Post-fire Conifer Regeneration Research Articles

Forest resilience and post-fire conifer regeneration in the southern Cascades, Lassen Volcanic National Park California, USA

Contemporary fire regimes in Californian forests are shifting, with fires becoming larger, more frequent, and increasingly severe. As landscapes transition back to active fire regimes, understanding how the physical environment and biological legacies of past disturbance interact with and determine forest development becomes an increasingly important management concern. We surveyed post-fire regeneration in a wilderness area of Lassen Volcanic National Park (LAVO) with a three-decade history of fires. We tallied conifer seedlings , and measured shrub and ground cover and mature trees in the field, and used geospatial data and water balance models to account for the possible effects of , terrain, live seed sources, vegetation characteristics, and immediate post-fire climate as measured by water balance on conifer regeneration. We used a Random Forest machine learning technique to model how landscape and local factors affect post-fire regeneration stocking and species regeneration occurrence. Post-fire conifer regeneration in LAVO was common: Median post-fire seedling density was close to desired stocking density, 73% of our plots had at least one conifer seedling, and only 16% of plots lacked tree regeneration or mature trees. Likelihood of stocking and individual species’ occurrence was most strongly related to time since fire, distance to forest, elevation, and species of nearest seed source. Tree regeneration increased with time since fire, but this effect was more pronounced at high elevation suggesting an interaction between terrain and time since fire on regeneration. Stocking was most likely on cooler (northeastern) slopes and at lower (<1950 m) elevations, nearer (within 200 m) to live forest patches, and where shrub cover was present but low (<20%). Climatic conditions as represented by water balance for the first five years after a fire did not influence likelihood of stocking but it did influence species occurrence. Distance to conspecific trees was important for species’ regeneration occurrence, and the landscape pattern of species occurrence was consistent with the elevational zonation of forest dominants in LAVO. Adequate tree regeneration of forests with an active fire regime in LAVO indicate that California montane forests are resilient to prescribed fire and managed wildfire and that expanded use of fire to reduce potential for severe fire promotes resilience in California forest landscapes.

Patterns and drivers of early conifer regeneration following stand-replacing wildfire in Pacific Northwest (USA) temperate maritime forests

Tree regeneration is a critical mechanism of forest resilience to stand-replacing wildfire (i.e., where fire results in >90 % tree mortality), and post-fire regeneration is a concern worldwide as the climate becomes warmer. Although post-fire tree regeneration has been relatively well-studied in fire-prone forests across western North America, it is less understood in fire regimes characterized by large patches of stand-replacing fire at long intervals, such as the nominally infrequent, high-severity fire regimes of the western Cascades of Washington and northern Oregon, USA (northwestern Cascadia) where some of world’s highest-biomass forests reside. Recent wildfire activity (2015–2020) in northwestern Cascadia provides a unique opportunity to build understanding of the mechanisms underpinning forest resilience to fire, with implications for forest management and the myriad socio-ecological values associated with these landscapes. Here, we asked: how is post-fire conifer regeneration across northwestern Cascadia affected by pre-fire stand age, burn-patch size, and topo-climatic conditions? We established 39 1-ha long-term monitoring plots across four recent wildfires in northwestern Cascadia to measure post-fire conifer abundance and species composition. We used generalized linear models to test how rates of conifer regeneration (seedlings ha−1 yr−1 of all heights) and establishment (seedlings ha−1 yr−1 with heights >10 cm) respond to differences in pre-fire stand age/seral stage (young, mid-seral, late-seral), distance to the nearest live seed source, and post-fire topo-climate setting. In general, initial post-fire seedling densities suggest tree regeneration is abundant following stand-replacing fire in most areas across the region. For example, at 3–5 years post-fire, established seedlings were present in 100 % of plots, and regeneration and establishment levels exceeded state forest practices minimum density thresholds (470 seedlings ha−1) in 82.1 % and 64.1 % of plots, respectively. Post-fire conifer regeneration and tree species richness were greater when pre-fire stand age was older, and was more dominated by shade tolerant species than when pre-fire stand age was younger. Regeneration decreased with farther distances to the nearest live seed source, though seedlings were still moderately abundant at distances as far as 400 m from edges of high-severity patches. Conifer establishment was greater in areas characterized by cooler and wetter macrosite (e.g., greater post-fire precipitation) and microsite (e.g., greater bryophyte ground cover) conditions. By providing information on the drivers and conditions in which land managers can expect natural regeneration to occur or not, our findings are directly relevant to pre- and post-fire forest management objectives under future climate and increased fire activity in systems structured in part by infrequent and severe wildfires.

Spatially explicit models of seed availability improve predictions of conifer regeneration following the 2018 Carr Fire in northern California

For many conifer species in dry conifer forests of North America, seeds must be present for postfire regeneration to occur, suggesting that seed dispersal from surviving trees plays a critical role in postfire forest recovery. However, the application of tree fecundity and spatial arrangement to postfire conifer recovery predictions have only recently become more common, and is often included at relatively coarse scales (i.e., 30 meters). In this study, we mapped surviving trees using lidar and created a spatially explicit estimate of seed density (seed shadows) with 10 m, 50 m, and 100 m median dispersal distances. We estimated the number of seeds produced by each tree using allometric relationships between tree size and fecundity. Along with the seed shadows, we used a suite of topographic variables as inputs to negative binomial hurdle models to predict conifer seedling abundance in 131 plots following the 2018 Carr Fire in northern California, USA. We compared models using each of the seed shadows to each other as well as to a model using the distance to the nearest surviving tree, which served as a baseline. All model formulations indicated that estimated seed availability was positively associated with conifer regeneration. Despite the importance of seed availability plays in regeneration and the substantial differences in seed availability represented by the different seed shadows in our analysis, we found surprisingly little difference in model performance regardless of which seed shadow was used. However, the models employing seed shadows outperformed the models with distance to the nearest live tree. Although we have demonstrated a modest improvement in predicting postfire conifer regeneration, the uncertainty in our results highlights the importance of tree detection and classification in future studies of this kind. Future studies may find it useful to consider other factors such as predation, site suitability, and seed mortality as potential drivers of discrepancies between total and realized dispersal kernels.

Reduced fire severity offers near-term buffer to climate-driven declines in conifer resilience across the western United States

Increasing fire severity and warmer, drier postfire conditions are making forests in the western United States (West) vulnerable to ecological transformation. Yet, the relative importance of and interactions between these drivers of forest change remain unresolved, particularly over upcoming decades. Here, we assess how the interactive impacts of changing climate and wildfire activity influenced conifer regeneration after 334 wildfires, using a dataset of postfire conifer regeneration from 10,230 field plots. Our findings highlight declining regeneration capacity across the West over the past four decades for the eight dominant conifer species studied. Postfire regeneration is sensitive to high-severity fire, which limits seed availability, and postfire climate, which influences seedling establishment. In the near-term, projected differences in recruitment probability between low- and high-severity fire scenarios were larger than projected climate change impacts for most species, suggesting that reductions in fire severity, and resultant impacts on seed availability, could partially offset expected climate-driven declines in postfire regeneration. Across 40 to 42% of the study area, we project postfire conifer regeneration to be likely following low-severity but not high-severity fire under future climate scenarios (2031 to 2050). However, increasingly warm, dry climate conditions are projected to eventually outweigh the influence of fire severity and seed availability. The percent of the study area considered unlikely to experience conifer regeneration, regardless of fire severity, increased from 5% in 1981 to 2000 to 26 to 31% by mid-century, highlighting a limited time window over which management actions that reduce fire severity may effectively support postfire conifer regeneration.

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

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.

Microenvironment characteristics and early regeneration after the 2018 Spring Creek Wildfire and post-fire logging in Colorado, USA

BackgroundWildfires are increasing in size and severity in forests of the western USA, driven by climate change and land management practices during the 20th century. Altered fire regimes have resulted in a greater need for knowledge on best practices for managing burned landscapes, especially in instances where a return to a previous forested ecosystem is desired. We examined a large wildfire from 2018 in southern Colorado to understand how fire severity and post-fire logging influenced stand structure, fuels, vegetation, and soil microsite conditions.ResultsTwo years post-fire and 1 year post logging, there was no difference in understory vegetation response. Logged plots demonstrated lower daily average temperature and minimum soil moisture and higher fuel loading across most fuel size classes relative to unlogged plots, which also corresponded with a loss of dead standing wood and little to no canopy cover. Early post-fire conifer regeneration was low across all plots, but lower soil moisture and higher soil temperature negatively impacted the density of regeneration.ConclusionsSuccessful tree regeneration is mediated by multiple factors from the microsite to landscape scale. Here, we demonstrate the importance of those microsite conditions such as soil moisture and temperature in predicting conifer tree establishment in the early post-fire period. Careful consideration of soil impacts and the associated changes to forest conditions should be taken when conducting post-fire logging to prevent detrimental effects on microsite conditions and forest recovery.

Effects of postfire climate and seed availability on postfire conifer regeneration.

Large, severe fires are becoming more frequent in many forest types across the western United States and have resulted in tree mortality across tens of thousands of hectares. Conifer regeneration in these areas is limited because seeds must travel long distances to reach the interior of large burned patches and establishment is jeopardized by increasingly hot and dry conditions. To better inform postfire management in low elevation forests of California, USA, we collected 5-yr postfire recovery data from 1,234 study plots in 19 wildfires that burned from 2004-2012 and 18yrs of seed production data from 216 seed fall traps (1999-2017). We used these data in conjunction with spatially extensive climate, topography, forest composition, and burn severity surfaces to construct taxon-specific, spatially explicit models of conifer regeneration that incorporate climate conditions and seed availability during postfire recovery windows. We found that after accounting for other predictors both postfire and historical precipitation were strong predictors of regeneration, suggesting that both direct effects of postfire moisture conditions and biological inertia from historical climate may play a role in regeneration. Alternatively, postfire regeneration may simply be driven by postfire climate and apparent relationships with historical climate could be spurious. The estimated sensitivity of regeneration to postfire seed availability was strongest in firs and all conifers combined and weaker in pines. Seed production exhibited high temporal variability with seed production varying by over two orders of magnitude among years. Our models indicate that during droughts postfire conifer regeneration declines most substantially in low-to-moderate elevation forests. These findings enhance our mechanistic understanding of forecasted and historically documented shifts in the distribution of trees.

Impacts of Postfire Management Are Unjustified in Spotted Owl Habitat

In mixed-conifer forests inhabited by California spotted owls, land managers hypothesize that without human intervention natural conifer regeneration will take many decades or longer to begin within interior areas of large high-severity fire patches, due to long distances from live tree seed sources. As a result, widespread post-fire logging, followed by sprayed application of herbicides and planting of conifer seedlings, are used to create tree plantations. These are activities routinely conducted in spotted owl territories following fires, despite current data that indicate this approach has adverse impacts on spotted owl occupancy. Land managers acknowledge such impacts, but continue these forest management practices, assuming they are a necessary harm, one that is warranted to ensure the later return of mature conifer forests used by spotted owls for nesting and roosting. However, few data have been gathered to test this hypothesis. At 5 years post-fire, we surveyed field plots on a grid within large high-severity fire patches in spotted owl habitat within the Rim fire of 2013 in the Sierra Nevada, California. In our analysis the percentage of plots lacking conifer regeneration decreased significantly with larger plot sizes, a finding contrary to previous studies which assumed vast “deforested” areas in wildland fires, a bias created by small plot size. We found higher conifer regeneration closer to live-tree edges, but we consistently found natural post-fire conifer regeneration at all distances into interior spaces of large high-severity fire patches, including &amp;gt;300 m from the nearest live trees. Distance from live-tree edges did not affect pine dominance in post-fire regeneration. The post-fire natural conifer regeneration reported in our results suggests that the adverse effects of current post-fire management in spotted owl habitat are not necessary practices that can be justified.

Tracking rates of postfire conifer regeneration vs. deciduous vegetation recovery across the western United States.

Postfire shifts in vegetation composition will have broad ecological impacts. However, information characterizing postfire recovery patterns and their drivers are lacking over large spatial extents. In this analysis, we used Landsat imagery collected when snow cover (SCS) was present, in combination with growing season (GS) imagery, to distinguish evergreen vegetation from deciduous vegetation. We sought to (1) characterize patterns in the rate of postfire, dual-season Normalized Difference Vegetation Index (NDVI) across the region, (2) relate remotely sensed patterns to field-measured patterns of re-vegetation, and (3) identify seasonally specific drivers of postfire rates of NDVI recovery. Rates of postfire NDVI recovery were calculated for both the GS and SCS for more than 12,500 burned points across the western United States. Points were partitioned into faster and slower rates of NDVI recovery using thresholds derived from field plot data (n=230) and their associated rates of NDVI recovery. We found plots with conifer saplings had significantly higher SCS NDVI recovery rates relative to plots without conifer saplings, while plots with ≥50% grass/forbs/shrubs cover had significantly higher GS NDVI recovery rates relative to plots with <50%. GS rates of NDVI recovery were best predicted by burn severity and anomalies in postfire maximum temperature. SCS NDVI recovery rates were best explained by aridity and growing degree days. This study is the most extensive effort, to date, to track postfire forest recovery across the western United States. Isolating patterns and drivers of evergreen recovery from deciduous recovery will enable improved characterization of forest ecological condition across large spatial scales.

Topography and post-fire climatic conditions shape spatio-temporal patterns of conifer establishment and growth

BackgroundConcern is mounting that larger, stand-replacing forest fires may accelerate compositional shifts or conversions to non-forested states under a warming climate. Post-fire climatic conditions influence system trajectories by facilitating or hindering juvenile recruitment. But without an accurate, long-term understanding of where, when, and how climatic variability and other ecological factors affect regeneration, our ability to predict post-fire trajectories is limited. I quantified multiple, interacting facets of post-fire conifer regeneration—including annual establishment rates and growth—one decade after stand-replacing fire on the eastern slopes of the North Cascades, Washington, USA. Sites were stratified across topographic settings to specifically capture the potential for topography to moderate seasonal and interannual climatic conditions.ResultsRecruitment of juvenile conifers occurred every year since fire, with considerable species-specific variability across topographic settings and distance to seed source. Juveniles of all species, except lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Watson), were rarely observed when live, conspecific seed source was more than 75 m away. Lodgepole pine was the only species for which greater distance to live seed source was not associated with lower densities, a pattern attributable to serotiny. Annual establishment rates were strongly correlated with post-fire conditions: rates were highest when growing seasons were relatively cool and moist. A lagged climate signal was apparent in annual growth rates, but standardized climate–growth relationships did not vary across topographic settings, suggesting that topographic setting did not decouple site conditions from broader climatic trends to a degree that affected growth patterns.ConclusionsThese results underscore the importance of favorable post-fire climatic conditions in promoting robust establishment and growth while also highlighting the importance of topography and endogenous, stand-level processes (e.g., seed availability and delivery) in shaping recovery over time. Furthermore, these results suggest that, while the growing concern of post-fire regeneration failure may indeed be warranted under some conditions, failure is not yet the rule in all places and at all times. A more detailed understanding of recovery dynamics through long-term monitoring and by examining multiple, interacting facets of regeneration across scales will improve our predictions of where and when regeneration failure or, conversely, robust recovery may occur under a changing climate.

Mixed-Severity Fire Fosters Heterogeneous Spatial Patterns of Conifer Regeneration in a Dry Conifer Forest

We examined spatial patterns of post-fire regenerating conifers in a Colorado, USA, dry conifer forest 11–12 years following the reintroduction of mixed-severity fire. We mapped and measured all post-fire regenerating conifers, as well as all other post-fire regenerating trees and all residual (i.e., surviving) trees, in three 4-ha plots following the 2002 Hayman Fire. Residual tree density ranged from 167 to 197 trees ha−1 (TPH), and these trees were clustered at distances up to 30 m. Post-fire regenerating conifers, which ranged in density from 241 to 1036 TPH, were also clustered at distances up to at least 30 m. Moreover, residual tree locations drove post-fire regenerating conifer locations, with the two showing a pattern of repulsion. Topography and post-fire sprouting tree species locations further drove post-fire conifer regeneration locations. These results provide a foundation for anticipating how the reintroduction of mixed-severity fire may affect long-term forest structure, and also yield insights into how historical mixed-severity fire may have regulated the spatially heterogeneous conditions commonly described for pre-settlement dry conifer forests of Colorado and elsewhere.

It matters when you measure it: using snow-cover Normalised Difference Vegetation Index (NDVI) to isolate post-fire conifer regeneration

Abstract. Landsat Normalised Difference Vegetation Index (NDVI) is commonly used to monitor post-fire green-up; however, most studies do not distinguish new growth of conifer from deciduous or herbaceous species, despite potential consequences for local climate, carbon and wildlife. We found that dual season (growing and snow cover) NDVI improved our ability to distinguish conifer tree presence and density. We then examined the post-fire pattern (1984–2017) in Landsat NDVI for fires that occurred a minimum of 20 years ago (1986–1997). Points were classified into four categories depending on whether NDVI, 20 years post-fire, had returned to pre-fire values in only the growing season, only under snow cover, in both seasons or neither. We found that each category of points showed distinct patterns of NDVI change that could be used to characterise the average pre-fire and post-fire vegetation condition Of the points analysed, 43% showed a between-season disagreement if NDVI had returned to pre-fire values, suggesting that using dual-season NDVI can modify our interpretations of post-fire conditions. We also found an improved correlation between 5- and 20-year NDVI change under snow cover, potentially attributable to snow masking fast-growing herbaceous vegetation. This study suggests that snow-cover Landsat imagery can enhance characterisations of forest recovery following fire.

Patterns of conifer regeneration following high severity wildfire in ponderosa pine – dominated forests of the Colorado Front Range

Many recent wildfires in ponderosa pine (Pinus ponderosa Lawson & C. Lawson) – dominated forests of the western United States have burned more severely than historical ones, generating concern about forest resilience. This concern stems from uncertainty about the ability of ponderosa pine and other co-occurring conifers to regenerate in areas where no surviving trees remain. We collected post-fire conifer regeneration and other data within and surrounding five 11–18year-old Colorado Front Range wildfires to examine whether high severity burn areas (i.e., areas without surviving trees) are regenerating, and how regeneration density is related to abiotic and biotic factors such as distance from surviving forest, elevation, and aspect. We found that some conifer regeneration has occurred in high severity burn areas (mean and median of 118 and 0stemsha−1, respectively), but at densities that were considerably lower than those in unburned and in low to moderate severity burn areas. Generalized estimating equation analyses revealed that distance from surviving forest was the most important predictor of conifer regeneration in high severity burn areas, with regeneration declining as distance from surviving forest increased; estimates of conifer regeneration were 211stemsha−1 immediately adjacent to surviving forest but only 10stemsha−1 200m from surviving forest. These analyses also revealed that conifer regeneration densities declined as elevation decreased. Regression tree analyses likewise showed that distance from surviving forest and elevation were important predictors of conifer regeneration in high severity burn areas; within 50m of surviving forest mean (median) regeneration was 150 (0)stemsha−1 at elevations ⩽2490m and 1120 (1000)stemsha−1 at elevations >2490m, but at distances ⩾50m from surviving forest mean (median) regeneration was only 49 (0)stemsha−1, regardless of elevation. Applying regression tree results spatially to the 2002 Hayman Fire, Colorado’s largest and most severe known wildfire, we found that 70% of the area without surviving forest exceeded this 50m threshold. These patterns of conifer regeneration suggest that Colorado Front Range ponderosa pine – dominated forests may not be resilient to high severity wildfire, particularly where surviving forest is not in close proximity. We recommend that land managers consider planting conifers within the interiors of large high severity burn patches, as well as implementing treatments to reduce the risk of uncharacteristic high severity wildfire in unburned forests, where maintaining a forested condition is desired.

Fire legacies impact conifer regeneration across environmental gradients in the U.S. northern Rockies

An increase in the incidence of large wildfires worldwide has prompted concerns about the resilience of forest ecosystems, particularly in the western U.S., where recent changes are linked with climate warming and 20th-century land management practices. To study forest resilience to recent wildfires, we examined relationships among fire legacies, landscape features, ecological conditions, and patterns of post-fire conifer regeneration. We quantified regeneration across 182 sites in 21 recent large fires in dry mixed-conifer forests of the U.S. northern Rockies. We used logistic and negative binomial regression to predict the probability of establishment and abundance of conifers 5–13 years post-fire. Seedling densities varied widely across all sites (0–127,500 seedlings ha−1) and were best explained by variability in distance to live seed sources (β = −0.014, p = 0.002) and pre-fire tree basal area (β = 0.072, p = 0.008). Beyond 95 m from the nearest live seed source, the probability of seedling establishment was low. Across all the fires we studied, 75 % of the burned area with high tree mortality was within this 95-m threshold, suggesting the presence of live seed trees to facilitate natural regeneration. Combined with the mix of species present within the burn mosaic, dry mixed-conifer forests will be resilient to large fires across our study region, provided that seedlings survive, fire do not become more frequent, high-severity patches do not get significantly larger, and post-fire climate conditions remain suitable for seedling establishment and survival.

A Mixed-Effects Heterogeneous Negative Binomial Model for Postfire Conifer Regeneration in Northeastern California, USA

Many western USA fire regimes are typified by mixed-severity fire, which compounds the variability inherent to natural regeneration densities in associated forests. Tree regeneration data are often discrete and nonnegative; accordingly, we fit a series of Poisson and negative binomial variation models to conifer seedling counts across four distinct burn severities and three forest types 10 years after the 23,000-ha Storrie Fire, a large mixed-severity fire in northern California. Despite the accessibility and power of the zero-inflated negative binomial mixture model, a flexible heterogeneous negative binomial model offered a superior fit. Incorporation of a random stand effect further improved model performance. A parametric bootstrap analysis was conducted to examine seedling distributions and stand stocking. Mean simulated seedling densities had an expansive range (272–29,257 ha−1). Stocking analyses suggest a high probability of deficient conifer coverage in the majority of lower-elevation high-severity burn stands. In addition, models were fit to fir and pine seedling counts. Only a minority of postfire stands were likely to be stocked in the pine-only analysis. These models will help land managers prioritize limited resources for artificial reforestation in mixed-severity burned landscapes.