Conifer seedling demography reveals mechanisms of initial forest resilience to wildfires in the northern Rocky Mountains

Highlights

• Conifer forests showed early resilience to two recent wildfires in Montana.

• Microclimate and microsite factors strongly influence post-fire seedling demography.

• Fine-scale heterogeneity in fire severity supports forest resilience to wildfire.

• A demographic framework applied here helps anticipate future forest resilience.

Abstract

Climate warming and an increased frequency and severity of wildfires are expected to transform forest ecosystems, in part through altered post-fire vegetation trajectories. Such a loss of forest resilience to wildfires arises due to a failure to pass though one or more critical demographic stages, or “filters,” including seed availability, germination, establishment, and survival. Here we quantify the relative influence of microclimate and microsite conditions on key stages of post-fire seedling demography in two large, lightning-ignited wildfires from the regionally extensive fire season of 2017 in the northern Rocky Mountains, U.S.A. We tracked conifer seedling density, survival, and growth in the first three years post-fire in 69 plots spanning gradients in fire severity, topography, and climate; all plots were limited to within 100 m of a seed source to assure seed availability. Microclimate conditions were inferred based on measurements in a subset of 46 plots. We found abundant post-fire conifer regeneration, with a median of 2,633 seedlings per hectare after three years, highlighting early resilience to wildfire. This robust regeneration was due in part to moderate post-fire climate conditions, supporting high survivorship (>50% on average) of all seedlings tracked over the study period (n = 763). A statistical model based on variables describing potential seed availability, microclimate, fire severity, understory vegetation, and soil nitrogen availability explained 75% of the variability in seedling density among plots. This analysis highlights the overarching importance of fine-scale heterogeneity in fire effects, which determine microclimate conditions and create diverse microsites for seedlings, ultimately facilitating post-fire tree regeneration. Our study elucidates mechanisms of forest resilience to wildfires and demonstrates the utility of a demographic perspective for anticipating forest responses to future wildfires under changing environmental conditions.

Introduction

Forest ecosystems and the services they provide are changing due to the combined impacts of warmer, drier climate conditions and increased wildfire frequency and severity, driven strongly by anthropogenic climate change (Abatzoglou and Williams, 2016, Holden et al., 2018, Parks and Abatzoglou, 2020). These changes stem in part from altered patterns of post-fire tree regeneration, which is a critical stage of community re-organization affecting forest composition and structure for decades to centuries (Tepley et al., 2013, Turner et al., 2016). Across western North America, numerous studies document a decline in post-fire tree regeneration in the 21st century (e.g., Stevens-Rumann et al., 2018, Turner et al., 2019, Coop et al., 2020), a trend expected to continue under climate warming (Kemp et al., 2019, Davis et al., 2020). These changes can indicate a loss of resilience to wildfire, used here to reflect the capacity of an ecosystem to recover following a disturbance and retain fundamental structures and functions (Gunderson, 2000). Forest resilience to wildfire is ultimately governed by complex interactions among climate, disturbance regimes, and species-specific responses to biophysical conditions (e.g., Rammer et al., 2021). Ongoing warming and increasing fire activity highlight the pressing need to understand mechanisms of forest resilience to wildfires, how these mechanisms vary among species and across biophysical gradients, and if, when, and where forest resilience to wildfire will be overcome in the future.

Anticipating forest resilience to wildfires is aided by considering key demographic filters that control fire-caused tree mortality and post-fire tree regeneration (Davis et al., 2018). Tree regeneration is the net result of mechanisms influencing seed production and dispersal, and seedling germination, establishment, and survival over multiple years after a fire (Karavani et al., 2018, Davis et al., 2018, Falk et al., 2022). The combined impacts of these processes depend on a suite of biotic and abiotic factors at each demographic stage, summarized by a demographic framework (Fig. 1).

Climate warming and changing fire regimes have the potential to impede post-fire tree regeneration by altering these biotic and abiotic factors that govern seedling demography. For example, numerous studies highlight fire effects on availability of viable seeds as a primary mechanism shaping post-fire seedling establishment (e.g., Kemp et al., 2016, Rodman et al., 2020b, Peeler and Smithwick, 2020, Stewart et al., 2021). Seed availability may become more limiting in the future due to shorter intervals between fires and thus time for trees to produce viable seed; through increased fire severity, which leaves fewer surviving seed trees; or through climate-driven reductions in tree fecundity (Enright et al., 2015, Clark et al., 2021, Gill et al., 2022). Vulnerability to these changes varies as a function of species traits. For example, species with serotinous cones can regenerate in the absence of surviving trees, from seeds stored in an aerial seed bank, but the production of serotinous cones is constrained by short fire-return intervals (Buma et al., 2013, Hansen et al., 2018).

Where seeds are available for post-fire germination, variability in fire effects and climate interact to shape the microclimate conditions experienced by seedlings. Post-fire climate conditions have well-recognized effects on tree regeneration (Fig. 1), with relatively cool, moist conditions generally most favorable (e.g., Harvey et al., 2016b, Andrus et al., 2018, Davis et al., 2019a, Rodman et al., 2020b). These regional climate conditions are mediated by local factors, including topography, forest structure and productivity, and understory vegetation (Dobrowski, 2011, De Frenne et al., 2019), which shape local microclimate conditions in post-fire environments (Ma et al., 2010, Carlson et al., 2020b, Crockett and Hurteau, 2022). With climate warming and increased fire activity, reductions in canopy cover and evapotranspiration are expected to result in less microclimatic buffering by vegetation in recently burned areas (Davis et al., 2019b, Wolf et al., 2021), increasing exposure to microclimatic extremes that inhibit regeneration (Carlson et al., 2020a, Hoecker et al., 2020).

Changes in fire severity can also alter microsite factors that influence seedling demography, independent of microclimate. Fires shape the competitive environment and increase the short-term availability of resources such as light, soil nutrients, and seed beds through changes in overstory, understory, and soil conditions (Fig. 1), which can play an important role in providing regeneration opportunities (Johnstone and Chapin, 2006, Parra and Moreno, 2017, Steed and Goeking, 2020). Responses to these varying factors differ among species and are difficult to tease apart, making it challenging to anticipate the impacts of changing fire severity (Urza and Sibold, 2017, Hill and Ex, 2020). For example, high-severity fire results in more stressful, warm-dry microclimate conditions (Wolf et al., 2021), but also a pulse of nutrient availability (Smithwick et al., 2005); sensitivity to these changes will depend on species traits (e.g., drought tolerance). The effects of fires on seed availability, microclimate, and other microsite factors can thus have contrasting effects on seedling demography, complicating our understanding of the influence of fire severity on post-fire tree regeneration.

Among recent work documenting tree regeneration in North America (see reviews by Stevens-Rumann and Morgan, 2019, Coop et al., 2020), post-fire demographic processes have rarely been tracked over time during the first several years after a wildfire, which is often a critical period for setting up long-term vegetation trajectories (e.g., through “priority” effects sensu Fukami, 2015, Tepley et al., 2013, Tepley et al., 2017, Urza and Sibold, 2017). This leaves a key research gap making it challenging to disentangle the relative importance of biophysical factors at early life stages and how these factors are impacted by changing climate and fire activity.

In this study, we use a demographic filter framework (Davis et al., 2018) to help elucidate mechanisms of forest resilience to two large wildfires in mid-elevation to lower subalpine conifer-dominated forests (hereafter “mixed-conifer” forests) of the northern Rocky Mountains, U.S.A. (Fig. 1). We build on prior research documenting post-fire near-ground atmospheric conditions (Wolf et al., 2021) to quantify the impacts of fire severity on regeneration through its effects on microclimate and other microsite factors, at sites where seed availability was not limiting. Specifically, we tracked seedlings in two wildfires in western Montana over three years post-fire, and estimated species-specific rates of seedling establishment, survival, and growth in study sites stratified by topographical position and fire severity to capture a range of abiotic and biotic conditions. Our objectives were to: 1) quantify the relative importance of climate, microclimate and fire severity at key seedling demographic stages for dominant conifer species; 2) disentangle the varying impacts of fire severity on seedling demography, including through changes in overstory and understory vegetation composition and structure and soil conditions; and 3) place our findings into the demographic framework to help assess what these examples imply about future forest resilience under climate change. Our results provide a detailed example of contemporary forest response to wildfire, and, more broadly, help to anticipate how ongoing climate warming and changing fire regimes will alter future forest resilience to wildfire.