Overstory–Understory Vegetation Cover and Soil Water Content Observations in Western Juniper Woodlands: A Paired Watershed Study in Central Oregon, USA

Abstract

The effects of western juniper (Juniperus occidentalis) control on understory vegetation and soil water content were studied at the watershed-scale. Seasonal differences in topsoil (12 cm) water content, as affected by vegetation structure and soil texture, were evaluated in a 96-ha untreated watershed and in a 116-ha watershed where 90% juniper was removed in 2005. A watershed-scale characterization of vegetation canopy cover and soil texture was completed to determine some of the potential driving factors influencing topsoil water content fluctuations throughout dry and wet seasons for approximately one year (2014–2015). We found greater perennial grass, annual grass, and shrub cover in the treated watershed. Forb cover was no different between watersheds, and as expected, tree canopy cover was greater in the untreated watershed. Results also show that on average, topsoil water content was 1% to 3% greater in the treated watershed. The exception was during one of the wettest months (March) evaluated, when soil water content in the untreated watershed exceeded that of the treated by <2%. It was noted that soil water content levels that accumulated in areas near valley bottoms and streams were greater in the treated watershed than in the untreated toward the end of the study in late spring. This is consistent with results obtained from a more recent study where we documented an increase in subsurface flow residence time in the treated watershed. Overall, even though average soil water content differences between watersheds were not starkly different, the fact that more herbaceous vegetation and shrub cover were found in the treated watershed led us to conclude that the long-term effects of juniper removal on soil water content redistribution throughout the landscape may be beneficial towards restoring important ecohydrologic connections in these semiarid ecosystems of central Oregon.

The Use of Low-Altitude UAV Imagery to Assess Western Juniper Density and Canopy Cover in Treated and Untreated Stands

“…Our ground-based tree canopy cover estimates (0.7% treated WS; 29.2% untreated WS) were similar to those by Ray et al [44] who, 10 years post-treatment, estimated <1% juniper cover in the treated WS and 30% in the untreated. Our results are also similar to those by Bates et al [50] for a similar western juniper ecosystem in southeastern Oregon; where juniper saplings occupied 0.8% of the treated plots and mature juniper occupied 29.6% of the control plots 12 years post-treatment.…”

“…and Schult.) Barkworth], Sandberg bluegrass (Poa secunda J. Presl), and Thurber's needlegrass [Achnatherum thurberianum (Piper) Barkworth] and some annual grasses, such as cheatgrass (Bromus tectorum L.) [43,44]. As reported by Ray et al [44], juniper canopy cover at the untreated WS is 31% and at the treated WS is less than 1%, this based on surveys in 2015.…”

“…Barkworth], Sandberg bluegrass (Poa secunda J. Presl), and Thurber's needlegrass [Achnatherum thurberianum (Piper) Barkworth] and some annual grasses, such as cheatgrass (Bromus tectorum L.) [43,44]. As reported by Ray et al [44], juniper canopy cover at the untreated WS is 31% and at the treated WS is less than 1%, this based on surveys in 2015. According to the juniper occupancy classification described by Miller et al [2], the untreated WS is considered at the highest level, Phase III, in which juniper is at nearly 30% occupancy and it is the dominant overstory species.…”

Treatment longevity and changes in surface fuel loads after pinyon–juniper mastication

“…As sagebrush‐bunchgrass communities transition to dense pinyon–juniper woodlands in the absence of periodic fire, there are many changes to wildlife habitat, ecosystem functions, and fuel loads. During this transition, shrubs, grasses, and forbs decrease due to competition with trees for water (Roundy et al 2014a, Ray et al 2019) and nutrients (Bates et al 2000, Rau et al 2011, Young et al 2014). These changes in vegetation reduce forage for ungulates such as cattle ( Bos taurus ; Miller et al 2005) and mule deer ( Odocoileus hemionus ; Rosenstock et al 1989) and reduce suitable habitat for sagebrush‐obligate species such as sage‐grouse ( Centrocercus urophasianus ; Baruch‐Mordo et al 2013, Bates et al 2017) and pygmy rabbits ( Brachylagus idahoensis ; Larrucea and Brussard 2008).…”

Soil Available Phosphorus Loss Caused by Periodical Understory Management Reduce Understory Plant Diversity in a Northern Subtropical Pinus massoniana Plantation Chronosequence

“…Human activities, climate, parent material, and topographic factors can influence the vertical and horizontal spatial heterogeneity of soil [12,13]. The vegetation cover and forest vertical structure have also been found to affect the physical properties of soil [14]. In turn, the heterogeneity of soil properties can influence plant composition [15,16].…”