Recurring wildfires provoke type conversion in dry western forests

Abstract

Recent wildfires across western North America have burned with uncharacteristically high severity, representing a substantial departure from natural fire regimes. In mixed‐conifer and pine–oak ecosystems of the southern Cascade Range, widespread shifts in stand structure and composition have led to a diversity of post‐wildfire vegetation responses. When recent wildfire “footprints” reburn in subsequent fires, their recovery pathways are complex. In order to understand the effects of overlapping mixed‐severity fires, we quantified changes in overstory and midstory structure and species composition for time periods prior to and following two large overlapping wildfires in the southern Cascades: the 2000 Storrie and 2012 Chips Fires. Plots were stratified into 16 severity combinations (unburned, low, moderate, and high in the Storrie Fire combined with the same four categories in the Chips Fire: e.g., moderate Storrie/high Chips) across the 9000‐ha overlapping burned area. Following the two fires, tree quadratic mean diameter and stand density declined for most species, but changes were species‐specific. Compared with preburn values, importance values for fire‐sensitive white fir (Abies concolor) were reduced by 66%, while resprouting fire‐resilient California black oak (Quercus kelloggii) importance values increased by 37% in severity combinations that included at least one high‐severity fire. Greatest shifts were documented in sites that burned twice at high severity, where resulting vegetation was dominated by oak sprout clumps and resprouting and fire‐stimulated montane chaparral species, while unburned and low‐severity strata retained a substantial component of Douglas‐fir (Pseudotsuga menziesii) and white fir. Results suggest that repeated moderate‐ and high‐severity fires can result in ecosystem state shifting toward fire‐resilient oak‐shrub communities in this fire‐prone landscape. Managers seeking greater landscape resilience can implement treatments such as thinning and prescribed burning, while taking advantage of fire‐created patches such as these in areas where the likelihood of a hotter and drier future makes the reestablishment of continuous forest cover unrealistic.

Higher severity fire increases the long-term competitiveness of pyrophytes in an upland oak-pine forest, Kentucky, USA

“…In this, at low burn severities mesophytic species were substantially more important (w/r/t IV) than pyrophytes and intermediates, but as burn severity increased, pyrophytes became more important than mesophytes. This nding contrasts assertions that wild res in the eastern US may accelerate mesophication (Reilly et al 2022) and is aligned with patterns observed in western US forests, where higher-severity re shifts ecosystems toward more re-resilient communities (Nemens et al 2022;Cocking et al 2014). The burn severity at which pyrophytes in our study became more important (CBI = ~ 2.0) is particularly noteworthy, because prescribed res in this area rarely exceed a CBI of 1.5 (Winkenbach 2020).…”

Remotely sensing the effects of managed wildfire programs on Sierra Nevada meadows

“…A 2015 post-fire vegetation change study found that low and moderate-severity fire had little impact on the vegetative composition of the basin, with high-severity fire being primarily responsible for shifts away from conifer cover and towards shrub and meadow cover (Naranjo 2015). This is congruent with best available science on vegetative succession post-fire, which suggests that high-severity fire is often required to create fire-driven forest conversion in California ecosystems (Coop et al 2020, Nemens et al 2022. Attempts to use prescribed fire, which burns at low-to-moderate-severity, to combat conifer encroachment in meadows have found that this kind of fire removes lodgepole pines that are smaller in diameter than 20 cm but that there is little effect on larger trees (Frenzel 2012).…”

“…Many studies have found that multiple consecutive fires are necessary to achieve conversion from forest to non-forest vegetation in the SN (White and Long 2019, Nemens et al 2022, Paudel et al 2022. The fires in question in the 3+ burn areas all occurred between 1973-1997, and from 1976-1998 meadow extent in those areas actually increased by 130.88%, the most marked increase seen in any of the meadow change analyses (See figure 8).…”