Areas of low natural regeneration potential post-fire in shrublands of southern California (selected years between 2008 and 2020)

Identifying locations where shrubland vegetation will not recover
naturally post-fire is a challenge given the vast areas that are regularly
burned in southern California. When shrublands are within the historic
fire return interval, e.g., 55 years for low-elevation shrubland (Keeley
and Safford 2016, Van de Water and Safford 2011), biomass accumulates and
shrub cover recovers after 10–14 years (Black et al. 1987, Bohlman et al.
2018). However, in many parts of southern California, the fire return
interval has decreased, often in conjunction with an increase in
non-native plant species, drought, and nitrogen deposition (Pratt et al.
2014, Allen et al. 2018, Syphard et al. 2019, Safford et al. 2022). Under
these conditions, post-fire biomass recovery can be impeded and, in some
cases, may result in type conversion from native shrubland to non-native
grassland (Syphard et al. 2019). We developed a repeatable method to
identify areas of low regeneration potential in southern California using
fire history data (FRAP 2021), using two rules guided by the published
literature (Zedler et al. 1983, Haidinger and Keeley 1993, Keeley and
Brennan 2012, Syphard et al. 2019, Underwood et al. 2021, Underwood and
Safford 2021). First, we set the threshold of the ‘number of fires in the
last 40 years’ to three or more fires, and second, we set the ‘time since
last fire’ to a threshold of <10 years.  We identified
pixels that met these criteria as having low natural regeneration
potential post-fire and, as a consequence, these areas could represent
candidate areas for post-fire restoration in shrublands.   The
rasters of low natural regeneration potential are a key input into the
online web mapping tool SoCal EcoServe, developed for US Department of
Agriculture Forest Service resource managers to calculate the long-term
impacts of wildfire on carbon lost. The tool is available at
https://manzanita.forestry.oregonstate.edu/ecoservices/ and described in
Underwood et al. (2022). The resulting rasters of low natural regeneration
potential provide a basis for integrating additional factors that might
affect the post-fire recovery of shrubands, including climatic water
deficit or nitrogen deposition.