Describing trends in past and future soil moisture in the clayey fan grassland ecosystem in Petrified Forest National Park

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 pro-file; 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 produc-tivity, 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 monitor-ing plots in 13 ecosystems in the Southern Colorado Plateau Network of the National Park Service. In Petrified Forest National Park, this included 30 vegetation monitor-ing plots within the clayey fan grassland 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 vegeta-tion 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 compari-son. Model simulations were also run for two time periods in the future: a near-term period (2020-2059) and a long-term pe-riod (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 emis-sions scenario. The soil-water model results provide a de-tailed look at historical patterns of soil water availability, and how soil water availability is projected to change in the future, both sea-sonally 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 inter-mediate (20–50 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 mois-ture by calculating correlations of annual NDVI (2001-2014) with soil water potential averaged over a range of moving windows. Key findings for the Petrified Forest NP clayey fan grassland ecosystem include the following: All of the climate models predicted increases in mean monthly and annual temperature in the near-term future (+2.3°C on average), and further increases in the long-term future (+4.9°C on average). Models had greater variation in their predictions of both the direction and the amount of future precipitation, especially during the summer and fall. Future soil water availability is projected to decline significantly in fall, winter, and spring. By the end of the 21st century, average fall/winter soil water potential is expected to drop to approximately -3.0 MPa, indicating conditions of extreme water scarcity, and approaching conditions observed in the spring now. Spring soil water availability will likely be less than summer’s, a pattern observed intermittently throughout the 20th century. The drying of soils in spring is predicted to occur dramatically earlier in the long-term scenario, and may eliminate the period when soils are moist during the spring growing season. This will likely have a substantial impact on plant species that rely on soil moisture during the spring. Productivity, as estimated by annual NDVI, is most strongly correlated to soil water potential in early summer, underscoring the importance of soil moisture during this time period.

Describing trends in past and future soil moisture in the clayey fan grassland ecosystem in Petrified Forest National Park

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