Long-term Ecohydrologic Response to Western Juniper (Juniperus occidentalis) Control in Semiarid Watersheds of Central Oregon : A Paired Watershed Study Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/12579w598

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  • Rangelands span over 50% of the globe and approximately 70% of the United States. Although livestock production is an important use of rangelands, the benefits of rangelands are highly diverse. Humans find intrinsic value in protecting these unique and variable landscapes for wildlife, vegetation, and recreation enthusiasts. Woodland plant encroachment has become a major concern for land management agencies and private landowners across the United States and many rangeland communities worldwide. Studies around the world are characterizing the effect that woody species may have on ecologic and hydrologic function, as well as the potential consequences of prolonged encroachment. This research is an addition to a central Oregon paired watershed study that began in 1993 as way of characterizing ecohydrologic effects of western juniper (Juniperus occidentalis) removal. The overarching goals of the study presented here were to: 1) Characterize vegetation-soil water interactions at the landscape scale; 2) Analyze long-term soil water and groundwater fluctuations for treated versus untreated watersheds; 3) Asses subsurface flow connections between upland watersheds and a downstream valley. A landscape-scale assessment (2014 - 2015) of shallow soil water content, for to top 12-cm of the soil profile, across both watersheds indicated the treated watershed as having a significantly higher (P < 0.05) mean value of soil water content for three (July, January, and May) out of five measurement periods (July, November 2014 and January, March, May 2015). The untreated watershed was 2% higher in March 2015, and no significant difference was found between the two sites in November 2014. Analysis of the structure of canopy cover (i.e. juniper dominated versus juniper removed) using linear regression models found juniper cover to be correlated with decreasing soil water content for three of five months, with the exception of the wettest months of March and May, when juniper canopy was correlated with increases in soil water content. Soil textural properties were also analyzed as an independent variable in the linear models, and clay content was found to be correlated with increases in soil water content during the three wettest months (January, March, and May) across both watersheds. The long-term (2004 - 2015) analysis of groundwater level and deep soil water content fluctuations showed there to be distinct seasonal and storm-event responses. Groundwater levels in the untreated watershed consistently displayed higher yearly maximum values when compared to the treated, however, groundwater levels in the treated watershed persisted longer into the dry out period. Similar findings were reported in relation to long-term soil water content where the untreated watershed often displayed higher maximum responses but declined back to dry status sooner than the treated watershed. Both watersheds responded to seasonal and storm-event precipitation through soil water content fluctuations in the deep soil profile and through groundwater level fluctuations. Precipitation event responses could be observed on the order of hours for the treated watershed and on the order of days for the untreated. Antecedent soil water content seemed to play a large role in the effectiveness of the storm events. Summer precipitation had little influence on the deeper soil profile and on groundwater response. This may be due to dryer antecedent soil conditions and flashy overland flow. Water content in the top measured soil profile, however, increased and stayed at high levels for up to two weeks following a summer precipitation event which yielded approximately 27 mm of rainfall. Soil water and groundwater response were greatly influenced by winter precipitation events and pre-storm soil water conditions. The hydrologic connectivity of subsurface water flows, through fluctuations in deep soil water and groundwater levels, was found to be an important process in these watersheds. Temporary subsurface hydrologic connections were observed between upland and valley wells as the wet season progressed. This connection was supported by the results of a stable isotope analysis, which indicated that the origin of the water may be the same. This was concluded given the similar values of oxygen-18 found for both the treated and untreated upland sites and for valley groundwater monitoring locations. Large-scale juniper manipulation projects are taking place across the western United States and around the world. Many projects have the objectives of increasing water availability and stream flow to no avail. This research provides baseline data towards understanding the importance of the valuable subsurface water resources in landscapes with limited precipitation availability.
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