Forest Ecology and Management 401 (2017) 264–272

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Forest Ecology and Management

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Annual dynamics and resilience in post-fire boreal understory vascularplant communities

http://dx.doi.org/10.1016/j.foreco.2017.06.0620378-1127/Crown Copyright � 2017 Published by Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: njday.ac@gmail.com (N.J. Day).

Nicola J. Day a,⇑, Suzanne Carrière b, Jennifer L. Baltzer aaBiology Department, Wilfrid Laurier University, 75 University Avenue West, Waterloo, ON N2L 3C5, CanadabWildlife Division, Environment and Natural Resources, Government of the Northwest Territories, Box 1320, Yellowknife, NT, Canada

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 March 2017Received in revised form 27 June 2017Accepted 29 June 2017

Keywords:Plant species compositionResilienceSubarctic boreal forestTemporal dynamicsUnderstoryWildfire

Boreal forests in western North America are considered to be resilient to wildfire disturbance, demon-strated by paleoecological evidence and adaptive regenerative traits possessed by many species.However, little is known about drivers of fine-scale temporal changes in understory communities in bor-eal forests immediately following fire. Knowledge of these changes, and their relationships with burnseverity and pre-fire forest stand conditions, could help us determine recovery of forests as wildlife habi-tat. Such information is urgently needed in the face of climate warming-induced changes in fire fre-quency and severity. We used a high-quality, long-term dataset of annual measurements of understoryvascular plant communities in sub-arctic boreal forest stands dominated by jack pine (Pinus banksiana),black spruce (Picea mariana), or a mix of the two in the Northwest Territories, Canada. Here, we describethe initial 10 years of annual post-fire understory plant community dynamics and assess the importantdrivers shaping understory composition during this critical period of post-disturbance communityassembly. First, we determined the relative importance of burn severity, pre-fire forest type, bare ground,woody debris, and number of years post-fire on understory species richness and composition dynamicsfollowing fire. Second, we explored annual dynamics in these communities and determined if composi-tional change was directional and predictable over time. We found that pre-fire forest type, burn severity,bare ground, woody debris, and number of years post-fire were important predictors of post-fire speciesrichness and composition. Pre-fire forest type explained the greatest variation in understory plant com-position, followed by burn severity. Across forest types, most species established within 1–3 years follow-ing fire and initial species composition determined directional changes in composition. Our resultssuggest that targeting monitoring efforts in the years immediately post-fire may be sufficient to under-stand forest successional direction with respect to composition and the important drivers of thosechanges over the first decade post-fire. However, the recent and ongoing impacts of climate change inboreal regions of western North America leads to uncertainty surrounding the continued ability of theseforests to demonstrate resilience under an altered fire regime so these interactions should continue to beconsidered across a range of forest types and burn severities.

Crown Copyright � 2017 Published by Elsevier B.V. All rights reserved.

1. Introduction

Disturbance determines forest community structure, dynamics,and ecosystem processes in many terrestrial biomes (Connell,1978; Turner et al., 2003). For example, coniferous forests experi-ence cycles of storms and insect outbreaks (e.g., Schelhaas et al.,2003) and forests on floodplains are regularly inundated resultingin plants being submerged for long periods (e.g., Vervuren et al.,2003; De Jager, 2012). Plant communities in these environmentsare often resilient to the disturbance cycles within which they have

evolved, demonstrated by plants possessing adaptive regenerationtraits such as the ability to resprout from belowground organs fol-lowing fire (Turner et al., 2003; Pausas and Keeley, 2014).

Boreal forests in western North America are considered to beresilient to fire disturbance, which occurs on average every50–150 years (Larsen, 1997; Johnstone et al., 2016). Post-fire suc-cessional trajectories in canopy species leads to self-replacementof the same tree species and are determined by interactions andfeedbacks between the environment and plant traits (Johnstoneet al., 2010). For example, cones of the dominant conifer speciesjack pine (Pinus banksiana) and black spruce (Picea mariana) areserotinous and semi-serotinous, respectively and require heat toopen cones, thereby enabling seed dispersal from aerial seed banks

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N.J. Day et al. / Forest Ecology and Management 401 (2017) 264–272 265

(Greene and Johnson, 1999; Lamont and Enright, 2000; Johnstoneet al., 2016).

The western North American boreal region is disproportionatelyimpacted by climate change, experiencing a 1 �C higher tempera-ture increase than the global average; models indicate that thistrend will continue (Romero-Lankao et al., 2014). Growing evi-dence suggests that climate change impacts have altered the fireregime and that this region will experience longer fire seasons,leading to more frequent and severe fires than have occurred his-torically (Kasischke and Turetsky, 2006; Soja et al., 2007). Whileit is understood that severe fires can alter successional trajectoriesfor trees, which are typically set during the first 3–7 years post-firein high latitude boreal forests (Johnstone et al., 2004, 2010),dynamics and drivers of understory plant community structurein the years post-fire are less clear, particularly at annual temporalscales (Wang and Kemball, 2005; Hart and Chen, 2006). Knowledgeof these temporal dynamics is valuable from both conservation andmanagement perspectives because understory vegetation containsthe majority of vascular plant diversity in these northern, high lat-itude forests and forms important food resources for wildlife andlocal human communities (Thomas and Kiliaan, 1998; Hart andChen, 2006).

Significant changes in canopy composition and successionalshifts in dominant species have been linked to burn severity inwestern boreal regions (Johnstone et al., 2010). Burn severity canalso be an important driver of understory community structurewith areas that experience greater severity having unique speciescomposition (Lecomte et al., 2005; Hollingsworth et al., 2013).However, in some cases pre-fire site characteristics and environ-mental gradients have been shown to be more important thanburn severity for structuring understory communities (Chipmanand Johnson, 2002; Hart and Chen, 2006; Boiffin et al., 2015). InQuébec, Purdon et al. (2004) found that post-fire understory com-position was largely related to dominant canopy species. Repeatedassessments in areas with different pre-fire forest types that haveexperienced different levels of burn severity could help us disen-tangle the relative importance of burn severity and site character-istics on community structure. Moreover, we could assess whetherchanges in the understory are predictable and directional based onthe initial stages of regeneration to help us understand key periodsto target monitoring programs to assess forest recovery.

Many studies investigating temporal changes and the impactsof fire on understory communities in boreal forests have used achronosequence approach, measuring areas that have burned atdifferent times based on historical records (e.g., Black and Bliss,1978; Thomas and Kiliaan, 1998; Légaré et al., 2001). While criticalfor capturing longer-term trajectories, these may miss informationon the immediate stages post-fire, which is a key time for commu-nity assembly processes to occur following disturbance (Peet,1992; Turner et al., 2003). Indeed, species assembly processesbegin immediately after fire when nutrients, such as phosphate,become more abundant (Certini, 2005) and sites are available forcolonisation (Peet, 1992). Moreover, biotic interactions are likelyto have already shaped species communities after one growingseason so immediate monitoring can optimise the ability to cap-ture effects of fire on understory composition and successional tra-jectories (Peet, 1992). In addition, it is difficult to link successionaltrajectories from different sites through chronosequence time,compromising our ability to fully understand relationshipsbetween current composition and previous fire events or site char-acteristics at the time of forest recovery.

Here, we describe the first decade of post-fire understory vascu-lar plant species community dynamics annually and assess driversof understory species richness and composition during this criticalperiod of community assembly. We established permanent vegeta-tion plots between zero and one year post-fire in three common

subarctic boreal forest types in the Taiga Shield ecozone of theNorthwest Territories, Canada. Understory vascular species inthese plots were re-assessed annually, providing a unique and highquality dataset to accurately investigate vascular plant communitydynamics during the initial period following fire. It is rare to gaininformation on the full vascular plant community at the annualscale, particularly in these remote, high latitude systems.

We used these data to investigate how understory vascularcommunities changed during the first 10 years post-fire andunderstand drivers of understory composition in boreal forests.Firstly, we investigated relationships between post-fire understorycomposition, burn severity, pre-fire forest type, bare ground,woody debris, and number of years post-fire. If burn severity wasan important driver of composition, this would be observed as sus-tained compositional shifts according to different levels of burnseverity, suggesting limited resilience of the system. Alternatively,compositional congruence within environments and forest typeswould indicate that regeneration in these communities was pre-dictable and resilient to fire over this time period. Secondly, wewere interested in exploring the annual dynamics in these vascularplant communities and determining if compositional change wasdirectional over time. This would indicate whether the initial spe-cies to regenerate in these forests were important for determiningfuture changes in composition or alternatively if the post-fire com-munity was ephemeral and relatively less important for determin-ing the direction of successional trajectories.

2. Methods

2.1. Sites and sampling

Sampling occurred in two locations that burned in natural wild-fires in 1998 on the Taiga Shield (Level II) ecozone (EcosystemClassification Group, 2008). These subarctic forests are typicallydominated by black spruce or jack pine, with patches of paperbirch (Betula papyrifera); trembling aspen (Populus tremuloides) israre in this area of the Taiga Shield (Ecosystem ClassificationGroup, 2008). Sampling occurred within a burn scar of a fire thatburned from June until September in 1998, burning a total areaof 160,000 ha. Sample locations at Tibbitt Lake was at the southend, while sample locations at Gordon Lake were at the northend of this burn scar (NT records: fire scar 1998ZF-029). Basedon climate records and 30 year averages from Yellowknife, NT forthe period 1977–2007, the mean annual temperature was �4.3and mean monthly temperatures ranged from �25.6 �C in Januaryto +16.8 �C in July. Annual precipitation ranged from 181 to388 mm (Environment and Climate Change Canada, 2017).

Transects were established in 1998 and 1999 along gradients ofburn severity in different pre-fire forest types. Six transects wereestablished in the Tibbitt Lake area (four transects established zeroyears post-fire, two transects established one year post-fire;62.55�N, 113.34�W) and three transects were established in theGordon Lake area (one year post-fire; 63.08�N, 113.15�W). Tran-sects were categorised by pre-fire forest type: black spruce, jackpine, or a mixture of the two, and underlying soil type differingin moisture-holding capacity from moist to dry: peat, clay, sand,or rock. Each of the nine transects was 55 m long with a 5 by1 m2 plot every 5 m for a total of six plots per transect (Fig. S1).Each plot was divided into 1 m2 contiguous subplots in whichobservations were recorded; our focus and analyses are at the plotscale. Measurements were recorded annually between July andSeptember until 2008 to provide information for 10 years post-fire for 54 plots for a total of 564 data points.

Presence of each vascular plant species, including seedlings,was recorded within each 1 m2 subplot of each plot. Species were

266 N.J. Day et al. / Forest Ecology and Management 401 (2017) 264–272

identified by the same observer in each year to ensure consistencyamong measurements (S. Carrière). Presence of woody debris, barerock, and bare ground were also recorded. Canopy openness, as apercentage, was measured in each subplot using a spherical crowndensiometer (Lemmon, 1956). Although canopy openness wasmeasured in most years, there are 105 instances of the 564 datapoints where it was not measured due to equipment failures. Burnseverity at each plot was measured by the composite burn index(CBI) in a 25 m2 area that encompassed the 5 m2 plot; CBI wasassessed in this larger area to monitor aspects other than under-story composition that are beyond the scope of this study. CBIwas assessed using an early version of the fire effects monitoringand inventory system (FIREMON) based on photos taken of eachplot in 1999, a method suggested in the protocol (Lutes et al.,2003). This is a standardised method to assess burn severity ofground substrates, understory, and overstory components allowingcomparison of burn severity between sites (Key and Benson, 2006).

2.2. Statistical analyses

The abundance of each vascular plant species, bare ground,woody debris, and bare rock in each plot in each year was calcu-lated by summing the number of subplots in which it occurredper plot, providing abundance values between 0 and 5. Large treestaller than 2 m were excluded from these analyses because ourfocus was on understory vegetation. There were a total of 564 datapoints over the 10 year period. Three of these plots contained novascular plants in the understory one year post-fire (T26, T31,T34) and one contained no plants four years after the fire (T31),reducing the species dataset to 560 data points for all composi-tional analyses. However, all 564 data points were retained forthe species richness analysis. All analyses were conducted at theplot scale in the open source statistical software program R version3.3.3, with packages where specified (R Core Development Team,2017).

Understory species richness was modelled as a function of abi-otic and biotic variables: burn severity, pre-fire forest type, bareground, woody debris, and number of years post-fire. These vari-ables were uncorrelated (r > 0.35; Table S1). Continuous predictorswere visually inspected to ensure they were normally distributedand standardised to a mean of zero and standard deviation ofone before being entered into the model to ensure comparabilitybetween coefficient values. Using a mixed-effects model approachto account for the spatial nestedness of the sampling design, therandom effect was plot nested within transect. In addition, the datashowed temporal autocorrelation so we specified the autoregres-sive order 1 (corAR1) term to account for this. While the speciesrichness response is a count variable, it is currently difficult toincorporate temporal autocorrelation terms with Poissonresponses in mixed effects models (Zuur et al., 2009). Therefore,this model was run with a continuous response in the nlme pack-age version 3.1-131 (Pinheiro et al., 2017). We also ran the modelin lme4 version 1.1-13 (Bates et al., 2015) specifying a Poissonresponse and necessarily without the temporal autocorrelationterm and the results were similar. We therefore present the resultsassuming a continuous response but with the correct temporalautocorrelation term.

Accumulation of species in plots over time within each pre-fireforest type were built using function ‘specaccum’ with method‘rarefaction’ in vegan version 2.4-2 (Oksanen et al., 2017). Principalco-ordinates analysis (PCoA) ordination was used to visualise spe-cies composition in each plot at each time. The PCoA was calcu-lated on Hellinger-standardised abundance data and specifyingthe Manhattan distance. The Hellinger standardisation gives lowweights to rare species and the Manhattan distance is recom-mended to reduce effects of species richness on ordination dia-

grams that can produce an arch (Podani and Miklos, 2002;Borcard et al., 2011). The arch effect proved difficult to removethrough recommended transformations, standardisations, andordination methods, including non-metric multidimensional scal-ing, which is likely due to a strong gradient in pre-fire forest typeand species richness (Podani and Miklos, 2002). Using the Hellin-ger standardisation and Manhattan distance minimised the archand maximised the variation explained. Data was Hellinger-standardised using ‘decostand’ and Manhattan distances calculatedusing ‘vegdist’ in vegan version 2.4-2 (Legendre and Legendre,2012; Oksanen et al., 2017). The PCoA was run using function‘cmdscale’ in base R.

A permutational multivariate analysis of variance (PERMA-NOVA; Anderson, 2001), which is not constrained by ordinationspace, was used to test the importance of burn severity, pre-fireforest type, bare ground, woody debris, and number of yearspost-fire for explaining variation in species composition in eachplot at each time point. Compositional dissimilarities wereassessed by Manhattan distance and P-values were estimated from1000 permutations restricted within years post-fire and transect toaccount for temporal and spatial autocorrelation. Uncorrelatedexplanatory variables were number of years post-fire, pre-fire for-est type, burn severity (CBI), woody debris, and bare ground. ThePERMANOVA was run using the ‘adonis’ function in vegan packageversion 2.4-2 (Oksanen et al., 2017).

We used time lag regression analyses to assess temporal betadiversity and investigate whether temporal patterns in understoryplant communities were directional, cyclical, or stochastic withinplots (Collins, 2000). This method provides formal, quantitativetests of changes in community composition that complements tra-ditional methods of looking for patterns with trajectories overlainon ordination graphs. For each plot, the Manhattan distance wascalculated between all pairs of time points. These distances wereused as response variables in regressions with the square-root ofthe time lag as a predictor variable, resulting in one regressionfor each of the 54 plots. The square-root transformation accountsfor fewer data points at longer time lags (Collins, 2000). The direc-tion of the regression slope indicates direction and rate of compo-sitional change. A significant and positive slope shows thatcomposition within plots has become more dissimilar over time,indicating that changes in composition are directional away fromthe initial composition. A slope not significantly different from zeroindicates that composition is stable or compositional change isstochastic. A negative slope shows that species composition hasbecome less dissimilar over time, indicating that the communityis converging on composition from an earlier time point. The R2

values for significant regressions are indications of the strengthof the signal of change. Significance of regressions were assessedby testing correlations between ecological distances (Manhattancompositional distances) and distance in time (square-root of timelag) using Mantel tests and 1000 Monte Carlo simulations usingfunction ‘mantel’ in vegan 2.4-2 (Bêche and Resh, 2007; Oksanenet al., 2017).

3. Results

These 54 plots encompassed the entire range of possible CBIvalues for burn severity between 0 and 3 (mean 1.72 ± 0.05;Fig. S2). A total of 74 understory vascular plant species (or speciesgroups) were recorded across the 10-year sampling period. Duringthis initial decade post-fire, 24 species colonised and 26 wentlocally extinct (Table S2). Species accumulation curves showedthat most species established within the first 1–3 years post-firein each forest type (Fig. S3). Mean species richness per plotincreased and plateaued approximately 3–4 years post-fire in pine

Fig. 1. Mean species richness per plot (± standard error of the mean) within each forest type according to time since fire. Forest types refer to pine = jack pine, spruce = blackspruce, or mixed = mixture of the two.

N.J. Day et al. / Forest Ecology and Management 401 (2017) 264–272 267

and mixed forest types (Fig. 1). In spruce forests, species richnesscontinued to increase until approximately seven years post-fire.The mixed model testing the relative effects of each variable onunderstory species richness showed that species richness differedsignificantly according to forest type, being higher in spruce forestscompared to pine and mixed forest types (Table 1). Burn severity,years post-fire, and woody debris were also significant and positivepredictors of species richness (Table 1).

The first two axes of the PCoA explained 24.80% of the variation(axis 1: 16.06%; axis 2: 8.74%), with the first axis relating to pre-fireforest type and the second axis relating to burn severity (Figs. 2and S4). PERMANOVA results showed that all explanatory variablesexplained a significant proportion of variation in understory spe-cies composition (Table 2). Forest type explained the greatestamount of variation in composition followed by burn severity.Time, in terms of number of years post-fire, had a significant effecton species composition but accounted for

Fig. 2. Site scores for principal co-ordinates analysis (PCoA) ordination with Hellinger-standardised abundance and Manhattan distance. Values in brackets on the axesrepresent the amount of variation in species composition explained by each axis. Symbols denote pre-fire forest type and greyscale indicates burn severity from CompositeBurn Index (CBI; where 0 = unburned). Forest types refer to pine = jack pine, spruce = black spruce, or mixed = mixture of the two.

Table 2Results from permutational analysis of variance (PERMANOVA) with Manhattan distance to test effects of explanatory variables on species composition in each plot. Permuationswere restricted within transect and years post-fire, with 1000 permutations.

Variable Variation explained (%) df SS MS Pseudo F P

Pre-fire forest type 19.42 2 62,789 30,894 76.43

Fig. 3. Time overlaid on site scores from principal co-ordinates analysis (PCoA) ordination, showing trajectories in plant species composition over time, according to pre-fireforest type and burn severity categorised from the Composite Burn Index (CBI; where 0 = unburned). Changes in species composition over time are shown by linking scoresfor each plot at each measurement with arrows. Plots are coloured according to transect: G = Gordon, T = Tibbitt. Forest types refer to pine = jack pine, spruce = black spruce,or mixed = mixture of the two.

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Our plots encompassed the full range of burn severity and CBIaccounted for 8.74% of the variation in understory composition(Table 2); burn severity was also highly correlated with the secondPCoA axis (R = �0.68; Fig. 2). We found varied responses in compo-sition to the burn severity gradient supporting the idea that theseplots are resilient to all levels of this type of disturbance. For exam-ple, if fire was critical for determining compositional change thenwe would expect unburned plots to show no significant directionalchanges in composition. However, one of the five unburned plotsshowed significant directional changes in composition (Fig. 3,Table S3). Similarly, Wang and Kemball (2005) found no differ-ences in dominant understory species along a gradient of burnseverity in mixed forests in boreal Manitoba.

There was a significant positive relationship between burnseverity and species richness, showing that areas that have experi-enced greater burn severity had higher species richness post-fire(Table 1). This is contrary to what we would expect if more severeburns caused mortality of underground regenerative plant struc-tures. However, some species are dependent on fire or heat toregenerate or release seed (Lamont and Enright, 2000) and it ispossible that severe burns exposed more mineral soil that are ideal

sites for seed germination (Hollingsworth et al., 2013); we found aweak positive correlation between bare ground and burn severity(R = 0.28, Table S1) but we did not record when mineral soil wasexposed, limiting our ability to infer this relationship. Our datadoes not enable us to be sure of the mechanism operating but thispattern supports the incredible regenerative ability of these borealunderstory vascular communities even after extreme firedisturbance.

Total annual precipitation was below average for 3–4 yearsprior to the 1998 fires (Fig. S6). However, 1998 was not consideredto be an extreme fire year in the Northwest Territories so it is pos-sible that fire weather and behaviour differed compared to otherstudies that have found burn severity to strongly affect post-firevascular communities (Mack et al., 2008; Hollingsworth et al.,2013). At a landscape scale, Boiffin et al. (2015) found no effectof burn severity on post-fire understory communities in borealQuébec after a severe fire year. They suggest that burn severityimpacts may be more detectable at local scales or in specific foresttypes. Our data supports this idea because burn severity was con-sistently a secondary driver of vascular plant composition withinforest types (Table 2, Fig. 2). This could explain why our results

Fig. 4. Time lag regression lines for each of the 54 plots with Manhattan distances against the square-root of the time lag between observations, displayed by pre-fire foresttype. Greyscale represents burn severity from Composite Burn Index (CBI; where 0 = unburned). Line type represents significance level based on Mantel tests with 1000permutations. See Table S3 for P-values and raw outputs of each time lag regression for each plot. Forest types refer to pine = jack pine, spruce = black spruce, ormixed = mixture of the two.

270 N.J. Day et al. / Forest Ecology and Management 401 (2017) 264–272

appear to contradict research suggesting burn severity to be astronger driver of post-fire understory community structure;research in Alaska (Hollingsworth et al., 2013) and Québec(Lecomte et al., 2005) have focussed only on black spruce-dominated forests while we also have jack pine and mixed foresttypes here. Our study suggests that responses of understory com-munities to fires in western Canada’s boreal forests may not begeneralizable from other boreal regions of North America and thatwe may need to account for differences in dominant canopyspecies.

The combination of trajectories on the ordination and the timelag analyses indicated that there were substantial changes inunderstory vascular species composition in the first 10 yearspost-fire (Figs. 3 and 4). The majority of plots demonstrated direc-tional change, indicating that initial species establishing in theunderstory immediately post-fire were important for determiningthe direction of the successional trajectory. This pattern supportsthe initial floristic composition concept in post-fire successions(Egler, 1954). However, compositional change was stochastic inmany plots, particularly those with low to moderate burn severity(Table S3), indicating that the initial species to establish in theseplots is less important for determining change than in severelyburned areas. The rapid establishment and regeneration of partic-ular species is likely because many plants rely on resprouting fromabove or belowground structures as opposed to seed banks or seeddispersal. Between 40% (Hollingsworth et al., 2013) and 70%

(Grandpré et al., 1993) of understory plants in boreal communitiescan resprout from belowground post-fire, contributing up to 70% ofbiomass two years post-fire (Mack et al., 2008). The evolved abilityof many species to resprout may contribute to the predictability ofcomposition from those initial colonisers and reduce competitivesorting experienced by dispersal-dominated communities post-disturbance (Peet, 1992; Pausas and Keeley, 2014). Moreover,many understory plants can colonise quickly from seed banks bur-ied in thick organic layers that are characteristic of boreal regions(Heinselman, 1981; Schimmel and Granstrom, 1996;Hollingsworth et al., 2013). Most of the species that went locallyextinct were present in very few plots or were species known tocolonise quickly after disturbance and then die out (Table S2).For example, Corydalis spp. and Geranium bicknellii are known tobe present only in the initial years post-fire, as they colonisequickly post-fire from seed banks in the humus layer(Heinselman, 1981). The loss of woody species that were in lowfrequency, such as Populus spp., were likely due to seedling mortal-ity (Table S2). Further research linking understory communitycomposition with regeneration traits across forest types and burnseverities would help us elucidate the relative importance of thesemechanisms in determining the resilience of these communities indifferent environments.

Forest type may determine understory composition in the bor-eal due to differences in light penetration over time (Grandpréet al., 1993; Légaré et al., 2001; Chipman and Johnson, 2002). While

N.J. Day et al. / Forest Ecology and Management 401 (2017) 264–272 271

we found no relationship between canopy openness and foresttype (Fig. S5), canopy openness was significantly correlated withburn severity, which had significant impacts on vascular commu-nity composition (Table S1). The increased burn severity andcanopy openness corresponded to increases in species such asChamerion angustifolium and Betula papyrifera, while low severityfires led to regeneration of rhizomatous species such as Ledumdecumbens and Vaccinium vitis-idaea (Fig. S4).

There are many metrics available to quantify burn severity(Ryan, 2002; Key and Benson, 2006; Boby et al., 2010). It is possiblethat there may have been a stronger relationship between burnseverity and understory composition if we had used a differentmetric but we consider this to be unlikely because of the clearcompositional distinction by forest type (Fig. 2; Table 2). However,it is worth noting that CBI does not quantify depth of burn or con-sumption of organic matter (Key and Benson, 2006; Boby et al.,2010). This is arguably the most important aspect of the firebecause deeper burning fires are more likely to cause death ofunderground structures or dormant seeds in the upper organic lay-ers from which plants can regenerate (Schimmel and Granstrom,1996; Wang and Kemball, 2005). Boby et al. (2010) proposed usingadventitious root scars on black spruce trunks, which grow into theorganic soil layer while the tree is alive (LeBarron, 1945). Theheight of the root from the organic layer surface therefore providesa minimum depth of the pre-fire organic layer (Boby et al., 2010).An effective method to measure depth of burn has yet to be devel-oped for non-black spruce forests and would be a worthwhile areaof research.

4.1. Implications for management of boreal forests post-fire

From a management perspective, our results show that moni-toring forests within the first 1–3 years post-fire can provide criti-cal information about how forests will recover over the longerterm. These immediate and annual-scale temporal dynamics arecritical to understand forest regeneration and are often missed inchronosequence approaches. Substantial compositional changestake many decades (e.g., Black and Bliss, 1978), so combiningknowledge of fine-scale temporal dynamics with the chronose-quence approach is an effective way to accurately ascertain howcomposition will change post-fire and the important drivers atmultiple spatial and temporal scales.

Monitoring and understanding drivers of post-fire forest assem-bly is especially critical now when climate warming, which is caus-ing more frequent or more severe fires (Kasischke and Turetsky,2006; Soja et al., 2007) could alter these dynamics and impactthe resilience and assembly of these communities. For example,Canada’s boreal forests have recently experienced two of its mostsevere fire years on record: in the Northwest Territories in 2014and in Saskatchewan in 2015 (National Forestry Database ofCanada, 2016). While our results demonstrate high resiliency ofthese understory communities, an altered fire regime may dramat-ically reduce the resilience of boreal forests (Johnstone et al., 2016).Therefore, continued monitoring of regeneration of understorycommunities over a landscape scale with different pre-fire vegeta-tion, environmental conditions, and burn severities needs to be pri-oritised for us to understand the impacts this altered disturbanceregime could have on northern biodiversity.

5. Conclusions

This study has provided us with rare insights into annualdynamics in boreal understory communities that occurred in the10 years immediately post-fire. The importance of forest typedemonstrates that these communities are highly resilient and

well-adapted to fire disturbance across environmental and burnseverity gradients. These communities were able to regeneraterapidly and initial colonisers post-fire determined successionaldirection in these forests. Our results suggest that targeting moni-toring efforts to the first 1–3 years post-fire may be sufficient tounderstand forest successional processes with respect to composi-tion and the important drivers of those changes. However, theincredibly recent and ongoing impacts of climate change in borealregions of western North America leads to uncertainty surroundingthe continued ability of these forests to be resilient under analtered fire regime so these interactions should continue to beconsidered.

Acknowledgements

We would like to thank the many students who helped collectand enter these data. Thank you to Kathleen Groenewegen of theGovernment of the Northwest Territories (GNWT) for providinginformation about the Tibbitt Lake Fire, AlisonWhite for discussionon aspects of the results, and to four anonymous reviewers whosesuggestions greatly improved the manuscript. This work wasfunded by the GNWT (SC). NJD was supported by a Natural Scienceand Engineering Research Council (NSERC) Postdoctoral Fellowshipand funding from the GNWT Cumulative Impacts MonitoringProgram awarded to JLB.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.foreco.2017.06.062.

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Annual dynamics and resilience in post-fire boreal understory vascular plant communities1 Introduction2 Methods2.1 Sites and sampling2.2 Statistical analyses

3 Results4 Discussion4.1 Implications for management of boreal forests post-fire

5 ConclusionsAcknowledgementsAppendix A Supplementary materialReferences