Long-term effects of tree expansion and reduction on soil climate in a semiarid ecosystem

Over a 13-year study period, growing degree days varied from near 30 (2018) to 100 (2016) across the SAGEsteppe plots. The number of wet degree days in spring on treated and untreated plots closely tracked October through June precipitation, rather than following a pattern of decreasing wet days with time since tree reduction. The additional wet days from tree reduction in the upper 30 cm (12 in) of soil were highest in drier years (2012, 2015 and 2018) and least in wetter years (2010, 2011, and 2016) (Fig. 4).  In the dry year of 2018 (12 to 13 years following treatment), there were 21 additional wet days in treated phase III plots compared to control.  Wet degree days generally followed the same response as wet days, which were driven more by wet days than soil temperature. The frigid/xeric regime had 19 ± 7.9 more wet days in spring than the mesic/aridic-xeric regime across the time interval of fall 2014 through spring 2018 (Table 1 and Appendix S1 Table S4 in Roundy 2020). In addition to lower soil moistures, soil temperatures were higher for the mesic/aridic-xeric than frigid/xeric regime for all seasons except winter. Burned plots also had greater cover of cheatgrass (Bromus tectorum) and annual forbs, especially for Phase III, than mechanically treated plots (19.7 vs 7.9%) (also see Williams et al. 2017 and Freund et al. 2020).