the relationship between climate and annual pollen deposition at northern tree-lines

The relationship between climate and annual pollen deposition atnorthern tree-lines

Sheila Hicks *

Department of Geology, University of Oulu, Linnanmaa, 90570 OULU, Finland

Accepted 30 July 1998

Abstract

Sixteen years (1981±1996) of results of monitoring annual pollen deposition (grains cmÿ2 yrÿ1) are presented for a

transect from the Betula woodland south across the northern tree-lines of Pinus and Picea in Finnish Lapland. The

annual variations in pollen deposition, which follow the same pattern all along the transect, are interpreted as a re-

sponse to regional climate. Calibration with a range of climate parameters is tested. Those which seem to best determine

the variation are temperature in the late summer of the previous year together with temperature in the spring and early

summer of the year of deposition, coupled with the presence or absence of frost days during these two periods. The

prevalence of southerly winds during the early summer may also be a factor. Ó 1999 Elsevier Science Ltd. All rights

reserved.

Keywords: Pollen deposition; Climate; Northern Finland; Pine; Birch

1. Introduction

Controlled experiments to measure empiricallythe response of plants to changes in a range ofclimate factors can only realistically be carried outfor time periods which are merely a fraction (2±5yr) of the actual time over which the changes arepredicted (10s or 100s of years). Proxy recordswhich reveal the same data but for a considerablylonger period of time are therefore very useful.Such proxy records consist of plant parts which

are preserved fossil and which contain within thema climate signal. The signal is usually in the formof annual variation and, consequently, for it to beof use the records must be long and continuousand have a good chronological control. The twomost obvious records are those provided by treetrunks (dendroecology, Lindholm, 1996) and pol-len. It is the pollen record which is considered here.

Pollen of the northern boreal tree species is welldispersed and easily recoverable from anaerobicdeposits such as peats and lake sediments. Suchsediments, which accumulate by the regular addi-tion of material from above, also provide a goodchronological record, the resolution of which isdependent upon the rate of accumulation. There

Chemosphere: Global Change Science 1 (1999) 403±416

* Corresponding author. E-mail: sheila.hicks@oulu.®

1465-9972/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.

PII: S 1 4 6 5 - 9 9 7 2 ( 9 9 ) 0 0 0 4 3 - 4

is, however, a very obvious annual variation inpollen production, which can be clearly seen in theaerobiological records (The Finnish Pollen Bulle-tin, 1986; Spieksma et al., 1995) and this variationis caused by climate. It follows, therefore, that thefossil pollen record can potentially reveal this an-nual climate signal. Four conditions need to beful®lled for the signal to be extracted:1. It must be possible to measure the actual

amount of pollen being deposited on the sedi-ment surface and incorporated into it.

2. The origin of the pollen in the sediment must beknown.

3. The fossil sample must correspond to a knownperiod of time, preferably a single year.

4. The relationship between the annual pollen de-position value of a species and the climate fac-tor(s) governing it must be known.These are by no means easily ful®lled condi-

tions. Point 1 is now fairly well established (Davis,1969; Bennett, 1994; Hicks et al., 1996), and sim-

ilarly much work has been done to establish point2 (Tauber, 1977; Prentice, 1986, 1988; Sugita,1993, 1994). Point 3 is clear in the case of annuallylaminated lake sediments but is more complicated,though gradually being achieved, in the case ofpeat deposits (Simmons, 1993). It is point 4 whichthe present paper focuses on.

2. Methods

Annual pollen deposition is monitored bymeans of modi®ed Tauber traps (Tauber, 1974;Hicks and Hyv�arinen, 1986; Hicks et al., 1996). Anetwork of 19 traps has been established fornorthern Finland but the results from only threeare presented here (Ke8, P9 and S22). These threemonitoring stations are on a north±south transectwhich crosses the northern limit of Pinus sylvestrisforest (Fig. 1). For precise details of the nature ofthe vegetation in which the pollen traps are situ-

Fig. 1. Location of the pollen trap sites with respect to the latitudinal vegetation zones in northern Finland.

404 S. Hicks / Chemosphere: Global Change Science 1 (1999) 403±416

ated see Hicks (1994, 1996). The pollen traps arelocated in the centres of small (< 200 m diameter)mires with their openings at ground level and areemptied once, yearly in September, at the end ofthe ¯owering season. Their contents are preparedby a standardised method (Berglund, 1986; Hickset al., 1996) and the quantity of pollen they con-tain is calculated by reference to a known quantityof added spores (Stockmarr 1971, 1973) so that theresulting values can be expressed as grains cmÿ2

yrÿ1.Values are available for the sixteen-year period

1982±1996 (intermittently for 1981±85 and con-tinuously for 1986±1996). Because of the terrestriallocation of the traps, the results are considered asbeing comparable to fossil values obtained fromhigh-resolution sampling of peat deposits. Acomparison with pollen in¯ux values obtainedfrom lake sediments is not necessarily to be ex-pected (Bonny, 1976; Davis, 1968, 1973; Davis andBrubaker, 1973; Davis et al., 1971), though, sur-prisingly, for this area, a correlation does seem toexist (Hyv�arinen, 1975).

The possible range of climate factors whichcould a�ect the quantity of pollen being incorpo-rated into a terrestrial sediment is preliminarilypredicted by analysing those factors which in¯u-ence pollen formation (within the plant), pollen

emission, pollen transport and pollen deposition(Fig. 2).

Records from the national meteorological sta-tions nearest to the three pollen localities (Kevo,Inari and Ivalo, for Ke8, P9 and S22 respectively,Fig. 1), for the 10 yr period, 1986±1995, providethe basis of the comparison (Meteorological Of-®ce, 1986). The following data are tested bygraphical comparison with the pollen record: av-erage monthly temperature in July and August,number of frost days in August and September,average monthly temperature in May and June,length and continuity of the warming period inMay and June, and prevalence of southerly andnortherly winds in May and June.

3. Results

The annual pollen deposition of the three majortree pollen types Betula, Pinus and Picea at thethree monitoring sites is shown in Fig. 3.

It should be remembered that all the sites arenorth of the northern limit of spruce forest (Fig. 1)and so all the Picea pollen recorded in the trapsmust have been transported by the wind fromfurther south. In addition to this two features arereadily observable.

Fig. 2. Climate variables which can potentially in¯uence pollen deposition.

S. Hicks / Chemosphere: Global Change Science 1 (1999) 403±416 405

Fig. 3. Annual pollen deposition for the three major tree species in northern Finland for the period 1981±1996 as recorded at sites Ke8,

P9 and S22.


1. At all three sites, irrespective of what the localvegetation is, the annual variation of the indi-vidual tree pollen types show the same patterns.

2. For each tree pollen type the actual quantity ofpollen is highest at the site where that tree spe-cies dominates (or in the case of Picea is closestto the site) and lowest at the site which is fur-thest from where the species is most abundant,i.e. at Ke8 Betula values are highest, at P9 theyare intermediate and at S22 they are lowest,while at S22 Pinus values are highest, at P9 in-termediate and at Ke8, lowest.These two features epitomise the two signals

embodied in pollen deposition. The ®rst can beinterpreted as illustrating the controlling role ofclimate on annual pollen variation, while thesecond illustrates the relationship between thegeneral quantity of pollen being deposited and thepresence/absence of the corresponding tree speciesin the surrounding vegetation. This is more clearlyseen in Fig. 4 where the average values for the 16yr period are plotted relative to the forest limits ofthe species in question (Hicks, 1994; Hicks et al.,1997).

It is the annual variation, the climate signalwhich is considered here. It is evident (Fig. 3) that1989 was an exceptionally high pollen year for allthree pollen taxa and, similarly 1986. In addition,1995 was a high pollen year for Pinus and Picea,and 1991 and 1993 for Betula. The year 1988 was a

low year for all species, and 1990 for Betula andPicea, while 1996 was also a low year for bothPinus and Picea. There is some very slight sug-gestion of a biennial ¯uctuation for Betula. Thishas been noticed by other workers (J�ager et al.,1991; Spieksma et al., 1995) although they pointout that it is not statistically signi®cant. It isthought that this is related in some way to thephysiology of the tree species and not to climate.

It seems clear that there is a relationship be-tween annual pollen deposition and climate butthat it is a complex one, and one which is alsooverlain by other factors which are related to long-term eco-climatological and/or eco-physiologicalconditions. The approach here has been to pickout the extreme years and to try to see which, ifany, of the selected possible climate factors un-ambiguously matches these. An initial attemptalong the same lines was published in Hicks(1996), but the factors are more speci®cally de®nedhere.

The meteorological data have been selected andgrouped on the basis of the situation postulated inFig. 2. Those factors which may a�ect pollen for-mation are considered as one group, namely av-erage temperatures in July and August, andnumber of frost days in August and September(Fig. 5).

Those factors which possibly a�ect pollenemission, such as average temperatures in Mayand June, and the nature of the beginning of thegrowing season, form a second group (Fig. 6), andthe wind factor which partly a�ects pollen dis-persal (Table 1), is a third.

In this study no individual additional factor isselected in connection with pollen deposition, al-though precipitation was considered in this respectin an earlier analysis (Hicks, 1996) and could berelevant (Norris±Hill and Emberlin, 1993).

The higher than normal temperatures in July1985 and July 1988, coupled with the relatively lownumber of frost days in August in each of theseyears (Fig. 5) could be linked with the high pollenvalues of the following years, 1986 and 1989 re-spectively, i.e. conditions for the formation ofpollen within the tree were good. Although thereare other years with similar frost-free conditions inAugust (1989, 1990, 1993 and 1995) these are not

Fig. 4. Average pollen deposition (1982±1996) for the three

major tree species in northern Finland on a north (Ke8) south

(S22) transect across the pine forest limit (P9).


Fig. 5. Climate factors which could a�ect pollen formation: late summer temperatures and frost.


matched by high temperatures in July and August.Admittedly the July temperatures in 1993 are rel-atively high but in that year there were a recordnumber of frost days in September. If this com-

bination of high temperature and few frost days isrelevant for high pollen values in the followingyear then the opposite should also hold good,namely that low July and August temperatures

Fig. 6. (a) Climate factors which could a�ect pollen emission: spring temperature and (b) Climate factors which could a�ect pollen

emission: the nature of the growing season.


and many frost days result in low pollen values inthe following year. The years with the lowest latesummer temperatures are 1987 and 1992. Of these1987 has the greatest number of frost days inAugust but 1992 has very few frost days. 1988 is,indeed, a signi®cantly low pollen year and 1993 a

low year for Pinus and Picea but not especially sofor Betula.

When the growing season during which pollenis emitted to the atmosphere is considered then thehighest May temperatures were recorded in 1989and 1992 and the highest June temperatures in

Fig. 6. (continued)


1986, followed by 1989 and 1992 (Fig. 6a). As hasalready been emphasised, 1986 and 1989 were bothhigh pollen years but 1992 is a low pollen year forboth Betula and Picea and only a moderately highone for Pinus. 1992 was also a year in whichtemperatures rose to high levels very quickly (Fig.6b). It took only 15 days for the cumulative tem-perature sum above 5°C to reach 40°C. A com-parable rapidity was seen only in 1995. It wouldappear, however, that these favourable conditionsat the beginning of the growing season only havean e�ect if the previous late summer has also beenfavourable. If little pollen was forming in the treethen it is immaterial how good the conditions foremission are because there is simply very littlepollen available to be emitted!

If the high and low pollen years are comparedwith wind direction then it is seen that the highpollen years, 1986 and 1989 are associated with

southerly winds during the months in which thetree species ¯ower, and the low pollen year of 1988with basically northerly winds (Table 1). This re-lationship is much stronger for the month of Junethan it is for the month of May.

Table 2 is an attempt to classify these di�erentclimate factors and their combined e�ects. All thedata from Figs. 5±7 and Table 1 are combined andgiven a signi®cance weighting on a two or threepoint scale (*, ** and ***). For each factor theyear(s) with the highest weighting (or lowest in thecase of the negative factor, frost) is highlighted(shaded in Table 2). The number of highlightedfactors for any one year are then summed to give aclimate ÔscoreÕ. On this scale the maximum wouldbe 9 and the minimum 0. This is done separatelyfor the two meteorological stations for which thewhole range of data are available, Kevo and Ivalo.These represent the northernmost and southern-most points on the transect and it can be expectedthat the Kevo situation is most relevant to Betulaand the Ivalo one to Pinus. The high pollen yearsof 1989 and the low pollen year of 1988 match wellwith the Ôclimate scoresÕ at both sites and for bothspecies (Figs. 7 and 8).

The best consistent match between the pollencurve and that of the Ôclimate scoreÕ is for Pinusat Ke8. Pinus is present at Kevo but only asscattered trees or forest patches, the real forestlimit being further to the south (Fig. 1). Thebiggest discrepancies between the curves forpollen values and Ôclimate scoreÕ are in 1991,1992 and 1993 for Betula when the curves actu-ally go in the reverse directions. This is not thecase for Pinus, however, except for the consecu-tive years 1993 and 1994 at Ivalo, where thetrend for Pinus goes in the opposite directionfrom that of the Ôclimate scoreÕ. As previouslynoted, the start of the summer in 1992 was, cli-matically, particularly favourable and this largelyaccounts for the high weighting of the ÔclimatescoreÕ in that year. This, however, may not besigni®cant in term of pollen deposition becausethe late summer of 1991 was bad and, therefore,pollen formation within the tree will have beenpoor. This may explain some of the non-agree-ment between the Betula pollen curve and theÔclimate scoreÕ curve in 1991±1993.

Table 1

Climate factors which could a�ect pollen dispersal. Dominant

wind direction at the time of pollen emission

Kevo Ivalo

Dominant wind direction in May

1985 N NE

1986 S N

1987 N N

1988 NW N

1989 W SW

1990 NW N

1991 N,S NE

1992 S SW

1993 N S

1994 N,S N

1995 S N

1996 N N

Dominant wind direction in June

1985 N N

1986 N,S S,W

1987 N N

1988 N N

1989 S SW

1990 N N

1991 N NE

1992 N N

1993 N N

!994 N N

1995 N N

1996 N N


Table 2

Nature of the di�erent climate factors which could a�ect pollen formation, emission and dispersal and their variation during the period

1986±1995


4. Conclusions

More analysis needs to be made of other cli-mate factors and the data also need to be treatednumerically to test the relationships objectively,and to assess whether the correlations are statis-tically signi®cant. A bigger data set (results areavailable for the same period for 14 pollen traps)

could also be considered. The results presentedhere must, therefore, be treated as preliminary.At the moment the range of variation inherent inthe calculation of the pollen deposition values(Maher, 1972) has not been assessed, neither havere®nements of the climatic variables ± dailytemperature rather than average monthly tem-perature, the number of dry sunny days during

Fig. 7. Climate scores for Kevo and Ivalo in relation to Betula pollen deposition at Ke8 and S22 respectively.


the spring warming period and the dominance ofsoutherly winds during just those days, etc. All ofthese would increase the resolution of the corre-lations, pin point areas in which their validitymay be questioned, and ultimately help in con-®rming the precise nature of the climate signalwhich can be derived from pollen depositionvalues.

The results presented here appear to indicatethat the conditions at the end of the precedinggrowing season are more important than those inthe early summer of the year in which the pollenis emitted to the atmosphere, although if boththese periods have favourable climatic conditionsthen pollen deposition can be really high (as in1989). A series of earlier than normal frost days

Fig. 8. Climate scores for Kevo and Ivalo in relation to Pinus pollen deposition at Ke8 and S22, respectively.


during the period of pollen formation can be di-sastrous. Wind direction is relevant in bringingextra pollen from outside the area where pollendeposition monitoring is taking place (Hicks etal., 1994, 1997). This pollen can either emphasisethe climate signal if conditions have been gener-ally favourable over an extensive area (100s or1000s of kilometres) but will blur the limate sig-nal if local climate conditions di�er substantiallyfrom those of areas further a®eld at the criticalperiod of pollen emission. Given the northernFinnish situation in which the forest vegetationbelts are broad and arranged in east±west ori-ented bands to provide a clear north±southtransect, southerly winds at this critical time maybring northwards up to 20% of the more north-erly areaÕs annual pollen value (Hicks et al.,1994).

In any case, pollen deposition data are bestreviewed from the point of view of annual varia-tion and long term trends. It is not possible toassign individual climate values to given amountsof pollen for speci®c species since the long termaverages of such ®gures tell more of the presence,absence and abundance of the tree species in thesurrounding vegetation. The pollen trap data arebeing extended backwards in time by analysing thepollen deposition in thin peat samples at a � an-nual resolution. The longer term annual variationrecords thus obtained will then be correlated withdendroecological series from the same sites. Theclimate signal obtained from these two indepen-dent but related series will allow a much morecon®dent prediction of future trends in pollenproduction in tree-line areas.

Acknowledgements

I would like to thank Raija±Liisa Huttunen forthe laboratory preparation of the pollen trapscontents and Hannele Ker�anen for helping to ab-stract the meteorological data from the monthlyreports. Comments from two reviewers, PierreRichard and Bas van Geel, are gratefully ac-knowledged. This publication is a contribution tothe EU-funded project (ENV4-CT95-0063) FOR-EST.

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the relationship between climate and annual pollen deposition at northern tree-lines

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