Patterns in recent and Holocene pollen influxes across Europe; the Pollen Monitoring Programme Database as a tool for vegetation reconstruction

The collection of modern spatially extensive pollen data are important for the interpretation of fossil pollen diagrams. Such datasets are readily available for percentage data but lacking for pollen accumulation rates (PAR). Filling this gap has been the motivation of the pollen monitoring network, whose contributors monitored pollen deposition in modified Taubertraps for several years or decades across European latitudes. Here we present this monitoring dataset consisting of 351 trap locations with a total of 2742 annual samples covering the period from 1981 to 2017. This dataset shows that climate parameters 5 correlating with latitude determine pollen productivity. A signal of regional forest cover can be detected in the data, while local tree cover seems more important. Pollen traps situated beyond 200 km of the distribution of the parent tree are still collecting occasional pollen grains of the tree in question. PAR’s of up to 30 grains cm-2 y-1 in fossil diagram should therefore be interpreted as long distance transport. Comparisons to fossil data from the same areas show comparable values. Comparisons often demonstrate that similar high values for temperate taxa in fossils sites are found further south or downhill. While modern 10 situations comparable to high PAR values of some taxa (e.g. Corylus) may be hard to find, CO2 fertilization and land use may case high modern PAR’s that are not documented in the fossil record. The modern data is now publically available in the Neotoma Paleoecology Database and hopefully serves improving interpretations of fossil PAR data.

results based on several initial time series (van der Knaap et al., 2001;Koff, 2001;Tinsley, 2001;Tonkov et al., 2001) as well as a first comparative study (Hicks et al., 2001). More individual results were published in the following years (e.g. Gerasimidis et al., 2006;Giesecke and Fontana, 2008;Hättestrand et al., 2008;Jensen et al., 2007;Kvavadze, 2001;Pidek, 2007) and comparative studies followed in a second special volume published in 2010 . The data produced by contributors to the PMP were analysed for different questions, including weather parameters determining the amount of pollen 15 production (van der  and its correlation to masting years in Fagus . However, no study has hitherto explored to what extent the PMP data collected provides modern comparisons to fossil situations as was originally intended. Since this dataset extends across the distribution limits of some major European trees, it can also provide information on the quantity of pollen dispersed over long distances, as an analogue for the interpretation of regional or local presence, which is difficult to achieve based on pollen percentages (Lisitsyna et al., 2011a). 20 The programme established a database collecting the original data for individual years, as well as general information on the pollen traps installed in the different regions (Fig. 1). The database was developed offline and was thus difficult to access by individual researchers. The paleoecology database Neotoma (Williams et al., 2018) offers a platform to store the PMP data and make it available to researchers worldwide, allowing them to interrogate the data and potentially identify modern analogues to interpret fossil pollen accumulation rates. The aim of this manuscript is to present an overview of the data in the 25 PMP database and explore its potential to provide analogues for fossil situations. We collected fossil datasets with estimates of pollen accumulation rates from the same regions where the pollen traps were installed to explore the relationship between modern and fossil pollen accumulation rates. 30 Sites in the PMP database were divided into 7 'trap regions' according to the latitude and altitude. These regions were further divided into 'trap areas', by grouping 2-10 trap locations according to their spatial proximity. The arctic/alpine region includes distant trap areas in Spitsbergen and Iceland, northernmost traps in Finland (Utsjoki) and Norway (Lofoten-Vesterålen) and

Data collection
The pollen traps used in the PMP network generally consist of a bucket or bottle large enough to contain the annual surplus in precipitation on a surface of usually 19.6 cm 2 (5 cm diameter opening) or similar. Many traps had a sloping collar inspired by the design of pollen traps by Tauber (1974), although few collars were truly aerodynamic. The collection of the trap content was generally carried out annually and any special circumstances potentially affecting the annual pollen deposition were noted 15 and stored in the database. For the analyses presented here and data overview we excluded traps where the pollen record is 2 years or less, as averages may be affected by high inter-annual variability. The only exception is the trap situated in Spitsbergen where there is a two-year record. Pollen accumulation from the two-year record shows little variation and, being the only analogue for a truly arctic and treeless environment, provides important information on long distance pollen transport. We also excluded annual samples with records shorter than 8 months and, in addition, traps or years with spurious values due to 20 particular events or local conditions (Table S1).
Most of the traps in the PMP network were placed in the open vegetation or in forest openings in order to avoid an unrepresentative contribution of individual trees e.g. due to anthers dropping into the traps. Traps were generally installed at ground level mimicking collection conditions relevant for sedimentary archives. In consequence tall herbs or grasses might overgrow or cover some of the traps potentially leading to higher pollen deposition. Traps not equipped with a mesh occasionally trapped 25 pollen collecting insects, leading to enormous counts of insect pollinated taxa e.g. Calluna, Erythranthe guttata. The presence of insects in the traps is usually noted for the collection year so that careful evaluation of the information in the database can also inform on herbaceous pollen types (Jensen et al., 2007). Including this information in comprehensive database queries is currently not possible and a manual screening of datasets is required when analysing herbaceous pollen types. This problem does not seem to occur in tree pollen taxa. The occurrence of phytophagous insects in the traps were not accompanied by 30 unusual peaks in tree pollen taxa, indicating that the insects inadvertently trapped were primarily collecting pollen from the herbaceous vegetation around the traps. Comprehensive database queries restricted to tree pollen, Poaceae and Cyperaceae should therefore not be affected by the occurrence of insects in the trap and mainly represent pollen transport via wind, the rainout of pollen from the atmosphere and the gravity component.
Concentrating the content of the traps was carried out either using filter paper or centrifugation and decanting the supernatant.
In many cases the trap content was washed onto a paper filter, which was later digested using acetolysis. Pollen quantity was assessed by adding Lycopodium spore tablets (Stockmarr, 1971) to each trap before processing. Pollen concentration was obtained from the ratios between pollen grains counted to Lycopodium spike counted and Lycopodium spike added. Details about Lycopodium spike data, as well as details of the pollen trap such as the exact size of the opening are stored in the 5 database. The PMP database was created in the PostgreSQL database system. Names of pollen taxa were unified using the accepted variable names from the European Pollen Database (Giesecke et al., 2019).

Investigated taxa and environmental parameters
We selected the common tree and shrub taxa of Europe. Pollen taxa generally refer to all the species within the genus. Pollen taxa allowing higher taxonomical resolution, which were consistently separated and excluded from the genus in the whole 10 dataset are marked as "excl.". Pollen taxa potentially including pollen grains from another genus are indicated by "incl.": Abies, Alnus (excl. A. viridis), Betula (excl. B. nana-type), Carpinus-type (incl. C. orientalis/Ostrya-type), Corylus, Fagus, Fraxinus (incl. F. ornus), Juniperus-type (incl. Cupressus, Tetraclinis, Thuja), Picea, Pinus (excl. P. cembra-type), Tilia, Quercus (incl. Q.robur-type, Q. cerris-type and Q. ilex-type). Pollen accumulation rates of trees and shrubs were summed as arboreal pollen accumulation (hereafter as "tree PAR"). We also included pollen from the plant families Cyperaceae and Poaceae (ex-15 cluding cereals). For the purpose of the analysis in this paper we refer to sum of tree PAR plus Cyperaceae and Poaceae as "total PAR".
The climate parameters Mean Annual Temperature (MAT) and Annual Precipitation (APrecip) for the trapping locations were obtained from WorldClim 2 (Fick and Hijmans, 2017). For site altitude we used the information supplied by the individual investigator. Comparisons between PAR and forest cover were conducted using the data of the Forest Map of Europe 20 (Kempeneers et al., 2012), which has a grid resolution of 1 km 2 . Forest cover was extracted at a spatial level of all grid cells within a 10 km radius. We used regression analysis to explore whether individual or combinations of these environmental parameters describing the trapping location can explain the variance in average pollen accumulation of the traps. To balance the contribution of high and low pollen producers in the assessment of the total PAR we applied correction factors (Table S2, Andersen, 1970). 25 Pollen deposition beyond the distribution area of the parent plant was studied by merging the distribution maps of the relevant species included in the pollen type described above (Caudullo et al., 2018;San-Miguel-Ayanz et al., 2016). These comparisons were not suitable for Alnus, Betula, Cyperaceae, Juniperus, Pinus and Poaceae as these taxa are widely distributed in Europe and few traps are located beyond their distribution area. We compared the amount of pollen accumulating from these taxa in pollen traps at 200 km from their mapped distribution limits. Pollen traps in the UK are situated beyond the natural distribution 30 limits of several of these trees but were excluded from the comparison as the taxa considered may be planted in the area.
For each trap location and each pollen taxon we calculated the distance to the nearest area of distribution using GIS (GRASS Development Team, 2018). Linear regression between this distance and the decadic logarithm of PAR was used to predict the threshold of long-distance transport (hereafter also as "LDT").

Comparison between modern and Holocene PAR
We searched for Holocene pollen records containing estimates of PAR in proximity to the locations of pollen traps. From the available sites we selected at least one and a maximum of three Holocene PAR records per trap region (Table 1). Holocene PAR 5 estimates often show high variation between samples due to changes in the sedimentary environment. To reduce this effect of the here conducted comparison Holocene data were averaged in 500-year bins.
Site and sample compilation resulted in PAR values from 354 Holocene and 271 modern samples. Average trap and fossil PAR values per taxon were submitted to one-dimensional clustering using the R-package Ckmeans.1d.dp (Song and Wang, 2011). The classes produced were used to facilitate the comparison between trap and fossil data and to match the modern 10 trap values with analogous situations in the past. The aim of this comparison was to find traps with similarly high values for individual taxa that compared to the highest average fossil PAR. However, averaging did not smooth out all spuriously high values variation.ignoring individual high fossil values as described above (Table S3). Thus, we aimed to find modern analogues for fossil situations represented by several bins (more than 500 years) representing a period of long-term vegetation stability.
We linked these periods with high fossil PARs to the closest pollen trap, using a distance matrix between fossil sites and pollen traps. All statistical analysis and data visualizations were produced in R (R Core Team, 2019). The PMP database version 02.02.2020 contains data from 351 trap locations with a total of 2742 annual samples covering the period from 1981 to 2017. Trapping sites cover a range of altitudes from 0 to 3000 m a.s.l. with annual precipitation ranging from 402 to 1549 mm. Mean annual temperature (MAT) for the sites fall between -5.7 to 14.1°C. The forest cover within 10 a 10 km radius of the trapping sites ranges from 0 to 98%. This range of environmental situations has yielded tree pollen accumulation rates from 5 to 86000 grains cm -2 y -1 , with a median value of 5400 grains cm -2 year -1 (Fig. 2). An overview of the taxonomic composition of the traps ( the south, where they also contribute much higher absolute amounts. The diversity of landscapes and forest types in central and southern Europe is well represented in the pollen composition of traps from this area. Differences in pollen composition and abundance between the high mountain forests of central and southern Europe and the boreal forest in the north are also noticeable. The pollen composition of the fossil sequences selected represents the same regional patterns as the traps (Fig. S2).
Temporal changes are most pronounced in both fossil sequences from the Alps, documenting forest compositional changes as 20 well as the suppression of the tree-line during the Holocene.
Total PAR is generally lower at high latitudes, with the lowest values in the arctic alpine region (trap area Spitsbergen).
However, the highest absolute values are not from the southernmost traps but from the lowland temperate region (trap area  (Jahns, 2004) Tver; Fig. 2). Nevertheless, latitude alone explains about 11% of the variance in log transformed total PAR, while MAT and forest cover within 10 km explain 21% and 72% respectively. In combination, these three variables explain 76% and with 25 addition of elevation 73% of the total variance in log transformed total PAR. Adjusting the PAR from individual taxa by Andersen factors reduces the bias of differential pollen production between different plants and thus increases the amount of variance explained by the regression model with all 4 explanatory variables to 82% (Table S4). This adjustment reduced the individual explanatory power of forest cover within 10 km, due to the inclusion of grasses in the total PAR. On the other hand, latitude alone explains 38% of the Andersen adjusted log transformed total PAR (Fig. 3a). 30 The regression models consider the full range of the data, while due to local factors there is often a spread of average trap values for different traps in the same region. Traps with maximum values per region do not follow a latitudinal pattern, the distribution of the minimum average trap values are more informative (Fig. 3a). These lower values closely follow a latitudinal trend. The average PAR south of 62°latitude and below the altitudinal treeline or close to forests are generally higher than 1000 grains cm -2 y -1 . An area with low PAR in the south is the coastal grassland in northern Bulgaria. The generally low PAR in this area can be explained by the sparse vegetation cover on thin rendzina soils formed on limestone rock. Adjusting the PAR values by Andersen factors increases the values for this region so that they fit the general latitudinal trend. (Fig. 3a). Traps with minimum average PAR values per region also correspond well to the forest cover within 10 km (Fig. 3b). Taking the traps 5 with minimum PAR within 3% wide bins of the forest cover data provides a regression model predicting a tree PAR of 3200 grains cm -2 year -1 at 80% forest cover within 10 km of the trap.
The comparison of PAR with the distribution limit of different tree taxa shows that PAR generally decline with distance ( Fig.   4). A gradual decline is best documented for Quercus where average values at the distribution limit scatter around 100 grains cm -2 y -1 . This analysis also documents the long-distance transport of many tree pollen, including the heavy pollen of Picea.
Where pollen numbers are very low in this comparison there is a degree of uncertainty since the likelihood of encountering a  (1) and minimum tree PAR per every 3 % of forest cover (2). grain depends on chance and is related to the number of pollen grains counted, which we did not consider in this analysis. For 5 better comparison of the absolute values between taxa we used regression analysis to estimate the amount of pollen at 200 km from the distribution limit (Fig. 4b). This comparison indicates that less than 30 grains cm -2 y -1 of Carpinus, Corylus, Fagus, Fraxinus, Quercus and Tilia are deposited beyond 200 km of the distribution of the parent trees. Only Picea shows less than 1 grain cm -2 y -1 at 200 km of the distribution range.
3.2 Recent and fossil PAR values at continental and regional level

Betula
Ignoring traps from the Caucasus and Turkey, trap sites cover the distribution of Betula pendula and B. pubescens. The highest 25 modern values between 11900-73900 grains cm -2 y -1 are found in Estonia and Russia as well as in one pollen trap from Hailuoto. As discussed in the main manuscript these modern PARs exceed values from fossil examples. The highest fossil PAR of around 10000 grains cm -2 y -1 in this comparison come from northern Poland and Estonia and are thus consistent with the area of high modern values. Whether high abundance of Betula is a characteristic of the eastern European forests or a result of frequent disturbance at the forest ecotone or due to recent land-use change is difficult to evaluate based on the available data. 30 Modern and fossil values agree for the sites in central Sweden at PARs between 1900-5600 grains cm -2 y -1 .

Corylus
Modern PARs stay below 2800 grains cm -2 y -1 except for two traps in north-central Poland. Values above 610 grains cm -2 y -1 are also found in pollen traps from the Baltic, Wales, Turkey and Georgia. The pollen type mainly comes from Corylus avellana, while C. maxima occurs in Greece, C. colchica in Georgia and C. colurna in Turkey and in plantations or as ornamental trees in the rest of the Europe. As discussed in the main text, the selection of fossil sites did not include studies from western and

Pinus
Highest modern PARs exceeding 43600 grains cm -2 y -1 are observed in traps on Cyprus, while the values in traps from the northern boreal forest often stay below 5400 grains cm -2 y -1 . Pinus PAR values increase from Finnmark (2000 grains cm -2 y -1 ) and central Sweden (5000 grains cm -2 y -1 ) to the Baltic and north-central Poland with 35000 grains cm -2 y -1 . In northern Europe nearly all Pinus pollen comes from P. sylvestris, while southern European trapping sites have a higher diversity of trees within Early Holocene situation. The lowest Pinus PARs (<125 grains cm -2 y -1 ) are found in taps from Iceland (2-5 grains cm -2 y -1 ), Lagodheki and most of the traps from Lofoten-Vesterålen (38-125 grains cm -2 y -1 ) and single traps from Lappskardet, Exmoor 10 England and Zermatt.

Quercus
Modern PARs of Quercus within the area of distribution of a parent tree in the genus range between 620-15000 grains cm -2 y -1 . Highest modern values (6000-15000 grains cm -2 y -1 ) are found in the traps from the UK, Poland and the southern Balkan.
The first two areas host only species belonging to the Q. robur-type, whereas the two latter also include species from Q. cerris-15 type and Q. ilex-type. The highest fossil values from Suminko seem too high for the region compared to other fossil samples, nevertheless modern values in pollen traps from the region provide comparable high PARs. Also, PARs in recent sediments from some lakes in north-eastern Germany show similar values (Matthias and Giesecke, 2014). PARs to about 1300 grains cm -2 y -1 are estimated for Rõuge Tõugjärv near the distribution limit of Quercus robur where the tree only became abundant after 7000 years ago. Quercus robur reached its maximum abundance even later in central Sweden where the PAR at Holtjärnen around 3200 years ago suggest its presence. The pollen seems to disperse well and is found in small amounts in most pollen https://doi.org/10.5194/bg-2020-217 Preprint. Discussion started: 27 July 2020 c Author(s) 2020. CC BY 4.0 License. Figure 11. See caption Fig. 6 traps beyond its distribution area and fossils samples far to the north of the distribution have also collected Quercus pollen during the Holocene, which may be partly due to the abundance of trees of this genus in Europe.

Tilia
The highest modern PAR 1500-4700 grains cm -2 y -1 was measured in Poland, European Russia (Tula) and Lagodheki in Georgia, however, this high range has no comparison in the fossil record. Lower values 120-1500 grains cm -2 y -1 were found 10 in traps from European Turkey and the Alps. Highest fossil PAR 270-1500 was measured in middle altitude and lowland temperate zone during the Middle Holocene. The closest trap analogues to them are in North Poland. Sites on the current edge of Tilia distribution and in the mountains (Central Sweden, Alps and Rila) show highest PAR range 120-270 grains cm -2 y -1 , also during the Middle Holocene. Trap records corresponding to the lowest class and above the LTD threshold (5-40 grains cm -2 y -1 ) appear in sites within distribution limit of Tilia (Baltic, European Russia, Poland), on its edge (Central Sweden, Wales, Black Sea Coast, Greece) or in the mountains (Šumava, Alps).    The latitudinal gradient in PAR is clearly visible in this dataset. Although data on plant biomass and primary productivity are not available for all trapping locations the regression analysis indicates that mean annual temperature has a strong influence on the quantity of pollen deposition. The July temperature of the previous year determines the amount of pollen production in Pinus near the tree-line (Autio and Hicks, 2004;McCarroll et al., 2003). Evidence from other European regions (van der Knaap et al., 2010;Nielsen et al., 2010) suggests that growing season warmth and other climate variables also explain the interannual 15 variability of pollen deposition. On the regional scale PAR corresponds to plant biomass of the parent tree (Matthias et al., 2015;Seppä et al., 2009). However, differences in plant biomass cannot explain the latitudinal gradient in PAR described here, which may, at least in part, result from the latitudinal gradient in primary productivity of trees (Gillman et al., 2015) as previously suggested (Matthias et al., 2015). An increase in primary productivity and pollen production has been shown in a carbon dioxide fertilization experiment (Wayne et al., 2002), which supports the interpretation that PAR of the same species 20 may vary due to environmental parameters determining its productivity.
Modern PARs from traps near the latitudinal limit of Pinus and Betula have been used previously to reconstruct past changes in the northern distribution limits of these trees (Seppä and Hicks, 2006). Here we evaluated larger distances and therefore had to ignore Pinus and Betula, while suggesting some general thresholds for other dominant European trees. The upper value of around 30 grains cm -2 y -1 for Picea agrees well with the fossil PAR value for the tree of 50 Picea grains cm -2 y -1 found in a 25 sample at Klotjärnen just after the occurrence of the first Picea bud scale (Giesecke, 2005b). However, these modern thresholds estimated here are likely to depend on the abundance of the parent tree in the larger region rather than properties of the pollen types. A larger threshold would be expected for Corylus compared to Fagus, based on the fall speed of pollen. However, Corylus is not very abundant near its northern limit in these more continental areas, while Fagus often dominates forests near its presumed limits. Similarly, the occurrences of taxa outside the mapped natural distribution can bias this estimate. 30 Linking the fossil to modern PAR values facilitates interpretation of the fossil record of individual sites. Unfortunately, the details cannot be discussed here. However, the central Swedish sites Holtjärnen and Klotjärnen provide excellent examples.
These sites are situated north of the modern distribution of Tilia, Corylus, Quercus and near the limit of Alnus glutinosa. The fossil PAR values are higher for these taxa than those found in pollen traps at or near these lakes (Giesecke, 2005a;Giesecke and Fontana, 2008 (Lotter, 1999) and at Meerfelder Maar (Kubitz, 2000) in western Germany 18000 grains cm -2 y -1 . Judging from pollen percentages even higher Early Holocene values should be found in more oceanic 10 situations and the Corylus PAR at Hockham Mere in eastern England may be as high as 40000 grains cm -2 y -1 for the early Holocene (Bennett, 1983). Modern values in pollen traps from Wales at around 2000 grains cm -2 y -1 are far below these early Holocene figures and it is likely that modern analogues of sites with high Corylus PARs no longer exist in Europe.
Conversely, the high modern PAR values for Pinus and Betula from Poland and Latvia are not found in the fossil examples.
Pinus PAR values around 30000 grains cm -2 y -1 were also obtained from 210 Pb dated modern lake sediment samples in 15 north eastern Brandenburg (Matthias and Giesecke, 2014). This study evaluated the PARs for the years 1993 and 2009. The increase in Pinus PAR values between the first and the second sampling period corresponded with an increase in the amount of standing pine volume in the region. Forestry practices aimed at increasing yield could account for the high Pinus values.
Pinus was extensively planted after the 1950s, even on soils where trees with a lower pollen production would have grown naturally. The fertilization due to increased nitrogen deposition, as well as increased atmospheric carbon dioxide, increase 20 the pollen production not only of Pinus. A carbon dioxide enrichment experiment of 19-year old Pinus taeda resulted in a twofold probability of reproductive maturity after 3 years (LaDeau and Clark, 2001). The continued experiment also showed that carbon dioxide fertilization increased the number of pollen cones and therefore pollen grains produced per tree (LaDeau and Clark, 2006).
In the case of Pinus, the modern dataset includes trap data from Cyprus, where Pinus brutia dominates at 1600 m a.s.l., 25 resulting in even higher Pinus PAR values compared to those found in the Polish and Baltic regions. The highest Betula values come mainly from Russia, where values frequently exceed 30000 grains cm -2 y -1 . We previously discussed such high fossil PAR values for Corylus, which is assumed to produce a similar amount of pollen. However, fossil Betula PARs in the examples considered here are consistently below 6000 grains cm -2 y -1 and published early Holocene values rarely exceed 6000 grains cm -2 y -1 (but see Theuerkauf et al., 2014). Pollen diagrams from the forest steppe ecotone in European Russia 30 are often characterized by high Betula percentage (Nosova et al., 2019;Shumilovskikh et al., 2018). However, there are no suitable diagrams with reliable PAR estimates from that region. It is thus difficult to judge whether high modern trap values are associated with recent land-use change or are characteristic of eastern European forests.
The comparison of regional PARs between traps and fossil estimates indicates higher fossil PAR of Picea, Fagus and Abies in middle altitudes of the temperate zone (Fig. 5), which, in the case of Abies, represents the Europe-wide decline in A. alba 35 (Tinner et al., 2013). in the cover abundance of Picea and Fagus (Abraham et al., 2016;Carter et al., 2018). Within a 60 km radius of the fossil sites Picea decreased in abundance from 70% during the Middle Holocene to 43%, compared to modern abundance. Fagus and Abies declined from Late Holocene values of 22% and 3% to currently 20% to 1% respectively (Abraham et al., 2016). The abundance of Abies in the Roztocze region (SE Poland; Fig. 6) provides a good analogue for the past abundance of the tree in Šumava with maximum PAR of 1000-3000 grains cm -2 y -1 . Abies disappeared from the Czech Republic during the Mediaeval 10 Age due to forest management methods (Kozáková et al., 2011), which were not practiced in SE Poland.

Limitations and problems
There are significant differences between the accumulation of pollen in traps and on peatlands and lakes (Lisitsyna et al., 2011b;. Differences in pollen trap design and placement in the landscape will influence the values. Trap values are also affected by modern processes that have no impact on the fossil signal. The large consistency of the data collected 15 in the PMP database indicates that these effects may be small. Nevertheless, some traps or individual years have unusual values and were removed from the comparison (Table S1). Despite this, the uncertainty of fossil PAR values is much greater than pollen traps, which is primarily due to the added uncertainty coming from sampling a sediment core, combined with the uncertainty of the age model (Maher, 1981). PAR from lake sediments has additional biases due to differential sedimentation of pollen grains in lakes (Davis and Brubaker, 1973), sediment redeposition, focussing and catchment erosion (Davis et al., 20 1984;Pennington, 1979). Although we carefully selected the best available fossil sites PARs from lake Suminko and Rõuge Tõugjärv may be biased by lake internal processes and the addition of stream borne pollen respectively. Nevertheless, their fossil PAR estimates are in the range of values found in pollen traps. Where detailed knowledge on the sedimentation process is available the bias of sediment focussing may be reduced as in the example of Hockham Mere cited above (see also Bennett and Buck, 2016;Bennett, 1983). Peatlands may thus seem the better choice for obtaining fossil PAR, which may be the case 25 in northern Scandinavia (Barnekow et al., 2007;Finsinger et al., 2013), but frequent changes in the rate of peat growth lead to difficulties assessing the time represented in individual samples at many sites.
The problem of traps being contaminated by pollen brought by insects and small animals was mentioned in the method section and for this reason only Poaceae and Cyperaceae pollen were included in this comparison. However, pollen from these two families is also often overrepresented in the pollen traps (Lisitsyna et al., 2011b), as the plants may overhang the Detailed comparisons of vegetation data to PARs hold potential for a better understanding of the spatial representation and processes shaping the pollen signal (Matthias and Giesecke, 2014) and allow estimates of absolute pollen productivity (Sugita et al., 2009). However, forest inventory data with the detail essential for this type of study is not available for all traps. The forest cover data presented here presented here has a resolution of 1 km 2 , so that it was insufficiently detailed to consider the distribution of trees within hundreds of meters of the traps. Moreover, without information on standing volume or age structure 5 the percentage cover used here is a crude measure of the vegetation producing the pollen and it is almost surprising that this variable has some explanatory power. Forestry practices like harvesting trees that start flowering at a later age (e.g.  years) reduce the number of trees producing pollen (Matthias et al., 2012) and bias the search for modern analogues.

Conclusions
Comparison of the mean annual PAR from traps and fossil sites showed similar ranges for Abies, Alnus, Betula, Carpinus, grains cm -2 y -1 should therefore be regarded as potentially of long distance origin. The application of these threshold values holds potential to refine and adjust reconstructions of tree distributions. Andersen, S.: The relative pollen productivity and representation of north European trees, and correction factors for tree pollen spectra.,