Less fuel for the next fire? Short‐interval fire delays forest recovery and interacting drivers amplify effects

Abstract

As 21st‐century climate and disturbance dynamics depart from historic baselines, ecosystem resilience is uncertain. Multiple drivers are changing simultaneously, and interactions among drivers could amplify ecosystem vulnerability to change. Subalpine forests in Greater Yellowstone (Northern Rocky Mountains, USA) were historically resilient to infrequent (100–300 year), severe fire. We sampled paired short‐interval (<30‐year) and long‐interval (>125‐year) post‐fire plots most recently burned between 1988 and 2018 to address two questions: (1) How do short‐interval fire, climate, topography, and distance to unburned live forest edge interact to affect post‐fire forest regeneration? (2) How do forest biomass and fuels vary following short‐interval versus long‐interval severe fires? Mean post‐fire live tree stem density was an order of magnitude lower following short‐interval versus long‐interval fires (3240 vs. 28,741 stems ha−1, respectively). Differences between paired plots were amplified at longer distances to live forest edge. Surprisingly, warmer–drier climate was associated with higher seedling densities even after short‐interval fire, likely relating to regional variation in serotiny of lodgepole pine (Pinus contorta var. latifolia). Unlike conifers, density of aspen (Populus tremuloides), a deciduous resprouter, increased with short‐interval versus long‐interval fires (mean 384 vs. 62 stems ha−1, respectively). Live biomass and canopy fuels remained low nearly 30 years after short‐interval fire, in contrast with rapid recovery after long‐interval fire, suggesting that future burn severity may be reduced for several decades following reburns. Short‐interval plots also had half as much dead woody biomass compared with long‐interval plots (60 vs. 121 Mg ha−1), primarily due to the absence of large snags. Our results suggest differences in tree regeneration following short‐interval versus long‐interval fires will be especially pronounced where serotiny was high historically. Propagule limitation will also interact with short‐interval fires to diminish tree regeneration but lessen subsequent burn severity. Amplifying driver interactions are likely to threaten forest resilience under expected trajectories of a future fire.

Peeking under the canopy: anomalously short fire‐return intervals alter subalpine forest understory plant communities

“…However, understory percent cover and diversity did not differ with fire interval, suggesting some resilience of understory communities to more frequent fires. We found changing FRI affected the understory via reduced postfire tree densities (which are a direct result of short‐interval fire; Braziunas et al ., 2023), indicating changes in postfire forest structure that result from anomalously short‐interval fire cascade to the understory (Fig. 5).…”

“…Conceptualization of key effects of anomalously short fire‐return intervals (FRIs) on understory plant communities in the Greater Yellowstone Ecosystem. (a) Several key environmental conditions vary between areas burned at either short or long FRI that drive changes in community composition, including lower postfire tree regeneration (Turner et al ., 2019; Braziunas et al ., 2023) and warmer, drier soils (Hoecker et al ., 2020). (b) Example quadrats illustrate primary changes to community composition, including the presence of species unique following each short‐ and long‐interval fire and a greater abundance of shade‐intolerant species, species adapted to lower moisture availability, and species capable of persisting in lower vegetation zones following short‐interval fire.…”

“…To compare understory responses to short‐ vs long‐interval fire in space, we sampled communities during summer 2021 across 20 plot pairs in which postfire tree regeneration and fuels were assessed by Braziunas et al . (2023). Briefly, sites were selected among recent (1994–2018) fires well‐distributed across Greater Yellowstone that burned young (< 30‐yr‐old) and mature (> 125‐yr‐old) lodgepole pine‐dominated forest at high severity (> 92.5% basal area mortality or 675–2000 RdNBR; Miller & Thode, 2007, Harvey et al ., 2016a; Fig.…”

“…elevation and aspect) was captured across pairs, we ensured environmental conditions were consistent within pairs using propensity scores accounting for elevation, aspect, and slope during site selection (Butsic et al ., 2017). Finally, all plots were dominated prefire by lodgepole pine, and postfire lodgepole pine densities were an order of magnitude lower in plots following short‐interval fire (Braziunas et al ., 2023).…”

“…(Hoecker et al, 2020;Wolf et al, 2021). Reduced tree regeneration following anomalous short-interval fire has been increasingly observed (Turner et al, 2019;Whitman et al, 2019;Busby et al, 2020;Braziunas et al, 2023) with more expected (Halofsky et al, 2020;Nolan et al, 2021;Rammer et al, 2021;Turner et al, 2022) as climate and fire regimes continue to change (Rogers et al, 2011;Westerling et al, 2011;Henne et al, 2021). However, few studies have focused on the potential effects of short-interval fires on understory vascular plant communities.…”