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2014 AES New Research Projects – 726

COL00726 – Ecohydrological Mechanisms and Consequences of Juniper Expansion into Rangelands in northwest Colorado

Sponsoring Institution: National Institute of Food and Agriculture

Project Director:

Troy Ocheltree
Assistant Professor
(970) 491-5658
Department of Forest, Rangeland & Watershed Stewardship

Non-Technical Summary

Many of the sagebrush steppe ecosystems of the western United States are being converted to juniper woodlands, causing huge changes to the ecosystem services provided by these ecosystems. The conversion of sagebrush steppe to juniper woodlands changes the amount and quality of habitat for wildlife, forage for grazing, carbon uptake and storage, and water availability and streamflow. Overgrazing and fire suppression are often implicated in juniper expansion but does not explain this phenomenon in all regions, suggesting that other mechanisms must be involved. Competition for water among plants is the dominant factor controlling vegetation found in different regions, but we currently don’t understand how competition for this limited resource may be involved in the expansion of juniper into sagebrush steppe. This research will investigate how the competition for water among species is contributing to the conversion of sagebrush steppe to juniper woodlands in northwest Colorado and across the western US. An advantage of providing a mechanistic understanding of ecological issues, like this project, is that the data often also be used to quantify and predict the impact of different management strategies. For this study, the data collected in this research will also be used to quantify the impact that juniper expansion will have on water availability in northwest Colorado and the western US, which affects forage production and streamflow in these regions.

In order to reveal how the competition for water contributes to the expansion of juniper we will utilize a suite of unique measurements made on individual plants to understand how the different species involved use water through space and time. Measurements of growth and water-use of the juniper (Juniperus osteosperma) and sagebrush (Artemesia tridentata) will be made to understand how much water is being used by these two species. We will also utilize a unique method that involves installing small probes into the stems of these species that can measure the rate and amount of water being used by entire plants. Although this can tell us the relative ability of the different species to extract water from the soil, it does not tell us if these two species are directly competing for water. If these species are using water from different depths in the soil, then they may be able to co-exist even in water-limited environments. To determine where these species are getting their water we will measure the stable isotopic composition of water in the plant tissue, which tells us the depth at which the plants are getting their water within the soil. The stable isotopic composition of water in the soil changes with depth, and so the water extracted from the plant tissue will tell use if the species are using primarily shallow or deep water. Finally, this data will be combined into a mathematical model describing how water moves through an ecosystem, which will allow us to estimate how the encroachment of juniper will affect how much water will be availability for the growth of other plants (besides juniper), which has a big effect on two major ecosystem services: 1) The productivity of forage for grazing operations and beef production, and 2) The amount of streamflow that will be available for fish and wildife, irrigation, and human consumption. This project will provide a better understand of why juniper is expanding into sagebrush steppe allow natural resource managers and land-owners to make more efficient and sustainable decisions about how to manage our resources.

Goals / Objectives

The major goals of this project are to identify how the competition for water is facilitating the expansion Juniperus osteospermainto sagebrush steppe ecosystems and evaluate the impact of juniper expansion on the hydrological cycle in these ecosystems. In order to identify how competition for water resources contributes to juniper expansion we will: 1) quantify the amount and spatial distribution of water extraction by J.osteosperma and Artemesia tridentata (the dominant species in the sagebrush steppe in NW Colorado), 2) determine the seasonal pattern of water use of J. osteosperma and A. tridentata, and 3) assess the drought tolerance of these two species in order to identify the lower soil-moisture threshold of survivability. The data collected to achieve the previous objectives can then be utilized to parameterize a land-surface model to assess how the water-use by J.osteosperma is directly impacting the hydrologic cycle in regions sensitive to juniper expansion.


The methods for this project will rely on ecophysiological techniques used to measure the growth and water-use of individual plants.


We will establish 3 sites along a gradient of juniper density; from not juniper present to high density of young juniper so that we capture the front boundary of juniper encroachment. At each site we will make measurements on five individuals of; J.osteosperma, A. tridentata and a dominant grass species. On each individual we will measure the following:

  1. We will measure leaf-level photosynthesis and transpiration to understand the relationship between growth and water-use under different soil moisture conditions.
  2. We will measure whole-plant water-use using sap-flow sensors, which measure the total rate of water-flow through the plant stems to quantify and understand the total amount of water the different species are capable of extracting from the soil.
  3. Whole-plant water-use has been measured before, but rarely combined with an understanding of where plants are extracting water within the soil profile. By measuring the stable isotopic composition of water in plant tissue we can estimate where the depth at which plants are using water to investigate the amount of niche-partitioning or direct competition for water resources involved in the expansion of juniper.
  4. Finally, we will also utilize a unique analysis of physiological drought tolerance that quantifies the ability of plants to maintain the supply of water to leaves (“xylem vulnerability), which explains the ability of plants to grow and survive when water is limiting.


Using the measurements described in the ‘Efforts’ section we will be able to quantify the amount of sources of water used by the species involved in the juniper encroachment phenomenon. This will provide us with a better understanding of how directly the competition for water is involved in the encroachment of J. osteosperma into sagebrush steppe ecosystems. Furthermore, we will develop a simple model of water-resource competition amount species combined with a quantified estimate of drought tolerance in order to understand the conditions that would promote the competitive advantage of either a juniper dominant woodland or a sagebrush dominant shrubland. We will also use the data collected above to paraterize a land-surface model that can scale-up canopy level measurements of water-use to the landscape scale. By parameterizing this model across a gradient of juniper density we will be able to estimate the effect of different levels of juniper encroachment on the hydrologic cycle in these semi-arid ecosystems.

Target Audience

There are three target audiences that will benefit from this project: 1) Research scientists, 2) Natural resource managers for city, state, and federal agencies, and 3) land-owners that are experiencing or are in danger of experiencing juniper expansion. Understanding the competition for water resources between these two species will provide information to help understand plant-plant interactions to help understand what plant strategies are more competitive in water limited systems. Furthermore, species ranges are changing (ie. expanding and contracting) all around the globe and we currently have little information on the mechanisms involved in many of these changing ecosystems, so this research will help us better understand the mechanisms that may be involved in other semi-arid systems. By understanding the mechanisms involved in the range expansion of juniper, natural resource managers can make better decisions about implementing strategies to achieve the management goals. Finally, land-owners are concerned about how the expansion of juniper will affect the natural resources; our research will provide quantitative estimates of how water resources will be affected by the expansion of juniper onto their property.


Activities – We will be collecting data in the field across a gradient of juniper expansion in Northwest Colorado; from sites where junipers are the dominant cover to sites not yet encroached upon by J. osteosperma. We will be measuring: 1) the water use of entire plant canopies using sap-flow sensors, 2) where plants are accessing water using water isotope analyses, and 3) quantifying the drought tolerance of these species by measuring the physiological capacity of these species to conduct waterwhen soil moisture is limiting (ie. cavitation resistance).

Products – We will publish our results in peer-reviewed journals that are relevant to the field (eg. Rangeland Ecology & Management). We will also produce model output of how juniper expansion will affect the hydrologic cycle under different environmental conditions (ie. wet vs. dry years) to help managers evaluate the impact of the management issue on water resources in these semi-arid ecosystems. Finally, we will meet with natural resource managers and extension in the region to get feedback throughout or project and to convey our results as they become available.

Expected Outcomes

We expect to quantify the soil moisture thresholds for when J. osteosperma can out compete A. tridentata for water resources, which will allow us to understand the mechanisms involved in the current juniper expansion phenomenon and predict the extent of expansion in the future as the frequency of drought may increase. By understanding where, when, and how much water the dominant species involved in juniper expansion extract from the soil profile we can provide quantitative estimates of how changes in vegetation structure will impact the limited water resources in northwest Colorado. Furthermore, by understanding the sensitivity of these two species to limited water resources across the year more targeted managing strategies can be established to achieve management goals in the face of these dynamic changes in ecosystem structure and function.

Keywords: Juniper expansion, sagebrush steppe, water resources, drought, juniper woodlands, northwest Colorado, plant ecophysiology

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