Conifer seedling demography reveals mechanisms of initial forest resilience to wildfires in the northern Rocky Mountains
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.
Section snippets
Methods
We characterized post-fire seedling demography by tracking seedling density, survival, and vertical growth in the first three years after fire occurrence to elucidate controls of post-fire tree regeneration. We monitored seedling regeneration in 69 sites across two fires. These include a subset of sites (22 burned and 11 unburned) from a previous study where we monitored microclimate to quantify the impacts of moderate- and high-severity fire on below-canopy atmospheric conditions (Wolf et al.,
Results
Regeneration was abundant by the third year after fire, with a median density among all sites of 2,633 seedlings per hectare (IQR 750 to 7500 ha−1). All burned sites (n = 47) had at least one live seedling, four sites (8.5%) had densities < 100 ha−1, and 12 sites (26%) had densities > 10,000 ha−1. Seedling density was significantly greater in burned than unburned sites on average for P. contorta, P. ponderosa, and L. occidentalis (Fig. 3). Comparing post-fire seedling densities with
Discussion
Our findings of abundant tree regeneration provide an important example of how forests can exhibit early resilience to contemporary fire activity, even in the context of increasingly stressful climate conditions for post-fire tree regeneration over the 21st century (e.g., Davis et al., 2019a). Biophysical aspects of the pre- and post-fire environments helped explain over 75% of the variability observed in post-fire conifer density in the two large fires we studied, supporting predictions based
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank undergraduate assistants A. Hendryx, M. Miller, R. Kirk-Davidoff, D. Darter, and S. Ammentorp for their help with data collection and entry. We thank Z. Holden for sharing climate data, A. Shaw and C. Cleveland for methodological instruction, and V. Archer for support for the project and insights on the study region. We are grateful to Kurt Wetzstein and the Lolo National Forest for cooperation with site selection and sampling. We thank S. Dobrowski for contributions to our study of
Open research statement
Data and code are publicly available via Dryad: https://doi.org/10.5061/dryad.9s4mw6mhj.
References (94)
- et al.
Canopy structure and below-canopy temperatures interact to shape seedling response to disturbance in a Rocky Mountain subalpine forest
For. Ecol. Manage.
(2020) - et al.
Patterns of conifer regeneration following high severity wildfire in ponderosa pine – dominated forests of the Colorado Front Range
For. Ecol. Manage.
(2016) - et al.
Mechanisms of forest resilience
For. Ecol. Manage.
(2022) - et al.
Accuracy of node and bud-scar counts for aging two dominant conifers in western North America
For. Ecol. Manage.
(2018) - et al.
Microsite conditions in a low-elevation Engelmann spruce forest favor ponderosa pine establishment during drought conditions
For. Ecol. Manage.
(2020) - et al.
Topographic position amplifies consequences of short-interval stand-replacing fires on postfire tree establishment in subalpine conifer forests
For. Ecol. Manage.
(2020) - et al.
A short-interval reburn catalyzes departures from historical structure and composition in a mesic mixed-conifer forest
For. Ecol. Manage.
(2022) - et al.
Spatial variability in microclimate in a mixed-conifer forest before and after thinning and burning treatments
For. Ecol. Manage.
(2010) - et al.
Review on fire effects on ectomycorrhizal symbiosis, an unachieved work for a scalding topic
For. Ecol. Manage.
(2017) - et al.
Impact of anthropogenic climate change on wildfire across western US forests
Proc. Natl. Acad. Sci.
(2016)
Moisture availability limits subalpine tree establishment
Ecology
February 24
Ion Exchange Resin Bag Method for Assessing Forest Soil Nitrogen Availability
Soil Sci. Soc. Am. J.
The Impacts of Changing Disturbance Regimes on Serotinous Plant Populations and Communities
Bioscience
Interactions Between Fire Refugia and Climate-Environment Conditions Determine Mesic Subalpine Forest Recovery After Large and Severe Wildfires
Frontiers in Forests and Global Change 5
Wildfire and spruce beetle outbreak have mixed effects on below-canopy temperatures in a Rocky Mountain subalpine forest
Journal of Biogeography:jbi.13994
Effects of fire on properties of forest soils: a review
Oecologia
Continent-wide tree fecundity driven by indirect climate effects
Nat. Commun.
Contributions of fire refugia to resilient ponderosa pine and dry mixed-conifer forest landscapes
Ecosphere
Wildfire-Driven Forest Conversion in Western North American Landscapes
Bioscience
Post-fire early successional vegetation buffers surface microclimate and increases survival of planted conifer seedlings in the southwestern United States
Can. J. For. Res.
Wildfires and climate change push low-elevation forests across a critical climate threshold for tree regeneration
Proc. Natl. Acad. Sci.
Microclimatic buffering in forests of the future: the role of local water balance
Ecography
Fire-catalyzed vegetation shifts in ponderosa pine and Douglas-fir forests of the western United States
Environ. Res. Lett.
Anticipating fire-mediated impacts of climate change using a demographic framework
Funct. Ecol.
Global buffering of temperatures under forest canopies
Nat. Ecol. Evol.
A climatic basis for microrefugia: the influence of terrain on climate
Glob. Change Biol.
Regeneration of montane forests 24 years after the 1988 Yellowstone fires: A fire-catalyzed shift in lower treelines?
Ecosphere
Visualizing fit and lack of fit in complex regression models with predictor effect plots and partial residuals
J. Stat. Softw.
Historical Contingency in Community Assembly: Integrating Niches, Species Pools, and Priority Effects
Annu. Rev. Ecol. Evol. Syst.
Is initial post-disturbance regeneration indicative of longer-term trajectories?
Ecosphere
Limitations to Propagule Dispersal Will Constrain Postfire Recovery of Plants and Fungi in Western Coniferous Forests
BioScience:biab139.
Ecological Resilience—In Theory and Application
Annu. Rev. Ecol. Syst.
The nature of the beast: examining climate adaptation options in forests with stand-replacing fire regimes
Ecosphere
It takes a few to tango: changing climate and fire regimes can cause regeneration failure of two subalpine conifers
Ecology
Drivers and trends in landscape patterns of stand-replacing fire in forests of the US Northern Rocky Mountains (1984–2010)
Landscape Ecol.
High and dry: post-fire tree seedling establishment in subalpine forests decreases with post-fire drought and large stand-replacing burn patches
Glob. Ecol. Biogeogr.
Decreasing fire season precipitation increased recent western US forest wildfire activity
Proc. Natl. Acad. Sci.
Topofire – Regeneration Mapper
The Forest Soil of the Douglas Fir Region, and Changes Wrought Upon it by Logging and Slash Burning
Ecology
The Earliest Stages of Tree Growth: Development, Physiology and Impacts of Microclimate
Changing disturbance regimes, ecological memory, and forest resilience
Front. Ecol. Environ.
Effects of Soil Burn Severity on Post-Fire Tree Recruitment in Boreal Forest
Ecosystems
Fire-induced deforestation in drought-prone Mediterranean forests: drivers and unknowns from leaves to communities
Ecol. Monogr.
Cited by (7)
Snow-cover remote sensing of conifer tree recovery in high-severity burn patches
2024, Remote Sensing of EnvironmentMesic mixed-conifer forests are resilient to both historical high-severity fire and contemporary reburns in the US Northern Rocky Mountains
2023, Forest Ecology and ManagementDifferences in regeneration niche mediate how disturbance severity and microclimate affect forest species composition
2023, Forest Ecology and Management