POLLEN METHODS AND STUDIES | Stand-Scale Palynology

POLLEN METHODS AND STUDIES | Stand-Scale Palynology

Stand-Scale Palynology R H W Bradshaw, University of Liverpool, Liverpool, UK ã 2013 Elsevier B.V. All rights reserved. Introduction Conventional pol...

1MB Sizes 0 Downloads 43 Views

Stand-Scale Palynology R H W Bradshaw, University of Liverpool, Liverpool, UK ã 2013 Elsevier B.V. All rights reserved.

Introduction Conventional pollen analysis records vegetational change at spatial and temporal scales that are largely unfamiliar to the plant ecologist. Forest succession, or the predictable replacement of one group of plants by another, is usually observed at the scale of the woodland stand (102–103 m2) during the lifetime of the ecologist, but pollen analysis of lakes and bogs reconstructs vegetation cover from far larger areas (104–106 m2), typically over time periods of thousands of years. Forest hollows and other ‘closed-canopy’ sites, by contrast, are dominated by pollen and spores that have traveled only a few meters from source plants and thus resolve vegetation dynamics at the scale of the woodland stand. Vegetation can be reconstructed with very high spatial resolution that can be easily compared with plot-based surveys of modern vegetation. Thus, stand-scale palynology provides a key link between paleobotanical and contemporary ecological studies. Ecological processes and properties that can be more easily studied using stand-scale rather than conventional palynology include the following: (1) the effects of disturbance on vegetation, either events such as fire, disease, or storm damage, or chronic disturbance such as animal browsing; (2) species invasions, succession and its relationship to climatic change, and anthropogenic impact; and (3) vegetation structure and openness. Stand-scale palynology is well suited to complement archaeological investigations where the study sites are often small and spatially well defined.

History Stand-scale studies began in Scandinavia, as did many methodological developments in palynology. The approach was adopted and developed in the British Isles and former colonies and then the initiative passed to North American scientists where many of the recent developments have taken place. Iversen (1964) in Denmark was the first to fully appreciate the benefits of local-scale palynological studies even though isolated earlier studies had taken place in Norway and Sweden. Iversen realized the potential of stand-scale studies to yield specific insights into dynamic ecological processes and in 1949 established a field laboratory within the 200 ha Draved Forest, Denmark, with the aim of developing the subject. Pollen–vegetation relationships, vegetation monitoring, and pollen analysis from 30 stand-scale sites were among the ongoing studies before these unique observations and experiments were terminated due to lack of funding in 2003. Special mention must be given to the related topic of soil palynology, which was largely developed by Dimbleby (1957, 1985). Soil palynology has developed more within archaeological than Quaternary science, yet shares some of the special features of stand-scale palynology of wetland and mor humus sites. Andersen (1970) was a landmark paper in development

846

of the relationship between source vegetation and stand-scale pollen deposition and the detailed history of Løvenholm Forest was a showcase for the power of the technique (Andersen, 1984). Iversen used mor humus profiles to describe the development of Draved Forest over recent millennia, while Andersen sampled forest hollows from Løvenholm Forest that covered most of the Holocene. These forests today are important reference areas for the study of undisturbed forest processes, yet the pollen studies showed how most of the forest area had been significantly altered by human activity beginning thousands of years ago. Such studies have improved our detailed understanding of the past relationships between people and vegetation in Northwest Europe. Andersen also demonstrated a strong empirical relationship between pollen deposition at a single point on the forest floor and the present vegetation within a 20–30 m radius. This finding was supported by Bradshaw (1981) but was later modified by thorough field investigations in the 8500 ha Sylvania Wilderness, Upper Michigan, USA (Calcote, 1995; Sugita, 1994). A review of the subject was made by Bradshaw (1988), and later, Mitchell (1998) covered some further developments and showed how stand-scale studies contributed to discussion about the openness of Northwest European forest structure (Mitchell, 2005). Syntheses of case studies involving suites of stand-scale sites are the best illustration of how standscale palynology provides an important interface to issues within dynamic ecology (Bradshaw and Lindbladh, 2005; Bradshaw et al., 2005; Davis et al., 1998). Although forest peats are found in the tropics and subtropics, almost all of the published stand-scale sites are from glaciated regions within boreal and temperate forest areas.

Pollen Representation Theory Andersen (1970) adopted a largely empirical approach to pollen representation at the stand-scale and developed correction factors to relate pollen counts more closely to source vegetations that have been widely adopted. He observed that within a forest, pollen seemed to land closer to its source than was predicted by physical laws describing the dispersal of small particles. He ascribed this observation to special properties of turbulent air movement under the forest canopy. Prentice (1985) developed a pollen representation theory that accounted for the special properties of stand-scale sites, and his approach laid the foundation for later models that consider pollen production and dispersal issues and allow quantitative landscape reconstruction. The POLLSCAPE model assumes that all pollen disperses through the air as single grains and this assumption is often violated at the stand scale. POLLSCAPE predicts that stand-scale sites have a somewhat larger source area for pollen (>50 m radius) than that observed by Andersen. The differences are not great, however, and there is a general consensus that stand-scale sites sample vegetation at a different scale compared

POLLEN METHODS AND STUDIES | Stand-Scale Palynology

to conventional sites, which are usually chosen so as to yield a regional record. Stand-scale sites are much smaller than conventional sites and ideally are located under a closed tree canopy. Plant macrofossils, insect remains, and charcoal can also be preserved in stand-scale sites, and the basis for their interpretation is increasing as more sites are investigated. Stand-scale sites are best suited for detecting severe, infrequent fires using charcoal fragments larger than 150 mm (Higuera et al., 2005).

Types of Sites Pollen is preserved in a variety of settings beneath forest canopies. If the forest canopy is temporarily or permanently removed, a considerably increased proportion of the pollen preserved at any one point derives from vegetation so far from that point that stand-scale resolution is effectively lost (Jacobson and Bradshaw, 1981). Suitable sample sites fall into three broad categories: small, wet, forest hollows (Figure 1); accumulations of mor humus; and soils of low biological activity (Table 1). The sediment in forest hollows may be mud or peat, and the pollen is preserved by waterlogging. Sedimentation can be continuous over thousands of years, but hiatuses are rather common, and the sites are sensitive to fluctuating water tables. Mor humus is a dry, acidic accumulation of partially decomposed plant litter of very low biological activity. This humus type is typically associated with acidic, free-draining soils such as podzols and is found under trees producing a leaf litter of high polyphenolic content. Little or

Figure 1 Small forest hollow in Suserup Skov, Denmark. Photograph by Gina Hannon. Table 1

847

no mixing occurs, and pollen is prevented from downwashing by becoming trapped in aggregates of humic material. Downwashing and mixing does tend to occur in soils, but where pollen concentrations are high, information about past vegetation can be extracted, for example, in soil horizons buried under archaeological monuments (Andersen, 1997). Each type of site has its own assets and problems for the palynologist. The major asset of all these sites is their small pollen source area and hence their good spatial resolution. They can provide information about taxa that produce sparse amounts of poorly dispersed pollen such as Arbutus unedo (strawberry tree), Ilex aquifolium (European holly), and Tilia cordata (small-leaved lime), whose past history is hard to determine from conventional pollen sites. Frequently, high temporal resolution can be attained to match the spatial resolution, especially in mor humus. Pollen grains from closed-canopy sites are more exposed to oxidation and microbial action than in conventional sites, posing potential preservation problems. Soil pollen samples tend to be the worst preserved, and large numbers of deteriorated grains or high values of resistant types, such as Polypodium (polypody fern) spores, indicate a probable preservation bias. Preservation can be excellent, and several forest hollows with plant macrofossil records have been reported (Figure 2;

Figure 2 Acorn cup of Quercus petraea (sessile oak) about 6000 years old recovered from a forest hollow in Suserup Skov, Denmark. Photograph by Peter Warna-Moors.

Properties of stand-scale, closed-canopy sites

Length of record (years) Continuity of record Temporal resolution (years/sample) Mixing or downwash of pollen Pollen preservation Macrofossils Charcoal Site availability

Small hollows

Mor humus

Soils

103 May be patchy 10–100 Minimal Usually good Occasionally present Present Restricted

102–103 Good 1–10 Minimal Good No Usually present Restricted

101 Poor – Yes Poor No Usually present Widespread

848

POLLEN METHODS AND STUDIES | Stand-Scale Palynology

Hannon et al., 2000). Macrofossils are very rare in humus or soil samples, although charcoal fragments can be found in all site types.

Insights from Case Studies Sylvania, Upper Michigan, USA Davis et al. (1998) used pollen analysis of forest hollows to investigate the late Holocene invasion of mixed deciduous stands by Tsuga canadensis (eastern hemlock) in Sylvania, Upper Michigan, USA. They asked whether the timing of the invasion was simultaneous throughout the study area and whether the invasibility of the area was affected by species composition. These are questions of importance to biologists that cannot be addressed using traditional palynological techniques. They compiled a dataset comprising ten sediment cores collected along a 10 km transect. Sediment accumulation began at different times from the Late Glacial until only 1200 years ago, but once initiated, apparently continued until the present day. They believed this was attributable to a continuously rising water table, but it does illustrate the special sensitivity of forest hollow sites to fluctuating water tables. Sediment accumulation rates were typically 1–3 cm per century with high pollen concentrations and generally good preservation (Davis et al., 1998). Windstorms were recorded in the sediments by wood layers and sharp increases in pollen abundance of Betula (birch), but the invasion of eastern hemlock was not facilitated by disturbance. The vegetation has retained a patchy structure throughout the Holocene, and Tsuga preferentially invaded stands previously dominated by Pinus strobus (eastern white pine) around 3000 years ago (Figure 3 H1–H4). The invasion process took a few hundred years. Once Tsuga had invaded a stand, it maintained its dominance until today with a gradually increasing abundance, probably reflecting the regional increase in moisture. Stands that today are dominated by Acer saccharum (sugar maple) and Tilia grandifolia (large-leaved lime) had multiple origins (Figure 3 M1–M4). Some were converted from Tsuga stands following disturbance, while others gradually lost Quercus (oak) during long periods of time. The spatial and numerical precision of these vegetation reconstructions is striking and quite distinct from interpretations that can be made from conventional, regional palynological studies. The 3000-year stability of most of the Tsuga stands is in contrast to the greater dynamics observed in the mixed hardwood stands and gives new insight into the development of patchiness in vegetation.

Southern Scandinavia Several forest hollows have been investigated in southern Sweden and Denmark that permit analyses and insights into vegetation dynamics and drivers at a range of spatial scales. Maps of southern Sweden have been prepared that compare stand-scale composition with regional forest composition interpreted from conventional lake and peat sites (Figure 4; Lindbladh and Foster, 2010; Lindbladh et al., 2000). Deciduous forest types dominated the central and western landscapes at 1250 BC (calendar years), while Pinus (pine) was abundant

toward the east. Deciduous taxa such as Alnus (alder), birch, Corylus (hazel), oak, and lime were all common, but no single taxon was a clear regional dominant. Fraxinus excelsior (European ash) and Ulmus (elm) were less abundant than the other major tree taxa. The stand-scale records from 1250 BC confirm the dominant role of deciduous trees with only a single site recording abundant Pinus. Single taxa could dominate individual stands, but although one taxon comprised more than 50% of the basal area in four stands, it was a different taxon (alder, birch, pine, and oak) in each case. A kaleidoscope of forest stands was probably found in a diverse, deciduous forest landscape. By AD 500 European beech had become established at the landscape scale, particularly in the southeast and western regions (Figure 4). Pinus sylvestris (Scots pine) maintained its importance in the east. Norway spruce was becoming a component of the northern forests, and increased importance of Betula and Juniperus communis (common juniper) in the central part of the region indicated widespread disturbance of forest cover. At the stand scale, the pie diagrams show an increasing importance of birch and pine at the expense of alder and lime. Stand-scale dominance patterns could differ from the regional abundance. For example, beech dominated a stand in the northwest in a region that only supported 2% beech, although beech was more abundant in the regional vegetation to the north. The maps from the present forest landscape show the widespread dominance of conifers, chiefly spruce but with abundant pine, particularly in the east and southern parts of the region (Figure 4). The present-day map shows a good general agreement with recent forest inventory data, although there is rather little spruce recorded from southern parts of the boreal– temperate forest ecotone. Spruce is close to its natural southern limits here, and there has been much planting during the present century. Young, dense plantations produce little pollen in relation to forest volume, and there is also a possible temporal mismatch between the pollen and tree data. The pollen data are from bioturbated sediments and can integrate pollen input spanning 50–100 years, especially in the soft, unconsolidated surface sediments that are difficult to sample with accuracy. Birch and beech are the commonest deciduous taxa, and this pattern is also observed in the stand-scale records. Spruce, birch, and pine, the three major boreal taxa, comprise over 70% of the forest volume in 9 out of 13 stands. Three of the remaining stands are northern beech outliers, and one record is from a recently clear-felled area. Both the regional and local records show the same general tendency of declining abundance of deciduous taxa (except birch and beech) and an increasing tendency for one or two taxa to dominate both stands and landscapes. The northern beech-dominated stands, however, differ from the surrounding regional composition showing that forest hollows can detect stands that are unrepresentative of the landscape. Nevertheless, most stands broadly reflect the landscape-scale shifts in composition during the last 3250 years, although individual taxa show greater ranges of abundance in the stand-scale records.

Stand-scale compositional trends The detrended correspondence analysis (DCA) of the calibrated pollen data from the individual forest stands for 1250 BC, AD 500, and the present, portrays the similarities

POLLEN METHODS AND STUDIES | Stand-Scale Palynology

H1: 6000 yrs

849

H2: 6000 yrs Hemlock Pine

CVA axis 2

Pine

SM–Hem

SM–Hem Oak Ash

Oak Ash

S. Maple

H3: 6000 yrs

CVA axis 2

Pine

(a)

CVA axis 2

Pine

Hemlock

SM–Hem Oak Ash

S. Maple

CVA axis 1

S. Maple

CVA axis 1 M2a: 4200 yrs

Hemlock

M2b: 1100 yrs Hem.

Pine

SM–Hem

Hem.

Pine

SM–Hem

SM–Hem

Oak

Oak

Oak

Ash

S. Maple

M3: 6000 yrs Pine CVA axis 2

Hemlock

SM–Hem

M1: 6000 yrs

Ash

S. Maple

M4a: 3900 yrs

Hemlock

Pine

Ash

S. Maple

M4b: 2400 yrs

Hem.

Pine

Hem.

SM–Hem

SM–Hem

SM–Hem

Oak Oak Ash

(b)

S. Maple

H4: 4300 yrs

Oak Ash

Pine

Hemlock

S. Maple

Oak

Ash

CVA axis 1

S. Maple

CVA axis 1

Ash

S. Maple

CVA axis 1

Figure 3 Trajectories through time of pollen assemblages (a) in Tsuga hollows and (b) in hardwood hollows projected into canonical variate analysis (CVA) space defined by 66 surface samples. Dots show the position of the trajectories interpolated to 100-year intervals. The oldest sample in each series is shown by a large diamond and the youngest by a circle with a crosshair. Large squares denote 3000 calendar years ago. The outlined areas include surface pollen assemblages from six major forest communities. Reproduced from Davis MB, Calcote RR, Sugita S, and Takahara H (1998) Patchy invasion and the origin of a hemlock-hardwoods forest mosaic. Ecology 79: 2641–2659.

and differences between the 16 individual stand histories (Figure 5). All stands move away from the rich deciduous forest indicated by pollen from alder, hazel, oak, and lime with some ash and elm. The commonest successional pathway is from birch and Carpinus betulus (European hornbeam) to forest comprising spruce and pine, although two stands became dominated by beech. The role of birch as a prominent taxon in forests undergoing transition from deciduous to coniferous was emphasized in the DCA, and many stands were at

this intermediate stage at AD 500. The same tendency was observed at the regional scale especially in the central region of the study area (Figure 4). The DCA shows that unique successional pathways can occur and be resolved by stand-scale palynology, but the general pattern in southern Sweden during the last 3250 years has been along the pathways that lead to spruce – and to a lesser extent to beech-dominated stands. The spreading rate of spruce and beech into southern Scandinavia can be estimated from

850

POLLEN METHODS AND STUDIES | Stand-Scale Palynology

6

Std. DCs

5 4 3 2

1250 BC

AD 500

1 0 1000

500

0

500

1000

1500

Years (BC/AD)

Mixed deciduous Alder hazel Birch Pine Spruce Beech Present

Figure 6 The mean values for rate-of-change analysis for 13 standscale sites in southern Scandinavia. The analysis included the calibrated tree pollen data, shrub, and herbaceous taxa that exceeded 2% of the pollen sum in at least one sample. Std DCs – chord distance per 100 years. Reproduced from Lindbladh M, Bradshaw RHW, and Holmqvist BH (2000) Pattern and process in south Swedish forests during the last 3000 years, sensed at stand and regional scales. Journal of Ecology 88: 113–128.

100 km

changes in climate. In contrast, the spread of beech was most likely controlled by site properties (Bialozyt et al., 2012). Figure 4 Forest maps of southern Sweden from 1250 BC, AD 500, and the present based on regional pollen sites. The pie diagrams show data from stand-scale sites. Reproduced from Lindbladh M, Bradshaw RHW, and Holmqvist BH (2000) Pattern and process in south Swedish forests during the last 3000 years, sensed at stand and regional scales. Journal of Ecology 88: 113–128.

Fagus 2.5

Axis 2

2.0

Tilia

Ulmus E CAlnus L Corylus D

1.5

G

1.0

A Betula

Fraxinus

Quercus

I Carpinus

0.5 B Pinus

0.0

K Picea 0.0

0.5

1.0

1.5 2.0 Axis 1

2.5

3.0

3.5

Figure 5 Detrended correspondence analysis (DCA) of 13 stand-scale sites from southern Scandinavia. Nine of the sites are represented by three time points (1250 BC, AD 500, and the present). Four sites cover just AD 500 to the present. The arrows show the direction in time. Reproduced from Lindbladh M, Bradshaw RHW, and Holmqvist BH (2000) Pattern and process in south Swedish forests during the last 3000 years, sensed at stand and regional scales. Journal of Ecology 88: 113–128.

the arrival times at the individual stands sampled by small hollows. Data–model comparison of species spread showed that the late Holocene spread of spruce was rather rapid (250 m year 1) throughout the last 4000 years and was only limited by biological processes of dispersal, despite significant

Rate of vegetational change Analysis of the rate of vegetational change indicates those time periods when rapid reorganization of forest composition occurred as well as periods of relative stability (Jacobson et al., 1987). Each stand-scale site in the study of records from Scandinavia has its own unique pattern, but certain common features are apparent when all stands are combined into a mean value (Figure 6; Lindbladh et al., 2000). A major increase in rate of change began around AD 1000 and has continued until the present day, with the last 1000 years showing an increase four times greater than that recorded during the preceding 2500 years. These recent changes thus completely overshadow the earlier record. The changes recorded from the last 150 years were the most rapid but represent the culmination of a transformation that was initiated 850 years earlier. Lindbladh and Foster (2010) combined data from 25 small hollows with six regional sites and confirmed the recent increased rate of change in south Scandinavian vegetation. They showed that while south Scandinavian oak populations had been in continuous decline for at least 2000 years, the majority of this decline took place during the last 300 years, corroborating historical records.

Anthropogenic or climatic control of recent dynamics of T. cordata and Fagus sylvatica in Denmark and southern Sweden? Much debate focuses on the origins of beech forests in Northern Europe, primarily owing to their current dominance of large areas and their often presumed ‘natural’ status. At the continental scale, European beech is arguably in equilibrium with climate and thus is an expected component of modern southern Scandinavian forests. Paleoecological investigation of sediments in forest hollows in southern Sweden, however, indicates that present-day beech stands often show signs of

851

Tr ee s

C ha rc oa l

Pi

ce a

Fa gu s

POLLEN METHODS AND STUDIES | Stand-Scale Palynology

0

Age years BP

500

1000

1500

2000

2500

3000

20

40

20

40

100 200 300 % total pollen

20

40

60

80 100

Figure 7 History of beech establishment and its relationship to large charcoal fragments recorded from a forest hollow at Siggaboda, southeast Sweden. Reproduced from Bjo¨rkman L and Bradshaw RHW (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.

90

90

80

80

70

70

60 50 40 30 20 10 0 –10

Simulated Observed

Fagus

Percent (forest biomass/tree pollen)

Percent (forest biomass/tree pollen)

Tilia

60 50 40 30 20 10 0

500

1000

1500

2000

Calendar year (AD)

–10

500

1000

1500

2000

Calendar year (AD)

Figure 8 Data model comparisons for lime (Tilia) and beech (Fagus) in Draved Forest, Denmark. Simulated data are percentage of total forest biomass, and observed data are calibrated pollen percentages from a mor humus profile. Reproduced from Cowling SA, Sykes MT, and Bradshaw RHW (2001) Palaeovegetation-model comparisons, climate change and tree succession in Scandinavia over the past 1500 years. Journal of Ecology 89: 227–236.

anthropogenic disturbance immediately prior to the replacement of T. cordata (small-leaved lime) by beech. Significant population expansions of beech were closely associated with fire episodes linked to anthropogenic activities at a forest hollow in Siggaboda, southeast Sweden, (Bjo¨rkman and Bradshaw, 1996; Hannon et al., 2010; Figure 7) and the Siggaboda record is repeated at other sites in the region (Bradshaw and Lindbladh, 2005). Controversy, therefore,

concerns not so much whether beech should be present on a continental scale, but rather whether or not its dominance in southern Sweden and Denmark is a ‘natural’ feature of the landscape (i.e., having nonanthropogenic origins). Do climate-driven simulations of forest composition predict beech dominance for southern Scandinavia? Simulations with a climate-driven forest stand simulation model in Draved Skov, Jutland, did not support the hypothesis

852

0

sa (c Pi / f) nu s Q s ue y rc lve us st ris (s /l) Q ue r Ti cu lia s ro bu ra nd pe Ti tr a lia ea Fa co (b gu rd /b a s ta r/f / fl an /l/ d cu pl Fa ) a gu ty ph s C s yll or ylv os ylu a (f) s tica (b r/f / fl /s ) C or ylu s av el la na (b r)

Al nu

Pi s g nu lu s ti

s Al nu

C

De

pt

h(

cm

no

) al y C r (B ha C rc /AD oa ) l(> 20 0

m)

POLLEN METHODS AND STUDIES | Stand-Scale Palynology

Phase

–2000

15 30

–1500

5

–1000 45

–500

4

0 500 1000 1500

3

60

2000 2500 75 90 105

2

3000 3500

120

1

4000 135

4500

200

400

600

20

20

20

20

40

20

20

40

% total pollen

Figure 9 Suserup forest hollow, Denmark. Summary diagram showing selected trees recorded as pollen (filled curves) and macrofossils (•). The charcoal fragments are expressed as number per cm3. Reproduced from Hannon GE, Bradshaw RHW, and Emborg J (2000) 6000 years of forest dynamics in Suserup Skov, a seminatural Danish woodland. Global Ecology and Biogeography 9: 101–114.

that historical changes in climate were favorable for stand-scale dominance of beech (Figure 8; Cowling et al., 2001). The results indicated that, following cooling during the Little Ice Age, lime tree populations should have rapidly reestablished their dominance in deciduous forests. Observation, based on a stand-scale pollen analysis from mor humus in Draved Forest, recorded a replacement of lime by beech (Figure 9; Aaby, 1983). Based on the results of climate-driven simulations, beech should not dominate modern-day northern temperate woodlands in Europe, whereas lime should be present in significantly greater quantities than are observed today (Figure 8). The minor presence of lime has been a long-standing feature of forests in Northwest Europe and can best be explained by anthropogenic interference. Forest clearance for agriculture followed by subsequent abandonment of fields favors the replacement of lime by beech, as does the introduction of domestic livestock into the forest (Bjo¨rse and Bradshaw, 1998). Thus, the widespread late Holocene replacement of lime by beech in Northwest Europe was driven by anthropogenic forces even though forest changes of comparable scale earlier in the Holocene were climate driven.

Charcoal and macrofossils Stand-scale forest structure and the importance of burning and forest grazing in the past are issues of conservation and management interest. Plant macrofossil and charcoal studies from southern Scandinavia have helped inform the debate about the degree of forest openness in the past, and relevant data have come from small forest hollows (Bradshaw and Hannon, 2004; Bradshaw et al., 2010; Hannon et al., 2010). The record from Suserup Skov, Denmark, covered the last 6000 years and contained abundant large fragments of charcoal from 5000 years ago until 1000 years ago (Hannon et al., 2000; Figure 9). The charcoal gave evidence of local burning, but the fires were almost certainly ground fires that maintained rather open conditions in the forest and kept out particularly fire-sensitive species. European beech, a very fire-sensitive species, only became abundant when the burning ceased while Scots pine, which tolerates moderate fire, was a natural component of the vegetation only during the periods of most frequent burning. The plant macrofossil record at Suserup considerably broadens our knowledge of mid- to late Holocene forest composition in Denmark. It shows that both small-leaved lime and Tilia platyphyllos (large-leaved lime) grew on site about

POLLEN METHODS AND STUDIES | Stand-Scale Palynology

5000 years ago as did Quercus robur (pedunculate oak) and Q. petraea (sessile oak). At this time, there is stand-scale evidence for at least 16 tree and shrub taxa, which is a unique record of diversity for Northwestern Europe that is unmatched in present woodlands. Suserup is a fertile, base-rich site, but its mid-Holocene record was probably characteristic of other similar as yet uninvestigated sites. Understanding the loss of this diversity is a research challenge that has relevance for the protection of European forest biodiversity at present.

Conclusions Stand-scale palynology permits spatial resolution that provides detailed insight into dynamic vegetation processes that cannot be resolved using conventional pollen analysis. The case studies presented in this article give information about invasion mechanisms, forest structure, local diversity, succession, fire, and disturbance processes that link directly to topical issues in long-term ecological research and conservation biology. The recognition of the former importance of lime in northern and central European forests is one important conclusion from stand-scale studies. Stand-scale sites also link to conventional regional sites, where the research focus is on reconstruction of past climate and land cover. Thus, stand-scale sites provide a pivotal link between Quaternary science and ecology. The understanding of stand-scale sites has benefited from recent developments in pollen representation theory. A promising future application is their increased use in the validation and development of stand-scale forest simulation models.

See also: Pollen Methods and Studies: Numerical Analysis Methods; POLLSCAPE Model: Simulation Approach for Pollen Representation of Vegetation and Land Cover.

References Aaby B (1983) Forest development, soil genesis and human activity illustrated by pollen and hypha analysis of two neighbouring podzols in Draved Forest, Denmark. Danmarks Geologiske Undersøgelse II 114: 1–114. Andersen ST (1970) The relative pollen productivity and pollen representation of North European trees, and correction factors for tree pollen spectra. Danmarks Geologiske Undersøgelse II 96: 1–99. Andersen ST (1984) Forests at Løvenholm, Djursland, Denmark, at present and in the past. Biologiske Skrifter Kongelige Danske Videnskabernes Selskab 24: 1–208. Andersen ST (1997) Pollen analyses from early Bronze age barrows in Thy. Journal of Danish Archaeology 13: 7–17. Bialozyt R, Bradley L, and Bradshaw RHW (2012) Modelling the spread of Fagus sylvatica and Picea abies in southern Scandinavia during the late Holocene. Journal of Biogeography 39: 665–675. http://dx.doi.org/10.1111/j.13652699.2011.02665.x. Bjo¨rkman L and Bradshaw RHW (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.

853

Bjo¨rse G and Bradshaw R (1998) 2000 years of forest dynamics in southern Sweden: Suggestions for forest management. Forest Ecology and Management 104: 15–26. Bradshaw RHW (1981) Modern pollen-representation factors for woods in Southeast England. Journal of Ecology 69: 45–70. Bradshaw RHW (1988) Spatially-precise studies of forest dynamics. In: Huntley B and Webb T III (eds.) Vegetation History, pp. 725–751. Dordrecht: Kluwer Academic Publishers. Bradshaw RHW and Hannon GE (2004) The Holocene structure of North-West European temperate forest induced from palaeoecological data. In: Honnay O, Verheyen K, Bossuyt B, and Hermy M (eds.) Forest Biodiversity: Lessons from History for Conservation, pp. 11–25. Wallingford: CAB International. Bradshaw RHW and Lindbladh M (2005) Regional spread and stand-scale establishment of Fagus sylvatica and Picea abies in Scandinavia. Ecology 86: 1679–1686. Bradshaw RHW, Lindbladh M, and Hannon GE (2010) The role of fire in Southern Scandinavia during the late Holocene. International Journal of Wildland Fire 19: 1040–1049. Bradshaw RHW, Wolf A, and Møller PF (2005) Long-term succession in a Danish temperate deciduous forest. Ecography 28: 157–164. Calcote R (1995) Pollen source area and pollen productivity: Evidence from forest hollows. Journal of Ecology 83: 591–602. Cowling SA, Sykes MT, and Bradshaw RHW (2001) Palaeovegetation-model comparisons, climate change and tree succession in Scandinavia over the past 1500 years. Journal of Ecology 89: 227–236. Davis MB, Calcote RR, Sugita S, and Takahara H (1998) Patchy invasion and the origin of a hemlock-hardwoods forest mosaic. Ecology 79: 2641–2659. Dimbleby GW (1957) Pollen analysis of terrestrial soils. New Phytologist 56: 12–28. Dimbleby GW (1985) The Palynology of Archaeological Sites. Orlando, FL: Academic Press. Hannon GE, Bradshaw RHW, and Emborg J (2000) 6000 years of forest dynamics in Suserup Skov, a seminatural Danish woodland. Global Ecology and Biogeography 9: 101–114. Hannon GE, Niklasson M, Brunet J, Eliasson P, and Lindbladh M (2010) How long has the ‘hotspot’ been ‘hot’? Past stand-scale structures at Siggaboda nature reserve in southern Sweden. Biodiversity and Conservation 19: 2167–2187. Higuera PE, Sprugel DG, and Brubaker LB (2005) Reconstructing fire regimes with charcoal from small-hollow sediments: A calibration with tree-ring records of fire. The Holocene 15: 238–251. Iversen J (1964) Retrogressive vegetational succession in the Postglacial. Journal of Ecology 52(supplement): 59–70. Jacobson GL and Bradshaw RHW (1981) The selection of sites for paleovegetational studies. Quaternary Research 16: 80–96. Jacobson GL, Webb T III, and Grimm E (1987) Patterns and rates of vegetation change during the deglaciation of Eastern America. In: Ruddiman WF and Wright HE Jr. (eds.) North America and Adjacent Oceans During the Last Deglaciation, pp. 277–288. Boulder, CO: Geological Society of America. Lindbladh M, Bradshaw RHW, and Holmqvist BH (2000) Pattern and process in South Swedish forests during the last 3000 years, sensed at stand and regional scales. Journal of Ecology 88: 113–128. Lindbladh M and Foster DR (2010) Dynamics of long-lived foundation species: The history of Quercus in Southern Scandinavia. Journal of Ecology 98: 1330–1345. Mitchell FJG (1998) The investigation of long-term successions in temperate woodland using fine spatial resolution pollen analysis. In: Kirby KJ and Watkins C (eds.) The Ecological History of European Forests, pp. 213–223. Oxford: CAB International. Mitchell FJG (2005) How open were European primeval forests? Hypothesis testing using palaeoecological data. Journal of Ecology 93: 168–177. Prentice IC (1985) Pollen representation, source area, and basin size – Toward a unified theory of pollen analysis. Quaternary Research 23: 76–86. Sugita S (1994) Pollen representation of vegetation in Quaternary sediments – Theory and method in patchy vegetation. Journal of Ecology 82: 881–897.