EXPLORING INTERACTIONS BETWEEN POST-FIRE VEGETATION AND SOIL RECOVERY FOLLOWING THE 2020 BIGHORN FIRE IN THE SANTA CATALINA MOUNTAINS, ARIZONA

The Santa Catalina Mountains (SCM), a 2,789-meter elevation Sky Island in southeastern Arizona, experienced several major wildfires in the 21^st century. The 2002 Bullock Fire burned the eastern SCM, and the 2003 Aspen Fire burned the west and central SCM, abutting the Bullock burn perimeter to the east. Combined, these fires burned 115,313 acres. The 2020 Bighorn Fire burned 119,978 acres, reburning much of the Bullock and Aspen burn areas. We conducted a three-year study from 2021 to 2023 where we performed vegetation and soil surveys at 22 plots spanning nine burn histories (i.e. burn severity from the Bullock or Aspen Fire and burn severity from the Bighorn Fire) and three vegetation Ecological Response Units (ERUs) (i.e. Madrean pine oak, ponderosa pine, and mixed conifer). The vegetation surveys consisted of canopy, understory and seedling assessments, and soil surveys consisted of estimating soil hydraulic properties using a tension infiltrometer, as well as analyzing soil biogeochemical composition and bulk density. We compared vegetation recovery indices to trends in soil recovery as part of an initial assessment to understand if and how post-fire changes in vegetation could influence of soil recovery and vice versa. Our initial approach was to compare shrub coverage and soil nitrate concentrations. Wildfires result in a release of nutrients (e.g. nitrate) that can stimulate vegetation growth, and larger releases of these nutrients occur as fire burn severity increases. Therefore, we anticipated sites burned at high severity to have greater shrub recruitment. However, our preliminary results show that the larger release of nitrate in plots burned at high severity did not result in the expected greater recruitment of understory (e.g. shrubs). Our findings also suggest that sites burned at high severity in the Bighorn Fire exhibited decreasing soil nitrate concentrations from 2021 to 2023, whereas sites burned at low severity exhibited either small positive or negative changes in nitrate concentrations, and unburned sites had negligible change. We observed substantial changes in vegetation and soil hydraulic properties over the monitoring period and continue to analyze data from the SCM to further explore correlations between vegetation and soil recovery.