- Western juniper (Juniperus occidentalis) encroachment has been associated with negative ecological and hydrological consequences including reductions in herbaceous production and diversity, deterioration for wildlife habitat, and higher erosion and runoff potentials. As a result, western juniper removal is a common and accepted rangeland management practice. Although studies evaluating the ecological repercussions and removal benefits of western juniper are increasing, quantitative evidence regarding the implications of juniper encroachment and removal on ecosystem carbon and nitrogen pools are lacking. In addition, it is unknown if the hydrologic system (transpiration in relation to soil moisture content) is altered by with or without juniper. This dissertation, as an addition to a central Oregon paired watershed study that began in 1993, addresses the effects of western juniper encroachment and removal on ecosystem processes: carbon, nitrogen, transpiration, and soil moisture. Our study site was a paired watershed in central Oregon where western juniper trees were eliminated in one watershed (treated, 116 ha) and were left intact in an adjacent watershed (untreated, 96 ha). This research was unique because it involved a paired study approach to monitoring changes in these variables post western juniper removal.
The overarching goals of the study presented here were to: 1) determine ecosystem carbon stocks in an encroached juniper watershed and an adjacent watershed where juniper removal occurred 13 years prior to determinations; 2) quantify and compare major pools of nitrogen in an encroached juniper watershed and an adjacent watershed where juniper removal took place 13 years prior to quantification; 3) determine seasonal transpiration for western juniper in relation to soil moisture in one juniper-dominated watershed and in another watershed where juniper has been removed, with a major goal of improving scientific understanding of the effects of juniper encroachment and removal on hydrology.
Thirteen years after western juniper elimination, we quantified aboveground carbon stocks for western juniper trees, shrubs, grasses, and litter in both the treated and untreated watersheds. We also quantified belowground carbon stocks (roots and soil) in both watersheds at two soil depths (0-25cm and 25-50cm). Aboveground carbon stocks were 5.8 times greater in the untreated than in the treated watershed. On the other hand, root carbon stocks were 2.6 times greater in the treated than in the untreated watershed. Soil carbon stocks at both 0-25 cm and 25-50 cm depth were not affected by juniper cutting. Overall, total ecosystem carbon stocks (average 137.6 Mg C ha-1) were not different between watersheds. Most carbon resided belowground (soil 0-50 cm and roots); 84% and 97% of the total ecosystem carbon, respectively, was found in the untreated and treated watershed.
Nitrogen stocks followed a similar pattern to that observed for carbon stocks. As a result of greater aboveground biomass, aboveground nitrogen storage in the untreated watershed (425.4 kg N ha-1) was substantially greater than that in the treated watershed (61.9 kg N ha-1). On the other hand, root nitrogen storage was 3.1 times greater in the treated than in the untreated watershed due to the gain of understory root biomass associated with western juniper elimination. Soil nitrogen stores at both 0-25 cm and 25-50 cm depth were not affected by juniper removal. Overall, total ecosystem nitrogen storage (average 1283.2 kg N ha-1) was not different between watersheds. Most nitrogen resided belowground (soil 0-50 cm and roots); 69% and 95% of the total ecosystem nitrogen, respectively, was found in the untreated and treated watershed.
We measured juniper transpiration using sap flow sensors for mature and juvenile growth stages in the untreated watershed and for saplings stage in the treated watershed where juniper trees were removed in 2005 but regrowth has occurred. Leaf water potentials were monitored for juniper trees to support the data of transpiration. We examined how seasonal transpiration is related to soil moisture. Results indicate that transpiration was greater in mature followed by juvenile and then saplings, supporting our expectation that water use consumption is a function of juniper’s stage of development. The significant differences between predawn and midday leaf water potentials for all juniper stages indicate that some degree of water was lost over the course of the day. The maximum range between these values in July months corresponds with significantly higher transpiration for all juniper stages. Our findings also indicate that annual and seasonal precipitation was highly variable over the course of the study (2017-2019), which was reflected in the mean soil water content (0-80 cm). This resulted in considerable intra- and interannual variation in transpiration. Intra-annually there were two distinct seasonal pulses of transpiration by juniper: spring and summer. In years with heavy winter precipitation (2017 and 2019), juniper exhibited higher transpiration during summer seasons followed by spring and finally the fall months. In the year with a lower summer precipitation (2018), juniper was capable of greater transpiration during the moist period (spring).
This research serves as a basis to indicate the benefits of juniper removal can be attained without substantially affecting the potential for ecosystem carbon and nitrogen pools. Hydrologically, our data suggest that considerable amount of water can be saved in areas with juniper elimination after regrowth with respect to areas with intact mature juniper encroachment. In addition, our study highlights the sensitivity of western juniper woodlands to variations in seasonal precipitation and soil moisture availability.