Western Juniper Woodlands
(of the Pacific Northwest)
Science Assessment
October 6, 1994
Lee E. Eddleman Professor, Rangeland Resources Oregon State University Corvallis, Oregon
Patricia M. Miller Assistant Professor Courtesy Rangeland Resources Oregon State University Corvallis, Oregon
Richard F. Miller Professor, Rangeland Resources Eastern Oregon Agricultural
Research Center
Burns, Oregon
Patricia L. Dysart Graduate Research Assistant Rangeland Resources Oregon State University Corvallis, Oregon
TABLE OF CONTENTS
Page
- EXECUTIVE SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- i
- WESTERN JUNIPER (
- Hook. ssp.
- )
- occidentalis
- Juniperus occidentalis
WOODLANDS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
Current Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Distribution of Western Juniper . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
Holocene Changes in Western Juniper Woodlands . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prehistoric Expansion of Juniper . . . . . . . . . . . . . . . . . . . . . . . . . . Historic Expansion of Juniper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44469
- Biology of Western Juniper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 11
Physiological Ecology of Western Juniper and Associated Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Western Juniper — Patterns in Biomass Allocation . . . . . . . . . . . . Western Juniper — Allocation Patterns of Carbon and Minerals . . Western Juniper — Biomass and Leaf Area Estimators . . . . . . . . . Western Juniper — Leaf Morphology of Adult Foliage . . . . . . . . . . Western Juniper — Seasonal Ring Growth . . . . . . . . . . . . . . . . . . Western Juniper — Physiological Ecology . . . . . . . . . . . . . . . . . . Western Juniper — Measured and simulated leaf conductance and transpiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Western Juniper — Modeling Leaf Conductance and
17 17 17 18 20 26 26 27
33
Transpiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gray Rabbitbrush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green Rabbitbrush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sandberg Bluegrass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Idaho Fescue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34 35 38 38 40
Ecology of Western Juniper Woodlands . . . . . . . . . . . . . . . . . . . . . . . . . .
Geology and Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Woodland Maturity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant Associations and Communities . . . . . . . . . . . . . . . . . . . . . . .
41 41 47 47 48
TABLE OF CONTENTS (Cont'd.)
Page
Range Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Woodland Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interactions and Succession . . . . . . . . . . . . . . . . . . . . . . . . . Fire Influences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Grazing Influences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Microbiotic Soil Crusts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of Disturbance on Microbiotic Crusts . . . . . . . . . . . . . Crust Influence on Soil Properties and
51 52 55 55 57 60 62 62 63
Seedling Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spatial Heterogeneity Within Microbiotic Crusts . . . . . . . . . .
67 70
Hydrologic Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Infiltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Runoff, Erosion, and Sedimentation . . . . . . . . . . . . . . . . . . . . . . . .
73 73 75 78
Wildlife in Western Juniper Woodlands . . . . . . . . . . . . . . . . . . . . . . . . . .
Large Herbivores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Birds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Small Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amphibians and Reptiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83 83 88 89 90 90
- Conversion of Western Juniper Woodlands . . . . . . . . . . . . . . . . . . . . . . .
- 91
Grazing Management of Western Juniper Woodlands . . . . . . . . . . . . . . .
Grazing Young Woodlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Grazing Management of Young Closed
98 99
Western Juniper Woodlands on Shallow Soils . . . . . . . . . . . . .
Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grazing Young Western Juniper Woodlands on Deep Soils . . . . . Grazing Management Following Western Juniper Control . . . . . . . Grazing Management on Sites Susceptible to Western
100 100 103 105
- Juniper Encroachment and Dominance . . . . . . . . . . . . . . . . . . .
- 107
- 108
- BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LIST OF TABLES
- Table
- Page
12
- Relationships between tree components (y) and tree age (x) . . . . . . . . . .
- 19
Dimensions of live junipers on Horse Ridge: n' = measured
- and n = the 10 destructively analyzed . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 22
- 3
- Regression equations for estimating component biomass, volume,
surface area, and biomass treatment for western juniper with basal circumference (cm) as the independent variable. The first 12 follow the form ln(Y) = A + B*ln(X) with variances (S2y,x) in logarithmic units. The last four are linear, untransformed
- equations with variances in arithmetic units . . . . . . . . . . . . . . . . . . . . . . .
- 23
24 25
456
Average dimensions of western juniper estimated by double sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Regression equations, standard error of estimate (Sy,x) and correlation coefficients for estimating leaf biomass and leaf area . . . . . . .
Daily total CO2 assimilated in grams per tree and daily water transpired in liters per tree, during a full sun day, for the average sized large-adult western juniper, 4.5 m tall with a
- basal circumference of 66 + 4 cm (Miller et al. 1992) . . . . . . . . . . . . . . . .
- 29
- 7
- Comparison of Juvenile and large adults classes of
- for the Tintic, Utah, population,
- Chrysothamnus nauseosus
June 1989, presented as mean ± SD: pre-dawn xylem pressure potential ( pd), midday xylem pressure potential ( md), photosynthesis (Amax), stomatal conductance to water vapour (gmax), instantaneous ratio of internal to ambient CO2 concentration (ci/ca), transpiration (E), instantaneous A/E, carbon isotope discrimination ( ), integrated ci/ca estimated from , integrated A/E estimated from and the leaf-to-air vapour pressure deficit, leaf nitrogen, potential photosynthetic nitrogen use efficiency (PPNUE), and plant height. All differences between juvenile and large adult classes were significantly different (P<0.05) based on a two-tailed Student's t-test comparison of juveniles and adults, with the exception of the instantaneous ci/ca which was based on a one-tailed Student's t-test since the direction of the difference was predicted. Sample sizes ranged from 18 to 20 for the juvenile class, and from 19 to 20
- for the large adult class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 37
LIST OF TABLES (Cont'd.)
- Table
- Page
43
- 8
- Some physical and chemical characteristics of western juniper
woodland soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 9
- Relationship of grazed and ungrazed conditions to
- cover of crusts, grasses, and bare soil . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 66
10 Permeability, infiltration and sediment production
- associated with microbiotic crusts on the Colorado Plateau . . . . . . . . . . .
- 68
11 Watershed and hillslope-scale hydrologic studies in
- pinyon-juniper environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 80
12 List of wildlife species observed in western juniper communities
- (from Puchy and Marshal 1993) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 84
13 Aboveground biomass production response to western juniper removal in central Oregon in 1984 (Vaitkus 1986). Values are for
- the second year and only for trees with >5 m crown diameter . . . . . . . . . .
- 93
14 Aboveground biomass production response to western juniper removal in central Oregon in 1983 and 1984. Values are weight for percent of area in each zone (sub-canopy and interspace)
- and include all size classes of trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 93
95
15 Estimated biomass and mineral content of a western juniper tree 11 meters (36 ft) tall with a basal diameter of 44 cm (17.3 in) (from Larsen 1993, Kramer 1990, and Miller et al. 1990) . . . . . . i
SCIENCE ASSESSMENT
EXECUTIVE SUMMARY
WESTERN JUNIPER (Juniperus occidentalis Hook. ssp. occidentalis)
WOODLANDS
- of Western Juniper (
- ssp.
- Juniperus occidentalis
- ) woodlands is
- occidentalis
Distribution
across a broad variety of vegetation types, soils, landscapes and moisture regimes. Area occupied by western juniper woodlands in the Pacific Northwest is nearly 4 million acres (about 1.6 million ha). Over 2.3 million acres of woodland are in Oregon with approximately 10% of the area in old woodlands. Nearly 90% of the area in Oregon is composed of young woodlands with trees less than 150 years old. Data and observation indicate the species continues to expand its range into new areas.
and retraction of western juniper took place over long time
Prehistoric Expansion
periods. Data indicate that where it did occur, its density was less than that at present. of western juniper woodlands has originated from open, sparse
Historic Expansion
and savannah-like stands of old juniper trees many of which are today 400 to 600 years of age or even older. Many of these stands were growing on topographic situations where fire would have been restricted. Factors which may have influenced the rapid expansion were a combination of mild winters with greater precipitation, reduced fire frequency and livestock grazing that contributed to increases in big sagebrush reduced production of grasses. Development of present day woodlands occurred under a different combination of variables than prehistoric expansion, therefore present day woodland communities have different plant composition and structure than prehistoric woodland plant communities.
attributes of western juniper that play a significant role in expansion,
Biological
competition and dominance are numerous. Big sagebrush plants are implicated as major contributors of safe germination and establishment sites and as locations for successful completion of the juvenile phase of growth for western juniper. Growth is usually very slow the first few years. Young juniper begin to overtop big sagebrush plants when they are about 20 to 30 years old. Trees, eight to nine decades old, may grow six to seven inches (15-18 cm)/year in height. A strong lateral root system is developed at an early age and some of these lateral roots or even a taproot may penetrate very deeply into soil and rock cracks.
Height and diameter growth rates of western juniper trees beyond 35 years of age have not been studied. General observation indicates a variation in growth rate depending on topographic position, soil type and woodland tree density.
Trees begin to produce abundant seed crops at 60 to 75 years of age. Seed vectors include a variety of birds and other animals, gravity and water. As stands of western ii juniper mature they commonly reach a canopy cover of 20% to 30% and densities of 40 to 100 trees/ac (about 100 to 240/ha). Within Oregon there is about 0.28 million acres(114,000 ha) of western juniper woodlands with an average canopy cover of 45%. Tree densities frequently exceed 400/ac (1,000/ha) and densities in excess of 4,000/ac (10,000 ha) have been recorded.
The primary mortality factor affecting western juniper was fire. However stand conditions with little understory fuel to carry a fire, fire risk factors, and policies which contribute to fire control have limited the influence of fire as a mortality factor. Insects and diseases as agents of juniper mortality are poorly studied. Observation indicates that individual trees and small groups of trees have been killed by these agents.
studies of western juniper show an age related trend toward increasing
Physiological
allocation of energy to construction of aboveground tissues. Adult foliage reaches a nitrogen content of about 1.2%. Carbon dioxide (CO2) assimilation per gram of foliage tissue appears to decline with age from a peak value of 587 mol/g/day in juvenile foliage to a peak of 324 mol/g/day for adult foliage.
Models are available for estimating juniper aboveground biomass, leaf biomass and leaf area from a variety of measured parameters including basal circumference. Both CO2 assimilation and transpired water loss have been measured for juvenile and adult foliage and these measurements can then be projected to trees of various size classes. For example, for the estimated total daily CO2 assimilated and total daily water transpired an adult western juniper tree 33 feet (10 m) in height with a basal diameter of 18 inches (46 cm) and a basal circumference of 57 inches (145 cm) ranged from a high in August of 661 grams of CO2 assimilated and 137 liters (36 gal) of water transpired into the atmosphere to a low in November of 10 grams of CO2 assimilated and 5 liters (1.3 gal) of water transpired.
A model is available for assessment of water use by developing stands of western juniper based on stand density, basal area, temperature, radiation and precipitation.
Physiological information is rather extensive for western juniper but almost no data is available for other associated plant species associated with western juniper woodlands with the exception of a few species that have been studied in plant communities outside the woodland.
studies of western juniper woodlands are incomplete in many respects.
Ecological
Woodlands exist on a variety of soils derived from a wide range of parent materials. However, studies correlating soil conditions to western juniper populations and production and to associated plant community structure and function are limited. Spatial distribution patterns of soil nutrients develop in the presence of western juniper. This pattern tends toward greater patchiness with tree age.
Successional changes in understory vegetation associated with increasing western juniper dominance are not sufficiently documented. Spatial measurements and general iii observations indicate that certain shrub and forb species present in communities without juniper are absent or appear as dead individuals on similar ecological sites where closing young woodlands of juniper exist. Plant species numbers, density and production, other than juniper, appear to decline and patterns of plant distribution change as trees grow and woodlands develop over time. Many young woodlands are in successional transition between states, therefore future plant communities are largely unknown for the many ecological site on which they exist.
Fire is an effective control agent of western juniper to prevent expansion into associated plant communities, to reduce stand densities, and to create a mosaic of plant communities on a landscape. Both grasses and shrubs, particularly sagebrush, are needed to supply sufficient fire fuel to cause death in larger individual trees therefore fires may not carry in a well developed woodland where these fuels are in low supply.
Grazing studies on western juniper woodlands are absent from the literature, however observations and data from other types of juniper woodlands indicate a heightened sensitivity of understory plants to herbivory. The effects of tree competition and herbivory have not been generally separated.
structure and function have not been studied in western juniper
Microbiotic crust
woodlands. Studies from southwestern pinyon-juniper woodlands showed that cover of crusts under certain kinds of grazing was reduced but that species numbers could respond either positively or negatively to grazing activity. Tree spacing and forb and grass cover may influence crusts at the species level. Both tree canopy cover as well woody and needle debris may reduce crust cover.
processes in western juniper woodlands are influenced by tree canopy and
Hydrologic
from the litter layer. Canopy interception losses may exceed 12% of the annual precipitation. Water runoff and sediment loss are influenced by canopy cover of all plants, plant density and spatial distribution of plants. Water and sediment loss in overland flow appears to be very high for some juniper sites. However runoff as a function of the interaction of soils, plants and climatic events at the site level have been studied insufficiently to develop acceptable standards. Extrapolation of hydrologic data from small-plots to watershed level predictions is risky. Data for the fall, winter and early spring, a period that is hydrologically active, are not available.
use and impact of management activities in western juniper woodlands are
Wildlife
poorly documented and little researched. Key unresolved questions relative to wildlifewestern juniper habitat interactions are; what is the level of importance of the woodlands, what structural components of woodlands are important. What landscape level vegetation patterns are important, what will be the effect of successional changes in these woodlands since they are in transition to different communities. Management guidelines for species using western juniper woodlands has largely been derived from observation and information gleaned from other vegetation types. Exceptions are mule iv deer and bird species such as Townsend's solitaire where a reasonably good data base exists.