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Avhandling - LUNDQUA Thesis 39


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Min avhandling

Björkman, L. 1996: The Late Holocene history of beech Fagus sylvatica and Norway spruce Picea abies at stand-scale in southern Sweden. LUNDQUA Thesis 39, 1–44. (+4 app.)

Min avhandling i kvartärgeologi, som försvarades den 6 december 1996, är en sammanläggningsavhandling. Den består av fyra artiklar och en syntes. De fyra ingående artiklarna är:

Björkman, L. & Bradshaw, R. 1996: The immigration of Fagus sylvatica L. and Picea abies (L.) Karst. into a natural forest stand in southern Sweden during the last 2000 years. Journal of Biogeography 23, 235–244. (Appendix I)

Björkman, L. 1996: Long-term population dynamics of Fagus sylvatica at the northern limits of its distribution in southern Sweden: a palaeoecological study. The Holocene 6, 225–234. (Appendix II)

Björkman, L. 1997: The role of human disturbance in the local Late Holocene establishment of Fagus and Picea forests at Flahult, western Småland, southern Sweden. Vegetation History and Archaeobotany 6, 79–90. (Appendix III)

Björkman, L. 1997: The history of Fagus forest in southwestern Sweden during the last 1500 years. The Holocene 7, 419–432. (Appendix IV)

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Abstract

High resolution pollen analysis was carried out on five peat profiles from small forest hollows at four sites in southern Sweden. The general aim was to investigate the establishment of Fagus sylvatica and Picea abies at stand-scale. Additionally, the aim was also to reveal the composition of the forests before these species immigrated. The sites used for this study (Bocksten in Halland; Flahult, Mattarp, and Siggaboda in Småland) were all located within the area where the present distribution limits of Fagus and Picea overlap each other. All forest hollows used to reconstruct the past vegetation have relatively small pollen source areas. The results from the different sites are presented in separate papers (Appendices I–IV). These results are compared and discussed in more detail in this synthesis.

Viewed on a continental scale the migration pattern of Fagus can be correlated with climate and its change over the millennia, but at finer scales such a correlation is weaker. At stand-scale there are factors other than climate that are crucial for the establishment of Fagus (e.g., disturbance, seed dispersal, human activities). The establishment of Fagus does not show a regional coherence in southern Sweden, and this may imply that climate was not the limiting factor for its establishment.

The present day distribution of Fagus in southern Sweden suggests a migration with a discontinuous front with outlying populations, and this model probably applies to its past distribution. This type of migration means that the landscape becomes infilled by dispersal from outpost stands. The timing of stand-scale establishment is then largely influenced by site-specific factors and chance. Fagus may still be migrating northwards in Sweden. It grows well in its outpost area, and it seems that present day land-use, not climate, is the limiting factor for local Fagus expansion. The northern distribution limits of Fagus probably still represents an active front, and outlying stands acts as "infection centres."

Fagus seeds are highly dependent on ground disturbance for successful establishment, and an undisturbed forest then consequently would be able to resist Fagus invasion for some time. A semi-open cultural landscape may be optimal for Fagus establishment, as cultural activities may create conditions particularly suitable for its regeneration. At two of the studied sites cultural activities seems to have created conditions that favoured establishment of Fagus (Mattarp at 400 BP, Flahult at 900 BP). At the other studied sites the local forest stands seem to have been relatively unaffected by cultural activities prior to the establishment of Fagus (Siggaboda at 950 BP, Bocksten at 1450 BP), at least no semi-open vegetation was present locally. At Siggaboda the pre-Fagus vegetation was dominated by Quercus, but Corylus and Tilia were also present. At this site fires (human-induced?) probably facilitated establishment and expansion of Fagus. At Bocksten the pre-Fagus vegetation was dominated by Quercus, Tilia, Alnus, and Corylus.

Picea invaded southern Sweden from the north during a period when the cultural landscape had already been evolving for some time. Picea is a dominant tree with an effective seed dispersal, and the relatively open and probably grazed forests in the area were not particular resistant to Picea invasion. An intensive grazing regime may not affect Picea, as grazing animals normally avoid Picea.

The timing of local Picea establishment seems to be mostly controlled by its migration, i.e., it became established when its front reached the studied sites. Picea invaded Mattarp c. 800 BP, but did not expand much locally until 400 BP, in connection with the establishment of Fagus. Picea invaded Siggaboda at about 200 BP. The population increase for Picea was very rapid and within c. 50 years it co-dominated the local forest stand together with Fagus. The establishment of Picea at the other studied sites was late. At Flahult Picea became established in the local forest stand at c. 100 BP, but it has not yet come to dominate the vegetation. At Bocksten Picea was probably planted, or self-sown from nearby plantations.

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Svensk sammanfattning

Denna avhandling är resultatet av min doktorandutbildning vid Lunds universitet. Avhandlingen baseras på pollenanlytiska undersökningar av ett antal lokaler i södra Sverige. I denna syntes sammanfattas resultaten av undersökningarna.

Introduktion och bakgrund

Avsikten med avhandlingen har varit att studera etableringen av bok (Fagus sylvatica) och gran (Picea abies) på beståndsnivå i södra Sverige. Dessa träd har invandrat i relativt sen tid. Boken har invandrat från söder, och de första bokarna nådde troligen Skåne för lite mer än 3500 år sedan. Det var dock först långt senare, för omkring 1500 år sedan, som boken blev ett vanligt träd i sydligaste Sverige.

Granen har däremot invandrat från norr. De första granarna vandrade in i norra Sverige för 4000–5000 år sedan. Granen har därefter vandrat söderut, och först ganska nyligen nått ned till sydligaste Sverige. Utifrån historiskt källmaterial, t ex kartor och ägobeskrivningar, har man lyckats belägga när de första granarna började att uppträda i norra Skåne. Man har visat att detta skedde under slutet av 1500-talet och början av 1600-talet.

Som källmaterial för min undersökning har jag använt fossila pollen som bevarats i små torvmarker (dvs i små mossar och kärr). Att jag har använt mig av små torvmarker medför att den lokala skogsutvecklingen kan rekonstrueras. De pollenkorn som deponeras på en liten torvmark har i allmänhet inte transporteras så långt från källan. Större lokaler har tyvärr den nackdelen att de pollen som deponeras har transporterats från större avstånd, och dessutom deponeras pollen från många olika vegetationstyper samtidigt. Därför visar ett pollendiagram från en större lokal i allmänhet vegetationsutvecklingen för ett större område. Om man vill beskriva den lokala vegetationsutvecklingen inom ett speciellt skogsbestånd måste små lokaler användas.

Mitt huvudsakliga undersökningsområde har varit den region i södra Sverige där bokens och granens nutida utbredningsområden överlappar varandra. Inom detta område har fyra lokaler utvalts. Lokalerna är Siggaboda, Flahult och Mattarp i Småland, och Bocksten i Halland (se fig. 1). På dessa lokaler har pollendiagram upprättats för att man skall kunna beskriva och diskutera den lokala vegetationsutvecklingen. Kritiska nivåer i pollendiagrammen har daterats med hjälp av 14C-metoden. Resultaten av dessa undersökningar finns redovisade i de bifogade artiklarna (Appendix I–IV). Fullständiga pollendiagram finns också redovisade (Plate 1–5). I de ursprungliga artiklarna har endast förenklade pollendiagram presenterats.

Resultat

I Appendix I redovisas resultatet av en undersökning av Siggaboda naturreservat (tidigare Stensjönäs domänreservat) i sydligaste Småland (fig. 1, se också fig. 4–8). Reservatet domineras i dag av bok och gran, antingen växande i blandbestånd, eller i mer eller mindre rena grupper. Reservatet har tidigare uppmärksammats för sin rika lavflora och insektsfauna, där flera sällsynta och hotade arter indikerar lång skoglig kontinuitet. I reservatets centrala delar finns knappast några tecken på att mänskliga ingrepp gjorts under de senaste århundradena. I reservatets kantzon finns en del stubbar som visar att det förekommit en begränsad trädfällning under detta århundrade. Förekomsten av sällsynta och hotade skogsarter i reservatet står i stark kontrast till avsaknaden av sådana i det omgivande landskapet, som till största delen utgörs att rationellt skötta gran- och tallbestånd.

Målsättningen med studien var dels att försöka datera den lokala etableringen av bok och gran, dels att försöka utröna om den nuvarande skogstypen har lång kontinuitet. En pollenanalys utfördes på en lagerföljd från en liten torvmark som ligger centralt i reservatet (Plate 1). Resultatet av analysen visar att boken etablerades på lokalen för ungefär 950 år sedan i samband med en brand. För ungefär 350 år sedan inträffade ytterligare en brand som i sin tur ledde till att skogsbeståndet kom att domineras av bok och ek under en period av ungefär 150 år. För ungefär 200 år sedan började det lokala ekbeståndet att minska, och kort tid därefter försvann eken helt från området. Inga gamla ekar finns kvar i det nuvarande beståndet vilket gör det troligt att den försvann som en effekt av selektiv huggning. Vid ungefär samma tidpunkt som eken började att minska etablerades granen på lokalen. De äldsta granarna i reservatet är ca 200 år gamla, och dessa etablerades troligen när den framryckande granfronten nådde fram till området.

Den nuvarande skogstypen med en blandning av bok och gran utvecklades först under de senaste 200 åren. Pollendiagrammet från den analyserade delen av lagerföljden täcker ungefär 2800 år. Under hela denna period domineras pollenspektrumen av trädpollen, vilket indikerar att reservatet haft lång skoglig kontinuitet. Denna kontinuitet är sannolikt en av förklaringarna till den nutida förekomsten av sällsynta och hotade arter. Den långa skogliga kontinuiteten innebär dock inte att det rådit stabila förhållanden i skogen. Reservatet har utsatts för åtminstone två bränder och genomgått kraftiga förändringar av trädartssammansättningen under de senaste 1000 åren.

I Appendix II redovisas resultatet av en undersökning av ett litet utpostbestånd av bok vid Mattarp (Mattarps bokdunge) i norra Småland (fig. 1, se också fig. 9–10). Målsättningen med denna studie var att datera bokens och granens lokala etablering, och att avgöra om boken fortfarande håller på att expandera. Om boken fortfarande befinner sig under spridning på det Småländska höglandet utgör knappast klimatet en begränsande faktor för dess vidare expansion norrut.

I direkt anslutning till den studerade bokdungen finns en mindre torvmark, vars lagerföljd har använts till en pollenanalys (Plate 2). Resultatet av analysen visade att boken etablerades på lokalen för ungefär 400 år sedan. Bokens etablering skedde i samband med en lokal röjning av skog. Det verkar varit lokala faktorer som kontrollerade etableringen av bok på denna lokal. Klimatet kan knappast ha varit en bakomliggande faktor för bokens etablering, eftersom det troligen varit gynnsamt för boken i detta område under lång tid. Det är fullt möjligt att enstaka bokar fanns spridda i skogarna på det Småländska höglandet redan för 1000 år sedan, men boken lyckades inte sprida sig till den studerade lokalen förrän för 400 år sedan.

Granen etablerades på lokalen för ungefär 800 år sedan, men en expansion ägde inte rum förrän för 400 år sedan i samband med bokens etablering. Det är fullt möjligt att den lokala markanvändningen gynnade både boken och granen vid denna tidpunkt. Antalet bokpollen som deponerats i den studerade lagerföljden har ökat kraftigt under hela den period som boken varit etablerad på lokalen. Detta förhållande pekar på att bokbeståndet hela tiden har ökat i omfattning, och troligen skulle det fortsätta att öka om detta inte förhindrades av den nuvarande markanvändningen.

I Appendix III redovisas resultatet av en undersökning av ett skogsområde nära Bocksten i centrala Halland (fig. 1, se också fig. 11–14). I detta område, som tillhör det sk Centralhalländska bokskogsområdet, finns stora sammanhängande bokskogar. Skogarna vid Bocksten domineras i dag till stor del av rationellt skötta bok- och granskogar. Granskogarna är i de flesta fallen mycket unga. Området låg under början av detta sekel strax utanför granskogens naturliga västgräns.

Målsättningen med denna studie har varit att undersöka när boken etablerades, samt att studera hur områdets skogar såg ut innan boken invandrade. I det studerade området finns många torvmarker som är lämpliga för paleoekologiska studier. På grund av detta förhållande kom två närbelägna torvmarker att undersökas för om möjligt påvisa beståndsmässiga skillnader i skogens struktur och sammansättning.

De två pollendiagrammen (Plate 3 och 4) visade att området dominerades av en rik nemoral skogstyp innan boken etablerades för ungefär 1500 år sedan. Ek, lind, al och hassel var viktiga arter i denna skogstyp. Bokens expansion var snabb och inom loppet av 100–200 år dominerade den det lokala beståndet. Det är fullt möjligt att den snabba expansionen på något sätt gynnades av mänskliga aktiviteter. Granen etablerades mycket sent på lokalen, troligen skedde detta under slutet av 1800-talet eller början av detta århundrade. Granens etablering skedde troligen i samband med att ett närbeläget torp övergavs. Det är högst troligt att huvuddelen av granbestånden i området är planterade. Dagens homogena bokbestånd (och granbestånd) i området är troligen en produkt av skogsskötsel under detta århundrade.

Det ena av de upprättade pollendiagrammen (lokal A, Plate 3) visar att det fläckvis funnits kvarvarande nemorala skogsbestånd så sent som för 200 år sedan. De två pollendiagrammen påvisade också att det förekommit skillnader i sammansättning mellan näraliggande bestånd, och att människans påverkan på vegetationen skiftat från bestånd till bestånd. Vid lokal A (som låg längre ifrån en torpbebyggelse än lokal B) växte boken i ett blandbestånd tillsammans med lind, ek och hassel under nästan 1200 år, fram tills beståndet röjdes för ungefär 200 år sedan. Vid lokal B (Plate 4) blev boken nästan helt dominerande redan snabbt efter att den etablerats. Lind och hassel försvann också i och med boketableringen.

I Appendix IV redovisas resultatet av en undersökning av ett bokbestånd vid Flahult, strax öster om sjön Bolmen i västra Småland (fig. 1, se också fig. 15–16). I dag är bokbestånd relativt vanliga i detta område, dessutom visar historiska dokument att boken var vanlig i området under sen medeltid. Målsättningen med denna studie var att datera den lokala etableringen av bok och gran, samt att studera hur det nuvarande bokbeståndets struktur uppkommit.

Centralt i det undersökta bokbeståndet finns ett litet alkärr vars lagerföljd har använts för en pollenanalys (Plate 5). Resultatet av analysen visade att boken etablerades i ett halvöppet kulturlandskap för ungefär 900 år sedan. Expansionen av kulturmarker inleddes ungefär samtidigt som bokens lokala etablering. Det var troligen den intensifierade markanvändningen som skapade förutsättningar för boken att etableras på denna lokal. Boken fanns möjligen i regionen redan tidigare, men den lyckades inte etableras sig i det studerade beståndet förrän lämpliga förutsättningar skapades för ungefär 900 år sedan.

Boken dominerar helt det nutida beståndet, men denna dominans verkar ha ett sentida ursprung. Bokpollenfrekvensen stiger kraftigt först under de senaste 50-100 åren. Det nutida beståndets sammansättning och struktur är troligen en effekt av markanvändningsförändringar under de senaste 100 åren. Granen etablerades i det lokala beståndet för ungefär 100 år sedan, men den har ännu inte fått någon större betydelse. I dag finns endast ett fåtal granar spridda i bokbeståndet. Granen har däremot fått större betydelse i områdets skogar under senare tid, troligen som en effekt av skogsbruk under de senaste 50 åren.

Diskussion

De storskaliga holocena migrationsmönstren för träd i Europa verkar vara styrda av klimatet och dess förändringar över tiden. Detta gäller troligen också för bokens och granens storskaliga migrationsmönster. När man däremot betraktar bokens etablering och expansion på beståndsnivå verkar helt andra faktorer, t ex störningar, vara betydelsefulla.

Redan innan boken expanderade i södra Sverige var det regionala klimatet gynnsamt. Därför kan klimatet knappast anses vara den enda faktor som fick boken att etableras i södra Sverige. Bokens etablering påverkas av en rad faktorer där t ex den vegetationstyp som invaderas, störningsregimen (bränder, stormar, betestryck mm) och mänsklig påverkan, är betydelsefulla. Tidpunkten för etableringen av bok i ett bestånd påverkas av lokala faktorer, där mänskliga aktiviteter kan ha stor betydelse. Slumpfaktorer kan även spela in, t ex när bokollon råkar spridas till ett bestånd där det råder goda förutsättningar för etablering. Perioder med snabb bokexpansion sammanfaller ofta med perioder då intensiteten i markanvändningen avtar, eller då den förändras efter en mera intensiv period.

Vid tidpunkten för bokens invandring till södra Sverige var skogarna redan till stor del påverkade av människan. I en del områden (t ex Mattarp, Flahult) invandrade boken i ett halvöppet kulturlandskap, där vegetations sammansättning till stor del kontrollerades av mänskliga aktiviteter. Bokens etablering underlättades sannolikt där av den rika tillgången på störda marktyper. I det halvöppna kulturlandskapet fanns givetvis också skogsbestånd (som dominerades av björk, ek och hassel), men deras sammansättning styrdes till stor del av mänskliga aktiviteter (t ex genom betestrycket).

I en del områden (t ex Bocksten, Siggaboda) verkar skogsvegetationen varit relativt opåverkad av mänskliga aktiviteter innan bokan invandrade, men lokala störningar (naturliga, eller inducerade av människan) underlättade troligen etableringen. Innan boken etablerades dominerades de relativt opåverkade skogstyperna i Småland och Halland av ek, lind och hassel. Den exakta sammansättningen och strukturen varierade dock en del mellan bestånden på grund av lokala faktorer.

Det är fullt möjligt att boken fortfarande befinner sig på vandring norrut i Sverige. Det verkar som om bokpopulationerna på nordliga utpostlokaler är mycket livskraftiga och kan expandera lokalt om de tillåts till detta. Det verkar snarare som att det är den nuvarande markanvändningen, och inte klimatet, som sätter stopp för en vidare expansion. De nordliga utpostlokalerna kan sägas utgöra spridningskällor varifrån boken kan spridas till det omgivande landskapet.

Granens expansion påverkas också av lokala faktorer, men den regionala spridningen i södra Sverige verkar mera styrd av dess spridningshastighet än av förändringar i t ex markanvändningen. Den lokala expansionen av gran sker vid den tidpunkt då dess front når fram till området. När granen nådde sydligaste Sverige mötte den ett halvöppet kulturlandskap som sedan lång tid tillbaka var påverkat av människan. Den ursprungliga skogstypen var då till stor del redan ersatt med betesmarker och relativt glesa och betade skogar. Denna vegetation var inte speciellt motståndskraftig mot granen, varför den snabbt kunde expandera när den väl nådde fram till ett nytt område.

Boken och granen är båda dominanta trädarter och konkurrensen dem emellan blir därför mycket intensiv när de möts i samma bestånd. Tyvärr har de ännu inte vuxit tillsammans under någon längre tid på de studerade lokalerna, varför det är svårt att säga något om utgången av konkurrensen på längre sikt. På den sydligaste av de studerade lokalerna (Siggaboda) har de vuxit tillsammans under ca 200 år. Det är också på denna lokal som de i nutid konkurrerar intensivt med varandra. Åtminstone i beståndet allra närmast provpunkten (fig. 8) kan man notera att granen konkurrerat ut boken, men detta gäller troligen inte generellt för hela lokalen.

Syntes

Introduction

The composition and structure of Swedish woodlands have changed considerably throughout the Holocene. These changes have been caused by several factors that include the immigration of species, climatic change, competition, succession, soil processes, disturbance regimes, and more recently, cultural activities. In southern Sweden these changes have been highly dynamic and complex, especially since the latter part of the Middle Holocene. During this period two very competitive and potential forest dominants – beech, Fagus sylvatica, and Norway spruce, Picea abies – immigrated into Sweden from different directions. The immigration of these species has contributed to the great shift of woodland composition during the last few thousand years (e.g., Björse et al. 1996). This period also witnessed the development of the present-day cultural landscape. The reasons for these rather recent vegetation changes can be difficult to understand fully as they may have been caused by a combination of natural and cultural factors.

This thesis focuses on local forest history, particularly on the immigration and establishment of Fagus and Picea at the spatial scale of the forest stand in southern Sweden. Many earlier regional-scale pollen studies have established when these species became regionally abundant (e.g., Nilsson 1964; Berglund 1966, 1991; Digerfeldt 1972, 1982; Göransson 1977; Regnéll 1989; Lagerås 1996), but we still lack detailed information about their establishment, particularly in which type of vegetation they became established, and how this came about. This is difficult to interpret with any certainty from regional-scale pollen diagrams because of poor spatial resolution.

However, an alternative way to study this problem is to use small forest hollows, i.e., small sites with organogenic deposits (peat or gyttja) in close proximity to forest stands of interest. Another possibility is to use deposits of mor humus on the forest floor, but such deposits are not available in all forest types. Small sites, especially closed-canopy sites, have the advantage that the majority of the pollen that becomes deposited has not travelled very far (e.g., Andersen 1970; Jacobson & Bradshaw 1981; Heide & Bradshaw 1982; Bradshaw 1988; Chen 1988; Yazvenko 1991; Jackson & Wong 1994; Sugita 1994; Calcote 1995). A pollen diagram from a small site therefore gives a more local picture of the vegetation than diagrams from lakes or bogs. Small sites with peat or gyttja deposits, or mor humus, have been used successfully elsewhere to study local forest history, for instance in Denmark (e.g., Iversen 1964, 1969; Aaby 1983; Andersen 1984, 1988, 1989), Ireland (e.g., Mitchell 1988; Hannon & Bradshaw 1989), and USA (e.g., Bradshaw & Miller 1988; Davis et al. 1992, 1994; Sugita et al. 1994). This type of site has until recently not been used very often in Sweden. Some Swedish studies are however, worth mentioning (e.g., Bradshaw & Zackrisson 1990; Bradshaw & Hannon 1992; Bradshaw 1993; Segerström et al. 1994, 1996; Lindbladh & Bradshaw 1995), as they clearly have shown the potential of local-scale forest studies.

In order to investigate and discuss the immigration and establishment of Fagus and Picea at stand-scale in southern Sweden four sites with small forest hollows were selected for high resolution pollen analysis (cf. Bradshaw 1988). These sites are Siggaboda, Flahult, and Mattarp in Småland, and Bocksten in Halland (Fig. 1). At these sites five small hollows in total were investigated (two adjacent hollows were studied at Bocksten, in order to evaluate stand-scale differences within a small area). All forest hollows selected for this study lie in close proximity to present Fagus stands, or stands where Fagus is abundant. In addition, Picea is more or less common in the vicinity of all selected hollows, and at least single Picea individuals occur near to all hollows. A further reason why these sites were selected is that they are all situated within the area of overlap for the present distributions of Fagus and Picea. Within this area of overlap, Fagus and Picea have had the possibility of competing with each other for some time. Moreover, Fagus and Picea have immigrated into southern Sweden from different directions, and they have both reached their present distribution limits rather recently. The geographical distribution of the selected sites therefore gives a gradient throughout the area with overlap where these species most likely became established at different times.

To avoid confusion I will clarify that I have used the terms forest and woodland interchangeably throughout this synthesis as well as in the appendices, i.e., the term forest has not been used in the same strict sense as suggested by Rackham (1976). Additionally, all dates in this thesis are given in uncalibrated 14C years BP, unless otherwise stated.

Fig. 1. (A) The forest regions of southern Sweden according to Sjörs (1965). (B) Detailed map of southernmost Sweden showing the distribution limits for Fagus and Picea forests according to Lindquist (1931, 1959). Note that isolated Fagus stands occur north of the northern limits of Fagus-Picea forest as indicated in Fig. 1A. The studied sites are also indicated, as well as other local- and regional-sce pollen sites referred to in this synthesis (A-Y, see also Table 2).

Small vs. large palaeoecological sites

This section of the synthesis briefly summarises characteristics of different types of sites that may be useful for palaeoecological reconstruction.

As mentioned in the introduction, small sites have the advantage of having a relatively small pollen source area and they therefore give a more local-scale picture of the vegetation. Investigations of the relationship between pollen assemblages and vegetation have been an important task for paleoecologists, but it was not until rather recently that these relationships were examined in a quantitative manner (Prentice 1985).

Today, the opinion that the size of the sedimentary basin reflects the size of the pollen source area is well established (i.e., a forest hollow has a small source area and a large lake a large one). Many theoretical studies have indicated this relationship (e.g., Tauber 1965; Jacobson & Bradshaw 1981; Prentice 1985, 1988; Sugita 1993, 1994), which has recieved confirmation from studies of pollen/vegetation relationships (e.g., Berglund 1973; Tauber 1977; Bradshaw & Webb 1985; Jackson 1990; Yazvenko 1991; Sugita et al. 1994; Calcote 1995).

However, even if the general relationship is clear, many researchers differ in opinion when they try to define specifically the radius or diameter of a given pollen source area. High precision in this matter is probably unobtainable and arguably not of critical importance. Even if the pollen source area for a small site has a radius of 20, 50, or even 100 m, the main part of the pollen grains are still derived from the local vegetation (i.e., the local pollen signal is strong), and the pollen diagram constructed is valid for stand-scale interpretations. Sugita (1994) has presented a model predicting that the "relevant pollen source area" for a small hollow (radius = 2 m) is around 50–100 m. A small lake (radius = 50 m) has according to the same model a somewhat larger source area around 300–400 m. The "relevant pollen source area" is defined as the distance from a sampling site beyond which the pollen represent a constant background pollen rain and within which differences in plant abundance will be recorded as variance in pollen analyses among sites. Only 30–45% of the total pollen loading comes from within this area, but this is enough for detection of the spatial variability in the vegetation. Calcote (1995) also found that the correlation between pollen and vegetation ceased to improve as the vegetation sampling radius was increased to 50–75 m. His study also implies that pollen grains deposited in small hollows record stand-scale vegetation.

The majority of pollen analytical studies published up to now have been based on studies of lake sediments from relatively large basins, i.e., sites which have a comparatively large pollen source area. Pollen grains are transported to this type of site from many different vegetation types, and these grains are mixed together forming the pollen assemblage. Pollen samples from lakes or bogs with a diameter of c. 1 km normally reflect the vegetation within a large area. Theoretical and empirical studies have indicated that pollen grains are transported to such sites from within a radius of several tens of kilometres (e.g., Jacobson & Bradshaw 1981; Bradshaw & Webb 1985).

One way to improve the spatial resolution in palaeoecological studies is to use small sites where the majority of pollen grains deposited have not travelled very far. Sites useful for such studies are often small basins where organogenic sediments are deposited, particularly when they are situated below a dense canopy. These sites are often referred to as closed-canopy sites (Bradshaw 1988). Closed-canopy sites normally provide both a temporal and spatial resolution that enable the pollen analysis to be a useful tool for stand-scale reconstruction. They can also reveal information about pollen taxa that are rarely found in regional-scale studies, especially for taxa producing few pollen grains or that are not dispersed very well. Far-travelled pollen (<1 km) normally only comprise a minor part of the pollen assemblage deposited at closed-canopy sites. When the canopy is opened up this immediately influences the pollen deposition and the pollen source area will increase. A larger opening (or a larger basin) simply means a larger pollen source area.

Andersen (1970) was among the pioneers investigating the relationship between pollen assemblages deposited on the forest floor and composition of the forest within 20–30 m from the sampling point. His investigation has been repeated and largely confirmed by several other studies (e.g., Bradshaw 1981a; Heide & Bradshaw 1982; Chen 1988; Yazvenko 1991; Jackson & Wong 1994; Calcote 1995), indicating that pollen/vegetation relationships are fairly well understood at this scale. However, the pollen source area for certain pollen types is probably somewhat larger than originally thought (e.g., Jackson & Wong 1994). Different pollen types have different source areas, as pollen grains vary in size and dispersal ability (Prentice 1985, 1988; Sugita 1994). This also affects the pollen source area giving light pollen types, or pollen types with sacci, a larger source area than heavy types.

Several small sites are needed to study stand-scale differences within an area, or differences in vegetation development on different soil types. Ideally, a net-work of small sites in different settings is preferred to reveal such differences. It is however, also possible to connect stand-scale studies with regional-scale ones. One example of such a study is that made by Andersen (1984), which described the forest history of Eldrup Forest in Denmark using pollen analyses based on sediments from small, wet hollows. Andersen was able to detect vegetation changes affecting the forest that were not discernible from a regional pollen diagram from a bog lying outside, but nearby the forest. He was also able to detect stand-scale differences within the forest, from small sites lying not more than 100 m apart.

Sites useful for local-scale studies can be divided into three major types (Table 1) according to Bradshaw (1988). Small wet hollows are small basins where organogenic sediments (peat or gyttja) are deposited. Temporal resolution is normally good (usually extremely good during the last 500 to 1000 years), and the sediments may be continuously deposited over several thousands of years, and sometimes spanning more or less the whole of Holocene (e.g., Sugita et al. 1994). Pollen grains are normally well-preserved in wet hollows, but less favourable preservation conditions may occur if the site dries up periodically.

Preservation of pollen is probably the largest problem when analysing sediments from small sites (Bradshaw 1988). Preservation conditions may vary from excellent to relatively poor within the same profile. Pollen grains are normally more susceptible to oxidation and microbiological activity when deposited in small sites than in lakes or bogs. A high concentration of degraded pollen grains, or pollen types (and spores) that are resistant to degrading, most likely indicate that preservation conditions in the sediment have been poor (Havinga 1971, 1984). However, these problems are mostly confined to soils and shallow, wet hollows that may dry out completely during the summer.

Table 1. Some important properties of small sites, slightly modified after Bradshaw (1988).

Fagus and Picea

This section of the synthesis gives a general background to topics and problems discussed in this thesis. It briefly describes what is known about the present (and historical) distribution of Fagus and Picea, as well as the migration and establishment of these species. It also deals with some ecological characteristics for these species that may be relevant for this study.

Holocene distributional changes for Fagus and Picea in Europe

It is beyond the scope of this thesis to describe and discuss in detail the migration patterns of Fagus and Picea in Europe as a whole. This has recently been done more thoroughly in Huntley & Birks (1983) and Huntley (1988). However, a brief summary of these changes can be useful as a background to the Late Holocene distributional changes that have affected southern Sweden. It should also be noted that two species are native to Europe for both the genus Fagus (F. sylvatica, F. orientalis) and Picea (P. abies, P. omorika). When Fagus and Picea are discussed or mentioned in this thesis, they refer to F. sylvatica and P. abies. The other two species, F. orientalis and P. omorika, have a restricted occurrence mainly in south-eastern Europe, and they are not considered in this thesis.

Fagus

The distribution changes for Fagus in Europe since the termination of the last Ice Age have been summarised by Huntley & Birks (1983). During the maximum of the Weichselian glaciation Fagus was probably restricted to the mountainous areas of the Balkans, Italy and the southern part of the Carpathians, even if the pollen evidence for this is scant. From Late Glacial time Fagus pollen grains are only found at some scattered sites, for instance in Italy. This distribution picture did not change much until about 9000 BP when high pollen percentages for Fagus are encountered at sites in the southern Balkans and in the Carpathians. During the earliest part of the Holocene, Fagus apparently migrated along the European mountain ranges. Fagus probably grew in mountain forests during this time. The rapid climatic amelioration at the transition to the Holocene most likely initiated the migration of Fagus.

At about 8500 BP Fagus rapidly migrated northwards through Italy and the northern part of the Balkans. This pattern was stable for more than 1000 years until about 7000 BP when eventually Fagus populations in the Carpathians also started to expand northwards. At about 6500 BP Fagus had extended its distribution further to the north and the west. At this time also an outpost area appeared in southern France. Continuing expansion with high rates of population movement of around 250-300 m/year eventually led to the merging together of all separate Fagus areas at about 5000 BP. Fagus was then more or less continuously distributed from the Massif Central in the west to the Carpathians in the east.

At about 4000 BP three, new, main features appeared in the migration patterns for Fagus: expansion into the Pyrenees, into Poland, and a recolonisation of the mountains in the southern Balkans. Continuing expansion at approximately the same rate as before brought Fagus rapidly into northern Germany, south-eastern England, northern Spain, and eventually also into southern Scandinavia.

In summary, the expansion patterns for Fagus in Europe during the Holocene show three main features (Huntley 1988): a comparatively slow expansion during the earliest part of the Holocene, an expansion only in the central and eastern part of Europe during the Middle Holocene, and a rapid expansion in the north and west during the Late Holocene, when Spain, England and Scandinavia finally became invaded.

Picea

The distribution limits for Picea in Europe have also changed considerably throughout the Holocene. These changes have been summarised by Huntley & Birks (1983). They have clearly shown that Picea occurred within two separate areas during the Late Glacial: one area in western Russia, centred around Moscow, and one in south-eastern Europe, extending from the Austrian Alps in the west to the Carpathians in the east.

During the beginning of the Holocene Picea populations in western Russia decreased considerably and almost vanished. At approximately the same time Picea expanded comparatively rapidly in the mountains of south-eastern Europe. In this area the expansion continued towards the west along the Alps and in the Carpathians until about 4000 BP. Later on, a slight recession occurred, and the area with Picea forests then became somewhat disrupted. After about 2000 BP a new, but rather small expansion has occurred, especially on the northern Alpine forelands.

In western Russia Picea became re-established about 7000 BP, and during the period 7000–5000 BP, a vast area with Picea forests, extending from eastern Finland to western Russia, was established. From about 4000 BP until the present day Picea populations have moved rapidly (with rates approaching 500 m/years) towards the west and south-west and expanded its distribution limits (Aario 1965; Aartolahti 1966; Moe 1970; Tallantire 1972a, 1977; Persson 1975; Huntley & Birks 1983; Tolonen 1983; Hafsten 1985, 1992; Huntley 1988). However, Picea individuals occurred in Scandinavia at a few favourable outpost sites as early as during the Middle Holocene (Kullman 1995), but it was not an ecologically important tree until the main expansion westwards. Today almost the whole of Fennoscandia lies within the present distribution limits for Picea, except parts of the extreme north, south and west.

Climate vs. human influence as causes for distributional changes

Fagus
There has been a long and lively debate trying to determine why Fagus apparently failed to migrate into the lowlands of Northwest Europe during the middle part of the Holocene. The traditional explanation has been that Fagus was unable to invade the pre-existing dense woodlands in northern and central Europe, so long as they were undisturbed (Iversen 1973; Godwin 1975; Behre 1988). During the earliest part of the Holocene Fagus probably invaded rather open forest types in mountainous areas, and these types did not significantly hinder the invasion (Huntley & Birks 1983). When the pre-existing woodlands in Northwest Europe started to become disturbed by human interference, expansion of Fagus seems to have occurred rapidly, and with migration rates as high as during earlier parts of the Holocene. Iversen (1973) proposed that the late establishment of Fagus in, for example Denmark, was probably related to a colonisation of podsolic soils in previously cleared forests.

Huntley & Birks (1983) argue that the northwards migration of Fagus in north and west Europe after about 5000 BP coincides with the first significant indication of human interference with the vegetation, and that this coincidence probably has significant implications. This coincidence has led to the postulate that Fagus acted as an opportunistic tree species when cleared areas eventually became available. Even if such conditions favour establishment of Fagus, this postulate only rests on a temporal coincidence and does not take into account the possibilities for Fagus to invade undisturbed forests (Huntley 1988). Moreover, the migration of Fagus was apparently more gradual than the expansion of the Neolithic culture, and this migration has continued until the present day. In Switzerland, north of the Alps, the possible influence of human activity, or natural disturbance, on the timing of establishment and expansion of Fagus has also been discussed (Richoz et al. 1994).

The alternative explanation is that Fagus migrated simply as a response to climatic change. It is possible that a decreasing continentality in climate and milder winters during the latter part of the Holocene, facilitated the migration towards the north. This postulate also gains support from distribution changes for Fagus grandifolia in North America (a species of similar ecological requirements), where this species has migrated to the north and west throughout almost the entire Holocene (Davis 1976, 1981; Bennett 1985; Dexter et al. 1987), at a time when apparently the continentality of the interior parts of the continent decreased. Prehistoric humans in North America are also believed to have had only a minor effect on tree migration. The great resemblance in migration patterns for Fagus in North America and Europe may indicate that Fagus has migrated purely as a response to climatic changes throughout the Holocene (Huntley & Webb 1989; Huntley et al. 1989). In Europe milder winters in the area north of the Alps have been postulated as a cause for the Late Holocene expansion of Fagus (Huntley 1988). A similar change in winter temperatures has also hypothetically been assigned as a cause for the Late Holocene expansion of Fagus grandifolia in eastern North America (Bartlein et al. 1986; Huntley 1988).

Iversen (1973) and other authors have strongly argued against the importance of climate for the migration of Fagus, as the invasion into Northwest Europe occurred after the Holocene climatic optimum. They instead assumed that a northward migration ultimately has to be driven by increasing temperatures. Evidence from other species, e.g., Corylus avellana, has indicated a southwards regression in Scandinavia after 6000 BP. This led to an interpretation that Fagus apparently migrated northwards during a time when temperatures were decreasing. Iversen concluded that the expansion northwards of Fagus evidently not was determined by a climatic response. However, with the benefit of hindsight Iversen's interpretation is certainly a simplification of the climatic changes that really have occurred. These changes comprise not only changes in temperature, but also alterations in seasonality, as well as independent changes in both summer and winter temperatures, and precipitation, and these changes are ultimately driven primarily by orbital forcing. These complex alterations can cause contradictory changes in distribution limits for different species (Webb 1986; Huntley & Webb 1989). The southwards recession in Corylus distribution can still be explained by a decreasing temperature, but this decrease is only applicable to the summer temperature, while the contemporaneous northwards expansion of Fagus may be explained by milder winters.

The hypothesis that Fagus was unable to invade woodlands in Northwest Europe until cultural activities created suitable conditions for establishment takes no account of the fact that natural disturbances, such as fires and storms, occur in all forest types. Fagus is dependent on ground disturbance for successful regeneration (Watt 1923; Bjerregaard & Carbonnier 1979; von Röhrig et al. 1978), and natural disturbances can equally well create suitable seed beds. Huntley et al. (1989) therefore conclude that human disturbances alone cannot have been the ultimate cause for the Holocene distributional changes for Fagus in both Europe and North America.

Picea
Factors lying behind the expansion of Picea are also uncertain, and varying migration rates in different areas during different times are difficult to explain. The westward migration of Picea along the Central European mountains during the Early Holocene may be explained by a climatic amelioration occurring at the transition to the Holocene, which enabled Picea expansion from Late Glacial refuges (Huntley & Birks 1983).

The distribution changes for Picea in the northern part of its European extension are also difficult to understand. The decrease in area during the Late Glacial and the earliest part of the Holocene may have been caused by a warmer climate. The re-establishment and expansion after about 7000 BP probably reflects a trend towards more suitable conditions for Picea. Its seedlings are rather sensitive to summer drought, and they also require sufficient insulating snow cover during the winter. An alteration in the ratio precipitation/evaporation may be a more important climatic factor than a direct change in temperature (Huntley & Birks 1983).

In several regional pollen diagrams from Fennoscandia the establishment and expansion of Picea coincides with indications of cultural disturbances, for example forest clearance or agriculture (e.g., Almquist-Jacobson 1994). These cultural activities may have facilitated the expansion of Picea. However, as pointed out by Tallantire (1972a), Picea also increases in several pollen diagrams without contemporaneous cultural activities. This may indicate that human activity is not the only cause explaining the migration of Picea in Fennoscandia (Huntley & Birks 1983). Huntley (1988) has postulated three factors explaining the migration of Picea, and these factors do not necessarily involve human interference:

1. Picea, particularly its seedlings, are intolerant to frost (and drought) during early spring. These factors inevitably favour Picea regeneration in areas with a sufficient snow cover during the winter, and a rapid increase in temperature during the spring without any major oscillations.

2. In North America ecologically related Picea species show similar migration patterns to Picea in Europe, i.e., an expansion and maximal extension during the latter part of the Holocene. In North America the influence of prehistoric humans on Picea migration can be neglected. The migration patterns for Picea in North America is also in accordance with simulated climatic changes. Climate then is the most likely major factor causing continental distribution changes.

3. The westward migration of the boreal vegetation zone is probably most easily explained by climatic changes, particularly with decreasing temperatures in Northeast Europe and northern Siberia. These changes may have led to an increase in winter precipitation over Fennoscandia, and eventually triggered Picea expansion westwards.

Before the introduction of modern forestry with planting of Picea outside its present distributional limits, and introduction of foreign provenances, Tallantire (1972a, 1972b) argues that the migration patterns for Picea were driven purely by climate. In addition, Picea has probably not been affected during prehistoric times by selective removal or forest grazing, as many other deciduous trees have been. The expansion of Picea may in some areas during later periods have been somewhat controlled by humans, for instance by slash-and-burn activities, or through active removal from forest-meadows, as it was unwanted in this vegetation type (e.g., Weimarck 1953).

Holocene history of Fagus and Picea in southern Sweden

To be continued ......