IARC-JAXA Research Review Meeting Proceedings (December 17, 2007) Japan
Aerospace Exploration Agency. pp. 16-19

Effects of wildfire on revegetation in interior Alaska
Shiro TSUYUZAKI1 & Kenji NARITA2
1
Graduate School of Environmental Earth Science, Hokkaido University
2
Faculty of Education and Human Studies, Akita University
1. INTRODUCTION
Wildfires often take place in taiga, due to lightning(Engelmark 1999). The initial stages after thewildfires greatly determine the forest dynamics. In
Alaska, ordinary wildfire has been crown fire that
promotes taiga regeneration (Bonan & Shugart 1989),‘Crown fire’ is that the ground- surface cover isincompletely burned.
Picea mariana develops aerial seedbank, anddisperses more seeds after fire. In addition, P.
mariana is adapted to establish wet and nutrient-poor
habitat that is distributed more on north slopes inmountains (van Cleve et al. 1986). In discontinuous
permafrost regions, the scenarios of succession differ
between north and south slopes, due mostly to thepresence of permafrost on the north slopes, and viceversa. Wildfire is likely to become larger and more
intense (Dale et al. 2001). To predict large-scaled
wildfire on revegetation, we have researched thefollowings: 1) Detecting plant community recovery
patterns with different fire damages, and 2)
Characterizing regeneration dynamics after wildfire,in particular, for tree species.

2. STUDY AREA AND METHODSWe selected a typical P. mariana forest on thenorth slope of Poker Flat, ca. 50 km north of
Fairbanks, interior Alaska, USA, for monitoring
revegetation, because of large and intensive fire,
northward slope, and accessibility. Large wildfires
stroke this region in summer 2004.
The field surveys were conducted in May (spring)and July-August (summer) in every year from 2005 to
2007. The total censuses were 6 times.
At the first census, we established 16 10 m × 10 m
plots. Based on fire severities, plots were assigned
as less-(L), moderately-(M), and heavily-(H) burned.
Higher stem survival and moss cover characterized L.
M was situated between L and H, i.e., most trees weredead but unburned plant cover remained somehow.
H is totally burned, i.e., all stems were killed and >80% ground surface was burned out. On each plot,
we measured height and diameter at breast height(DBH) for all alive and dead stems ≥ 1.3 m in height,
to reconstruct pre-fire forest structure. Therefore,
fallen trees were included in the measurement.
Outside the plots, 22 tree cores were sampled by aborer or clippers in summer 2005.
Two seed traps (52 cm × 26 cm in surface area)
were set up on each plot in summer 2005. Seeds
were collected at every census. Seed germination
test was performed in an incubator [15oC/25oC (12hr/12 hr)] with continuous or 12-hr light.
In each plot, 6 1 m × 1 m quadrats were randomly
set up. Canopy openness was estimated by photos
taken on each quadrat at 1.3 m above the groundsurface by a fish-eye lens in summer 2005 (Frazer etal. 1999). Albedo was measured on each quadrat(see Hydrology Team report), and duff thickness wasestimated by a steel stake.
At every census,
vegetated area was estimated on each quadrat. The
cover of each species was recorded on burned(hereafter, i.e., BS) and unburned (US) groundsurfaces, separately, at summer census. When tree
seedling was observed in the quadrats, the seedlingwas marked by a numbered flag, and was recorded onheight, crown area and location. In summer 2007, >20 seedlings for P. mariana, Populus tremuloides andBetula neoalaskana were excavated to investigate
allometry on stem height and biomass.
(Generalized) linear models were used to detect
relationships between dependent variables such asstem height and independent variables such as habitatand stem diameter. The differences in stem heightand crown area were examined between habitats andbetween species by repeated measures ANOVA (Zar
1999). The numbers of captured and germinatedseeds were compared between three habitats andbetween seasons by Tukey’s test. Determinants on
albedo were examined by a multiple regression with abackward procedure using incident radiation, canopy
openness, burned area and plant cover. Allometricdifferences were compared by ANCOVA between
habitats and between species.

3. RESULTS
3.1 Environments and fire severities
On the 16 plots, the altitude ranged from 244 m to437 m, and the slopes faced 7.0o-43.5o from north towest with 4.8o-19.0o slope angles. Before thewildfire, 5-54 P. mariana stems ≥ 1.3 m, that
accounted for 97% of total, had established in theplots, and 81% of them were burned and dead. The
maximum height was 13.0 m. Alnus crispa, B.
neoalaskana and Salix spp., established with low

3.2 Seed immigration on tree species
Seed traps constantly captured P. mariana seeds, ofwhich total was 1069, until summer 2007 (Fig. 1).
B. neoalaskana was second dominant in the traps, i.e.,
756 seeds captured. The other seeds were recorded
from forbs and grasses. P. mariana showed 21%
seed germination percentage, except quite low
percentage in summer 2007. P. mariana seeds
immigrated even to H, showing that seed immigration
was not restricted in the surveyed area.
3.3 Plant communities in herb layer
For 3 years, there were > 43 taxa, consisting of 31
vascular plants (4 trees, 13 shrubs, 12 herbs, and 2
ferns) on the 96 quadrats. As expected, Sphagnum
spp. were predominant on US.
Plant coverfluctuated little for trees, herbs and mosses on US,
Unburned

14

Total

Density

12
10

Number of seeds/trap

5

0

4
3
2
1
0

Emerged in 2005
Recruit

2

Death

Less

3.4 Seedling emergence, survival, and growth

Picea mariana (148)

In total, 592, 509 and 79 seedlings emerged in
Burned 4

2

2

0

0

4

Moderate
Fire severity

while shrub cover significantly increased on both US
and BS. By field observations, we confirmed thatshrubs, e.g., Ledum groenlandicum, Vaccinium
vitis-idaea, and V. uloginosum, came from legacies
through the last fire. Since there were few seedlings
on shrubs, vegetative reproduction was mostimportant for shrub regeneration. Trees slightly
increased in cover on BS, mostly by broad-leaved
seedlings. Two herbs, Epilobium angustifolium andCalamagrostis canadensis, and a moss, Ceratodon
purpureus, significantly increased in cover on BS.

Populus tremuloides (271)4Salix spp. (62)Betula neoalaskana (40)200Aug 05 May 05 Aug 06 May 07 Aug 07

4

Heavy

Figure 1. Numbers of Picea mariana seeds captured byseed traps, and germinated in lab-experiment. Mean isshown with SE. Duration: ■ = summer 2005-spring 2006,□ = spring-summer 2006, ■ = summer 2006-spring 2007,
and ■ = spring-summer 2007. Number of captured seeds
was not different between three fire severities (Tukey’s test).
The number of germinated seeds was different between
seasons, and is not significantly different between severities.

2

8
6

4

Density

16 Picea mariana (656)105Number of seeds
germinated/trap

frequencies. On the forest floor, the survived plant
cover ranged from 0% to 100%, indicating that thefloor was patchily burned with various scales andthus was suitable for this study.
The maximum age of tree cores was 174, and age
was linearly correlated to stem height.
The
stem-diameter growth suggested that drastic events,
such as fire, occurred 60 and 130-140 years before
now. The forest regeneration before the 2004 fire
was not simultaneously, and thus the previous fires
were presumably crown fire.
The canopy openness ranged from 57% to 95% on
the 80 quadrats. A forest unburned by the 2004 fire,of which height was comparable with the burned
forest, had < 54% canopy openness. Plant cover onforest floor decreased down to 40% by the fire evenon L, showing that the surveyed area received firedamage more or less. Duff thickness ranged from 3
cm to 73 cm, and was negatively correlated to burned
area. The albedo was mostly determined by total
plant cover, indicating that the albedo seemed not toreturn to the pre-fire conditions until vegetation cover
becomes higher enough to reflect radiation.

Aug 05 May 05 Aug 06 May 07 Aug 07

Figure 2. The fates of seedlings emerged from Summer 2005 to Summer 2007.
Light gray lines indicate P. mariana seedlings emerged in 2005. Dark gray lines
indicate total number of seedlings for each species at each census. Open and solid0columns indicate number of recruits and deaths. Total number of seedlings is shown
Aug 05 May 05 Aug 06 May 07 Aug 07 in parentheses. On unburned surface, there were no seedlings for P. tremuloides andSalix spp., and were 3 for B. neoalaskana.

3

1

0

Seedlings emerged in 2006

10

Aug 06 May 07 Aug 07

10

10

1

1

0.1

Aug 05 May 05 Aug 06 May 07 Aug 07

Aug 05

Aug 07

Burned surface between species
Intercept: NS
1000
Slope: P < 0.01

1

1

10

100

1000

Belowground biomass (mg)10000

Aug 07

4.1 Seed immigration and seedling establishment
P. mariana seeds were captured fluently by seedtraps after 2005 and the seedlings emerged well. P.
mariana provides viable seeds for several years, byproducing semi-serotinous cones (Bonan & Shugart
1989). The seed immigration was unlikely to belimited for P. mariana regeneration immediately after
wildfire, although viable seeds decreased in 2007.
The decline in viable seeds suggested that more seeds
came from the aerial seedbank on the canopy.
Between Yukon and British Columbia, seedling

10000

P. mariana between habitats
Intercept: NS
Slope: NS

Aug 06

4. DISCUSSION

Burned surface between species
Intercept: NS
Slope: P < 0.001
1000

10

0.1

BS (Fig. 4). Broad-leaved trees allocated more to
belowground than to aboveground, and to stem thanto leaf. Furthermore, broad-leaved trees allocatedaboveground biomass more to height growth than P.
mariana. These results indicated that broad-leaved
trees acquired light more effectively than P. mariana,
and did not have to allocate more to abovegroundorgans. One cause seemed to be lower allocation
plasticity of P. mariana, showing from the biomass
allocation did not differ between the two habitats.
In conclusion, the establishment of broad-leaved trees
proceeded if the ground was burned completely.

10000
Leaf biomass(mg)Aboveground biomass(mg)Aug 06

Seedlings emerged in 2006

Figure 3. Changes in stem height and crown area on tree seedlings for 3 years.
○ = P. mariana on burned surface (BS), ● = Picea mariana on unburned surface(US), ● = Populus tremuloides on BS, ● = Salix spp. on BS, and ● = Betula
neoalaskana on BS. Height and crown area are significantly different for P.
mariana between BS and US at P < 0.01 (repeated measures ANOVA), and are
different between P. mariana and broad-leaved trees at P < 0.01.

2005, 2006 and 2007, respectively (Fig 2). P.
mariana was most abundant, i.e., 68% of total. P.
mariana seedlings emerged more on US, i.e., 82% ofthem emerged there.
From broad-leaved tree species, P. tremuloides, B.
neoalaskana and Salix spp., 376 seedlings wererecorded. All of these seedlings established on BS,
except 3 B. neoalaskana seedlings on US. Stems ≥DBH for broad-leaved tree species were less in thesurveyed plots, indicating that these seeds came fromthe external environments.
On P. mariana seedlings emerged in 2005, seedlingsurvival averaged 85% on BS and 82% on US (Fig.
2), indicating that the survival was better on US. On
the survival rates on the BS by 2007, P. tremuloides
showed 58%, Salix spp. 80%, and B. neoalaskana
85%. Therefore, the survival rates were not low for
all the species.
Stem growth, evaluated by height and crown area,
was significantly higher for the three broad-leaved
trees than for P. mariana (Fig. 3). Furthermore, on P.
mariana, stem growth was significantly slower on the
US than on BS.
The significant differences in the slopes ofallometry indicated that resource allocation differed
between P. mariana and broad-leaves tree species on

100

100

100

100
10
1
0.1

P. mariana between habitats
Intercept: NS
Slope: NS

1

10

100

1000

Stem biomass (mg)Stem height (cm)Stem height (cm)2

Seedlings emerged in 2005

100

Crown area (cm2)

Seedlings emerged in 2005

10000

Burned surface between species
Intercept: NS
Slope: P < 0.01

101

P. mariana between habitats
Intercept: P < 0.01
Slope: P < 0.01

0.1
1

10

100

1000

10000

Aboveground biomass (mg)Figure 4. Allometry between belowground and aboveground biomass, between stem and leaf biomass, and between
aboveground biomass and stem height. ● = Picea mariana on burned surface (BS), ● = Picea mariana on burned surface (US),● Populus tremuloides on BS, and ● = Betula neoalaskana on BS. The differences are investigated by ANCOVA.

recruitment on P. mariana is highest in the first 5
years after fires, while additional establishment is notobserved after 10 years (Johnstone et al. 2004). On
Poker Flat, number of seedling recruits declined in
2007, 3 years after the fire. Tree regeneration
patterns on P. mariana, should be determined for the
first several years after fire.
All broad-leaved trees observed in Poker Flatproduce long-distance wind-dispersed seeds. Most
seeds for broad-leaved trees immigrated from theexternal environments, since the mature stems wereless distributed in the surveyed area. These meanthat seed supply will persist for broad-leaved trees.
Furthermore, the intensive forest fire, that removes
duff layer, promotes the seedling establishment ofbroad-leaved trees.
4.2 Environmental changes
If moss mat, mostly consisting of Sphagnum,
remains on the ground surface after wildfire, soil
temperature keeps low. However, the complete
removal of moss mat promotes soil temperature
increase and/or melting permafrost (Yoshikawa et al.
2002). Albedo also declines by fire, and is mostly
determined by plant cover on the ground surface inPoker Flat. Fire severity, in particular, on the forest
floor, must be mentioned to predict regeneration.
Surface temperature increases after fire, due toincrease in canopy openness and decrease in albedo
on a P. mariana forest (Chambers et al. 2005). In
addition, the Sphagnum cover seems to influencepermafrost distribution (see Hydrology Team report).
A trigger on environmental changes is considered to
be in Sphagnum layer.
In P. mariana forest, the ground surface is often
dominated by Sphagnum spp., that explain 80-90%
aboveground biomass (Bonan & Shugart 1989). To
predict carbon balance as well as regeneration pattern,
we have to mention Sphagnum layer more details.
The keystone species on the north slopes of interior
Alaska must be Sphagnum spp.
4.3 Regeneration patterns
The seedling survival did not differ greatly
between P. mariana and broad-leaved trees, and thegrowth was extremely higher on broad-leaved trees.
Mean annual biomass increment is higher in dry sites
than in wet sites along a chronosequence on P.
mariana forests in Manitoba, Canada, but carbon
pools in bryophyte, understory and forest floor areless for the dry sites (Wang et al. 2003). The
allometric relationships indicated that P. mariana has
lower allocation plasticity than broad-leaved trees,
and thus the growth is limited on the burned groundsurface.
On Poker Flat, plant-cover removal on the groundsurface determines colonization patterns on trees.

The recovery of a few shrubs and herbs were also
determined by the presence/absence of Sphagnum
mat. Canopy openness influences the distribution
pattern and productivity of herbaceous plants on theforest floor (Whigham 2004). However, initial treecomposition after fire had little effects on understory
composition in the coniferous forests of eastern
Canada, while soil burn severity affected temporalchanges in understory species (Lecomte et al. 2005).
Those suggest that forest regeneration afterlarge-scaled fire differs from weak fire, including
crown fire. Fire severity may change successional
sere on P. mariana forests towards mixed conifer
and/or broad-leaved forests.
To clarify plant community-climate interactions
with micro-and macro-spatiotemporal changes, weneed further observations on: 1) changes in plantcommunity structures for longer term, 2) seedlingsurvival and growth related to Sphagnum recovery,and 3) determinants on Sphagnum recovery.

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