Effects of short‐interval disturbances continue to accumulate, overwhelming variability in local resilience

Abstract

Increasing rates of short-interval disturbances have the potential to rapidly transform ecosystems via shifts in post-disturbance regeneration. While research has explored compound events in multiple biomes, we know little regarding how local site conditions interact with short-interval disturbances to influence post-disturbance regeneration. Furthermore, questions remain regarding the consequences of continued high frequency events: What happens when emerging new communities are themselves subject to short-interval disturbances? To investigate effects of ongoing short-interval fires on regeneration, we examined post-fire forest regeneration in two locations in interior Alaska. We established 50 plots across a mosaic of fire histories (one, two, or three fires in <70 yr) in an upland and lowland site in interior Alaska. To investigate how shifts in community driven by short-interval fires differ according to local site conditions, we quantified abundance, proportion, and density of conifer and deciduous regeneration in a drier upland site and a wetter lowland site. Both sites were dominated by black spruce prior to burning. In the drier upland site, black spruce (Picea mariana) presence declined sharply after two fires, while paper birch (Betula neoalaskana) became increasingly abundant with each additional fire. In the wetter lowland site, less organic soil was consumed by fire and presence of black spruce persisted through an initial single reburn (two fires), indicating local topography may temporarily buffer reburning impacts. However, after three burns, conifers were effectively eliminated in both upland and lowland stands. Deciduous regeneration differed with site: Birch dominated in upland plots, while willow (Salix spp.) and aspen (Populus tremuloides) dominated in lowlands. These results offer strong empirical evidence of the divergence of boreal successional trajectories from previous historic norms. Furthermore, results from this study demonstrate shifts in post-fire succession in forested ecosystems continue to accumulate with additional short-interval disturbance events, overwhelming the interactive effects of local site conditions.

Disturbance ecology and the problem of n = 1: A proposed framework for unifying disturbance ecology studies to address theory across multiple ecological systems

“…Comparatively, both systems demonstrate community stability except for high frequencies and magnitudes, an important commonality. Contrast with Hayes and Buma (2021), a similar multi-disturbance system in boreal forests with similar resilience mechanisms as Chileen et al, but where f values are >1. In that case, substantial ecosystem shifts were noted.…”

Future fire regimes increase risks to obligate‐seeder forests

“…However, many miss the feedback between climate, fuels and future fire regimes (Franklin et al., 2014). There is also the potential for interacting and synergistic impacts, with shifts in fire regimes occurring alongside shifts in demographic traits and or climate change, which may increase extinction risk (Enright et al., 2015), cause shifts in forest composition (Hayes & Buma, 2021), and reduce biodiversity or ecosystem resilience (Stevens‐Rumann et al., 2018). Further work will concentrate on combining powerful fire regime simulations with population‐level demographic data to explore these complex feedbacks with greater realism.…”

“…Such demographic changes coupled with more frequent fire could narrow the interval for plant reproductive maturity and survival (“interval squeeze”; Enright et al., 2015), leading to decreased chances of population persistence and increased likelihood of extinction (Enright et al., 2015; Hoecker et al., 2020). Ecosystem‐level changes arising from too frequent fire can cause shifts from forest to non‐forested states (Bowman, 2000) or change forest structure and function (Hayes & Buma, 2021) with evidence that frequent fire favours grass over shrub species in some forest understoreys (Andersen et al., 2005; Watson et al, 2009; Murphy & Bowman, 2012; Hammill et al., 2016; Fairman et al., 2017). Understanding where such forest changes are most likely to occur could be used to prioritize management and resource allocation for activities such as fire suppression, pre‐emptive management and post‐fire restoration.…”

Short-interval fires increasing in the Alaskan boreal forest as fire self-regulation decays across forest types

“…Briefly, if a second fire occurs prior to the reconstruction of serotinous cones, there is little seed supply available for regeneration 16 , 18 and dispersal from unburned edges is generally less likely due to the relatively heavy seeds of conifers 16 . There is ample empirical evidence for this; short-interval fires with intervals less than 30-year intervals (less than the typical maturity time of black spruce of 30–40 years 19 have repeatedly resulted in a near or complete loss of black spruce seed and seedlings (e.g., 15 , 20 ). Other serotinous species show similar population declines after short-interval fires as well 13 , 14 , 18 .…”