An improved understanding of the ecohydrologic relationships in semiarid rangelands is imperative for the development of effective rehabilitation and land management practices. This thesis addresses the ecohydrologic relationships of two significant issues concerning semiarid rangeland ecosystems: western juniper encroachment and increasing stream temperatures.
This thesis is divided into two chapters; each chapter is a manuscript reflecting a separate research site and project. The first chapter compares the use of ground and UAV-based measurements to assess vegetation and juniper characteristics in a juniper-dominated ecosystem. The second chapter describes the results of a preliminary investigation into stream temperature relationships of a semiarid riparian system in northcentral Oregon. Both manuscripts are currently being prepared for journal submission.
Western juniper encroachment is a concern across many areas of the western United States and is associated with ecohydrological changes such as increased erosion and reduced intercanopy vegetation. The first research study took place at the Camp Creek Paired Watershed Study (CCPWS), as part of a long-term research project into the ecohydrological impacts of juniper encroachment and removal. The study sought to assess differences in vegetation cover between two watersheds with different densities of western juniper and to examine the accuracy of data collected using low-altitude Unmanned Aerial Vehicles (UAVs) to characterize canopy cover and vegetation cover.
Based on ground-based measurements, some significant differences in vegetation cover were found between the two watersheds. Shrub cover was higher in the treated watershed than in the untreated watershed, although bare ground was similar. Herbage production in the treated watershed was also significantly greater in the treated watershed.
Canopy cover estimates using UAV-based data were similar to ground estimates when multispectral vegetation indices were used. Additionally, supervised classification that utilized multispectral imagery and Normalized Difference Vegetation Index (NDVI) values yielded more accurate indications of overall vegetation cover than using multispectral imagery alone, but was only successful at differentiating between juniper from other vegetation when fall imagery was used.
The second chapter of this thesis addresses stream temperature, a concern in many regions of the world because of its impact on cold-water species and biochemical processes. However, published research regarding stream temperature dynamics in arid or semiarid rangeland systems is limited. The research for the second manuscript took place in a semiarid rangeland system in northcentral Oregon along Fifteenmile Creek, which has been found to exceed suggested maximum stream temperatures. This study took place between 2014 and 2017, and examined stream temperature relationships associated with riparian shade, groundwater inflows, and ambient conditions.
Stream temperatures generally followed the longitudinal gradient, with higher stream temperatures corresponding to lower elevations. During the summer, a difference of up to 5°C in the 7-day moving average stream temperature was observed between the highest and lowest elevation sites, while stream temperatures during the fall and winter seasons were more similar between sites. Air temperature was shown to be highly correlated to both shaded (r=0.960) and non-shaded (r=0.961) stream temperatures. In general, no significant difference was found between areas with riparian shading and non-shaded areas.
Shallow groundwater temperatures showed less variability than stream temperatures. Groundwater was also generally cooler in the summer and warmer in the winter when compared to surface flow conditions. Differences between shallow groundwater temperatures and stream temperatures of up to 8°C in the summer and 10°C in the winter were observed, indicating that shallow groundwater inputs may have a moderating input on stream temperatures.
Ecohydrologic connectivity, particularly concerning the relationship of vegetation and hydrologic characteristics, was an important consideration in both research studies. While additional research is necessary, this research provides insight into an improved understanding of how these connections can influence semiarid rangeland ecosystems.