Long- e m dec easing end in o es i es
in no hwes e n Canada
TUOMO H. WALLENIUS,
1,
JUHO PENNANEN,
2
AND PHILIP J. BURTON
3,4
1
Finnish Fo es Resea ch Ins i u e, Van aa Resea ch Uni , P.O. Box 18, FI-01301 Van aa, Finland
2
Depa men o Biosciences, Uni e si y o Helsinki, P.O. Box 65, FI-00014 Helsinki, Finland
3
Canadian Fo es Se ice, P ince Geo ge, B i ish Columbia V2N 4Z9 Canada
4
Ecosys em Science and Managemen , Uni e si y o No he n B i ish Columbia, P ince Geo ge, B i ish Columbia V2N 4Z9 Canada
Abs ac . The annual a ea o o es bu ned has dec eased in ecen cen u ies o e la ge a eas o
Fennoscandia, Sibe ia and empe a e No h Ame ica. To de e mine i his same end ex ends o a spa sely
popula ed egion o no he n Canada, i e sca s on li ing and dead ees, o es s and ages and cha ed
wood we e sys ema ically sampled in 85 s udy plo s in an a ea o 564 000 km
2
in no hwes e n Canada. A
signi ican nega i e end in he occu ence o o es i es was obse ed: a e age a ea bu ned pe yea
dec eased om 2.0%in he i s hal o he 19 h cen u y o 0.33%in he la e hal o he 20 h cen u y.
Annually bu ned a eas co ela ed signi ican ly wi h a local ee ing based index, July mon hly d ough
code and he Paci ic decadal oscilla ion bu no wi h June-Augus mean empe a u e, dis ance o he
nea es oad, o he yea o oad building. None o he clima ic indica o s o access his o y (indica i e o he
s a o local i e supp ession) could explain he long- e m nega i e end in i es. Ea lie in e p e a ions
ha humans domina ed he causes o o es i es in he pas , e en in spa sely popula ed egions, dese e
u he a en ion as a possible explana ion o he dec easing end in i es.
Key wo ds: annually bu ned a ea; bo eal o es ; Canada, clima e; i e cycle; i e supp ession; o es i e, human
in luence.
Recei ed 1 Ma ch 2011; accep ed 4 Ma ch 2011; inal e sion ecei ed 18 Ap il 2011; published 4 May 2011.
Co esponding Edi o : D. P. C. Pe e s.
Ci a ion: Wallenius, T. H., J. Pennanen, and P. J. Bu on. 2011. Long- e m dec easing end in o es i es in no hwes e n
Canada. Ecosphe e 2(5):a 53. doi:10.1890/ES11-00055.1
Copy igh : Ó2011 Wallenius e al. This is an open-access a icle dis ibu ed unde he e ms o he C ea i e Commons
A ibu ion License, which pe mi s es ic ed use, dis ibu ion, and ep oduc ion in any medium, p o ided he o iginal
au ho and sou ces a e c edi ed.
E-mail: Tuomo.W[email p o ec ed]
INTRODUCTION
The ecological li e a u e ypically desc ibes he
No h Ame ican bo eal o es as a biome ha
equen ly bu ns in na u al ligh ning igni ed
o es i es (Rowe and Sco e 1973, Johnson
1992, Paye e 1992). P io o he mid-19 h cen u y
i e cycles ha e a ied om 30 o 130 yea s in
Canada’s bo eal o es s (Zhang and Chen 2007).
Howe e , he majo i y o i e his o y s udies in
No h Ame ica ha e been conduc ed ei he in he
sou he n pa o bo eal zone (Wei e al. 2000,
Be ge on e al. 2001) o in mo e empe a e
coni e ous o es s (Heinselman 1973, Heye dahl
e al. 2001). Excep o he esea ch based on i e
s a is ics and sa elli e images spanning he las
ew decades (S ocks e al. 2002, Kasischke and
Tu e sky 2006), ela i ely ew la ge-scale i e
his o y s udies (Ya ie 1981, La sen 1997) ha e
been conduc ed in no he n bo eal No h Ame -
ica o in ‘‘ he ue bo eal’’ as de ined by B and
(2009).
The gene al iew is ha clima e dic a es he
a ea annually bu ned, a leas in emo e a eas
such as he bo eal o es s o No h Ame ica. I
has been sugges ed ha as a consequence o
clima ic wa ming, i es ha e (and will con inue
o) become mo e equen (Gille e al. 2004,
www.esajou nals.o g 1May 2011 Volume 2(5) A icle 53
Flannigan e al. 2005, Soja e al. 2007), which
could be he eason o he inc ease in he a ea
annually bu ned seen in he i e s a is ics.
Howe e , he e ec o clima e on i e equencies
is appa en ly complex and spa ially and empo-
ally a iable. Clima e change in he 20 h cen u y
has no led o mo e high- isk i e wea he
e e ywhe e in he bo eal o es s. Du ing he
20 h cen u y in sou hwes e n and sou heas e n
Canada, he clima e has shi ed o condi ions less
conduci e o o es i es, bu o e he same
pe iod, o example, he Eu asian aiga has
become d ie (Gi a din e al. 2009).
Se e al wea he ac o s, including empe a-
u e, p ecipi a ion and wind, a ec daily i e isk.
Fo example, summe empe a u e and p ecipi-
a ion oge he wi h o he clima ic a iables
explained 79%o he annual a ia ion in bu ned
a ea du ing he la e hal o he 20 h cen u y in
Alaska (Du y e al. 2005). I has been demon-
s a ed ha oscilla ion o he sea su ace empe -
a u es could explain he long- e m ends in i e
egimes (Ki zbe ge e al. 2001). In bo eal No h
Ame ica, he Paci ic Decadal Oscilla ion (PDO)
co ela es wi h o es i es (Du y e al. 2005,
Fau ia and Johnson 2008). Ano he clima ic index
ha has been shown o co ela e well wi h
annually bu ned a ea is he mon hly d ough
code (MDC) o July in Canada (Gi a din e al.
2009).
In addi ion o clima e he e a e also human
ac o s ha ha e been hough o a ec i e
egimes. No ably, i e supp ession is associa ed
wi h a dec ease in i es in Fennoscandia (Zack-
isson 1977) and in No h Ame ica (Heinselman
1973). Howe e , in no he n Canada, o es i es
ha e no been supp essed as e ec i ely o o as
long a pe iod o ime as in sou he n Canada o in
Fennoscandia. Fi es in Canada’s emo e no he n
egions we e no well documen ed be o e he
1950s o 1960s (S ocks e al. 2002) and o es i es
which occu ed ou side o p o ec ion zones
be o e abou 1980 we e nei he epo ed no
supp essed (Sima d 1997). La ge a eas o Cana-
da’s bo eal o es emain zoned o ‘‘ modi ied
supp ession’’ by p o incial, e i o ial, and na-
ional pa k agencies, meaning ha he o es is
allowed o bu n unless communi ies o o he
in as uc u e is a isk (S ocks e al. 2002).
In con as o ecen i e supp ession e o s,
p esumably esul ing in a educ ion in he a eas
bu ned, o he human ac ions may ha e aug-
men ed he occu ence o i es. In he 19 h
cen u y a common no ion was ha humans we e
he p edominan cause o o es i es (Blomq is
1888, Bell 1889). Cu en ly, his is no conside ed
ue o mos o he bo eal No h Ame ica, as
80%o he bu n a ea and mo e han 70%o he
numbe o i es ( hose .200 ha) in compiled
s a is ics o no he n ecozones a e caused by
ligh ning-igni ed i es (S ocks e al. 2002, Ka-
sischke and Tu e sky 2006). Ne e heless, i has
been p oposed ha de elopmen ac i i ies and
indus ial o es y ha e inad e en ly p omo ed
ligh ning i es by inc easing he a ailabili y o
lammable uels (A ien i e al. 2009, Lindenmaye
e al. 2009).
Du ing he las 150 yea s, o es i es and
biomass bu ning ha e declined globally (Ma lon
e al. 2008). This can be clea ly seen om i e
his o y ield s udies in Fennoscandia, sou he n
Canada and he wes e n Uni ed S a es, whe e
annually bu ned a eas ha e s eeply dec eased
o e he las cen u y o longe (Zack isson 1977,
Flannigan e al. 1998, Niklasson and G ans o¨m
2000, Zhang and Chen 2007). Howe e , in
no he n No h Ame ica he i e his o y is no
clea . While some ield s udies show leng hening
o he i e cycles (La sen 1997), o he s poin o a
conside able empo al a iabili y (Cy e al.
2009). Indi ec econs uc ions sugges ha he e
has been no end o e he las h ee cen u ies
(Gi a din and Sauchyn 2008), and compiled i e
s a is ics a es o conside able sho ening in i e
cycles (S ocks e al. 2002, Kasischke and Tu e sky
2006). These os ensible con lic s be ween s udies
undoub edly e lec egional di e ences, di e -
en ime ho izons o e which ends a e assessed,
and p obably some me hodological di e ences.
Fu he ield s udies a e clea ly needed o cla i y
he complex empo al and spa ial pa e ns in he
occu ence o o es i es in he no he n No h
Ame ica.
The aim o his s udy is o cla i y he i e
his o y o he pas wo cen u ies o e a la ge
egion o no hwes e n Canada. Speci ic esea ch
ques ions a e: (1) How o en ha e o es s bu ned
in he pas ? (2) Ha e he e been any signi ican
ends in he i e cycle du ing he las wo
cen u ies? (3) Wha a e he causes o any
obse ed empo al pa e ns in i e cycles?
www.esajou nals.o g 2May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.
METHODS
S udy a ea
The s udy a ea is loca ed in no heas B i ish
Columbia, no hwes Albe a, sou heas Yukon
Te i o y and sou hwes No hwes Te i o ies in
he Canadian No hwes (Fig. 1). The app oxi-
ma ely ec angula s udy a ea s e ches abou 730
km om no h o sou h be ween la i udes 578N
and 63.58N, and be ween 1258W and 1118W
longi ude. The a ea encompasses abou 564 000
km
2
o which G ea Sla e Lake and smalle wa e
bodies co e 10%. Topog aphy is mos ly la o
gen ly con ou ed excep o he wes e n ma gin
o he s udy a ea. Ele a ion anges om 120 m a
he Mackenzie Ri e o 2942 m on Moun Syl ia,
bu mos ly a ies be ween 150 and 500 m.
Clima e o he egion is con inen al. A e age
empe a u e in July is be ween 158C and 178C
and in Janua y is be ween 198C and 278C
(En i onmen Canada, Na ional Clima e Da a
and In o ma ion A chi e 2010). A e age annual
p ecipi a ion is be ween 280 and 450 mm. The
majo i y o he a ea belongs o he Taiga Plains
ecozone bu segmen s o he Taiga Shield and
Bo eal Plains ecozones a e included in he eas e n
pa o he s udy a ea.
Picea ma iana (Mill.) B i on, S e ns & Poggen-
bu g, Picea glauca (Moench) Voss, Pinus con o a
Douglas, Pinus banksiana Lamb., La ix la icina (Du
Roi) K. Koch, Populus emuloides Michx. and
Populus balsami e a L. a e he mos common ee
species. The ield laye is ypically domina ed by
dwa sh ubs belonging o genus Ledum,Vacci-
nium,andEmpe um. The pan-bo eal mosses
Pleu ozium sch ebe i (B id.) Mi . and Hylocomium
splendens (Hedw.) B.S.G. and di e en species o
Sphagnum ypically co e he o es loo . Based
on digi al opog aphic maps om Na u al
Resou ces Canada, muskeg and o he ypes o
we lands co e app oxima ely 20%o he s udy
a ea.
Humans ha e inhabi ed no he n Canada o
housands o yea s (F ancis e al. 2002). The i s
humans in he Ame icas we e hun e -ga he e s
who in luenced hei en i onmen by hun ing
la ge he bi o es and by ini ia ing a leas some
landscape i es. The e ec o hese ea ly people
on o es i e egimes is unce ain bu po en ially
e y signi ican : e en a small popula ion o
humans, i i wishes o do so, is capable o
bu ning o es s mo e equen ly han he o es s
would na u ally bu n (G ans o¨m and Niklasson
2008). The human popula ion densi y in he
s udy a ea is oday p obably a eco d high
le els, bu is s ill less han 0.1 inhabi an s pe
km
2
. The e we e p ac ically no oads be o e he
1940s. In he No hwes Te i o ies, he i s oads
we e buil in he 1960s. A la ge po ion o he
oad ne wo k was cons uc ed o suppo he oil,
gas and mine al explo a ion ha s a ed in he
1950s and has expanded and con inued o he
p esen day. In he sou he n pa o he s udy
a ea, a modes le el o comme cial o es y has
been p ac iced in he la e hal o he 20 h cen u y
(Schneide 2002). Mos o he s udy a ea emains
un oaded wilde ness, domina ed by p ime al
o es and li le human use. Fi e supp ession
ac ions in he a ea ha e a ied h ough ime and
space bu he longes policy has been o p o ec
p ope y and li es in p oximi y o ci ies, high-
ways and majo i e s. Conside ing ou sam-
pling s a egy (see Me hods: Fi e his o y
econs uc ion), he oad building da es cons i-
u e he likely s a o i e supp ession, al hough
i may ha e become e ec i e only much la e .
Fi e his o y econs uc ion
Fi e his o y was econs uc ed using (1) i e
sca s on ees, (2) o es s and ages and (3) i e
killed and cha ed snags in 85 s udy plo s wi hin
a bu e zone o 0.2–1.0 km om he exis ing oad
ne wo k in he a ea. The i s plo was andomly
loca ed wi h A cGIS so wa e. Addi ional plo s
we e loca ed wi h a p ocedu e whe eby he nex
plo was placed on he opposi e side o he oad
40 km away as he ae ial poin - o-poin dis ance
(Fig. 2). The o hogonal dis ance o each plo
cen e om he oad was andomized using 0.1
km s eps o be 0.2–1.0 km. A e he i s 30 plo s
he dis ance om he oad was limi ed o 0.5 km
a maximum because o ime limi a ions. The
plo s we e loca ed wi h he help o a GPS de ice.
I a plo cen e ell in a wa e body, i was ejec ed
and a new plo was andomized o be 2–9 km
o wa d along he oad on he opposi e side o i .
The adius o sampling o each ci cula plo was
100 m, encompassing an a ea o 3.1 ha. This plo
size was conside ed o be small enough o se e
as a poin -wise measu e o he occu ence o
i es, ei he bu n o no bu n in any i e, while
being la ge enough o ha e a good chance o
www.esajou nals.o g 3May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.
include i e sca ed ees. Al oge he 453 samples
om li ing ees and 45 samples om dead ees
we e collec ed, polished wi h ex a ine sandpa-
pe and b ough o he labo a o y o analysis by
mic oscope.
Fi e sca s we e used as a p ima y sou ce
because hey yield ela i ely accu a e da es o
i es. Disks o wedges including i e sca s we e
sawn om up o i e li ing and dead ees pe
plo . Fi e sca s a e ypically loca ed a he base o
Fig. 1. Loca ions o he s udy a ea and s udy plo s in No hwes Canada.
Fig. 2. Schema ic diag am po aying he loca ing o s udy plo s.
www.esajou nals.o g 4May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.
he unk and ha e a sha ply o med inju y
showing no lesion ha g ows yea a e yea . The
la e is ypical o sca s made by alling ees and
ungi. In wo plo s i e sca s we e collec ed mo e
han 100 m om he plo cen e . Howe e , in
bo h cases i e sca s we e s ill conside ed o ha e
o igina ed om he same i e ha had bu ned he
plo cen e as he o es age and s uc u e we e
simila in he plo cen e and a he addi ionally
sampled loca ion u he away.
Fi e sca s om li ing ees we e da ed by
coun ing he annual ings backwa ds om he
las yea o g ow h. Possible missing ings can,
howe e , cause an e o o a couple o yea s in
da ing. This e o was minimized by selec ing he
adius ha ing he b oades annual ings in each
sample, and by c osschecking i e sca da es om
he same plo . Fi e sca s om 9 dead ees we e
dend och onologically c oss-da ed (see he la e
pa o his sec ion).
Li ing ees we e sampled o hei age in
e e y plo . Fi s , h ee dominan ees (no
g owing unde he canopy o o he ees) nea es
o he plo cen e we e sampled. Secondly, one o
h ee ees ha subjec i ely seemed o be he
oldes wi hin 20 m om he plo cen e we e
sampled. Samples we e ex ac ed om as close o
he base o he ee as possible, on a e age less
han 0.3 m abo e he g ound le el. Ages o he
sampled ees we e es ima ed by coun ing he
annual ings o he pi h and by adding a ixed
numbe o yea s o accommoda e he ime ha is
needed o open-g own ees o each he
sampled heigh . Based on ou sampling, one
yea was added o all b oadlea ed ees, wo
yea s o Pinus and 5 yea s o Picea, La ix and
Abies. In a ew cases whe e pi h was no p esen ,
he missing annual ings we e es ima ed based
on cu a u e o he ings nea he pi h (A no and
Sneck 1977).
The ee ages we e used o in e ing i e da es
i no i e sca s co esponding o he ee ages
could be ound. Fi e was de e mined o ha e
occu ed one yea be o e an es ima ed yea o
ge mina ion o he oldes sample ee in he plo .
To a oid assigning alse i es, he e had o be a
leas one o he sample ee bo n wi hin 30 yea s
o indica e a egene a ion coho ollowing a i e.
T ees killed and cha ed by i e we e used as
addi ional e idence o ensu e ha he s udied
o es s we e ac ually bu ned.
The samples om dead ees we e dend o-
ch onologically c oss-da ed, i possible. Samples
om li ing ees wi h he longes sequences o
undis u bed g ow h we e used o build mas e
ch onologies o Picea glauca and Pinus banksiana.
Inaddi ion,da ap o idedbyH.F i s,M.
Hu on, T. Knowles, C. La sen, D. Meko, K.
Mose , F.H. Schweing ube , C. S ock on and J.
Szeicz (Wo ld Da a Cen e o Paleoclima ology,
In e na ional T ee Ring Da a Bank 2010) we e
used o building he mas e ch onologies. Da es
o he samples we e de e mined based on a
dis inc ly high co ela ion wi h he mas e se ies
using COFECHA so wa e (G issino-Maye 2001)
and e i ied ag eemen wi h he known i e yea s
and ee age s uc u e in he plo in ques ion. In
all cases, he isual appea ance o he sample was
also conside ed o exclude e oneous da es.
C oss-da ing p o ed o be challenging because
we we e dealing wi h mul iple ee species in a
e y la ge a ea whe e one mas e ch onology pe
species is no enough. O he 45 samples om
dead ees only 15 could be dend och onologi-
cally c oss-da ed. The samples ha emained
unda ed we e mos ly cha ed snags. Cha coal
was ound especially on b anch ips bu no on
he unk, indica ing ha he ee had p obably
been killed by he p e ious i e. Snags ha had
been dead o a long pe iod be o e he i e would
ha e e y likely been wi hou ba k and d y, and
he e o e would ha e bu ned mo e ho oughly.
Hence, he dea h o ees wi h cha ed b anch
ips was da ed o a pe iod immedia ely (one
yea ) be o e he las i e in he plo in ques ion.
Ve y low se e i y i es ha do no kill ees, o
e en cause sca s on hem, may no be de ec ed
wi h he me hods desc ibed abo e. Howe e ,
such low-se e i y i es end o sp ead slowly and
emain small. The g ea majo i y o he a ea
bu ned in his egion is om la ge powe ul i es.
The econs uc ed i e his o y was compa ed
wi h i e s a is ics, including all documen ed
o es i es, p o ided by Na u al Resou ces
Canada and he i e managemen agencies o
B i ish Columbia, Albe a and No hwes Te i-
o ies.
S a is ical analysis
The a e age annually bu ned p opo ion o a
pe iod om yea
0
o yea
1
(including
1
)was
es ima ed as
www.esajou nals.o g 5May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.

a 0; 1¼1
1 0þ1X
1
¼ 0
bð Þ
cð Þð1Þ
whe e b( ) is he numbe o bu n plo s and c( )
he numbe o ac i e plo s in yea . A plo was
de e mined o be ac i e (i.e., eco ding i es)
since he i s de e mined i e yea o since he
i s yea in he annual ing ch onology o he
plo in ques ion, whiche e came i s .
The i e cycle o he same pe iod om yea
0
o yea
1
(including
1
)is
0; 1¼1=a 0; 1ð2Þ
The i e cycle equals o he ime needed o bu n
a cumula i e a ea equi alen o ha o a la ge
e e ence landscape (on a e age once by sepa-
a e, small i es) and i is he in e se o he
a e age o he annually bu ned p opo ions
(Johnson and Gu sell 1994). The i e cycle also
equals he expec ed poin -wise a e age i e
in e al.
We assume ha in a gi en yea , he s udy plo s
beha e independen ly. Then, he numbe o
obse ed i es o yea ollows a binomial
dis ibu ion, b
;B(c( ), p( )), whe e p( ) is he
p obabili y ha a plo bu ns, gi en he condi-
ions du ing he yea . When b( ) and c( ) a e
known, he pos e io p obabili y o p
is he be a
dis ibu ion Be a(b( )þ1, c( )b( )þ1). The e o e
he 0.025 and 0.975 quan iles o he be a
dis ibu ion gi e he 95%con idence in e al o
p( ).
We used logis ic eg ession o e alua e he
e ec o se e al ac o s on i e occu ence, and
he magni ude o a possible esidual end. The
dependen a iable is he obse ed yea ly num-
be o i es and he co a ia es a e he yea , oad
building da e and a se o clima ic indica o s. The
a e age empe a u e o e June–July–Augus
(JJA), July mon hly d ough code (MDC), and
he annual a e age Paci ic decadal oscilla ion
(PDO) index we e e alua ed o bes explain
o es i es in ou s udy a ea based on ea lie
esea ch in no he n No h Ame ica (Du y e al.
2005, Fau ia and Johnson 2008, Gi a din e al.
2009). We used he annual a e age PDO alue,
which is e y s ongly posi i ely co ela ed wi h
he Janua y–Feb ua y ( ¼0.79), and sp ing-
summe ( ¼0.96) PDO alues used by Du y e
al. (2005) and Fau ia and Johnson (2008),
espec i ely. In addi ion, an index de i ed om
a h ee-cen u y long econs uc ion o ee
g ow h in he Taiga Plains eco egion by Gi a din
and Sauchyn (2008) was used as a p oxy o
clima ic o cing on i e egimes in he egion.
Thei econs uc ion (he ea e e e ed o as GiR)
is based on he clima ically egula ed componen
o a ia ion in ee- ing wid hs in he egion and
no di ec ly o bu ned a eas. The GiR index
ep esen s an unde ined se o wea he a iables
ha co ela e bo h wi h he ee g ow h and he
occu ence o i es.
His o ical ins umen ally measu ed PDO indi-
ces we e acqui ed om he Royal Ne he lands
Me eo ological Ins i u e Clima e Explo e (2010).
PDO index alues a e sea su ace empe a u e
(8C) anomalies oscilla ing a ound ze o; he
alues used in ou analysis anged om a
minimum o 2.3 o a maximum o 3.08C. July
d ough codes (MDC; Gi a din e al. 2009) o he
s udy a ea we e compu ed using clima e da a
om he Clima e Resea ch Uni o he Uni e si y
o Eas Anglia and so wa e SimMDC p o ided
by Ma in Gi a din (Canadian Fo es Se ice,
Saul S e. Ma ie, On a io). The his o ical June-
Augus mean empe a u es in Fo Simpson and
Hay Ri e du ing 1895–2008 we e acqui ed om
he Royal Ne he lands Me eo ological Ins i u e
Clima e Explo e (2010) and he En i onmen
Canada, Na ional Clima e Da a and In o ma ion
A chi e (2010b). A ew missing da a en ies we e
assigned an a e age o he mon h and s a ion in
ques ion.
Road building da es we e de e mined om
maps o a ious da es, o he publica ions (G een-
wood 1992), di e en in e ne sou ces, and
pe sonal communica ion wi h local au ho i ies.
Despi e hese e o s, exac da es could no be
ound o e e y oad. Fo hese oads he
building da es we e assigned a midpoin da e
be ween a known poin o ime when he oad
was no buil ( o example, om an old map
wi hou he oad) and when i appa en ly was
p esen ( o example, a mo e ecen map, o he
yea o ield sampling).
We model he haza d o bu ning h( ) in a yea
o plo by
logi ðhð ÞÞ ¼ aþb þq ; þX
i
cixið Þð3Þ
www.esajou nals.o g 6May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.
whe e x
i
( ) a e he clima ic co a ia es ques ion,
2 0,1gindica es i he oad nea es o plo
was buil be o e , and a,b,q, and c
i
a e
pa ame e s. Fo cla i y, we index he c
i
co e-
sponding o each clima ic indica o as c
JJA
,c
MDC
,
c
PDO
and c
GiR
. The clima ic indica o s we e
no malized so ha he mean alue was 0 and
s anda d de ia ion 1. Yea was scaled o ake
alues be ween 0, o he i s , and 1, o he las
analysed yea .
Since we assume ha bo h plo s and yea s a e
independen o each o he , he abo e model is a
s aigh o wa d gene alized linea model. How-
e e , he unce ain y in he da ing o he i es is
signi ican , p obably a ies in ime, and di e s
be ween he i es da ed om s and age (age-
da ed) and i es dend och onologically c oss-
da ed om i e sca s (sca -da ed). We he e o e
used a Bayesian amewo k o he eg ession
modeling, which allows inco po a ing he da ing
e o , which is no exac ly known, di ec ly in he
analysis. We assume he ac ual da e o a i e
eco ded o yea is dis ibu ed acco ding o a
no mal dis ibu ion N( þd
j
,
j
( )
2
), whe e j¼a
and j¼s, o age-da ed and sca -da ed i es,
espec i ely. We model
a
by
að Þ¼ðk1ð  0Þþk0ð 1 ÞÞ=ð 1 0Þð4Þ
whe e
0
and
1
a e he i s and he las yea s in
he da a, espec i ely, and k
0
and k
1
a e pa am-
e e s. The sys ema ic e o d
j
, as well as
s,
¼k
2
a e assumed o be independen o and we
assume ha
a
¼0.
To calcula e he likelihood o he obse ed
numbe s o each ype o i e obse a ion, we also
need o know he p obabili y s( ) o he i e being
obse ed only as an age-da ed i e. We assume
ha his nuisance pa ame e is a unc ion o ime
since he i e in ques ion, and es ima e i by
sð Þ¼~
mað Þ=ð~
mað Þþ~
msð ÞÞ ð5Þ
whe e s( ) is he p obabili y ha a i e a yea is
obse ed only as age-da ed, and ˜
m
a
( ) and ˜
m
s
( )
a e ke nel-smoo hed yea ly numbe s o obse ed
age-da ed and sca -da ed i es, espec i ely.
Now he comple e model o he p obabili y o
eco ding a i e on a plo a yea is
ha; ð Þ¼sð ÞX
1
0¼ 0
Nð þda; að Þ2Þð 0Þlogi 1
ðaþb 0þq ; þX
i
cixið 0ÞÞ ð6Þ
o he age-da ed i es, and
hs; ð Þ¼ð1sð ÞÞX
1
0¼ 0
Nð þds; sð Þ2Þð 0Þlogi 1
ðaþb 0þq ; þX
i
cixið 0ÞÞ ð7Þ
o he sca -da ed i es.
Finally, he likelihood unc ion is gi en by
L¼Y
;
q ð Þðga; ð Þha; ð Þþð1ga; ð ÞÞ  ð1ha; ð ÞÞÞ
Y
;
q ð Þðgs; ð Þhs; ð Þþð1gs; ð ÞÞ  ð1hs; ð ÞÞÞ
ð8Þ
whe e q
( ) is an indica o unc ion o whe he
plo is ac i e in yea , and g
a,
( ) and g
s,
( )
indica e i an age-da ed (sca -da ed) i e was
eco ded on plo in yea .
Fo he ac ual pa ame e s o in e es , a,b,q,
and c
i
, we used non-in o ma i e p io s. In o ma-
i e p io s we e used o he pa ame e s o he
da ing unce ain y ke nel o he age-da ed i es.
We used log-no mal p io s o k
0
,k
1
, and k
2
, wi h
meanso 7.4,2.7,and0.6yea s,and95%
con idence limi s o 2.7–20.1, 1.0–7.4, and 0.37–
1.0 yea s, espec i ely. A no mal p io was used
o d
s
, wi h mean ¼0 and s anda d de ia ion ¼
0.5. The pos e io dis ibu ions o he eg ession
pa ame e s we e calcula ed wi h an i e a i e
me hod using an adap i e Ma ko chain Mon e
Ca lo algo i hm (Haa io e al. 2005) implemen ed
in Cþþ.
RESULTS
Signs o i es we e omnip esen in he s udied
landscape; all he 85 s udy plo s had been bu ned
a leas once du ing he p e ious 220 yea s (Fig.
3). Sca e ed i e sca s could be ound in
app oxima ely hal o he s udy plo s, while
pos - i e egene a ion coho s and cha ed wood
p o ided e idence o s and- eplacing i es in he
es o he plo s. None o he sample plo s
happened o all in ecen ly bu ned a eas ( hough
some we e obse ed be ween plo s), wi h he
mos ecen i e sampled occu ing in 1981. The
a e age annually bu ned a ea o e he whole
www.esajou nals.o g 7May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.
s udy a ea du ing he las wo cen u ies has been
1.3%, co esponding o a i e cycle o 77 yea s
(Fig. 4).
Theoccu enceo o es i eshas a ied
conside ably du ing he las wo cen u ies. The
o emos change was he dec easing end in he
Fig. 3. Annual ee ing and i e ch onologies o s udy plo s a anged acco ding o he ea lies yea o in e ed
i e occu ence. The age o oldes li ing ees is shown by da k g ay ba s, and i e yea s a e indica ed by black
diamonds. Ligh g ay ba s deno e dead wood samples wi h cha ed b anch ips belonging o a p e ious ee
gene a ion.
www.esajou nals.o g 8May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.
annually bu ned a ea o e he whole pe iod.
A e age annually bu ned a ea dec eased om
2.0%in he i s hal o he 19 h cen u y o 0.33%
in he la e hal o he 20 h cen u y, co espond-
ing o i e cycles o 50 and 300 yea s, espec i ely.
Fi es dec eased especially a e he mid-19 h
cen u y. In he 1940s he e was an in e media e
peak in i es a e which a apid u he decline in
annually bu ned a eas ollowed (Fig. 4).
In he sou he n pa o he s udy a ea (in
B i ish Columbia and Albe a) ou econs uc ion
is in good ag eemen wi h i e s a is ics o he
a ea excep o he 1940s whe e ou econs uc-
ion shows conside ably highe bu ned a eas
(Fig. 5A). In he whole s udy a ea he 1960s and
1970s s a is ics and econs uc ions show simila
alues bu in he 1990s s a is ics epo a e age
annually bu ned a eas ha a e highe han ou
uppe con idence limi (Fig. 5B).
The ull eg ession model was i s i ed o
he pe iod 1900–2000, o which all he clima ic
ac o s we e a ailable. O he clima ic ac o s,
GiR and MDC we e posi i ely, and he PDO
index nega i ely, co ela ed wi h he bu ned
a ea, indica ed by co esponding eg ession
pa ame e s ha we e posi i e (o nega i e,
espec i ely) wi h a leas 95%pos e io p oba-
bili y (Fig. 6). Since all clima ic p edic o s we e
no malized o ha e a a iance o 1, he magni-
udes o he co esponding eg ession pa ame-
e sshow he ela i eimpo anceo he
p edic o s. The GiR index appea s o be he
single bes clima ic p edic o o bu ned a ea.
Acco ding o he pos e io dis ibu ions, he
eg ession pa ame e s o JJA (c
JJA
) and oad
building da e (q) we e equally likely o be
posi i e o nega i e, and hence did no show an
e ec on he bu ned a ea (Fig. 6A). Also, sample
plo dis ance om oads (0.2–1.0 km) did no
co ela e wi h he bu ned p opo ions o he a ea
(p¼0.7, Spea man’s ank es ).
Despi e he co ela ion be ween he clima ic
ac o s and bu ned a ea, he clima ic ac o s did
no explain he dec easing end in i es. The
esidual end ep esen ed by bwas clea ly
nega i e bo h o he pe iod 1900–2000 and
1800–2000 (wi h pos e io p obabili y .0.9999,
Fig. 6A and 6B, espec i ely). Indeed, none o
GiR, MDC, and PDO co ela e signi ican ly wi h
yea (p .0.25 o each, Spea man’s ank es ). I
is no ewo hy ha he summe empe a u e
index JJA does ha e a signi ican posi i e end
(p ,10
6
), bu i does no ha e explana o y
alue in he eg ession model. This indica es ha
on a yea ly le el, JJA is no nega i ely co ela ed
wi h i es, which would be necessa y o i o
explain he end in i es.
Fig. 4 shows he a e age yea ly bu ned
p opo ion o he pe iod 1800–2000, as p edic ed
by he eg ession model. The p edic ion uses he
maximum pos e io densi y alues o he
pa ame e s, es ima ed o he pe iod 1800–2000
(Fig. 6B). Road building da e and JJA we e le
ou o he p edic i e model, based on he analysis
o he pe iod 1900–2000, and PDO and MDC
we e no used o he 19 h cen u y o which hey
we e no a ailable. The p edic ion alls well
inside he 95%con idence en elope o he
empi ical econs uc ion.
Rega ding he da ing e o , ep esen ed by k
0
,
k
1,
k
2,
and d
s
he analysis ga e clea addi ional
in o ma ion only abou he sys ema ic sca
da ing e o , d
s
, ha had i s highes pos e io
densi y a 0.5 (Figs. 6A and 6B). This means ha
he i es da ed based on i e sca s a e equally
likely o ha e occu ed du ing he p e ious as
he cu en ly gi en yea .
DISCUSSION
Signs o pas i es we e ubiqui ous in he la ge
ac s udied in no hwes Canada despi e he
la ge p opo ion o we lands (20%). In he 19 h
cen u y a e age annually bu ned p opo ion o
he landscape a ied be ween 3%and 1%in
di e en decades, co esponding o i e cycles o
30 o 100 yea s (Fig. 3); o e he en i e 19 h
cen u y, he a e age i e cycle was app oxima ely
60 yea s. These alues all well wi hin he ange
o his o ical i e cycles epo ed in bo eal No h
Ame ica, despi e he mos ly mo e sou he ly
loca ion o such s udies (Zhang and Chen
2007). Mos s udies om bo eal Fennoscandia
o Cen al Sibe ia ha e also sugges ed pas i e
cycles o be ween 30 and 100 yea s, o somewha
longe (Niklasson and G ans o¨m 2000, D oby-
she e al. 2004, Wallenius e al. 2011). I seems
ha despi e di e ences in dominan ee species,
his o ical i e e u n in e als ac oss la ge pa s
o he ci cumbo eal o es s ha e been less han
100 yea s, wi h he long- e m and la ge-scale
a e ages being a ound 50–80 yea s. Howe e , i
www.esajou nals.o g 9May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.
Fo es Resea ch 40:2027–2035.
Wallenius, T., M. La ja aa a, J. Heikkinen, and O.
Shibis o a. 2011. Declining i es in La ix domina ed
o es s in no he n I ku sk Dis ic . In e na ional
Jou nal o Wildland Fi e in p ess.
Wei , J. M. H., E. A. Johnson, and K. Miyanishi. 2000.
Fi e equency and he spa ial age mosaic o he
mixed-wood bo eal o es in wes e n Canada.
Ecological Applica ions 10:1162–1177.
Wo ld Da a Cen e o Paleoclima ology, In e na ional
T ee Ring Da a Bank. 2010. hh p://www.ncdc.noaa.
go /paleo/ ee ing.h mli
Ya ie, J. 1981. Fo es i e cycles and li e ables: a case
s udy om in e io Alaska. Canadian Jou nal o
Fo es Resea ch 11:554–562.
Zack isson, O. 1977. The in luence o o es i es in he
No h Swedish bo eal o es . Oikos 29:22–32.
Zhang, G. F., and W. J. Chen. 2007. Fi e cycle o he
Canada’s bo eal egion and i s po en ial esponse o
global change. Jou nal o Fo es y Resea ch 18:55–
61.
www.esajou nals.o g 16 May 2011 Volume 2(5) A icle 53
WALLENIUS ET AL.