lightning- caused and human- induced for...

Studia Quaternaria, vol. 30, no. 1 (2013): 29–40. DOI: 10.2478/squa-2013-0003

LIGHTNING- CAUSED AND HUMAN- INDUCED FOR EST FIRESAS EVI DENCED BY PTE RID IUM SPORES IN SE LECTED

QUA TER NARY REC ORDS FROM PO LAND

Krzysz tof Bi ñka1, Jerzy Ni tycho ruk2

1 Bi ñka, K., War saw Uni ver sity, In sti tute of Ge ol ogy, De part ment of Pa le on tol ogy, Al. ¯wirki i Wig ury 93, 02- 089, Warszawa, Po land, email: [email protected] War saw Uni ver sity, In sti tute of Ge ol ogy, De part ment of Cli mate Ge ol ogy, email: j.ni tycho [email protected]

Ab stractThe oc cur rence of Pteridium spores – the com mon fire-adapted plant – was ob served in Po land’s Pleis to cene (threeinterglacials and nu mer ous interstadials) and Ho lo cene pol len se quences. Un til the on set of the mid dle Ho lo cene,bracken was re corded rel a tively rarely. This marks cli ma tic con di tions with spo radic wildfires, fol lowed by quick re -moval of the clones (or main te nance of the clones in a sup pressed state) in the sub se quent, post fire successionalstages. In each inter gla cial we can also iden tify short pe ri ods (most of ten syn chro nous) of some what higher fre quencyof Pteridium, in di cat ing a pos si ble in crease in nat u ral fires as an ef fect of stormy con di tions. These short pe ri ods areplaced mainly within phases dom i nated by co nif er ous wood lands. The very high con tent of bracken found from areanalysis of the long Saalian se quence at Ossówka as three reg u lar cul mi na tions just above three non-tree phases,east ern Po land is the unique ex cep tion. To ex plain this we found the close mod ern an a logue of the above phe nom e nonin the Alas kan re gion, at the bound ary be tween the tun dra and the bo real zone where in a very nar row for est zone, es -pe cially with the con ti nen tal sig na ture, light ning-ini ti ated fires are very fre quent (the edge ef fect). Pleis to cene re cordsof bracken dur ing for est pe ri ods might in di cate that thun der storms and light ning strikes were re spon si ble for its highercon tent. This is not to ex clude the pos si bil ity that inter gla cial fires were set by Palaeo lithic hu mans. How ever, it ismore likely that the wildfires were uti lized and to some ex tent con trolled, es pe cially at Ossówka, where the palaeolake ex isted for a long time af ter the inter gla cial; and this surely at tracted the at ten tion of game and hu mans. Our in ves ti ga -tions show that rare, more abun dant Pteridium in the Pleis to cene se quences can be traced through out the cor re spond -ing pe ri ods even at dis tant sites. This might be in di rect ev i dence of a cli ma tic pat tern that pro mote stormy con di tionsand fires at that time. In the Ho lo cene, dif fer ent fac tors seem to be re spon si ble for the long-term dy nam ics in Pteri-dium clones. Both in Po land and in ad ja cent ar eas bracken peaked in the mid dle Ho lo cene from 8000 to 5000 BP,when this ter ri tory was oc cu pied by de cid u ous wood lands. There is agree ment that this is due to burn ing of for ests bythe hunter-gath erer so ci et ies of the Late Mesolithic. How ever, in spite of in creased clear ings in the sub se quent phasesbracken sub stan tially de creased in abun dance. This means first of all that fires rather than clear ings were re spon si blefor the rise in Pteridium spores in the Ho lo cene pol len se quences.

Key words: bracken spores, fires, Qua ter nary, Po land

IN TRO DUC TION

Pteridium aquilinum is a com mon plant grow ing on ev -ery con ti nent ex cept in Arc tic/Ant arc tic re gions. For manyrea sons – first of all eco nomic – it is an im por tant plant, of tenin va sive and dif fi cult to erad i cate; and be cause of this the lit -er a ture con cern ing ev ery as pect of its mod ern ex is tence isvo lu mi nous. In Eu rope, it oc curs both in open-can o piedwoods as well as in open hab i tats (mostly in west ern, mar i -time re gions) such as heaths, grass lands and moors (Marrs,Watt 2006 and ref er ences).

Win ter frosts are the crit i cal fac tor con trol ling the ab -sence of bracken in north ern Eu ro pean tree less ar eas (Marrs,Watt 2006). Hence in north ern Fin land in the tran si tionalzone of birch for est, siz able patches be come smaller and scat -tered and they have no sporulation (Oinonen 1967). In Po -land, clones of bracken are noted com monly in for ested ar eas

with higher light in ten sity, es pe cially at the for est edges, inopen wood lands and can opy gaps where they are very vig or -ous plants. Ephem eral, short-lived stands in open ar eas af terclear-cut ting where Pteridium is lim ited by drought, are alsode scribed (Markowski 1971). The Pteridium spores noted inQua ter nary se quences seem to be a good in di ca tor of fire.Oinonen (1967) pointed out that ger mi na tion of spores ispos si ble ex clu sively in the pres ence of ash.

There are re cords for bracken spores in Po land from tem -per ate pe ri ods dur ing the Qua ter nary. In some se quenceshow ever Pteridium is noted more fre quently or even veryabun dantly – e.g. in the Ho lo cene – whereas in the Pleis to -cene units it is re corded spo rad i cally or its spores form lowcurves. In ex cep tional cases only de pos its yielded moreabun dant spores than in the Ho lo cene cores from the in ves ti -gated area.

Views about the cause of the Ho lo cene max ima ofbracken are only partly co in ci dent. It is ev i dent that in ear lierpe ri ods hu mans in ten sively de stroyed wood lands, which in -flu enced the abun dance and dis tri bu tion of this light de -mand ing plant. As sev eral au thors have pointed out, dis-turbances such as burn ings or clear ings formed new hab i tatsfor bracken, which re sulted in a higher fre quency of spores(Doll ing 1999, Pakeman et al. 2000, Mars, Watt 2006). How -ever, which of the two fac tors is re spon si ble for this, is lessclear. Fur ther more, some work ers see cli mate as an ad di -tional con tri bu tion to its Ho lo cene abun dance. In older in ter -vals, when hu man im pact on veg e ta tion is hard to iden tify inpol len spec tra, bracken has not at tracted at ten tion of re -search ers, be cause spores are rather barely rep re sented. Inthis case the main causes of the bracken oc cur rence seem tohave been nat u ral for est re gen er a tion cy cles and a fire re gime that has been cli mat i cally in duced.

The im pe tus for this study was the un usual cy clic oc cur -rence of three very reg u lar, high cul mi na tions of bracken inthe se quence at Ossówka (Podlasie re gion, east ern Po land),yield ing pol len flora from the Holsteinian/Early Saalian age(Fig. 1). This very long, mo not o nous core of lake marl ac cu -mu lated over sev eral thou sand years is the key pro file for theMid dle Pleis to cene in Cen tral Eu rope. Its post-inter gla cialpe riod was re ex am ined. This high per cent age of bracken isun usual, be cause it is noted three times, just af ter tree-lessphases. This is atyp i cal both for its mod ern dis tri bu tion,where Pteridium near the north ern for est limit is a rare plantand shows poor or no sporulation, and for its very frag men -tary fos sil re cord in such an ini tial for est. This gave rise toclose scru tiny of the avail able pol len se quences of dif fer entages for map ping the oc cur rence of bracken. This might alsoal low a broader spec trum of prob lems to be cov ered with re -spect not only to fire re gimes but also in di rect fac tors, whichit gen er ates – e.g. storm sys tems – con fined to some stages ofinter gla cial suc ces sion. Let us – from a palynological per -spec tive – look into the Pleis to cene and the Ho lo cene oc cur -rence of Pteridium in the stud ied area.

MA TE RI ALS AND METH ODS

The start ing point for the anal y sis was the well-knownHolsteinian/Saalian se quence at Ossówka (Krupiñski 1995),with a num ber of bo real interstadial events and tree lessstadials. The newly an a lyzed pro file was taken from a dee-per, cen tral part of the palaeolake (Nitychoruk et al. 2006)and sam pled with a higher res o lu tion. The 22 m long core (24m – 2 m) con sists of lake marl rich in di a toms at most lev elsan a lyzed. Only at the top (4.5 m – 2 m) the de pos its pass into a non-cal car e ous silt. The core was sam pled for pol len anal y sis at 10 cm in ter vals and af ter that, we used stan dard palynolo-gical prep a ra tion meth ods with hy dro flu oric acid, Erdtman‘sacetolysis and Lycopodium spores (Stockmarr 1971). Be -cause of fre quency vari a tions, counts range from 300 to 1000 pol len grains in each sam ple.

Pol len di a grams of Po land and to a lesser ex tent of ad ja -cent ar eas were used as sup ple men tary ma te rial to the ex am i -na tion. In many ways they rep re sent a source of data, whichhas a dif fer ent value. We tried to pick out deep-wa ter lac us -trine se quences (Fig. 2), be cause suc ces sion in peat bogs is

of ten dis turbed and sporomorphs are badly pre served. Alsoin peat de pos its, be cause of the prox im ity to the source area,bracken is of ten overrepresented in the spec tra (Greisman,Gaillard 2009). This may de form the pat tern of floristicchange in ferred from the pol len count.

Un doubt edly, long Ho lo cene se quences with short sam -pling in ter vals – es pe cially these from the pro ject IGCP 158b– are pos i tive ex am ples. Pleis to cene re cords with re li ablepol len data and suf fi cient sam pling, are rather rare. Usu allypol len di a grams are frag men tary, show ing only a part of suc -ces sion. In a large num ber of se quences bracken spores arenot noted – prob a bly be cause of ed i to rial re quire ments (di a -grams have to fit on a page). We have at our dis posal mainlypol len di a grams rep re sent ing the ‘true’ interglacials: the old -est one – the Ferdynandovian (Cromerian III and IV) – twoex ten sive pol len di a grams, with inter gla cial flora and thecom plex of ad ja cent stadials/interstadials, the Mazovian(Holsteinian) inter gla cial (about 340 000 – 325 000 yrs BP) –a num ber of sites, some times with the long Saalian suc ces -sion; and the Eemian – nu mer ous di a grams.

For tu nately, a large part of the an a lyzed se quences is lo -cated in a rel a tively small area in cen tral and east ern Po land,where af ter suc ces sive glaciations, palaeo-lakelands wereformed. This al lows more re li able com par i sons be tween pol -len suc ces sions of dif fer ent ages, which were sit u ated in thesame geo graph ical re gion with rel a tively sim i lar cli matecon di tions and hab i tat tol er ance for bracken. This fact is im -por tant, be cause e.g. in the UK Pteridium fa vors equally of -ten open ar eas af ter for est clear ings (pas tures, heaths) (Pake-man et al. 2000), whereas in Po land it is a very rare vis i tor insuch places. Hence, the rea sons for bracken peaks in se -quences from such ar eas might have been dif fer ent.

The pos si bil ity of map ping Pteridium at sites dis persedin time and space de pends on the high res o lu tion of pol len di -a grams, in which, un for tu nately, spores of bracken are of tenomit ted and not in cluded in the list of iden ti fied types. In thePleis to cene re cord only the high to tal sum (about 1000 pol len counted) per mits a re con struc tion of more sub tle trends inbracken oc cur rence. Also spores of bracken seem to be easyiden ti fied. From a side view and when the pol len slide is toothin, which can lead to a rup ture of the exine along leasuresand some spores can be over looked dur ing counts. Thus, cor -rect iden ti fi ca tion needs some pro fi ciency.

RE SULTS

The Pleis to cene

The Ferdynandovian Inter gla cial

In this inter gla cial (e.g. at Zdany, Pidek 2003, Fig. 2)bracken spores was found rarely. How ever, higher val ues ofPteridium oc curred in a few sam ples be fore each of the twochar ac ter is ti cally warm tem per ate phases for this suc ces sion(about 3% with a max i mum up to 5–7%). Their cul mi na tionco in cides with ex pan sion of the ini tial pine-birch for est (with an ad mix ture of oak be fore the sec ond op ti mum).

Holsteinian Inter gla cial and the Early Saalian

In many Holsteinian se quences a bracken is noted spo -rad i cally through out the inter gla cial (e.g. at Nowiny ¯u-

30 K. BIÑKA & J. NITYCHORUK

LIGHT NING-CAUSED AND HU MAN-IN DUCED FOR EST FIRES 31

Fig. 1. Sim pli fied pol len di a gram of the Ossówka core.

kowskie – Hrynowiecka-Czmielewska 2010; at Brus – Pidek 2003). At some sites higher val ues of Pteridium (2–7 spores/1000 or more) were found dur ing the tran si tional pine zonebe tween yew and horn beam-fir phase (at Raków 1 and 2,Mal ice 3 and 2 – Nita 2009; at Woskrzenice – Biñka andNitychoruk 1995; at Kali³ów, about 2% – Biñka and Nity-choruk 1996; at Ossówka and Komarno – Krupiñski 1995)and at the end of the tem per ate phase (at Bia³e £ugi 3 and 2, at Wielki Bór 2 – Nita 2009).

Of par tic u lar in ter est is the oc cur rence of three im pres -sive cul mi na tions of bracken in the Early Saalian with theper cent ages de cid edly higher than in the Ho lo cene se -quences. They show al most the same pat tern – the start ingpoint in the birch zone, the am pli tude, the cul mi na tion andthe grad ual de crease. The bracken ex pan sion is best ex -pressed in the long pol len se quence at Ossówka (Fig. 1)where three cy cles were found against two max ima in thecore, which was an a lyzed ear lier (Krupiñski 1995). Theamount at tained max i mum val ues of ca 13%. From the

nearby sites at Wilczyn, at a dis tance of about 5 km fromOssówka (Biñka et al. 1997) as well as at Kali³ów at a dis -tance of 10 km (Biñka and Nitychoruk 1996) very high per -cent ages of bracken were also re ported. How ever, the cha-racter of pol len spec tra in the cor re spond ing in ter vals sug -gests that they might have been pro duced in an in ter mit tentfash ion. Abun dant bracken ap pears soon af ter short pe riod of tree-less veg e ta tion dom i nated by grasses, sage brushes, sed-ges and a very char ac ter is tic peak in ju ni per (O-2). The lastspe cies marks al ready some cli ma tic ame lio ra tion. The fur -ther in crease in tem per a ture causes the ex pan sion of a grad u -ally more closed birch for est with larch and ju ni per (O-3 b).At that point spores of bracken be gin to rise pro nouncedlyand cul mi nate just be low the bound ary with the pine zone(O-3 c). Through out this phase its curve grad u ally de creases.This scheme is al most ex actly re peated af ter the suc ces sivecool ing (O-4) with the dif fer ence be ing that the grad ual de -cline of bracken in the pine zone was abruptly in ter rupted bya cold birch-ju ni per pe riod (O-5 e). This cy cle re peats again.


Fig. 2. Sites in Po land men tioned in the text.

In the suc ces sive Saalian zones bracken oc curred sparselyjust as at other sites of cor re spond ing age e.g. Wielki Bór 1, 2(Nita 2009) and Brus (Pidek 2003).

Eemian Inter gla cial and the Early Vistulian

In the Eemian Inter gla cial, if this in for ma tion is given,spores of bracken are noted rarely, but reg u larly (about 1–2spores/1000), e.g. at Imbramowice (Mamakowa 1989) and at many other sites. In sev eral pol len di a grams how ever, apartfrom this pat tern, a some what higher fre quency is ob servede.g. in the Picea pol len zone at Nidzica 12–19, max i mum27/1000 spores were noted (Biñka et al. 2011; Fig. 4), and atDziewule (Biñka and Nitychoruk 2001, 2003) – about 7–14/1000 spores. Also in other re cords in this zone we can see alit tle higher per cent age of bracken spores, which forms thecurve –a sev eral spores were prob a bly found in each sam ple– at Solniki (Kupryjanowicz 2008), Horoszki (Granoszewski 2003), Studzieniec (Krupiñski 2005) and Czaple (3–5 spo-res/1000; Biñka and Nitychoruk 2011). The abun dance ofPteridium was also re ported from the Picea zone from ad ja -cent ar eas e.g. at Hollerup (Björck 2000).

The Quercus pol len zone is the sec ond in ter val within theEemian where Pteridium spores show higher val ues. In somese quences an in creased fre quency of bracken is rec og niz able:at Czaple (through out the oak zone, at 8 spores per sam ple) aswell as at Solniki – at the be gin ning of the oak zone.

In the Early Vistulian only in the late Brörup sensustricto and in the late Amersfoort were a slightly higheramount of bracken spores some times re ported (e.g. at Czaple).

The Ho lo cene

In the Ho lo cene bracken com monly oc curred in a va ri ety of for est hab i tats and is noted more of ten in the pol len se -quences than in the Pleis to cene. In Po land, the in creas ing fre -quency of spores ex ceed ing 1% has been re ported since ca8000 BP and it grad u ally de clined at about 4500 years ago,with the high est con tent be tween 7000–5000 BP in ferredfrom isopollen maps (Madeja et al. 2004). Only non-sig nif i -cant re gional dif fer ences were ob served. The sim pli fied pol -len di a grams from B³êdowo (Biñka et al. 1991) (Fig. 3),cen tral Po land, and from many other sites e.g. Lake Suszek(Miotk-Szpiganowicz 1992), Lake Steklin (Noryœkiewicz1982), Lake Woryty (Pawlikowski et al. 1982) and Lake£ukcze (Ba³aga 1982), where the higher curve of brackenstarted as early as 8000 BP, show this pat tern. At B³êdowo inthe mid-Ho lo cene, bracken reached the max i mum of 1% to3% (even to 6% in one sam ple). In the se quences out side Po -land, e.g. in Fin land, which were sam pled with higher res o lu -tion, we can also see the same scheme (Ojala, Alenius 2005,Heikkilä, Seppä 2003).

DIS CUS SION

There are a few fea tures of bracken, which are im por tantfor palynological in ves ti ga tion. In the life cy cle of brackentwo fea tures are es sen tial – the veg e ta tive re pro duc tion ofPteridium by rhi zomes form ing con stantly ex pand ing pat-ches, and the ex pan sion into new ter ri to ries by spores dis -persed by wind, where new clones are es tab lished. Ad mit -

tedly, the ger mi na tion of spores in a lab o ra tory un der var i ousgrowth pa ram e ters is high (Marrs and Watt 2006), but Oino-nen (1967) pointed out that un der nat u ral con di tions this pro -cess is pos si ble al most ex clu sively in the pres ence of charredbio mass or ash al low ing ger mi na tion. For such re la tion ships,see Page (1986) and Oberdorfer (1990). This means that ger -mi na tion of spores in the pres ence of ash al lows the found ingof new pop u la tions and their fur ther veg e ta tive ex pan sion,and as an ef fect the higher pro duc tion and dis tri bu tion ofspores. This close link be tween spores of bracken and fire ises pe cially im por tant for palynological con sid er ation.

Re cent ob ser va tions of bracken have shown that it, oncees tab lished, re mains un changed un der the closed tree can opy for sev eral years (Doll ing 1999). Oinonen (1967) ob servedlong-lived clones, which were rel a tively static dur ing a 20years in ves ti ga tion. This sta tus quo can be dis turbed bychanges in the light con di tions, for ex am ple bracken in vadesnew grounds rap idly on clear-cut ar eas, how ever, fronds ex -pand only where rhi zomes are pres ent; oth er wise the ex pan -sion is slow (Doll ing 1999). Sim i larly, nat u ral dis tur bancessuch as fires or gaps cause an in crease in fronds den sity. In ar -eas with more hu mid cli mate e.g. in Eng land, Pteridiumforms per ma nent patches out side the wood land hab i tat,which are hard to erad i cate and to con trol. This is re garded asan in ter me di ate stage to wards wood land (Marrs et al. 2000).In Po land, bracken can per sist only a short pe riod in openspaces and is removed quickly in successional stages.

From a palynological point of view, it is no tice able thatthe shade from a dense frond can opy and thick leaf-lit terlayer may be im por tant bar ri ers to the es tab lish ment of young trees. Doll ing (1996) noted in Swe den a high mor tal ity ofpine in the bracken clones, both when seed ing ex per i mentswere con ducted and when seed lings were planted. The mor -tal ity of Picea abies in turn was very low. Marrs and Hicks(1986), Marrs (1987a,b), Marrs et al. (2000) ob served intheir stud ies in Eng land that seed lings of both Betula andPinus sylvestris per sisted un der a bracken can opy, but did somore will ingly when frond densities were lower.

The Pleis to cene pol len di a grams show that like to daythis plant did not oc cur in the non-tree pol len phases, bothwith the cold steppe and the tun dra sig na tures. Its spores aresome times found in in ter vals with open veg e ta tion but withthe pres ence of re worked pol len de rived from inter gla cialsed i ments. Also trans port via wind from the south to open ar -eas is not ex cluded, be cause spores are noted in mod ern pol -len rain (Lacey, McCartney 1994). Within inter gla cial sequ-ences spores are rather sparsely re ported (about 1–2 spores/1000 in some sam ples), and some times the con tin u ouscurves (about 1% or more) are rarely formed. Also the highfre quency of spores is a rare phe nom e non and lim ited only tothe Ho lo cene and a few Pleis to cene sites. Thus, in the inter -gla cial for est Pteridium is noted rather rarely. New cloneswere gen er ated due to lo cal wildfires and quickly af ter thesenat u ral clear ings were shaded out in the sub se quent succe-ssional tree stages. As a re sult, patches of bracken con sid er -ably de creased or were elim i nated if the new dis tur bancessuch as nat u ral fires, wind throws, and gaps dur ing the for estre gen er a tion cy cle did not taken place. These con clu sions are partly con sis tent with that drawn by Mc Vean (1958, alsoMarrs, Watt 2006) for the early Ho lo cene. Ac cord ing to his



Fig. 3. Sim pli fied pol len di a gram from Lake B³êdowo (Ho lo cene) af ter Biñka et al. (1991), mod i fied.

opin ion, de spite the suit able hab i tat and cli mate for bracken,cli max wood lands were un fa vor able for the es tab lish ment ofPteridium in any way and it was ex cluded by com pe ti tion.Thus, this plant can per sist only when nat u ral gaps fol low ingdis tur bances are formed. Nat u ral fires were, in our opinion,

the key process allowing the persistence of bracken as asubordinate component in the primeval forest.

Against this, the in ter vals of higher con tent of brackenspores are in trigu ing, es pe cially these at Ossówka, be causeof their cyclicity, their mag ni tude and their po si tion in suc-


Fig. 4. Sim pli fied pol len di a gram from Nidzica site (Eemian) af ter Biñka et al. (2011), mod i fied.

cessional stages. Two fac tors in flu enced the dy nam ics ofbracken at Ossówka: fires which trig gered the ini ti a tion ofyoung in di vid u als and their veg e ta tive in va sion in the birchzone (the in crease of spores in the spec tra), and di min ish ingof com pe ti tion from the pine for est, which grad u ally sup-pressed the ex pan sion of patches and fi nally elim i nated themfrom com mu ni ties (the con tin u ous de cline of the brackencurve). The ex per i ments men tioned above (Marrs, Hicks1986; Marrs 1987a, b; Marrs et al. 2000; Doll ing 1996) showthat the elimination of bracken, although difficult, ispossible.

In creased fire dis tur bances may have con trib uted tohigher Pteridium abun dances, though it is dif fi cult to as sessthe cause of this fire-re gime shift. The anal y sis of the dis tri -bu tion of light ning-ig nited fires in Fin land, in ar eas withmod ern veg e ta tion sim i lar to that at Ossówka in pol len zonesO-3 and O-5, showed that de spite strike den sity that wastwice as high in the north ern ar eas than in south ern ones, nat -u ral fires were reg is tered less of ten in north ern Fin land, be -cause of more ‘wet‘ fuel (Larjavaara et al. 2005). What ismore, in the for est of north ern Fin land, which serves as an an -a logue of the birch zone at Ossówka, the fre quency and sizeof the bracken patches clearly de creased and they showedpoor sporulation (Oinonnen 1967). This is not the only dif -fer ence. Oinonen (1967) ob served that sporulation wasrather rare even in warmer re gions of Fin land. Hence, it is notex cluded that new clones could have been ini ti ated fromspores trans ported by wind from more south ern ar eas. Thuscli ma tic con di tions in the birch zone at Ossówka would bedif fer ent to those in north ern Fin land. At this site, the onlyforc ing mech a nism seems to have been ex treme strikes andthe pres ence of larch, a ge nus un der es ti mated in the pol lenrain as a po ten tial source of fuel. Ex treme strikes and in tensefor est fires, lim ited to tem po ral and ter ri to rial scope werenoted, e.g. in north ern Fin land dur ing the sea son 1924–1927(Larjavaara et al. 2005). They caused a six times greater in -crease in wildfires than in later pe ri ods. This time span is tooshort to ex plain the long last ing pres ence of bracken atOssówka.

A close an a logue to the hy poth e sized sce nario at Os-sówka would be the dis tri bu tion of light ning strikes at in te -rior Alaska in the bound ary be tween the bo real zone and thetun dra (Dissing, Verbyla 2003), where wildfires af fect thelarge for ested ar eas (Wendler et al. 2010). Here the high estlight ning strike den si ties fall within a 10–25 km zone of bo -real for est close to the open veg e ta tion in this area. De cid edly lower light ning fre quen cies are ob served in the tun dra or inthe tran si tional zone of shrub veg e ta tion. How ever, also inthis case most of the strikes have taken place in the nar rowtree-less zone ad ja cent to the bo real for est. It is in ter est ingthat with the pro gres sion from a mar i time to a con ti nen tal cli -mate the num ber of strikes is mark edly higher in the for estedar eas and low and sta ble in the tun dra zone. The range ofmean tem per a tures for Alaska in te rior in clud ing bo real for -est, the area in ves ti gated by Dissing and Verbyla (2003), var -ies from about –14°C in the cold est month to about 13°C inthe warm est one in mar i time re gions (e.g. at Bethel), andfrom –20°C to –24°C and about 16°C re spec tively in thewestern areas with a more continental climate (Shulski,Wendler 2007).

On the ba sis of our pol len and spore ev i dence, es pe ciallyLarix, we thus hy poth e size that cli ma tic con di tions at Os-sówka were con ti nen tal with ex treme sea sonal vari a tion anddry sum mers. Be sides pro mot ing the avail abil ity of dry fu elsthis sit u a tion may have con trib uted to more light ning, in duc -ing more fires dur ing the ini tial stages of the two Saalianinterstadials (pollen zones O-3 and O-5).

Some sim i lar ity to these ex treme events can be seen inthe high level of charcoals re ported from the Late Gla cial. Inthe Neth er lands charcoals or even charred pine trunks, com -monly found at the top of AllerÝd soils (Usselo layer), arelinked with the ac cess to the large masses of dry, dead woodfrom the AllerÝd age, which re sulted from Youn ger Dryascool ing (Hammen and Geel 2008) and nat u ral fires. In Po -land, abun dant charcoals were found in the same po si tion,e.g. in Wolin Is land. At this site, how ever, the ac tiv ity of theLate Paleolithic man might have con trib uted to such an in -crease in the char coal in flux (Lata³owa and Borówka 2006).In North Amer ica sim i lar char coal peaks in the se quences atthe on set of the Youn ger Dryas are as so ci ated with a hy poth -e sized comet im pact and as a con se quence of the ex tinc tionof the Pleis to cene megafauna (Firestone et al. 2007). Closein spec tion of many sites by Marlon et al. (2009) showed thatthrough out the Late Gla cial an in crease in fire ac tiv ity iscom monly reg is tered in North Amer ica but large-scale fireevents rather co in cided with the rapid cli mate change, withthe ex cep tion of the AllerÝd/Youn ger Dryas bound ary.

We think, that cli ma tic pat tern (maitime?) dur ing theabrupt cli mate change and/or the mi gra tion rate of Pteridiumare the rea sons, that wild fire events at the scale ob served atOssówka based on bracken are not reg is tered in Po land dur -ing the tran si tion from tree-less phases to bo real for est in thelast 500,000 years. In Po land, such tran si tional veg e ta tion inthis time span is noted in the pol len di a grams at least 27times, based pre dom i nantly on the long pollen sequences.

A sub se quent more abun dant oc cur rence of Pteridiumspores is noted in the Holsteinian intra-inter gla cial cool ing,well de fined in the se quence at Kali³ów (Biñka and Nity-choruk 1996). How ever, a higher curve (about 2% in eachsam ple) ap peared in the late yew pol len zone, in flu enced bythe mar i time cli mate and con tin ued through out the wholelength of the pine zone, mark ings the shift to wards a morecon ti nen tal cli mate (Biñka and Nitychoruk 1996). In the lastcase the dry bio mass of the for est floor in the sum mer wouldnot have been the po ten tial rea son for nat u ral fires be causespores oc curred con tin u ously both in the more ‘wet’ and themore ‘dry‘ zones. Ob vi ously then, the ex treme strikes ratherthan the co nif er ous for est were likely to be the main forcewhich influenced the fire regime in this case.

In the Holsteinian the scarce ap pear ance of brackenspores in the Picea zone (be fore the yew phase) is sig nif i cant, be cause this means that wild fires do not show a clear pref er -ence for such a for est. In the Eemian in turn, an in creased fre -quency of bracken in the spruce zone (e.g. at Nidzica, Fig. 4)is es pe cially in ter est ing be cause in this in ter val the veg e ta -tive spread of patches of bracken seems to be more dif fi cult.Oinonen (1967) noted, that dense tree stands, par tic u larlyspruce are able to weaken and shat ter the bracken stands andgrad u ally per haps to erad i cate it com pletely from the site. Inthis in ter val a de crease in sum mer and win ter tem per a tures as


well as an in crease in pre cip i ta tion is hy poth e sized (Zagwijn1996; Kühl, Litt 2003). Be cause nat u ral fires are the prod uctof a few driv ing forces – fuel mois ture in the late spring/sum -mer, light ning den sity or the type of for est (de cid u ous andco nif er ous), there fore the last two forces seem to have beenthe only fea tures fa vor ing fires in the Eemian Picea zone.Also in this case it is worth stress ing that de spite the highmor tal ity of Pinus in patches of bracken (Doll ing 1996), theex pan sion of Pteridium was fi nally sup pressed by the pinefor est in the se quence from Nidzica. Spruce in turn, seed lings of which can per sist un der the bracken can opy and later shat -ter the patches, shows a curve de cline dur ing bracken cul mi -na tion. This de pres sion and sud den peak in Picea ver sus thegrad ual and qui es cent rise in Pinus speaks for the view that itwas the fires, which caused the re sult rather the spruce for est.The higher mag ni tude of light ning strikes in the Picea zone,be ing an ef fect of weather fa vor ing thun der storms, seems tohave been more broadly dis trib uted geo graph i cally, as wecan in fer from Andersen‘s pol len se quence from Den mark(Björck et al. 2000) with similar bracken dynamics to those at Nidzica.

The Eemian oak pol len zone is an other in ter val with a lit -tle higher fre quency of bracken spores. High sum mer tem -per a tures and the oak for est ac com pa nied by Pinus are surelysup port ing fac tors in fire de vel op ment. In the same veg e ta -tion sce nario in the old est inter gla cial (Ferdynandovian), asim i lar pro por tion of bracken spores is noted be fore the firsttem per ate cli ma tic op ti mum (Pidek 2003) with the for est re -sem bling Eemian oak-pine veg e ta tion. In the same po si tionwithin inter gla cial suc ces sion and sim i lar veg e ta tion of pine-oak-larch for est (Krupiñski et al. 1987) found a very highcurve, ap prox i mately 10% of bracken. The age of this tem-perate interval is however problematic.

Judg ing from the broader geo graph ical range of somebracken phases in the Pleis to cene, a nat u ral ig ni tion due tolight ning rather than hu man ac tiv ity seems to be re spon si blefor fires. The case of post inter gla cial fire events at Ossówkacould be ex cep tional. The large, deep lakes at this time arerarely noted. They must have been places where nu mer ousan i mals came from the vi cin ity look ing for food and water.

This, com bined with fires, must have at tracted Palaeo -lithic man. It is worth men tion ing that at the clas si cal site atHoxne there is def i nite ev i dence that peo ple were pres entclose to the inter gla cial lake (West and Mc Burney 1954).

There is no doubt, that in light of the Pleis to cene re cords,the mid-Ho lo cene max ima of bracken were due to hu man ac -tiv ity (burn ing). This con clu sion is based on sev eral grounds:

1 – the max i mum spread of bracken, hav ing taken placewhen the cli mate be came wet ter in the so called At lan tic pe -riod, an in ter val of rather fire proof con di tions,

2 – the dom i nance of the de cid u ous for est at this timewhich are also fire proof,

3 – the higher fre quency of Pteridium, which is rare inthe Pleis to cene inter gla cial se quences in the stud ied area, ex -cept from some sites an a lyzed in this work,

4 – the shape of the curves of Pteridium, which showssud den peaks in the Ho lo cene (Madeja et al. 2004) againstrel a tively smooth curves in the Pleis to cene, e.g. at Ossówkaor at Nidzica.

In Po land, the mid dle Ho lo cene peaks of bracken are

com monly re lated to the Late Mesolithic and the Early Neo -lithic econ omy, in which fire was a use ful tool for de for es ta -tion (Lata³owa 1992; Ralska-Jasiewiczowa and van Geel1998; Madeja et al. 2004). From the post Neo lithic on wardsthe con tin u ous in crease in herbs, which sug gests the pres -ence of open-can o pied wood lands or open spaces as a re sultof for est clear ing and clear ance for cul ti va tion, gen er ally didnot pro duce an in crease in bracken spores. This im plies theuse of clear ing meth ods other than fire. Where burn ing wasin volved the fi nal ef fect had to re sult in a greater area oc cu -pied by bracken as com pared to when only man ual clear ingswere prac ticed be cause, apart from re-veg e ta tion from un -der ground rhi zomes, new clones were es tab lished from ger -mi nat ing spores. This means that even an in creas ingly openfor est since the Neo lithic (a con tin u ous in crease in light de -mand ing pol len types – Biñka et al. 1991), which shouldhave en cour age veg e ta tive ex pan sion of this fern, did not re -strain the bracken de cline when the suit able hab i tats, rich inash, de creased. An other pos si ble rea son for the de crease inbracken spores might have been cat tle graz ing in the for est,which in Po land has been com mon prac tice as a sup ple men -tary ac tiv ity to farm land graz ing even af ter the Sec ond World War. Thus, tram pling im pacts and bracken fronds eaten bycattle could have been a means of controlling this plant in thepast (Marrs, Watt 2006).

In some ar eas of West ern Eu rope the above sce nariomight have been dif fer ent. Not only this, the scale of the dis -tur bance by hunter-gath er ers of the Late Mesolithic in thisarea to the cli max wood lands was, in con trast to Po land, lowas it is ev i denced by Pteridium (Hunt ley, Birks 1983). InEng land where bracken, form ing dense, long last ing patches, oc curs abun dantly in many open com mu ni ties such as grass -lands or heaths (Marrs, Watt 2006), the high amount ofspores in palynological re cords from the Neo lithic (Hunt leyand Birks 1983) can be both a re sult of for est clear ing (burn -ing) and/or in va sion into newly cleared ground af ter de for es -ta tion. In such open ar eas the pro duc tion of spores in non-shaded con di tions is high (Dring 1965) and the dis persalrange of bracken spores is wider than in wood lands. For these rea sons there can be an abun dance of air borne spores deposi-ted finally into the lakes.

It is clear, that the dis tance from the late gla cial refugia in south ern Eu rope (Hunt ley, Birks 1983), the type of burn ingac tiv ity, the ter rain mor phol ogy or a dif fer ent pat tern of set -tle ment phases are the real rea sons, the time span of brackenmax ima in se quences from Eu rope must be more dif fer en ti -ated. For ex am ple, in the south ern Alps a higher con tent ofPteridium is found around 6000 BP; how ever it peaks at alater time (Tinner et al. 1999; 2000). At a nearby site Lago diAnnone (Wick, Möhl 2006) spores were more abun dantsince 7000 cal BP and in the south ern Pre-alps (Lago diFimon) bracken cul mi nates like in Po land (Valsecchi et al.2008).

In the Ho lo cene, bracken spores were some times ac com -pa nied by charcoals. In Ger many, ac cord ing to Kalisa et al.(2003) just af ter the At lan tic pe riod, pro nounced peaks ofchar coal were noted de spite the es tab lish ment of fire proofde cid u ous for est at that time. The ob ser va tion of Tinner et al.(1999, 2000) as well as other re ports (Bos, Urz 2003) provethat the pres ence of Pteridium and char coal may show a pos i -


tive cor re la tion, a phe nom e non ob served by Odgaard (1992)in the case of Calluna, where a higher abun dance of pol lenlargely de pends on fire, but where the heathland is formedonly. Be cause the trans port and de po si tion of char coal par ti -cles is a more com plex pro cess (Condera et al. 2009) such aco in ci dence of char coal and bracken spores has not al waysbeen doc u mented. In south ern Swe den, the fire re gime in theHo lo cene which was iden ti fied us ing a char coal method(Greisman and Gaillard 2009; Olsson et al. 2010), is partlyco in ci dent with the one in Po land based on bracken. How -ever, in Swe den Pteridium is al most ab sent in phases of highfire ac tiv ity with the ex cep tion of one sam ple with an un usual frequency of spores, suggesting the location of plants justnear a peat bog.

It is in ter est ing that some work ers re con struct ing the Ho -lo cene fire re gime see cli mate as a pri mary force. Ac cord ingto Olsson et al. (2010) peaks of char coal re corded on sites insouth ern Swe den were con trolled by the cli mate (warmer and drier phases) of the early/mid dle Ho lo cene rather than by hu -man in duc tion. Oth ers, e.g. Hunt ley and Birks (1983) as wellas Tinner et al. (1999, 2000), con sider the com bined ef fectsof hu man ac tiv ity and cli mate as the trig ger ing fac tors forHo lo cene fires, based on bracken occurrences.

In Po land re cords for bracken from the Pleis to cene andHo lo cene clearly show that dur ing the lat ter for est fires maybe strongly con sid ered due to burn ing by hu mans. In morewest ern ar eas other for estry prac tices and the ex pan sion ofbracken into open grounds would be partly re spon si ble forbracken cul mi na tion (Pakeman 2000; Hunt ley, Birks 1983),which is, in our opin ion an ef fect of dif fer ent cli ma tic-flo-ristic conditions.

FI NAL CON CLU SIONAnal y sis of the Qua ter nary oc cur rence of Pteridium

spores in lac us trine se quences seems to point to a dif fer entsce nario to ex plain the ex pan sion and de cline of this plant inthe for ested ar eas of the Pleis to cene and the Ho lo cene. Gen -er ally, the spo radic ap pear ance of bracken spores through outthe inter gla cial – interstadial se quences un der the tem per ateand bo real cli mates as well as in the Early Ho lo cene is typ i calfor most sites in Po land. In the Pleis to cene the pe ri ods of ahigher (or high) pro por tion of Pteridium spores in the inter -gla cial/interstadial phases – some times with a broader geo -graph ical range – are rather lim ited to in ter vals with a highercon tent of co ni fers. Fires were ini ti ated by nat u ral ig ni tiondue to light ning strikes dur ing thun der storms un der a spe -cific cli mate, prob a bly with a con ti nen tal sig na ture, with sea -sonal vari a tions and dry sum mers. If this trig ger ing mecha-nism, which gen er ates new patches, was to dis ap pear, thebracken stage, be ing a tran si tional successional stage wouldfi nally be elim i nated from veg e ta tion. In the mid-Ho lo cenein turn, the cul mi na tion of bracken shows that it peaked incon junc tion with wood land clear ance by burn ing and not incon junc tion with fa vor able cli ma tic con di tions. There fore,hu man ac tiv ity was the main force be hind the higher occu-rrence of Pteridium at that time.

Ac knowl edg ment

This re search was funded by Min is try of Sci ence and HigherEd u ca tion grant No. N 307 508438 in 2010–2013.

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