Abstract
AbstractIn sagebrush ecosystems, pinyon and juniper tree expansion reduces water available to perennial shrubs and herbs. We measured soil water matric potential and temperatures at 13–30 and 50–65 cm soil depths in untreated and treated plots across a range of environmental conditions. We sought to determine the effects of tree expansion, tree reduction treatments, and expansion phase at time of treatment over 12–13 yr post‐treatment. Because the effects of tree reduction on vegetation can vary with the soil temperature/moisture regime, we also analyzed differences in soil climate variables between the mesic/aridic‐xeric and frigid/xeric regime classifications for our sites. Growing conditions during all seasons except spring were greatly limited by lack of available water, low temperatures, or both. Advanced tree expansion reduced wet days (total hours per 24 hr when hourly average soil water matric potential >−1.5 MPa), especially in early spring. Fire and mechanical tree reduction increased wet days and wet degree days (sum of hourly soil temperatures >0°C when soil is wet per 24 hr) compared with no treatment for most seasons. Burning resulted in higher soil temperatures than untreated or mechanically treated woodlands. Tree reduction at advanced expansion phases increased wet days in spring more than when implemented at earlier phases of expansion. Added wet days from tree reduction were negatively associated with October through June precipitation and vegetation cover, rather than time since treatment, with more wet days added on drier sites and years. The longer period of water availability in spring supports increased growth and cover of not only shrubs and perennial herbs, but also invasive weeds on warmer and drier sites, for many years after tree reduction. We found that sites classified as mesic/aridic‐xeric had warmer soil temperatures all seasons and were drier in spring and winter than sites classified as frigid/xeric. Land managers should consider reducing trees at earlier phases of expansion or consider revegetation when treating at advanced phases on these warmer and drier sites that lack perennial herb potential.
Highlights
For more than a century, woody plants have been expanding into semiarid grasslands and shrublands worldwide (Archer et al 2017)
Seasonal effects As expected, wet days were highest in early spring, wet degree days were highest in early spring and late spring, and soil temperatures were highest in summer (Fig. 1)
Winter had a high number of wet days, it had limited wet degree days due to low soil temperatures
Summary
For more than a century, woody plants have been expanding into semiarid grasslands and shrublands worldwide (Archer et al 2017). The climate of sagebrush ecosystems in the Great Basin of the western United States consists of cool wet winters and warm dry summers (Leffler and Ryel 2012, Williams et al 2018, Miller et al 2019). Reduction in trees as major water users should result in longer periods of water availability for nutrient transport in the RGP to support growth of shrubs and herbs, and more water for maintenance of shrubs in the MP during dry summer periods (Bates et al 2000, Leffler and Ryel 2012, Mollnau et al 2014, Roundy et al 2014b). Tree reduction could increase soil water availability by reducing tree cover and interception of precipitation (Williams et al 2018)
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