Investigating plant-soil-water relationships in western juniper (Juniperus occidentalis) woodlands
The connections between upland water sources, groundwater, and downstream valleys influence the amount of water available to multiple ecohydrologic processes that drive many ecosystem services (e.g., forage and water provisioning, carbon/nutrient cycling, water quality, wildlife habitat, recreation, etc.).
Our long-term ongoing research study in central Oregon has provided critical information regarding vegetation and hydrology interactions in western juniper-dominated landscapes.
Project objectives are:
1) Characterize water-plant-soil interactions occurring in previously treated (juniper removed) and untreated watersheds,
2) Determine hydrologic connectivity between upland watersheds and downstream valleys and;
3) Determine juniper reestablishment effects on ecological and hydrological function ten years after treatment.
Various instrumentation, including weather stations, flumes, groundwater wells, soil moisture, and tree transpiration sensors, are being used to collect hydrological data. Intensive field monitoring of soil and vegetation variables, including soil texture, bulk density, carbon content, canopy cover, and herbaceous yield, to mention a few.
- Greater streamflow and spring flow levels were observed in the treated watershed. Peak streamflow at the treated watershed was 269 gallons per minute (gpm) vs. 182 gpm in the untreated site.
- Seasonal soil moisture and aquifer recharge in response to winter precipitation and snowmelt runoff inputs.
- Strong surface water-groundwater and upland-valley hydrologic connections were observed during winter precipitation and spring runoff seasons.
- Since juniper removal in 2005, we have observed a sustained improvement in vegetation in the treated watershed.
- Greater perennial grass, annual grass, and shrub cover were observed in the treated watershed ten years following juniper removal.
- Also, 13 years post-treatment, we found 30% more forage production in the treated watershed than in the untreated.
- Greater grass cover positively correlates with changes in soil moisture, whereas juniper cover negatively correlates with soil moisture content.
- On average, 41% of total annual precipitation is intercepted by juniper canopy cover.
- Mature juniper trees ranging from 12 to 28 inches in trunk diameter (mean value of 17.5 inches) used an average of 23 gallons per day (gpd) of water during a wet year (15 inches PPT) and 12 gpd during a dry year (8 inches PPT). On average, a mature juniper tree uses 4718 gallons per year. A juniper sapling (1-inch diameter) uses 71 gallons per year.
The improved soil and vegetation conditions observed in the treated watershed also contributed to higher belowground carbon storage. Thirteen years post-juniper removal, the increased presence of deep-rooted perennial grasses found in the treated watershed was also reflected in higher root carbon content. Grass-root carbon storage and total belowground carbon content were greater in the treated watershed than in the untreated.
Peer-reviewed publications related to this project:
Ecohydrologic connections in semiarid watershed systems of central Oregon USA
The seasonal water balance of western juniper-dominated and sagebrush-dominated watersheds.
A hydrogeologic framework for understanding local groundwater flow dynamics in the southeast Deschutes Basin, Oregon, USA
Overstory-understory vegetation cover and soil water content observations in western juniper woodlands: A paired watershed study in central Oregon, USA
Water use and soil moisture relationships on western juniper trees at different growth stages
Water use by mature and sapling western juniper (Juniperus occidentalis) trees
Ecosystem carbon in relation to woody plant encroachment and control: juniper systems in Oregon, USA
Effects of western juniper (Juniperus occidentalis) control on ecosystem nitrogen stocks in central Oregon, USA
The use of low-altitude uav imagery to assess western juniper density and canopy cover in treated and untreated stands
Ecology and Hydrology of Western Juniper - OSU Range Field Day 2016
The OSU Range Field day featured this 23-year research project being conducted in the heart of juniper country in central Oregon. The event that took place on 28 June 2016 at Brothers, OR included morning presentations on project research findings, followed by a tour to the paired-watershed study site.
This range field special report, which includes contributions from OSU and USDA-ARS researchers, illustrates some of the ecological and hydrological relationships found in western juniper woodlands of eastern Oregon.
Ecology and Hydrology of Western Juniper_Special Report_OSU Range Field Day 2016.pdf