Basin. Annual precipitation at the lower to 2000 m in elevation and were distributed 32 elevations averages 220-280 mm, increasing to km along the mountain range. Plots were situ > 700 mm at higher elevations (NOAA 1993). ated along an elevation gradient representing Most moisture is received as snow in Novem communities from the lower- to upper-eleva ber, December, and January and as rain in tion]. oceidentalis woodland belt. Dominant March through June. understory vegetation in the dispersed and ]. oeeidentalis woodlands on Steens Moun intermediate plots was A. tridentata spp. tain form a discontinuous belt between 1450 vaseyalW and Festuea idahoeWJis (13 stands), and 2100 m in elevation, Severe winter condi A. arbW3cula and F. UWhoeWJis (4 stands), and a tions probably restrict]. oeeidentalis from ex mosaic of A. arbW3eula and A. tridentata ssp. panding into higher elevations (Billings 1954, vaseyalW (2 stands). Understory vegetation in Mehringer 1987). Limited distribution below the closed stands (n = 3) comprised a few 1500 m is possibly due to a combination oflate remnant deep-rooted perennial grasses, skele spring frosts (Billings 1954) and limiting mois tons of dead A. tridentata ssp. vaseyana, and ture. Tree canopy cover varies from open to 70% bare ground (EOARC data file). 30% cover, except on mesic P. tremuloides sites An additional six O.i-ha plots were estab where]. oeeidentalis cover approaches 100%. lished in six separate E tremulaides stands. However, based on age structure and canopy Three stands were in advanced stages of J. leader growth, tree canopies are still actively occidentalis invasion with few to no adult E 1995] WESTERN JUNIPER EXPANSION 39 TABLE L Juniperus occidentalis stand maturity classes (2) obvious but not abundant, and (3) abundant. in Artemisia communities (modified from Blackburn and In each plot a lO-tree subsample was ran Tueller 1970). domly selected for aging in each offour height Closed Abundant adult trees generally >5 m tall classes: (1) <0.5 m, (2) 0.5-1.8 m, (3) 1.8-3 m, and usually several trees>130 yr ofage, with little understory, particularly on and (4) >3 m. In several ofthe dispersed plots, south slopes. sample size for trees >3 ill was smaller than 10, due to a lack of trees. We also sampled all Intermediate Abundant]. occidentalis ofall age classes, old trees on plots when they occurred (n = 0-5 with a more open tree canopy and an 1 understory beginning to decline; trees ha- ). Old trees were easily identified by their > 130 )IT ofage are rare. growth form, containing rounded tops and heavy limbs, and lacking strong terminal Dispersed Abundant young trees <2 ill tall, a few leader growth (Burkhardt and Tisdale 1969). A adult trees but old trees absent, and a well-developed understory. cross section was removed approximately 30 em above ground level from each tree >0.5 m tall and at ground level for trees < 0.5 m, and tremuloides trees and dead P. tremuloides then brought back to the lab for aging. Two trunks on the ground. The remaining three radii from each cross section were polished, stands were characterized by a dominant P. stained, and counted. Age was estimated by tremuloides overstory and an understory of averaging both radii and adding 10 yr to cor young]. oeeidentalis. Elevation for the]. ocei rect for the 30-cm base. Mean differences dentalis-P. tremuloides plots ranged from 1930 between radii were 4% for trees >50 yr and to 2000 m, all with a similar northeast aspect. 1% for trees < 50 yr of age. Adams (1975) Measurements reported that growth-ring characteristics of]. oceidentalis are useful in dendrochronological Prior to sampling, string was stretched along studies. The presence of false and missing the contour ofeach O.4-ha plot at1-m intervals rings was similar to that for Pinus ponderosa. to keep track ofmeasured trees.]. occidentalis Over 1200 trees were aged and approximately 1 density (trees ha- ) was recorded for trees 14,000 counted and measured. In the six P. <0.5 m tall, defined as adult, across the entire tremuloides stands, density of both]. oeciden plot. Tree height, minimal and maximal crown talis and P. tremuloides and age and height for diameters, and basal area just above the trunk ]. occidentalis were measured across the entire swell at the stem base near the litter layer O.l-ha plot. were recorded. Tree height was measured Evidence indicated minimal]. oeeidentalis with a tape for trees < 2 m and a clinometer mortality has occurred on Steens Mountain for trees >2 m tall. Tree canopy cover was during the past 120 years. We observed very estimated by adding crown area measure ments of all trees for each plot. Similar mea few dead or dying trees for all age classes surements were recorded on juvenile trees (excluding seedlings), except where individual (defined as trees < 0.5 m tall), but only those J. oecidentalis trees had been cut or burned. on the lower left quarter (0.1 hal of each 0.4 Mortality ofjuniperus species rapidly declines ha plot. Current-year]. oceidentalis seedlings following the seedling stage (Van Pelt et al. (any plant with cotyledons still attached) were 1990). juniperus has few pests that prove fatal not recorded. Establishment location of each to the tree (Lanner 1984). We avoided recently juvenile tree was recorded: beneath the canopy cut or burned stands, which constituted a of]. oecidentalis, Artemisia, other shrubs, tus small percentage of]. oceidentalis-occupied sock grass, or in the interspace. Less than 1% of stands. "There remains of dead trees were juveniles were located beneath other shrubs or observed, we noted they persisted for a long grasses; therefore, only]. oecidentalis, Artemisia, period of time. By recutting several stumps and interspace are reported. adjacent to one of our plots and aging and ]. occidentalis is considered submonoecious matching ring widths with adjacent live trees, (Vasek 1966). Male and female reproductive we determined these trees were harvested status was detennined by estimating abundance around 1920. Others have also observed the of cones and berries for each tree. Abundance persistence of juniperus stumps (Young and was ranked in four classes: (0) absent, (1) scarce, Budy 1979). 40 GREAT BASIN NATURALIST [Volume 55 Statistical Analysis characterized by an abundance of adult trees (> 3 m tall), a tree canopy cover of 18-28% Height growth data for adult trees were (Table 2), and the presence of a few old trees analyzed using a randomized complete block (130+ yr; 2 to 5 ha-I ).]. occidentalis densities design in PROC GLM of SAS (SAS 1986). began increasing in these stands between Means were separated using Duncan's 1878 and 1890. In the intermediate]. occidenr Multiple Range Test at p < .05 level. A split taUs stands, trees>130 yr were rare. Tree plot design was used in the analysis ofjuvenile canopy cover ranged from about 8 to 16%, and height growth. Main plots were sites and sub densities of adult trees varied from 35 to 100 plots were location ofestablishment (interspace, ha-I . Trees <3 m in height, particularly juve Artemisia,]. occidentalis). A Duncan's Multiple niles, were abundant.]. occidentalis expansion Range Test was used to separate the means. in these sagebrush-grassland communities began between 1890 and 1910. In the dis REsur;rs persed stands few trees were >60 yrs old, and Little change in j. oeeidentalis density we aged no trees > 100 yr, Tree canopy cover was usually <5% in the dispersed stands and appeared to occur between the early 1700s I and the 1880s (Fig. 1). We encountered old densities of large adult trees <35 ha- . trees (standing trees > 130 years old, large Invasion of]. occidentalis into these sage stumps, and burned-out trunks) on several A. brush-grassland communities began after arbuseula flats and A. tridentata ssp. vaseyana 1930. communities. However, data indicated preset Greatest densities of]. occidentalis trees tlement tree densities in these Artemisia com measured on Steens Mountain occurred in P. munities were < 5 trees ha-l , suggesting very tremuwides sites (Table 3). In the late stages of open]. oeeidentalis stands. The first evidence ]. occidentalis succession on these sites, tree of an increase in tree densities occurred in the canopy cover approached 100%. Live P 1880s, with relatively steady establishment tremuloides occurred only on one of the three ensuing into the 1950s, similar to that sites, and almost all trees were <0.5 m tall. In observed by Tausch and West (1988). In the the remaining two stands only the remnants of 1960s]. oeeidentalis establishment began large P tremuloides trunks decaying in the occurring at a geometric rate. understory were present. J. occidentalis inva Closed]. oecidentalis stands, which once sion in these P. tremuloides sites began be supported A. tridentata ssp. vaseyana, were tween 1910 and 1920. No]. occidentalis trees 7 -' « l- e 0 I- , u. 0 4 I- Z UJ 3 t.l a: UJ 2 a. 1 o +-;==T==r==;==r==r===;===;=~ ~~-----.-----,-l- 170017201~0176017801800182016401860188019001920194019601~O YEAR Fig. 1. Years nfestablishment for]uniperus occidentalis trees on Steens Mountain, Oregon (n = 1200). 1995) WESTERN JUNIPER EXPANSION 41 TABLE 2. General description ofclosed, intermediate, and dispcrsed]wliperus occidentolis stands OIl Steens Mountain in Artemisia tridentata ssp. vaseyanl1 and A arbuscula communities, and the percentage ofjuveniles located hene-ath]. occidentalis, Artemisia, and interspace. Canopy cover, basal area, and density means are followed by range ill paren !hese' ( ). Establishment site % Density (#ha-I ) (for juveniles) Canopy Basal area Adults Juveniles # sites cover % (m 2ha-1) >0.5 In ht <0.5 m ht J. occidentalis Artemisia Inlerspac.:e A. tridentata ssp. caseyana closed 6 22 (18-28) 5.2 (3.1-9.8) 296 (217-496) 580 (1l8-1226) 86~1 9b 5' intermediate 8 6 (5-10) l.8 (0.5-4.7) 95 (5Q...I65) 815 (335-1423) 29b 58' 13' dispersed 2 2 (1-3) 0.4 (0.2-().6) 52 (31-70) 188 (96-280) 3b 50" 47' A. arbuscula closed 3 15 (12-20) 3.5 (l.8-5.4) 158 (7....247) 99 (2Q...198) 27" 67' 6' intermediate 3 6 (4.5-6.7) l.8 (0.11-3.2) 104 (77-153) 375 (167-790) U b 61' 28" 'Sites ofestablishment means ('Jo) follnwed br ~;mir..r Iowe~ letters ,m: not signiflCaItdy different between <:J(.b1ishmenl sites within J. ocddlJlla!u stand lMurity c1it.1'St:!! (p" ,(5). >80 yr were encountered. In stands with a P. and female, and 30% contained neither fruits tremuloides overstory, P. tremuloides density of nor cones. J. occidentalis trees producing abun small shoots was greater than that ofJ. oeei dant crops of cones or berries were either delltalis. However, P. tremuloides size classes male or female dominant. No trees were mea between 0.5 m and large adults were absent, sured which contained an abundant crop of indicating a lack of P. tremuloides stand reju both berries and cones. Sixty-five percent ofJ. venation. On these sites]. occidentalis inva occidentalis trees with an abundant crop of sion began between 1930 and 1940. berries contained no male cones. The remain Height growth for young J. oceidentalis ing 35% contained only a scarce number of trees «20 yr) across all sites averaged cones. The majority of trees producing abun 2.9 em y,..-I. Based on growth rates and beight dant crops of male cones contained only oftrees between 10 and 20 yr ofage (n = 200) scarce numbers ofberries. Approximately 75% across all Artemisia sites, 90% of trees 15 yr of trees producing heavy crops of berries or old were < 1 m tall (64% were <0.5 m tall). cones were >50 yr old. Trees <20 yr old ex Surprisingly, height growth rates of juvenile pressing reproductive effort were rare and trees did not significantly differ between A. produced only a few cones or berries. The arbuscula and A. tddentato ssp. vaseyana ratio of trees producing large crops of cones communities. However, location of establish versus berries (cones:berries) increased from ment within communities significantly influ 1.7 in the scattered]. occidentalis stands to 3.8 enced growth rates of young j. oceidentalis in the closed stands. trees (Table 4). Trees establishing beneath an Artemisia canopy grew faster than young trees DISCUSSION growing in the interspace. Low densities and limited distribution of]. Shrub and tree canopies also significantly occidentalis trees> 130 yr and limited num influenced location ofJ. occidentalis seedling bers of dead trees or old stumps suggest ]. establishment in Artemisia communities. The occidentalis has greatly expanded on Steens largest proportion ofjuvenile trees was usually Mountain during the past 100 yr. Distribution located beneath canopies of A. tridentata ssp. of old trees was generally limited to rocky vaseyana or A. arbuscula and]. occidentalis ridges and A. arbuscula communities. Old (Table 2). Less than 20% ofjuveniles acrOss all trees were found only occasionally growing in 22 Artemisia sites established in the interspace. deeper, well-drained soils such as A. tlidentata On Steens Mountain, for trees >0.5 m tall, ssp. vaseyana grassland communities and 32% expressed predominantly only male or were absent in P. tremuloides l.'ommunities. In only female characteristics, 38% both male northeastern California, Barbour and Major 42 GREAT BASIN NATURAUST [Volume 55 TABLE 3. Mean densities (# ha-1) followed by range in () of Populus tremuloides and ]tmiperus occidentalis in P. tremuloides sites. P. IremuloUle, Stage of ]. occuIentalis SUCCesSl00 Adult Juvenile Adult Juvenile Lat. (n = 3) 17 1316 1392 9462 (0-50) (0-3952) (9"..!I--2203) (4327-18.791) Intermediate 1060 6553 1090 2816 (n = 3) (476-1670) (5266-G480) (632-1739) (622-5968) (1977) found a similar distribution of old and support this hypothesis. Fires set by Native young J. occidental., trees. A. tridentata ssp. Americans also declined in the 19th century """eyana and A. arbl/sclIla communities, which due to large reductions in their populations contained a low density ofJ. accidentalis trees caused by European diseases (Thompson prior to settlement. were the earliest sites to 1916, Cressman 1981) and relocation to reser initiate an increase in]. occidentalis. Dates of vations in the 1870s. initial establishment of closed and intermedi The invasion of conifers into P. tremuloides ate stands were similar to periods of early communities is a common occurrence through stand development reported by Young and out the western u.s. However, conifers report Evans (1981) in northeastern California and ed to typically invade P. tremllloides stands are Eddleman (1987) in central Oregon. species adapted to more mesic sites such as Expansion of]. occidentalis coincides with Pinus contO/ta ~odgepole pine), P. ponderosa, Euro·American settlement in this portion of Pseudotsuga menziesii (Douglas-fir), Abies the Great Basin. Although no direct cause-and concolor (white fir), Abies lasiocarpa (sub effect relationship can be drawn, we hypothe alpine fir), Picea engelmannii (Engelmann size that climate, altered fire frequencies. and spruce), and Picea pungens (blue spruce) (Bartos grazing in the late 1800s were primary factors 1973, Mueggler 1985). Invasion of the more initiating the recent expansion of]. occidentalis. drought-tolerant]. occidentalis into P. tremu Following the end of the Little Ice Age in the wides stands is not well documented. mid 1800s (Bryson 1989), winters became more P tremllioides is frequently considered a mild and precipitation increased above the fire-induced species, replaced by less fire tol present long-term average in tbe northern half erant conifers (Baker 1925, Daubenmire 1943, of the Great Basin between 1850 and 1916 Mueggler 1976). Prior to settlement, lightning (Antevs 1948, Graumlich 1985). Mild, wet win and human-set fires probably helped maintain ters and cool, wet springs promote vigorous many P. tremuloides communities. However, growth in J. occidentalis (Earle and Fritts the occurrence of fire in P. tremllloides slands 1986, Fritts and Xiangdig 1986). in the Rocky Mountains has been greatly Presettlement fire-return intervals in A. tri reduced since the late 1800s Gones and DeByle der/,tlIta ssp. vaseyana communities have been 1985). Mueggler (1985) suggested the combi reported to vary from 15 to 25 yr (Houston nation offire suppression and heavy grazing in 1973, Burkhardt and Tisdale 1976, Martin and P. ttr",wloides communities may favor the estab Johnson 1979). Burkhardt and Tisdale (1976) lishment ofconifers. concluded that fire-frequency intervals of An increase in Attemisia cover may also 30-40 yr would be adequate to keep]. occi enhance the invasion ofJ. occidentalis. As a dentalis from invading a sagebrush-grassland sagebrush-grassland community shifts towards community. Following settlement, frequency a greater dominance of sbrubs, the number of of fire in sagebrush grasslands has greatly safe sites for J. occidentalis seedling establish declined. The reduction of fine fuels by high ment increases. Others have also reported the densities ofdomestic livestock greatly reduced majority ofJ. accidentalis seedlings established the potential for fire in the Intermountian beneath Artemisia canopies (Burkhardt and Shrub Region (Burkhardt and Tisdale 1976, Tisdale 1976, Eddleman 1987). In west Texas, West 1988). Griffiths' (1902) observations of J. pinchotii frequently establishes beneath the overgrazed landscape on Steens Mountain mesquite plants (McPherson et al. 1988). 1995J WESTERN JUNIPER EXPANSION 43 TABLE 4. Mean growth rates for juvenile Juniperus occi ACKNOWLEDGMENTS dentalis trees (2-30 yr old) in three different establish ment sites. This is Technical Report 10,494 of the Establishment site em yr-1 Eastern Oregon Agricultural Research Center, Artemisia 3.3A Oregon State University. J. occidentalis 2.7AB LITERATURE CITED B Interspace 2.4 ADAMS, A. W. 1975. A brief history of juniper and shrub Means followed by similar uppercase letters are not significantly different (p populations in southern Oregon. Wildlife Research ~ .05). Report 6, Oregon State Wildlife Commission, Corvallis. ANTEVS, E. 1948. Climatic changes and pre-white man. Shading by nurse plants may benefit]. occi Pages 168--191 in The Great Basin, with emphasis on glacial and postglacial times. University of Utah dentalis seedlings (Johnsen 1962) by reducing Biological Service Bulletin 10(7). summer surface temperatures by 45-57% of BAKER, E S. 1925. Aspen in the central Rocky Mountain bare ground surface temperatures (Burkhardt region. USDA Bulletin 129l. and Tisdale 1976). Enhanced growth rates of BALDWIN, E. M. 1981. Geology of Oregon. Kendal/Hunt Publishing Co., Dubuque, IA. young trees growing beneath A. tridentata ssp_ BARBOUR, M.G., AND J. MAJOR. 1977. Terrestrial vegeta vaseyana suggest microclimates beneath shrub tion of California. Wiley Interscience, New York, NY. canopies are more beneficial than conditions BARNEY, M. A., AND N. C. FRlSHKNECHT. 1974. Vegetation in the interspace. Burkhardt and Tisdale changes following fire in the pinyon-juniper type of (1976) reported]. occidentalis seedling growth west-central Utah. Journal of Range Management 74,91-96. rates were correlated positively with Artemisia BARTOS, D. L. 1973. A dynamic model of aspen succes and correlated negatively with bare ground. sion. Pages 13-25 in Proceedings, IUFRO Biomass ]. occidentalis approached full reproductive Studies. IUFRO. potential near 50 yr. As]. occidentalis densities BILLINGS, W. D. 1954. Temperature inversions in the pinyon-juniper zone of a Nevada mountain range. increased, the proportion of trees became pre Butler University Botanical Studies 12. dominantly male across sites. Highly fecund BLACKBURN, W H., AND P T. TUELLER. 1970. Pinyon and female trees appeared to be most important in juniper invasion in black sagebrush communities in open stands where]. occidentali. was actively east-central Nevada. Ecology 51: 841-848. expanding. In central Oregon, Eddleman BRYSON, R. A. 1989. Late Quaternary volcanic modulation (1984) observed that trees in the interior of milankovith climate forming. Theoretical Applied Climatology 39: 115-125. woodlands were strongly dominated by male BURKHARDT, J. w., AND E. W TISDALE. 1969. Nature and cone production while trees growing in the successional status of western juniper vegetation in open produced more female cones. He also Idaho. Journal of Range Management 22: 264-270. reported trees did not produce significant __,-,' 1976. Causes ofjuniper invasion in southwestern quantities offruit until 50--70 yr ofage. Idaho. Ecology 76, 472-484. CARAHER, D. L. 1977. The spread of western juniper in Conclusion central Oregon. Pages 3-8 in R. E. Martin, J. E. Dealy, and D. L. Caraher, eds., Proceedings, Western Optimal climatic conditions around the Juniper Ecology and Management Workshop. turn of the century, reduced fire return inter USDA Forest Service, General Technical Report vals, and the indirect effect of livestock PNW-4. COTTAM, w: P, AND G. STEWART. 1940. Plant succession as through the reduction of fine fuels and an a result of grazing and of meadow desiccation by increase in Artemisia cover are probably pri erosion since settlement in 1892. Journal of Forestry mary factors that have contributed to the rapid 38,613--626. expansion of]. occidentalis in southeast CRESSMAN, L. S. 1981. The sandal and the cave. Oregon State University Press, Corvallis. Oregon during the late 1800s and early 1900s. CRONQUIST, A., A. H. HOLMGREN, N. H. HOL\IGREN, AND The accelerated increase in]. occidentalis J. L. REVEAL. 1972. Intermountain flora: vascular density and invasion during the last 30 years plants of the Intermountain West, U.S.A. Volume 1. into new communities is probably largely due Hafner Publishing Company, New York, NY. DAUBENMlRE, R. F. 1943. Vegetational zonation in the to the continued absence of fire, abundant Rocky Mountains. Botanical Review 9: 325-393. woody plant cover, and the large increase in]. DEALY, J. E., J. M. GEIST, AND R. S. DRISCOLL. 1978. occidentalis seed rain. Western juniper communities on rangeland of the 44 GREAT BASIN NATURALIST [Volume 55 Pacific Northwest. Pages 201-204 in D. E. Hyder, grasslands. American Midland Naturalist 102: ed., Proceedings, First International Rangeland 391-397. Congress, Denver, CO. MEHRINGER, P. J., JR. 1987. Late Holocene environments EARLE, C. J., AND H. C. FRITTS. 1986. Reconstructing on the northern periphery of' the Great Basin. Final river flow in the Sacramento Basin since 1560. report, Bureau ofLand Management, Portland, OR Report, California Department of Resources, Agree MEHRINGER, P. J., JR., AND P. E. WIGAND. 1990. Compari ment DWR B-55395. Laboratory of Tree-ring son of late Holocene environments from woodrat Research, University ofArizona, Tucson. middens and pollen: Diamond Craters, Oregon. Pages EDDLEMAN, L. E. 1984. Ecological studies on western 294-32S in J. L. Betancourt, T. R Van Devender, juniper in central Oregon. Pages 27--35 in T. E. Bedell, and P. S. Martin, eds., Packrat middens: the last ed., Proceedings, Western Juniper Management 40,000 years of biotic change. University of Arizona Short Course. Oregon State University and Extension Press, Tucson. Selvice, Corvallis. MITCHELL, V. L. 1976. The regionalization of climate in ___' 1987. Establishment and stand development of the western United States. 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