Post-fire Tree Research Articles

Wildfire facilitates upslope advance in a shade-intolerant but not a shade-tolerant conifer.

Wildfires may facilitate climate tracking of forest species moving upslope or north in latitude. For subalpine tree species, for which higher elevation habitat is limited, accelerated replacement by lower elevation montane tree species following fire may hasten extinction risk. We used a dataset of postfire tree regeneration spanning a broad geographic range to ask whether the fire facilitated upslope movement of montane tree species at the montane-to-subalpine ecotone. We sampled tree seedling occurrence in 248 plots across a fire severity gradient (unburned to >90% basal area mortality) and spanning ~500 km of latitude in Mediterranean-type subalpine forest in California, USA. We used logistic regression to quantify differences in postfire regeneration between resident subalpine species and the seedling-only range (interpreted as climate-induced range extension) of montane species. We tested our assumption of increasing climatic suitability for montane species in subalpine forest using the predicted difference in habitat suitability at study plots between 1990 and 2030. We found that postfire regeneration of resident subalpine species was uncorrelated or weakly positively correlated with fire severity. Regeneration of montane species, however, was roughly four times greater in unburned relative to burned subalpine forest. Although our overall results contrast with theoretical predictions of disturbance-facilitated range shifts, we found opposing postfire regeneration responses for montane species with distinct regeneration niches. Recruitment of shade-tolerant red fir declined with fire severity and recruitment of shade-intolerant Jeffrey pine increased with fire severity. Predicted climatic suitability increased by 5% for red fir and 34% for Jeffrey pine. Differing postfire responses in newly climatically available habitats indicate that wildfire disturbance may only facilitate range extensions for species whose preferred regeneration conditions align with increased light and/or other postfire landscape characteristics.

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

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.

Fire-caused mortality within tree neighborhoods increases growth of Pinus lambertiana more than growth of Abies concolor

Global increases in large-tree mortality and multiple disturbances such as fire, drought, and pestilence increase the importance of understanding how large-tree growth responds to changes in competitive neighborhoods. We used 34,175 spatially mapped trees within 25.6 ha that burned at low to moderate severity in 2013 to investigate how changes to tree neighborhoods affected growth of 3,652 surviving trees of two species, Abies concolor (white fir) and Pinus lambertiana (sugar pine). Using diameter measurements taken in 2014 and 2019, we estimated post-fire radial growth of each surviving tree. We modeled annual basal area increment as a function of diameter and extracted residuals to yield relative growth rate (RGR). We used a multi-model approach and AIC to compare the species-specific effects on RGR of all post-fire tree mortality within a neighborhood radius versus mortality broken down by cause. To represent density reduction around each surviving tree, we computed crowding metrics for neighborhood radii up to 20 m for all tree mortality combined and for each mortality cause. The best-fit model included terms for density reduction within 10 m of the surviving tree due to 1) fire or mechanical damage and 2) bark beetle mortality, as well as their interactions with species and diameter. Density reduction due to fire or mechanical damage had a positive effect on RGR that was nearly twice as strong for P. lambertiana as A. concolor. Tree diameter interacted with density reduction for just P. lambertiana. Small-diameter (10-cm) P. lambertiana had a fourfold larger increase in basal area increment than large-diameter (60-cm) P. lambertiana as density reduction due to fire or mechanical damage ranged from 0 to 100 %. The strong growth response of medium-diameter P. lambertiana to density reductions could eventually increase the abundance of large-diameter (≥60 cm) P. lambertiana. In conjunction with mortality rates, which were lower for large-diameter P. lambertiana (0.97 %) than A. concolor (3.31 %), this could increase the relative abundance of large-diameter P. lambertiana from 51.2 % in 2020 to 62.6 % by 2050. Changes to competitive neighborhoods can have species-specific effects on large-tree growth, an important consideration for long-term management of fire-prone forests.

Predictive accuracy of post-fire conifer death declines over time in models based on crown and bole injury.

A key uncertainty of empirical models of post-fire tree mortality is understanding the drivers of elevated post-fire mortality several years following fire, known as delayed mortality. Delayed mortality can represent a substantial fraction of mortality, particularly for large trees that are a conservation focus in western US coniferous forests. Current post-fire tree mortality models have undergone limited evaluation of how injury level and time since fire interact to influence model accuracy and predictor variable importance. Less severe injuries potentially serve as an indicator for vulnerability to additional stressors such as bark beetle attack or moisture stress. We used a collection of 164,293 individual tree records to examine post-fire tree mortality in eight western USA conifers: Abies concolor, Abies grandis, Calocedrus decurrens, Larix occidentalis, Pinus contorta, Pinus lambertiana, Pinus ponderosa, and Pseudotsuga menziesii. We evaluated the importance of fire injury predictors on discriminating between surviving trees versus immediate and delayed post-fire mortality. We fit balanced random forest models for each species using cumulative tree mortality from 1 to 5-years post-fire. We compared these results to multi-class random forest models using first-year mortality, 2-5-year mortality, and survival 5-years post-fire as a response variable. Crown volume scorched, diameter at breast height, and relative bark char height, were used as predictor variables. The cumulative mortality models all predicted trees that died within 1-year of fire with high accuracy but failed to predict 2-5-year mortality. The multi-class models were an improvement but had lower accuracy for predicting 2-5-year mortality. Multi-class model accuracies ranged from 85% to 95% across all species for predicting 1-year post-fire mortality, 42%-71% for predicting 2-5-year mortality, and 64%-85% for predicting trees that lived past 5-years. Our study highlights the differences in tree species tolerance to fire injury and suggests that including second-order predictors such as beetle attack or climatic water stress before and after fire will be critical to improve accuracy and better understand the mechanisms and patterns of fire-caused tree death. Random forest models have potential for management applications such as post-fire harvesting and simulating future stand dynamics.

Spatial and temporal drivers of post-fire tree establishment and height growth in a managed forest landscape

BackgroundIn seed-obligate conifer forests of the western US, land managers need a better understanding of the spatiotemporal variability in post-fire recovery to develop adaptation strategies. Successful establishment of post-fire seedlings requires the arrival of seeds and favorable environmental conditions for germination, survival, and growth. We investigated the spatiotemporal limitations to post-fire seedling establishment and height growth in dry to moist mixed conifer forests with and without post-fire forest management treatments (salvage logging, grass seeding) in areas burned from low to high severity. In 2011, we measured post-fire seedling establishment year, juvenile density (seedlings and saplings), and height growth (annual and total) in 50 plots with six conifer species in the School Fire (2005), Blue Mountains, WA, USA. In 2021, we remeasured the plots for post-fire juvenile density and height growth.ResultsPost-fire juvenile tree densities appeared sufficient for self-replacement of forest (> 60 stems ha−1) in 96% of plots in 2021 (median 3130 stems ha−1), but densities were highly variable (range 33–100,501 stems ha−1). Annual seedling establishment was positively correlated with cooler, wetter climate conditions during the summer of germination (July–September) and the growing season of the subsequent year (April–September) for multiple tree species. We found lower juvenile densities at greater distances to seed sources and with higher grass cover, while salvage logging had no effect. Annual height growth was shorter on warmer, drier topographic positions for three species, whereas annual height growth was associated with climate variability for one species. Shifts in height class structure from 2011 to 2021 were, in part, explained by differences among species in annual height growth.ConclusionsAbundant and widespread tree seedling establishment for multiple conifer species after fire was strong evidence that most burned sites in the present study are currently on a trajectory to return to forest. However, post-fire establishment may be constrained to brief periods of cooler, wetter climate conditions following future fires. Long-term monitoring of post-fire recovery dynamics is needed to inform management activities designed to adapt forests to climate change and future disturbances, which will collectively shape future forest structure and composition.

Wildfire affects boreal forest resilience through post-fire recruitment in Northeastern China

Climate change has increased the severity, frequency, and impact of fire disturbance on boreal forests worldwide. Unusual fire events can trigger a shift from needle-leaf dominated forests to broad-leaf dominated forests, altering the original succession trajectory. Thus, it is necessary to identify how altered fire regimes affect boreal forest resilience or capacity to recover after fire disturbance. In this study, we used the post-fire tree recruitment density, the aboveground biomass, the understory shrub cover, and the herb cover as indicators of forest resilience. We explored the links between patterns of resilience and burn severity across topographic positions in two post-fire forests with different time since fire. We quantified how burn severity, time since fire, and topographic position affect resilience indicators related to post-fire overstory and understory recruitment in the Great Xing’an Mountains of Northeastern China. We found that burn severity and topographic position exhibited strong influences on post-fire overstory tree recruitment and aboveground biomass. Severe fire promoted deciduous needle-leaf tree (larch) recruitment in lowland sites by removing a thick organic layer. In contrast, severe fire promoted broad-leaf tree (birch and aspen) recruitment in south-facing upland sites with relatively warmer and drier microclimates. Therefore, severe fire disturbance may trigger a post-fire species shift from larch dominated forest to broad-leaf forest, especially in south-facing upland sties. We also found that the understory may be more resilient to high severity fire than the overstory, because severity had short-term effects on post-fire understory recruitment. Our study suggested that burn severity was a crucial factor that affects boreal forest resilience through substantially altering resilience indicators related to post-fire overstory tree recruitment.

Fire interval and post-fire climate effects on serotinous forest resilience

BackgroundClimate change is eroding forest resilience to disturbance directly through warming climate and indirectly through increasing disturbance activity. Forests characterized by stand-replacing fire regimes and dominated by serotinous species are at risk when the inter-fire period is insufficient for canopy seed bank development and climate conditions for recruitment in the post-fire growing season are unsuitable. Although both factors are critical to serotinous forest persistence, their relative importance for post-fire regeneration in serotinous forests remains poorly understood. To assess the relative effects of each factor, we established plots in severely burned knobcone pine (Pinus attenuata Lemmon) forests in Oregon and California, USA, representing a range of past fire intervals (6 to 31+ years). Specifically, we evaluated effects of fire interval and pre-fire canopy seed bank (proxies for seed supply) and post-fire climate on three metrics of post-fire tree regeneration (seedling density, probability of self-replacement, percent population recovery).ResultsSeed supply consistently had the strongest effect on post-fire regeneration. Between 6- and 31-year fire intervals, post-fire seedling density increased from 1000 to 100,000 seedlings ha−1, while probability of self-replacement increased from ~ 0 to ~ 100% and percent population recovery increased from 20 to 2000% of the pre-fire population, respectively. Similarly, increasing the canopy seed bank by two orders of magnitude increased seedling density and percent population recovery by two orders and one order of magnitude, respectively, and increased the probability of self-replacement by > 50%. Greater post-fire climatic moisture deficit exacerbated the effect of seed supply; an additional 4–6 years between fires was required under high moisture stress conditions to reach similar regeneration levels as under low moisture stress conditions.ConclusionThe overriding effect of seed supply—strongly driven by pre-fire stand age—on post-fire regeneration suggests that altered fire frequency (an indirect effect of climate change) will have a profound impact on serotinous forests. Although direct effects of hot and dry climate are lower in magnitude, they can alter forest recovery where seed supply nears a threshold. These findings reveal how fire interval and climate combine to determine changes in forest cover in the future, informing management and vulnerability mapping.

Where are the trees? Extent, configuration, and drivers of poor forest recovery 30 years after the 1988 Yellowstone fires

Postfire recovery of fire-adapted forests remains uncertain as climate and fire regimes continue to change. Areas of poor postfire tree regeneration following late-20th-century fires may reveal characteristics associated with increased vulnerability to forest decline. However, sufficient time must have elapsed and pre- and postfire forest cover must be compared to distinguish areas that have not recovered. We used remotely sensed data and the Normalized Difference Vegetation Index (NDVI) to detect areas of poor forest recovery across > 250,000 ha of area burned as stand-replacing fire 30 years after the 1988 Yellowstone fires. We asked three questions: (1) What is the extent and configuration of sparse and reduced forest recovery? (2) How do vegetation characteristics compare between areas of sparse and reduced recovery vs. recovered forest? (3) What environmental characteristics explain the distribution and patch size of sparse and reduced recovery? We related postfire (2013–14) NDVI to field-measured stem density to establish an NDVI threshold of sparse tree regeneration, and we contrasted pre- (1986–87) and postfire (2018–19) NDVI as a proxy for pre- and postfire forest cover. Sparse and reduced forest recovery occupied ∼ 41,000 ha across the burned area, about half of which was ≥ 150 m from ex situ seed sources. Patches of poor recovery were generally large, with ∼ 13,400 ha in patches ≥ 50 ha and an area-weighted mean patch size of 97 ha. Vegetation was short (<2 m) in areas of sparse and reduced recovery and non-evergreen biomass was three times greater than in recovered forest. Sparse and reduced recovery was more likely at high elevations, on steep slopes, and far from ex situ seed sources, and patches were larger at high elevations and far from seed sources. It took 20 years for sparse and reduced recovery to be distinguishable from recovered forest using NDVI, suggesting a time lag before remotely sensed data can detect alternative pathways of postfire forest recovery. Now, 30 years after the 1988 fires, ∼16 % of the forest burned as stand-replacing fire has failed to recover. The extent and configuration of these areas suggest some may persist as sparse or non-forest for the foreseeable future, with implications for biodiversity and ecosystem processes.

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.

Assessing the effects of burn severity on post-fire tree structures using the fused drone and mobile laser scanning point clouds

Wildfires burn heterogeneously across the landscape and create complex forest structures. Quantifying the structural changes in post-fire forests is critical to evaluating wildfire impacts and providing insights into burn severities. To advance the understanding of burn severities at a fine scale, forest structural attributes at the individual tree level need to be examined. The advent of drone laser scanning (DLS) and mobile laser scanning (MLS) has enabled the acquisition of high-density point clouds to resolve fine structures of individual trees. Yet, few studies have used DLS and MLS data jointly to examine their combined capability to describe post-fire forest structures. To assess the impacts of the 2017 Elephant Hill wildfire in British Columbia, Canada, we scanned trees that experienced a range of burn severities 2 years post-fire using both DLS and MLS. After fusing the DLS and MLS data, we reconstructed quantitative structure models to compute 14 post-fire biometric, volumetric, and crown attributes. At the individual tree level, our data suggest that smaller pre-fire trees tend to experience higher levels of crown scorch than larger pre-fire trees. Among trees with similar pre-fire sizes, those within mature stands (age class: &amp;gt; 50 years) had lower levels of crown scorch than those within young stands (age class: 15—50 years). Among pre-fire small- and medium-diameter trees, those experiencing high crown scorch had smaller post-fire crowns with unevenly distributed branches compared to unburned trees. In contrast, pre-fire large-diameter trees were more resistant to crown scorch. At the plot level, low-severity fires had minor effects, moderate-severity fires mostly decreased tree height, and high-severity fires significantly reduced diameter at breast height, height, and biomass. Our exploratory factor analyses further revealed that stands dominated by trees with large crown sizes and relatively wide spacing could burn less severely than stands characterized by regenerating trees with high crown fuel density and continuity. Overall, our results demonstrate that fused DLS-MLS point clouds can be effective in quantifying post-fire tree structures, which facilitates foresters to develop site-specific management plans. The findings imply that the management of crown fuel abundance and configuration could be vital to controlling burn severities.

Interactions Between Fire Refugia and Climate-Environment Conditions Determine Mesic Subalpine Forest Recovery After Large and Severe Wildfires

Infrequent stand-replacing wildfires are characteristic of mesic and/or cool conifer forests in western North America, where forest recovery within high-severity burn patch interiors can be slow, yet successful over long temporal periods (decades to centuries). Increasing fire frequency and high-severity burn patch size, under a warming climate, however, may challenge post-fire forest recovery, promoting landscape-level shifts in forest structure, composition, and distribution of non-forest patches. Crucial to a delay and/or impediment to this shift, fire refugia (i.e., remnant seed sources) may determine forest recovery trajectories and potential forest state-transitions. To examine how fire refugia attributes (i.e. extent, composition, and structure) interact with local climate and environmental conditions to determine post-fire forest recovery responses, we developed fine-grain maps of fire refugia via remote sensing and conducted field-based assessment of post-fire conifer tree establishment largely originating (i.e., dispersed) from fire refugium in the Central Cascade Range of the Pacific Northwest United States. We found that limitations on seed availability, represented by the distance2-weighted density (D2WD) of fine-grain refugia extent, largely explained post-fire tree establishment responses within our relatively mesic and cool subalpine study sites. Interactions between seed availability, climate, and environmental conditions indicated that the structural attributes of refugia (e.g., tree height) and site abiotic/biotic environmental controls (e.g., climate water deficit, canopy cover, and coarse woody debris cover) interplayed to constrain or enhance species-specific tree establishment responses. Importantly, these interactions illustrate that when seed availability is critically low for a given area, climate-environment conditions may strongly determine whether forests recover following fire(s). Toward modelling and predicting tree establishment responses and potential forest state-transitions after large stand-replacing fires(s), our study demonstrates the importance of accurately quantifying seed availability via the fine-grain extent, configuration, and attributes of remnant seed source legacies.

Initial succession after wildfire in dry boreal forests of northwestern North America

Wildfires in the boreal forest of North America are generally stand renewing, with the initial phase of recovery often governing the vegetation trajectory for decades. Here, we investigated post-fire vegetation changes in dry boreal forests of the Northwest Territories, Canada, during the first 5 years following the unusually severe 2014 wildfire season. We sampled post-fire tree regeneration and the understory plant community at 1, 3, and 5 years post-fire across different stand types within fires that burned in 2014. Post-fire trajectories of tree recruitment, understory cover by plant functional types, and plant diversity varied widely among sampled stands, as well as among years post-fire. Tree seedling density reached relative equilibrium by 3-years post-fire, whereas trends in understory plant cover and understory species assemblages suggested an ongoing change that will extend beyond 5 years of observation. In almost half of sampled stands, the composition of recruited trees differed from that of the pre-fire stand, suggesting a change in tree species dominance. An analysis of regional climate revealed a significant, albeit spatially variable, warming and drying trend that will further accelerate forest stand transformation through both climate drivers of plant community composition and indirectly through increasing fire activity. While the 2014 wildfires enhanced the structural and compositional heterogeneity of the region, they also triggered vegetation changes that are likely to be persistent. As such, this study exemplifies the speed and variability that characterizes post-fire stand development in a strongly moisture-limited part of North America.

The Effects of Crown Scorch on Post-fire Delayed Mortality Are Modified by Drought Exposure in California (USA)

Accurately predicting the mortality of trees that initially survive a fire event is important for management, such as planning post-fire salvage, planting, and prescribed fires. Although crown scorch has been successfully used to predict post-fire mortality (greater than one-year post-fire), it remains unclear whether other first-order fire effect metrics (e.g., stem char) and information on growing conditions can improve such predictions. Droughts can also elevate mortality and may interact, synergistically, with fire effects to influence post-fire tree survival. We used logistic regression to test whether drought exposure, as indicated by summarized monthly Palmer Drought Severity Index (PDSI) over ten-years could improve predictions of delayed mortality (4–9 years post-fire) at the individual tree level in fire-affected forest inventory and analysis (FIA) plots in California (USA). We included crown scorch, bark thickness, stem char, soil char, slope, and aspect in the model as predictors. We selected the six most prevalent species to include in the model: canyon live oak, Douglas-fir, Jeffrey pine, incense-cedar, ponderosa pine, and white fir. Mean delayed mortality, based on tree count, across all FIA plots across all tree species and plots was 17%, and overall accuracy was good (AUC = 79%). Our model performed well, correctly predicting survivor trees (sensitivity of 0.98) but had difficulty correctly predicting the smaller number of mortality trees (specificity of 0.27) at the standard probability=0.5 mortality threshold. Crown scorch was the most influential predictor of tree mortality. Increasing crown scorch was associated with greater risk of delayed mortality for all six species, with trees exhibiting over 75% crown scorch having a probability of dying that exceeded 0.5. Increasing levels of stem char and soil char (first order indicators) were associated with increasing mortality risk but to less effect than crown scorch. We expected that greater drought exposure would increase delayed post-fire mortality, but we found that increasing drought exposure (median and minimum PDSI) was associated with a modest decrease in post-fire mortality. However, we did find that trees with high levels of crown scorch were less likely to survive with increasing drought exposure (median PDSI). Delayed mortality risk decreased as terrain slope increased. Taken together, our results suggest that trees with substantial crown damage may be more vulnerable to delayed mortality if exposed to drought and that crown scorch is an effective post-fire mortality predictor up to 10 years post-fire.

Post-fire tree regeneration from seed of the endangered Nothofagus alessandrii Espinosa in the Maule region of central Chile

The endemic ruil (Nothofagus alessandrii Espinosa) forest is one of the most endangered forest ecosystems in central Chile, where its regeneration is challenged by factors such as low seedling germination capacity and recently by fire. In this study, we sampled 36 plots (625 m2 each) within the very restricted distribution range of N. alessandrii and quantified regeneration of the species from seed after a catastrophic large-scale fire event in 2017. By means of logistic regression, Path analysis, and non-metric multidimensional scaling, we related ruil seedling regeneration with percentage vegetative cover, number of ruil individuals, richness of native species, density of seedlings of the introduced species Pinus radiata D. Don, and fire severity. Our results indicate that fire severity was the most important variable explaining the probability of regeneration of N. alessandrii. Seedling recruitment was present in 65% of the sites with low fire severity, but no recruitment was observed in sites severely burned by fire. The ecological implications of these findings suggest that after a high-severity fire, efforts to restore N. alessandrii should be concentrated on reforestation activities; however, in areas of low-severity burn, efforts must be focused on the management and protection of the new recruits.

Post-fire tree regeneration in forests of the Tikhvin Ridge, North-West Russia

We assessed the effect of ground fires on the success of advanced regeneration of tree species on gentle hilly ridges of terminal moraines of the North-Eastern part of the Leningrad Region. To study the juvenile generation of tree species, we carried out a reconnaissance survey of transects in the forest areas affected by wildfires. The forest stands were 10–12 years old. The stem numbers and heights of the regenerating tree species were recorded. The correlation analysis was used to determine the degree of influence of external factors on forest regeneration, including soil conditions and the burnt area size. We found differences in the species composition of stands with a predominance of deciduous and coniferous species depending on soil and hydrological conditions. We also found a relationship between soil factors and spruce and pine regeneration size in stands with a predominance of coniferous and deciduous species. The soil granulometric composition influenced proportions of coniferous and deciduous trees in post-fire stands. Using the knowledge obtained about the stand composition, we can forecast the future development of forest stands and prescribe the optimal forest management regimes.

Modifying regeneration strategies classification to enhance the understanding of dominant species growth in fire-prone forest in Southwest China

In fire-prone ecosystems, plant species having different post-fire regeneration strategies (PFRS) coexist at the local scale while showing different growth dynamics. To evaluate the effect of PFRS on species regeneration, we investigated the plant communities burned in different years in the central Yunnan Province, Southwest China. Several indicators describing plant growth and population importance were measured to compare the regeneration of four dominant tree species, including one facultative seeders (FS) and three obligate resprouters (OR). Partial correlation and mixed linear effect modeling were applied to disentangle the contribution of intrinsic and environmental factors to the interspecific variation in post-fire regeneration. We identified two subtypes of plant growth strategy in the early post-fire stage for OR species; i.e., number growth (OR ​+ ​N) priority and height growth (OR ​+ ​H) priority for sprouting stems. Generally, the FS and OR species varied in height growth rate with different temporal dynamics. The OR ​+ ​N species occupied post-fire space horizontally with more resprouts and larger coverage than OR ​+ ​H and FS species at the earlier stage. In contrast, the OR ​+ ​H species generally had far less resprouts per clump, more variations in stem height and basal diameter. Factor analysis showed that the variation of post-fire plant regeneration was not effectively explained by environmental factors (R2 ​< ​20%); however, the linear mixed models with the modified PFRS as a random effect substantially increased the explanation. The differentiated regeneration dynamics and growth priorities in the four dominant tree species indicated a critical dichotomy of habitat occupation strategy for the resprouter species during the early stage of post-fire forest restoration. Our study uncovered a trade-off between height versus number growth priority in the post-fire tree growth strategy and provides a novel perspective in understanding the living space occupying (niche partitioning) process and species coexistence in post-fire forest community assembly.

Tree spatial pattern and mortality prediction in burned patches of Dahurian larch (

Background Fire-caused tree mortality and spatial pattern are crucial for evaluating forest dynamics and developing management prescriptions. Aims We investigated direct fire effects on spatial distribution and mortality of Dahurian larch (Larix gmelinii Rupr.) and assessed the Ryan and Amman (R–A) model performance and the importance of immediate mortality predictors. Methods We analysed spatial patterns of fire-killed and surviving trees of three size classes in plots that burned at low- to high-severity using pair-correlation functions and tree mortality with the R–A model and generalised linear mixed models. Key results The mixed-severity fire caused strong density-dependent mortality and more aggregated surviving tree patterns at short distances. The R–A model generally performed acceptably, and crown scorch and bole char height were critical predictors determining post-fire tree mortality. Conclusions Fire-caused tree mortality and spatial patterns are controlled primarily by spatial variation in tree size and biological and structural characteristics. The prediction biases of the R–A model arose primarily from the intrinsic traits of Dahurian larch and the imbalanced dataset. Fine-scale neighbourhood density might be a fundamental priority for fire management and restoration. Implications This study could possibly improve mechanistic understanding of spatial pattern development and tree mortality in similar fire-prone conifer forests.

Relative Inference for Paired Data: More than Meets the Eye

Relative Inference for Paired Data: More than Meets the Eye

Small Area Estimation of Postfire Tree Density Using Continuous Forest Inventory Data

Wildfire activity in the western United States is expanding and many western forests are struggling to regenerate postfire. Accurate estimates of forest regeneration following wildfire are critical for postfire forest management planning and monitoring forest dynamics. National or regional forest inventory programs can provide vegetation data for direct spatiotemporal domain estimation of postfire tree density, but samples within domains of administrative utility may be small (or empty). Indirect domain expansion estimators, which borrow extra-domain sample data to increase precision of domain estimates, offer a possible alternative. This research evaluates domain sample sizes and direct estimates in domains spanning large geographic extents and ranging from 1 to 10 years in temporal scope. In aggregate, domain sample sizes prove too small and standard errors of direct estimates too high. We subsequently compare two indirect estimators—one generated by averaging over observations that are proximate in space, the other by averaging over observations that are proximate in time—on the basis of estimated standard error. We also present a new estimator of the mean squared error (MSE) of indirect domain estimators which accounts for covariance between direct and indirect domain estimates. Borrowing sample data from within the geographic extents of our domains, but from an expanded set of measurement years, proves to be the superior strategy for augmenting domain sample sizes to reduce domain standard errors in this application. However, MSE estimates prove too frequently negative and highly variable for operational utility in this context, even when averaged over multiple proximate domains.

A short-interval reburn catalyzes departures from historical structure and composition in a mesic mixed-conifer forest

Recent increases in fire frequency and severity across the western US are triggering abrupt changes in ecosystem structure and composition, especially in lower montane forests, but consequences of fire-regime change for mesic, mixed-conifer forests remain uncertain. Glacier National Park (Montana, USA) is characterized by a complex mosaic of species typical of Rocky Mountain, Pacific Northwest and boreal floras, creating opportunities for species composition to shift as species respond individualistically to environmental change. We quantified prefire and postfire stem density and composition in a recent anomalously short-interval fire (15 yrs) and historically typical long-interval fire (88–282 + yrs) to answer two questions: (1) How do the structure and composition of tree regeneration differ after long- and short-interval fires that burned in close proximity? (2) What drivers explain differences in postfire regeneration densities among species? We additionally compared understory cover in each fire to anticipate consequences of contemporary fire for the whole plant community. Mean postfire tree seedling density was 45 times higher after the long- than short-interval fire, and many stands in the short-interval fire lacked any tree regeneration. Postfire tree seedling densities were above prefire stem densities in the long-interval fire but well below in the short-interval fire. After both fires, fire-avoiding conifers (e.g., Picea engelmannii, Abies lasiocarpa) were generally replaced by fire-tolerating or fire-embracing species (e.g., Larix occidentalis, Pinus contorta var. latifolia). Some stands remained dominated by fire-sensitive conifers after long-interval fire but never after short-interval fire. Seedlings of Thuja plicata and Tsuga heterophylla were present after the long-interval fire but absent before and after the short-interval fire, despite historically occupying both burned areas. For all species, seedling density increased with indicators of proximate seed supply. Seedling density of dispersal-dependent species declined with distance to live trees, whereas P. contorta responded to serotiny. Seedlings of drought-intolerant species were more abundant on northerly aspects. Total understory cover was 50% lower after short-interval than long-interval fire, but shrub cover was twice as high. Our study suggests that persistent seed sources will allow L. occidentalis to expand following sequential high-severity fires, while dispersal-limited, drought-intolerant conifers may decline due to loss of seed sources and elevated postfire aridity. Increased fire activity will likely produce sparser forests that are dominated by fire-tolerating and fire-embracing conifers at the expense of fire-avoiding and fire-refuge species. Complete regeneration failure may be delayed by high fire-trait syndrome diversity, but the structure and composition of extant forests will shift.