Forest Burning Research Articles

Estimating Human Impacts on Soil Erosion Considering Different Hillslope Inclinations and Land Uses in the Coastal Region of Syria

Soils in the coastal region of Syria (CRoS) are one of the most fragile components of natural ecosystems. However, they are adversely affected by water erosion processes after extreme land cover modifications such as wildfires or intensive agricultural activities. The main goal of this research was to clarify the dynamic interaction between erosion processes and different ecosystem components (inclination, land cover/land use, and rainy storms) along with the vulnerable territory of the CRoS. Experiments were carried out in five different locations using a total of 15 erosion plots. Soil loss and runoff were quantified in each experimental plot, considering different inclinations and land uses (agricultural land (AG), burnt forest (BF), forest/control plot (F)). Observed runoff and soil loss varied greatly according to both inclination and land cover after 750 mm of rainfall (26 events). In the cultivated areas, the average soil water erosion ranged between 0.14 ± 0.07 and 0.74 ± 0.33 kg/m2; in the BF plots, mean soil erosion ranged between 0.03 ± 0.01 and 0.24 ± 0.10 kg/m2. The lowest amount of erosion was recorded in the F plots where the erosion ranged between 0.1 ± 0.001 and 0.07 ± 0.03 kg/m2. Interestingly, the General Linear Model revealed that all factors (i.e., inclination, rainfall and land use) had a significant (p < 0.001) effect on the soil loss. We concluded that human activities greatly influenced soil erosion rates, being higher in the AG lands, followed by BF and F. Therefore, the current study could be very useful to policymakers and planners for proposing immediate conservation or restoration plans in a less studied area which has been shown to be vulnerable to soil erosion processes.

Impacts of prescribed burning on urban forest soil: Minor changes in net greenhouse gas emissions despite evident alterations of microbial community structures

Knowledge of the impacts of prescribed burning on ecosystem functions could help to improve forest management practices. However, relative to forest safety and aesthetic values, changes in soil properties and processes after prescribed burning are not well understood. This study aimed to quantify changes in soil microbial communities and greenhouse gas emissions following prescribed burning in a subtropical suburban forest. The soil microbial communities were determined with the Illumina sequencing methods. Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) emissions were quantified under three different soil water contents, and net greenhouse gas emissions were derived from emissions of the three gases after 7 days of incubation and expressed in CO2-equivalent. Prescribed burning had negligible impacts on soil bacterial and fungal community diversities but significantly changed soil microbial community structures. Compared to the unburnt soil, the relative abundances of Chloroflexi increased by 86% and 119% one month and three months after the prescribed burning, respectively. However, the relative abundances of Mortierellomycota decreased by 94% one month after prescribed burning. At different soil water contents, The CO2 and N2O emissions during a 7-day laboratory incubation did not differ consistently or significantly between the soils collected before/after the prescribed burning. Simultaneously, there were no significant changes in net greenhouse gas emissions from soil in association with the prescribed burning. The results revealed that relative to the unburned soils, prescribed burning caused minor alterations in net soil greenhouse gas fluxes, despite short-term evident changes in soil microbial community structures.

Fruit provision from Berberis microphylla shrubs as ecosystem service in Nothofagus forest of Tierra del Fuego

Berry production is a non-wood product worldwide recognized by its nutritional value and taste, but the most studied species are non-native commercial plants in productive areas, leaving aside native berries. We propose that native berries (Berberis microphylla G.Forst) naturally growing in degradation forests areas could diversify livestock establishment production and complement traditional uses (e.g., livestock). The aims of this work were to 1) environmentally characterize (e.g., soil nutrient content and physical conditions, air conditions and photosynthetically active radiation) in three degraded Nothofagus antarctica Oerst. forest (due to past fires and livestock use) of Tierra del Fuego; and 2) evaluate berries production of B. microphylla in terms of quality production (e.g., fruit number and weigh per shrub) and in terms of quality (individual fruit weight, fruit pulp percentage, and soluble solids content) to assess provisioning ecosystem service of this native shrub in different degraded areas. Studied sites were defined as: 1) Severe soil degradation condition (SEV) (high frequency of horses, bulls and some native guanacos year round, severe soil erosion, and shorter herbaceous layer), 2) Moderate soil degradation condition (MOD) (cattle and guanaco year round, intermediate level of soil erosion and intermediate height of herbaceous layer), and 3) Slight soil degradation condition (SLI) (livestock only during winter, but high frequency of native guanacos, lower soil erosion and taller herbaceous layer). (SEV) had the highest air and soil temperature, least soils nutrients content, highest bulk density, the least soil water content and the poorest fruit production. (MOD) had the highest soil water content and nutrient-rich soils, while (SLI) had the highest relative air humidity and PAR. B. microphylla shrubs grow with similar morphology on the different soil degraded condition. The highest fruit production were at (SLI), however the (SEV) had the highest soluble solids. We conclude that calafate shrubs in degraded Nothofagus forests offer a provisioning ecosystem service through their excellent fruits quality. Livestock farms could diversify their production through native fruits taking advantage of the altered areas occupied by B. microphylla. However, we recommend avoiding intensive livestock use in burned forests since it could lead to an irreversible soil erosion. Proper livestock management in Nothofagus burned forest could keep over the time not only the recognized ecosystem provision services (fruits, meat, wood), but also those of regulation and support that calafate shrubs offer and that make the functionality of the ecosystem.

Evaluating Post-Fire Vegetation Recovery in Cajander Larch Forests in Northeastern Siberia Using UAV Derived Vegetation Indices

The ability to monitor post-fire ecological responses and associated vegetation cover change is crucial to understanding how boreal forests respond to wildfire under changing climate conditions. Uncrewed aerial vehicles (UAVs) offer an affordable means of monitoring post-fire vegetation recovery for boreal ecosystems where field campaigns are spatially limited, and available satellite data are reduced by short growing seasons and frequent cloud cover. UAV data could be particularly useful across data-limited regions like the Cajander larch (Larix cajanderi Mayr.) forests of northeastern Siberia that are susceptible to amplified climate warming. Cajander larch forests require fire for regeneration but are also slow to accumulate biomass post-fire; thus, tall shrubs and other understory vegetation including grasses, mosses, and lichens dominate for several decades post-fire. Here we aim to evaluate the ability of two vegetation indices, one based on the visible spectrum (GCC; Green Chromatic Coordinate) and one using multispectral data (NDVI; Normalized Difference Vegetation Index), to predict field-based vegetation measures collected across post-fire landscapes of high-latitude Cajander larch forests. GCC and NDVI showed stronger linkages with each other at coarser spatial resolutions e.g., pixel aggregated means with 3-m, 5-m and 10-m radii compared to finer resolutions (e.g., 1-m or less). NDVI was a stronger predictor of aboveground carbon biomass and tree basal area than GCC. NDVI showed a stronger decline with increasing distance from the unburned edge into the burned forest. Our results show NDVI tended to be a stronger predictor of some field-based measures and while GCC showed similar relationships with the data, it was generally a weaker predictor of field-based measures for this region. Our findings show distinguishable edge effects and differentiation between burned and unburned forests several decades post-fire, which corresponds to the relatively slow accumulation of biomass for this ecosystem post-fire. These findings show the utility of UAV data for NDVI in this region as a tool for quantifying and monitoring the post-fire vegetation dynamics in Cajander larch forests.

Pertanggung Jawaban Pidana Karyawan Korporasi Dalam Tindak Pidana Lingkungan Hidup (Studi Putusan No. 133/Pid.B/2013/PN. MBO)

Environmental Crimes committed by a person or corporate legal entity often occur around the environment where we live without us knowing it, especially in an environment full of companies that can damage the surrounding environment. The research method used in this study is juridical normative and descriptive analytical nature of the study. The legal regulation on environmental crime, especially regarding forest and land burning is regulated in Article 187 of the Criminal Code, Article 78 paragraph (3) of Law Number 41 of 1999 concerning Forestry, Article 69, Article 108 and Article 119 of Law Number 32 of 2009 Regarding Environmental Protection and Management, Article 48 of Law Number 18 Year 2004 Plantations. Criminal liability of corporate offenders in imprisonment for 3 (three) years and a fine of Rp. 3,000,000,000 (three billion rupiah), Determine if the fine is not paid replaced with imprisonment for 5 (five) months. Judge's consideration in this decision, that the Defendant violated Article 108 in conjunction with Article 69 paragraph (1) letter (h) jo Article 116 paragraph (1) letter (b) of Law Number 32 of 2009 concerning Environmental Protection and Management and jo Article 64 paragraph (1) of the Criminal Code has several considerations, namely incriminating circumstances and mitigating circumstances.

Ignition of Forest Combustible Materials in a High-Temperature Medium

The ignition of typical forest combustible materials (birch leaves) at temperatures characteristic of a crown forest fire was investigated. The dependence of the delay time of ignition of a birch-leaf sheet on the temperature of the surrounding medium was determined, and the mechanism of ignition of leaves in a high-temperature medium was revealed. A mathematical model of classification of forest combustible materials on the basis of experimental data on their properties has been developed.

Tree regeneration on industrial linear disturbances in treed peatlands is hastened by wildfire and delayed by loss of microtopography

The world’s forests are highly fragmented by linear disturbances, and many forests have failed to recover decades after abandonment. Lack of recovery is common in unproductive forests, such as treed peatlands, because of conditions that limit tree growth, including simplification of microtopography (loss of microsites). The persistence of these features affects biodiversity, but of particular concern in Canada is the detrimental effects on threatened woodland caribou. Although natural regeneration of trees in peatlands occurs in some places, it is not an effective recovery strategy for restoring the habitat of woodland caribou. This has led to restoration activities with costs exceeding CAD$12 500/km. However, current restoration does not consider wildfires, which can destroy planted trees but also initiate early seral conditions that favor natural regeneration. Here we compared tree regeneration on seismic lines and adjacent forest controls for burnt (75 sites) and unburnt (68 sites) treed peatlands in northeast Alberta, Canada. Tree regeneration (stems with a DBH < 1 cm/ha) varied from 28 500 in burnt lines, 11 440 in unburnt lines, and 18 210 in burnt forest, to 9520 in unburnt forest. Wildfires promoted denser regeneration in sites with a greater proportion of serotinous species and water table depth. Microtopography and terrain wetness explained regeneration on burnt lines, but not unburnt lines. In burnt and unburnt lines, sunlight, microtopography, and depth of water table most affected tree regeneration patterns.

Biochemical and functional responses of stream invertebrate shredders to post-wildfire contamination

Forests in Mediterranean Europe including Portugal are highly susceptible to wildfires. Freshwaters are often exposed to post-wildfire contamination that contains several toxic substances, which may impose risk to freshwater organisms and ecosystem functions. However, knowledge on the impacts of post-wildfire runoffs from different origins on freshwater biota is scarce. In forest streams, invertebrate shredders have a major contribution to aquatic detrital-based food webs, by translocating energy and nutrients from plant-litter to higher trophic levels. We investigated the leaf consumption behaviour and the responses of oxidative and neuronal stress enzymatic biomarkers in the freshwater invertebrate shredder Allogamus ligonifer after short-term exposure (96 h) to post-wildfire runoff samples from Pinus and Eucalyptus plantation forests and stream water from a burnt catchment in Portugal. Chemical analyses indicated the presence of various metals and PAHs at considerable concentrations in all samples, although the levels were higher in the runoff samples from forests than in the stream water. The shredding activity was severely inhibited by exposure to increased concentrations of post-wildfire runoff samples from both forests. The dose-response patterns of enzymatic biomarkers suggest oxidative and neuronal stress in the shredders upon exposure to increasing concentrations of post-wildfire runoffs. The impacts were more pronounced for the runoffs from the burnt forests. Moreover, the response patterns suggest that the energy from the feeding activity of shredders might have contributed to alleviate the stress in A. ligonifer. Overall, the outcomes suggest that the post-wildfire contamination can induce sublethal effects on invertebrate shredders with impacts on key ecological processes in streams.

Mice and Habitat Complexity Attract Carnivorans to Recently Burnt Forests

Faunal responses to wildfire depend on the fire effects on direct mortality, habitat structure, and resource availability for animals. Despite the importance of large predators in terrestrial trophic webs, little is still known about how fire affects carnivorans (the mammalian order Carnivora). To evaluate the responses of the carnivoran community to fire, we studied three recently burnt forest areas in the western Mediterranean basin. Line transects were used to quantify evidence of carnivorans (mainly feces) and to measure environmental variables and resources (small mammal abundance, fleshy fruit availability, and plant cover). Throughout the study, we found 212 carnivoran field signs, 93% of them produced by red fox and stone marten. Immediately after fire, carnivoran occurrence was more frequent close to the perimeter of the burnt area, where fire severity was low, and in places with greater small mammal abundance. Small mammal abundance and plant cover had the greatest effect on the frequency of occurrence of red fox in the burnt area surroundings, and this increased with time-since-fire in the burnt area. Furthermore, the presence of red fox did not affect stone marten occurrence. Stone martens were found around the burnt area perimeter, probably because of their preference for high plant cover, and they were not significantly affected by small mammal abundance. The scat frequency of occurrence of both species was not significantly related to fleshy fruit availability. Accordingly, rodents and carnivorans were more abundant where the habitat was more complex. Our results show that the responses of some carnivorans to fire are influenced, directly and indirectly, by habitat structure and resource availability.

Synergism of climatic variables and forest burns in the State of Acre

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Forest fires environmental impact study

The impact of the toxic components of the pyrolysis products of forest combustible materials, air temperature, and thermal radiation on the environment was studied. The data on the amount of solid particles contained in the smoke of forest fires and controlled fires are presented. The ratio of the mass of solid particles of smoke aerosol and the mass of burnt combustible material for different combustible materials is determined. The data on the toxicity of gases generated during the forest combustible materials burning and their effect on the human body are presented. The specific emissions (emission factors) of substances in a fire are determined and the methodology for their calculation is given. The dynamics of forest burnability in the Krasnoyarsk Territory over the past 48 years and the reasons for the increase in forest area covered by fires are presented. Measures to reduce harmful emissions from forest fires into the atmosphere and to reduce environmental pollution by improving the mechanisms for regulating forest protection activities and preventing the occurrence and extinguishing of forest fires are proposed.

Boreal blazes: biomass burning and vegetation types archived in the Juneau Icefield

The past decade includes some of the most extensive boreal forest fires in the historical record. Warming temperatures, changing precipitation patterns, the desiccation of thick organic soil layers, and increased ignition from lightning all contribute to a combustive combination. Smoke aerosols travel thousands of kilometers, before blanketing the surfaces on which they fall, such as the Juneau Icefield. However, many aerosols found in smoke plumes are also produced by other processes and therefore can be ambiguous indicators of fire activity. Here, we use the monosaccharide anhydrides levoglucosan, mannosan, and galactosan as specific indicators of biomass burning to unambiguously demonstrate that fire aerosols reach the Juneau Icefield and are integrated into the snowpack. Back trajectories and satellite observations demonstrate that smoke plumes originating in central Alaska and eastern Siberia affect the Juneau Icefield. These regional sources of fire differ from other combustion aerosols deposited on the Juneau Icefield, such as black carbon, that originate from local fossil fuel burning. Ratios of levoglucosan/mannosan (L/M) and levoglucosan/(mannosan + galactosan) (L/(M + G)) demonstrate that while the majority of fire aerosols reaching the Juneau Icefield originate from softwood burning, grasslands and hardwood forests are also sources. The presence of these hardwoods suggests that fire aerosols may reach the Juneau Icefield from locations as far away as East Asia.

Juridical Analysis of Environmental Law Enforcement in Forestry Crimes Regulation in the Regional Autonomy

This paper discusses the UUPLH as the basis of Indonesia's environmental policy to prevent and overcome environmental pollution and destruction. A good and healthy environment is created; ecological law enforcement uses state administrative law instruments, civil law, and criminal law. At the law enforcement level, there are many obstacles. Due to the unclear formulation of offenses and various sanctions, the proof is quite difficult, except in being caught red-handed. Strictly speaking, preventive and repressive law enforcement measures against forest and land fires cases and their ecosystem impacts are still not adequate. This fact can be seen from the lack of resolution of forest and land burning issues that have been submitted to the court, and even almost none of the perpetrators of forest and land logging were charged with the legal sanctions above. As for the culture of law culture, the cases of forest and land fires are certainly large-scale corporations, which can even control political power. The state, in this case, state administrators, should consistently target corporate crime and focus on law enforcement efforts using available instruments.

No room to move: bat response to rainforest expansion into long-unburnt eucalypt forest

The expansion of rainforest trees into long-unburnt open forests is widespread globally, including in high-rainfall regions of eastern Australia. Increased tree density can reduce insectivorous bat activity and species richness by constraining echolocation and foraging success. Yet it is unknown whether sclerophyll and rainforest trees differ in their effects on open forest bat communities. We sampled insectivorous bats and nocturnal flying insects at two heights (understorey, canopy) in dry sclerophyll forest of eastern Australia with contrasting fire histories and levels of rainforest pioneer invasion. We found that both time since fire and functional identity of midstorey trees influenced the local bat community, whereas insect biomass had little effect. Long-unburnt forests with a rainforest pioneer midstorey had lower bat activity (63% lower) and species richness (35% lower) than recently burnt forests with a more open midstorey. Bat species richness also declined beneath the dense sclerophyll midstorey in long-unburnt forests, although was unaffected in the canopy above. Strong negative correlations between rainforest tree density and bat community activity and richness indicate that rainforest trees, in contrast with sclerophyll trees, exert additional negative control over open forest bat communities. Our results show that habitat suitability for clutter-intolerant bats declined well before the often-recommended maximum fire-intervals for dry open forest, providing evidence of an upper threshold for fauna conservation in rainforest-invaded open forests. To conserve bat communities in dry open forests vulnerable to rainforest invasion, fire should occur with sufficient frequency to prevent rainforest pioneers developing a dense midstorey that displaces clutter-intolerant bats.

It’s time to stop pretending burning forest biomass is carbon neutral

Schulze et al. (GCB Bioenergy, 2020, 12, 186–197) argue against putting forests into conservation, alleging that managing forests, including for bioenergy, produces more climate mitigation than letting them grow. However, this conclusion is based on not counting emissions from burning biomass, and other carbon accounting sleight of hand. This is a letter regarding https://doi.org/10.1111/gcbb.12672

Water extractable organic C and total N: The most sensitive indicator of soil labile C and N pools in response to the prescribed burning in a suburban natural forest of subtropical Australia

Soil labile carbon (C) and nitrogen (N) pools are important indicators of soil fertility that are sensitive to environmental disturbance. However, few studies have periodically examined the long-term dynamics of soil labile C and N pools following prescribed burning and their implications on nutrient cycling in forest ecosystems. In this study, 0–5, 5–10, and 10–20 cm soil depths were sampled multiple times, spaning six years across a series of prescribed burning sites in a eucalypt natural forest of subtropical Australia. Water extractable organic C (WEOC) and total N (WETN), hot water extractable organic C (HWEOC) and total N (HWETN), microbial biomass C (MBC) and N (MBN) were analysed and correlated with the post-fire recovery time and other major environmental factors. Prescribed burning had long-lasting impact on WEOC and WETN for nearly 12 years following the burning. The lengthy recovery time was associated with the slow recovery in the forest floor of organic layers, which significantly diminished upon the burning. This was longer than the proposed 8–10 year burning cycle in Toohey Forest, and necessitated extending the burning cycle or reducing burning intensity to preserve the soil fertility. The WEOC and WETN were the most sensitive indictors of burning disturbances on soil labile C and N pools in this forest ecosystem. The MBC & MBN, and HWEOC & HWETN, however, did not show consistent patterns of recovery. More studies are needed to differentiate between the burning impacts and the seasonal impacts in the context of intensifying climate change and urban developments.

Improving forest burn severity estimations with partial least squares regression and orthogonal signal correction methods in Daxing’an Mountains, China

Several indices and simple empirical models and ratios of single band from pre- and post-fire Landsat images have been developed to estimate and/or map burn severity. However, these models and indices are usually site-, time- and vegetation-dependent and their applications are limited. The Daxing’an Mountains range has the largest forested area in China and is prone to wildfires. Whether or not the existing models can effectively characterize the burn severity over a large region is unclear. In this study, we used the orthogonal signal correction method based on partial least squares regression (PLSR) to select those variables that better interpret the variance of burn severity. A new index and other commonly used indices were used to construct a new, multivariate PLSR model which was compared with the popular single variable models, according to three assessment indices: relative root mean square error (RMSE%), relative bias (RE%) and Nash–Sutcliffe efficiency (NSE%). The results indicate that the multivariate PLSR model performed better than the other single variable models with higher NSE% (68.2% vs. 67.8%) and less RE% (3.7% vs. − 8.7%), while achieving almost the same RMSE%. We also discuss the spectral characteristics of the four selected variables for constructing the multivariate PLSR model and their correlation with the field burn severity data. The new model developed from this study should help to better understand the patterns of forest burn severity and assist in vegetation restoration efforts in the region.

The role of climate-fuel feedbacks on Holocene biomass burning in upper-montane Carpathian forests

Abstract Over the past few decades, mean summer temperatures within the Carpathian Mountains have increased by as much as 2 °C leading to a projected increased forest fire risk. Currently, there are no paleofire records from the Western Carpathians that provide the long-term range of natural variability to contextualise the response of upper-montane forests to future environmental change and disturbance regimes. We present the first high-resolution Holocene fire history record from the upper-montane ecotone from the High Tatra Mountains, Slovakia, as well as provide a regional synthesis of pan-Carpathian drivers of biomass burning in upper-montane forests. Our results illustrate that forest composition and density both greatly influence biomass burning, creating two different climate-fuel feedbacks. First, warmer conditions in the early Holocene, coupled with generally higher abundances of Pinus sp., either P. cembra and/or P. mugo/sylvestris, created a positive climate-fuel relationship that resulted in higher amounts of biomass burning. Second, cooler and wetter late Holocene conditions led to denser Picea abies upper-montane forests, creating a negative climate-fuel feedback that reduced biomass burning in upper-montane forests across the Carpathians. Given that warmer and drier conditions are expected across the entire Carpathian region in the future, our results illustrate how future climate change could potentially create a positive climate-fuel relationship within upper-montane forests dominated by Picea abies and Pinus cembra and/or P. mugo/sylvestris.

Soil fauna of cut and burnt forest areas in the Khibiny Mountains

In the framework of monitoring the restoration of anthropogenic disturbed soil cover and its invertebrates fauna in the polar mountains, a study of pine forest cut and burnt forest areas have been conducted in the inter-mountain valley of the Khibiny Mts. Changes of edaphic factors that determine the diversity and abundance of soil fauna were revealed five years after exposure the fire: decrease of soil pH (from 3.8 to 4.2), and content of total carbon and nitrogen, and a steady increase of humidity in the upper soil horizon in the felling and in the burned pine forest. In soil samples at four sites, representatives of 20 taxonomic groups of invertebrates have been taken into account, with greater diversity and minimal numbers in the burnt pine forest. A significant reduction of soil fauna number has been detected as a result of all types of anthropogenic impact, with insignificant differences in this parameter between disturbed sites (burnt pine forest, forest cutting, and burnt forest cutting). In a pair of control pine forest and a burnt pine forest an increasing differences in the structure of the soil fauna has been revealed in comparison with the study of these territories in 2015 or two years after a ground fire. The importance of a vegetation layer confirmed for preserving the soil fauna and maintaining the hydrothermal regime favorable for its recovery. The attraction of light- and thermophilic species of ground beetles and staphylinids, which are not typical for Khibiny Mts has been revealed on the burning felling and burning pine forest. Thus, burnt territories enrich the local fauna of this Polar Mountain Massif and the fauna of the Murmansk region as a whole.

Boreal tree regeneration after fire and fuelwood harvesting in coastal Nunatsiavut

Consecutive landscape-scale disturbances are known to influence boreal forest regeneration, yet few published data exist on the compounding effects of natural and anthropogenic disturbance on regeneration in subarctic forests. We conducted a dendroecological study of eastern coastal boreal forest regeneration two decades after fire at three subarctic forest stands that are important sources of fuelwood for the people of Nunatsiavut (Labrador, Canada). We quantified spruce (Picea spp.) and balsam fir (Abies balsamea (L.) Mill.) seedling regeneration, standing dead tree density, trees harvested within burned forests, and aged proximal unburned stands. Age of unburned forest varied with site; however, each exhibited continuous regeneration over several decades. Despite low seedling regeneration at each site (stems·m−2; less than 35% of pre-fire stem density), model results indicated harvesting post-fire did not impact seedling regeneration. Findings suggest a negative pressure on seedling abundance from increased presence of tall multi-stemmed shrubs, yet we argue that because stands exhibit a range in tree age, shrubs may be a set of early successional species rather than indicating a change in successional trajectory. Examination of a larger chronosequence within coastal forest stands of Nunatsiavut is warranted to further understand forest regeneration in the eastern subarctic under changing disturbance regimes.