Describing trends in past and future soil moisture in the desert sand shrubland ecosystem in the Bullfrog Region of Glen Canyon National Recreation Area

Andrews, Caitlin; Bradford, John; Norris, Jodi; Gremer, Jennifer; Duniway, Michael; Munson, Seth; Thomas, Lisa; Swan, Megan

Variation in the structure and function of Colorado Plateau ecosystems is driven by seasonal moisture availability in the soil profile; however, the availability of soil moisture is expected to change in the coming decades as a result of climate change. To understand how ecosystems will be affected by this, we need to understand in more detail how these soil moisture patterns relate to plant productivity, and what changes in soil moisture are expected, both seasonally and at the range of soil depths that plants are adapted to use. We used a soil water model (SOILWAT2) to estimate historical, current, and future soil water availability in 300 vegetation monitoring plots in 13 ecosystems in the Southern Colorado Plateau Network of the National Park Service. In the Bullfrog region of Glen Canyon National Recreation Area, this included six vegetation monitoring plots with-in the desert sand shrubland ecosystem. The SOILWAT2 model used plot-specific vegetation and soil core data collected as part of the Southern Colorado Plateau Network’s long-term upland vegetation monitoring program to represent the environment. Model simulations were run using climate data from the past (1916–2011) to provide historical context, the present (1980-2015) to create a current baseline for comparison. Model simulations were also run for two time periods in the future: a near-term period (2020-2059) and a long-term period (2060-2099). The future projections were made using nine general circulation models under two possible greenhouse gas emissions pathways: the 8.5 representative concentration pathway (RCP), and the 4.5 RCP. Results of the 8.5 RCP simulations are presented here; this pathway corresponds to a comparatively high greenhouse gas emissions scenario. The soil-water model results provide a detailed look at historical patterns of soil water availability, and how soil water availability is projected to change in the future, both seasonally and at different soil depths. Model output included daily soil water potential estimates at multiple depths within the soil profile. This report describes results at deep (50–100 cm) soil depths. A second goal of this work was to better understand how changes in seasonal soil water availability might affect vegetation. To do this we examined how vegetation today is affected by seasonal differences in soil moisture by calculating correlations of annual NDVI (2001-2014) with soil water potential averaged over a range of moving windows. Key findings for the Bullfrog region of Glen Canyon NRA desert sand shrubland ecosystem include the following: All of the climate models predicted increases in mean monthly and annual temperature in the near-term future (+2.4°C on average), and further increases in the long-term future (+5.1°C on average). Models had greater variation in their predictions of both the direction and the amount of future precipitation, particularly in the late summer and fall. Future soil water availability is projected to decline for all seasons under both the near and long-term scenarios. The largest decreases in soil moisture are expected in the fall/winter and spring. Fall/winter soil water potential is expected to approach -3.0MPa, indicating conditions of extreme water scarcity. Predictions of soil moisture for both the near and long-term future are very similar, suggesting that the worst effects of decreasing soil moisture may be felt in the near-term future. The drying of soils in spring is predicted to occur earlier, and will shorten the period when soils are wet during the winter and spring growing season. This will likely have a substantial impact on plant species that rely on this soil moisture.

Describing trends in past and future soil moisture in the desert sand shrubland ecosystem in the Bullfrog Region of Glen Canyon National Recreation Area

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