J. Range Manage. 53:119–126 January 2000 Understory dynamics in cut and uncut western juniper woodlands JON D. BATES, RICHARD F. MILLER, AND TONY J. SVEJCAR Authors are research associate, Oregon State University, Eastern Oregon Agricultural Research Center, Burns, Ore. 97720; professor, Rangeland Resources Department, Oregon State University, Eastern Oregon Agricultural Research Center, Burns, Ore.; and research leader, USDA-ARS, Eastern Oregon Agricultural Research Center, Burns, Ore. At the time of the research, the senior author was a research assistant, Rangeland Resources Department, Oregon State University, Eastern Oregon Agricultural Research Center. Abstract Resumen Expansion of western juniper (Juniperus occidentalis spp. occi - La expansión del "Western juniper" (Juniperus occidentalis dentalis Hook.) woodlands in the sagebrush steppe has the poten- spp. Occidentalis Hook) en las estepas de "Sagebrush" tiene el tial to change composition, structure, and productivity of under- potencial de cambiar la composición, estructura y productividad story vegetation. Cutting of western juniper woodland can poten- de la vegetación herbácea. La tala de bosques de "Western tially restore understory productivity and diversity. Understory juniper" puede potencialmente restaurar la productividad y responses were assessed after cutting a juniper woodland in diversidad de la vegetación herbácea. Se evaluó la repuesta de la southeastern Oregon in 1991. The experimental design was a vegetación herbácea después de cortar en 1991 un bosque de randomized complete block with eight, 0.8 ha sized blocks and 2 "Western juniper" en el sudeste de Oregon. El diseño experi- treatments, cut and uncut woodland. Understory cover, density, mental utilizado fue el de bloques completos al azar con 8 blo- diversity, biomass, and nitrogen (N) status were compared ques de 0.8 ha y dos tratamientos, con corte y sin corte. Se com- between treatments after cutting. Plants were separated into 5 paró entre tratamientos la cobertura, densidad, diversidad, bio- functional groups: bluegrass (P o a spp.), perennial bunchgrass, masa y el contenido de nitrógeno de la vegetación herbácea perennial forb, annual forb, and annual grass. Cutting of juniper después de la tala. Las plantas fueron separadas en 5 grupos fun- reduced belowground interference for soil water and N. Leaf cionales: "Bluegrass" (P o a. spp.), "Bunchgrass perenne", hier- water potentials were less negative (P<0.01) and understory N bas perennes, hierbas anuales y zacates anuales. El corte de concentration and biomass N were greater (P<0.05) in the cut "Western juniper" redujo la interferencia en el subsuelo del versus woodland treatment. Cutting of juniper trees was effective agua y el nitrógeno del suelo. Los potenciales de agua de las in increasing total understory biomass, cover, and diversity. In hojas fueron menos negativos (p<0.01) y la concentración de N the second year post-cutting total understory biomass and N de la biomasa de la vegetación herbácea fue mayor (p<0.05) en el uptake were nearly 9 times greater in cut versus woodland treat- tratamiento con corte de "Western juniper". El corte de árboles ments. Perennial plant basal cover was 3 times greater and plant de ‘Western juniper" fue efectivo para incrementar la biomasa diversity was 1.6 times greater in the cut versus woodland treat- total, cobertura y diversidad de la vegetación herbácea. En el ments. In the cut, perennial bunchgrass density increased by 1 segundo año después del corte, la biomasa de la vegetación her- plant m- 2 in both duff and interspace zones and bluegrass bácea y la absorción de nitrógeno fueron casi 9 veces mayores en increased by 3 plants m- 2 in interspaces. Plant succession was el tratamiento con corte que en el tratamiento sin corte. La dominated by plants present on the site prior to juniper cutting cobertura de plantas perennes fue 3 veces mayor y la diversidad suggesting that pre-treatment floristics may be useful in predict- de plantas fue 1.6 veces mas en las áreas con corte que en las ing early successional understory response. Early plant dynamics intactas. En las parcelas con corte, la densidad de "Bunchgrass" perenne se incremento a razón de 1 planta m-2 en los interespa- on this site supports the multiple entrance point model of succes- -2 sion as perennial grasses and bluegrass made up the majority of cios y el "Bluegrass" aumentó en 3 plantas m . La sucesión veg- total herbaceous biomass and cover. etal fue dominada por plantas que estaban presentes en el sitio antes de cortar el "juniper", sugiriendo que la composición florista pre-tratamiento puede ser útil para predecir la respuesta sucesional inicial de la vegetación herbácea. La dinámica inicial Key Words: interference, water potential, nitrogen content, de las plantas en este sitio sostiene el modelo de sucesión de diversity, restoration, tree cutting. punto de entrada múltiple conforme los zacates perennes y el "bluegrass" constituyen la mayoría de la biomasa y cobertura Displacement of sagebrush steppe communities by the recent total de la vegetación herbácea. expansion of western juniper (Juniperus occidentalis spp. o c c i - dentalis Hook.) woodlands has raised concerns over the effects of tree canopy development on plant community structure, composi- Oregon, northeastern California, and southwestern Idaho (Miller tion, and diversity. Since the late 1800's western juniper has and Wigand 1994). Prior to Euro-American settlement, western increased in density and spatial extent in central and eastern juniper was largely confined to areas with shallow rocky soils underlain by fractured bedrock (Burkhardt and Tisdale 1969, This manuscript was submitted as Technical Paper 11,265 for the Oregon Miller and Wigand 1994, Miller and Rose 1995). Recent juniper Agricultural Experiment Station. Manuscript accepted 28 March 1999. expansion has occurred in deeper more productive soils occupied JOURNAL OF RANGE MANAGEMENT53(1), January 2000 119 by mountain big sagebrush [Artemisia tri - Understory dynamics after tree cutting respectively, over the past 30 years. d e n t a t a spp. vaseyana Nutt.] grasslands, in a western juniper woodland were Soils on the site are rocky, 40 to 50 cm riparian zones, and quaking aspen [Po p u l u s assessed in this study. We hypothesized deep, and clay loam in texture. Soils are tr e m u l o i d e s Michx.] woodlands (Burkhardt that cutting of juniper would reduce underlain by a welded ash tuff of and Tisdale 1969, Eddleman 1987, Miller belowground interference for water and rhyolite/rhyodacite composition, which and Rose 1995). Reduced fire frequency is nitrogen, which would result in increased restricts root penetration. Soils were clas- the main causal factor attributed to the cover, biomass, density, and diversity of sified as clayey-skeletal, smectitic, frigid, expansion of juniper (Burkhardt and understory plants. Lithic Argixerolls. Tisdale 1976, Evans and Young 1985). The decrease in fire frequency began in the late Experimental Design 1800's with the suspension of fires set by Materials and Methods Experimental design was a randomized Native-Americans (Miller and Wigand complete block with 8 blocks and 2 treat- 1994). Reductions in fine fuel loads as a Study Site ments, cut and uncut juniper woodland. result of heavy livestock grazing at the turn The study site was on Steens Mountain Blocks were 0.8 ha in size and were of the century and additional fire suppres- in southeast Oregon (118°36' E, 42°55' N). selected for their similarities in soil type, sion have further reduced fire frequency in Elevation at the site is 1,525 m. Aspect is slope, and aspect, and overstory/understo- shrub steppe communities (Burkhardt and west facing with a 22% slope. The site ry density and cover characteristics. Tisdale 1976, Miller and Wigand 1994). was dominated by an 80-year-old juniper Measurements of baseline vegetation char- Succession to juniper-dominated communi- woodland (Bates 1996). Full occupancy of acteristics were recorded prior to tree cut- ties is accompanied by reductions in under- the site by juniper was indicated by the ting in July 1991. Half of each block was story productivity (West 1984, Vaitkus and limited leader growth on juniper trees, low cut with chainsaws in August 1991. All Eddleman 1987), cover (Driscoll 1964), herbaceous cover, and the majority of cut juniper trees were left in place. and diversity (Burkhardt and Tisdale 1969), mountain big sagebrush shrubs being Leaving cut trees on site is a standard increased site aridity (Angell and Miller dead. Juniper canopy cover averaged 24% practice in eastern Oregon. Three small 1994), and accelerated soil erosion and tree density averaged 228 trees/ha. juniper trees (<3 m) were left in each cut (Buckhouse and Mattison 1980). Bare ground accounted for 74% of the block for measurement of leaf water Juniper dominated communities are rel- area and rill erosion was evident through- potentials (Y ). Post-treatment measure- atively stable and resistant to all but the l out the site. Basal cover of understory ments of understory characteristics, Yl, most severe fire disturbances (Miller and perennials averaged less than 2.3% across and soil water content began in April 1992 Wigand 1994). Natural or prescribed fire the site (Bates 1996). In the early 1900's and were concluded in September 1993. is largely eliminated as a management tool this site was used as wintering grounds for Livestock were excluded during the study. for restoring understory vegetation in domestic sheep. Since the late 1940's the woodlands because of lack of fuels neces- site has been moderately grazed by cattle sary to carry fires through juniper stands. Understory Sampling Procedures in the early spring. Understory measurements were basal Consequently, understory restoration in The understory was dominated by many juniper dominated communities is cover (perennial plants), canopy cover, Sandberg's bluegrass (Poa sandbergii density, diversity, biomass, and nitrogen limited to mechanical treatments such as Vasey).