-Polen 10·39-19( 1999)

AEROPALYNOLOGY OF BIRCH (BETULA SP.) L~ SPAJN

Jato, V.'; Aira, M.J.2; Jgléslas, M.l.1; Alcázar, P.'; Cervigón , P.'; Fernández, D.'; Recio, M.'; Ruíz, L.' & Sbai, L. •

'Oapartal1'anlo de Bio!ogla Vagelal y C1enc as del Suelo, Universidad de VIgo, Facultad da Cianc·as, Campus Universitario As Lagoas. 32~-0urense. Spam.

' Departamento de Biolog·a Vegetal, Facultad de Farmacia. Unr1ers dad de Santiago de Compostela, 15706-Sant,ago da Compostela, Spa1n.

' Departamento de Blologla Vegetal, Campus de Rabana es. Universidad de Córdoba, 14004·Córdoba. Spain.

' Departamento de Biología Vagetalll, Facullad de Farmacia, Unr1arsidad Complutense de Madrid, 28040-Madrid, Spam. ' Departamento de Bioiogla Vegelai, Facullad de Biología,

Universidad de León, 24071-León, Spain. ' Departamento de Biología Vegetal, Facul:ad de Ciencias, Universidad de Málaga, Apdo. 59. 29080-Málaga, Soain.

' Departamooto de Biologla Vegetal, Facu ~ad de C1encias, Universidad de Granada, 18001·Granada,Spa'n.

• Unidad de Botánica, Facultad de C1ellC18S, Unoversidad Autónoma da Barcelona, 08193-Bc !aterra, Barce ona, Spain.

SU~1MARY : The aim of this study was 10 <lcterminc the bdta VJOI vr OiiCb pollell in 13 Sp:mish cilics. with di ffcr-cnt bioclirnauc :.ncJ planl char:u:tcristícs, nncl to dclimil the a.rcas in which unpor1lmt conc~ntr.:nioll!. nrc rcachcll lln\.h ¡JOlleu is prcsent lli Sp3in's :Hrno:,ph1:re fwm ~·t:uch to thc cnd of M ay, thc hig)1est valucs OOng rccordcd b.:twc .:n muJ- \ 1;u¡;h ;ullt thc end of April Dai ly mean concentr:uio:t! gr~:ncr thiln 100 polleo gr:nnVm' are only (O..JI\(.l in 1he northMltem comer of 1bc lbcrian l'tn1nsull (~.noab'll ~' Comt"'<ICI>. OurcnS< and Vigo). ll.crc is ó1 StttJnd ZO.lt CO\~riog the llOflhem balf o( thc COJntr¡, exclllfling thc :t(crcmcntiUI'tl"tl

3rt:a, ia wtuch daily mean \'3 lues of SO pollen grain5/m1 a.e rnrcly Cl:ceedOO. Lastly. in 1 te rcst or 1he coontry, Bctub pollen J~,-cls arl! \'t'C)' 1~ or practic-JII) r.on-e,.islcnl. Nrnilhk dttT~.;.rcnccs in 101.3.1 annual Bc1t.:la pollcn couniS au: r¡;;;;~.rdt.ood for UJC samc. Ctl}' in diffcrenl )c:lr~ Altha:Jb)l thc t.hHcrences beto,;.•een tbc onset of pollin:uioo in the samc city for dirfcrent yc:us :lre gfllernll}' not very significJnl, cootrasting m::tcorologicJI condition) recorded frOIIl ene ycar to thc next may produ::c dirfcrcnccs of up to 30 d:l):t

KEY WORDS: Bem!a. Birch. Spain, Ac robiolog)'

RESUMEN: En el p•csente lf:t})¡jo, se reJiiu un c.stuc:lio compomnivu c:uuc el COII•I>OtC3· n,iento del polen de abedul en 13 loc:alidadcs de Espaii:!. con ca.raclcrf~ticas bioclin\.i tic:\s y bios,eogr.Uicas diferen tes. El polen de :tb<dul e~tá í)ICSentc en la atmósfcr:a en Espana durante los meses Ce lr.:lrto a tU3.)'0, y alcanu sus concenu~cioocs más c:lcva<b~ cn11c l.1 seguntb qumceoa de mar1..0 y final c!c abril. lJaicamcntc en las loc3hdadcs siU;.:td:t,. en d extremo noroccidcntal de la r cniasula lbtrka (Santiago de Compostela, Vif'O y Our: nsc). se alcanzan conccnuacioncs mt:d ía~ diarias Sl.J)(riores a 100 gra(los/ntl. En el resto d¡;; !01 mi uu.J norte lo) valores med1os di.uios rJrameniC exceden los 50 gnr~QS/m ', " t:n las otras Joc:.lidades, cl [l()lcn de abedul aparece en ronccnlr.lciOIIC~ nmy bajas o estj au5c:nt( P.:~.ra

39

Jaro el al.

un3 misma ciucbd, exi~tcn imponantt:s diferencia$ cnuc l.u cantid:Jd~"S :muaks registradas ~n los dlfcrcm.cs _anos. En g~ncral, l:u di fcrenc1a.s en el inicuJ tk la cs ración pol ínica no son Importantes. SI b1en. en 3lgun año, llega :l. l\.'lber hasl:t JO dí:ts ele difcrcnciJ. PALAilRAS CLAVe: llttuln. Abedul. !!spaña, Acrobiologfa

INTROOUCl"ION

Birch is rcprcscmcd in Spain by two spccics: Bemla alba L. and /Jewla pe11d1tla Roth. (MORENO & PEINADO. 1990). The former is more widcly distribmcd, especially in the nonh, being more abundant towards lhe west where it is lhe dominan! o ce and fonns high mountain oro-Cantabrian acidophilic forcsts, with a clearly Euro..Sibcrian distribution. "Iñeyarefound atallitudes ofovcr 1,150 meters, as the last trcc i"ormalions of the altitudinal scquence, with mountain therrno-cl!mates and hyper-humid ombro-climatcs (IZCO, 1994). Thcir lim it , although the subjcct of sorne comroversy, is situated in the Galician mountain ranges of Anc:trcs and Caurel (COS­TA el al., 1990). In 1his sarnc arca. but on siliccous soils and with a greater Mcditerra­nean influence, therc are al so birch forests in thc Galician·Portugucsc hi gh mountain layer and the Ourense·Sanabria and sup1 a-Meditt>­rranean layer.

Galicia's mountains and foothills are populated by non-climactic birches. in place of mountain oak graves, which are located on acidic soils and with altitudinal limits bctween 600 and l, 100 meters.

In the Euro-Sibcrian rcgion, birch m:w form pan of riparian forcsts. togcther with Alnus gl111in osa. Salix alrocinern and Fran.~ula a/nus. Lastly, in othercharacteristic forcsts uf 1hc Cantabrian mountain range or E.1stern Pyrenees. birch is al~o found w1th beech and peLiolatc oak.

Thc dislribution of /Jcwla pcndula is more limitcd, forming part of Pyrcncan

40

mountain ash forests and of dense bushes in place of acidophi lic Pyrenean fir forests. lt can :tlso be found in the ccntcr of thc pcninsula and in thc southeasrem mountams as a species cultivated 111 differen! Spamsh nreas (RIVAS:v!ARTÍNEZ, 1987).

Birch is an ancmophilous tr"c with high pollcn produc1ion (MOORE & \VEBB. 1978; LEWIS el ni., 1983), whose allergenic capacity has been described by numerous authors (SP lEKS:v!A, 1990; NORRIS-H lLL & E:v!BERUN, l991;D'AMATO&SPTEKSMA 1992). lts polleo i~ cons1dcred to be the m a in cause of pollmos1s 111 Norlhern and Central Europe (WIHLclai .. 1998;SP1EKSMAe~al. ,

1995), not only during its polleo scason but even during prc1•ious and subsequent pcriods. sincc i1s polleo can be casily transponed 01·er long diswnces (WALLIN el al.. 1991; HJEL­lvll~OOS.l991). bJsuchcases,nntigcnic activi1v seems to be linkcd to allergcns ciJamclcri•l i~ of birch polleo grains 1ha1 are deposited on dust partJclcs inside houses, triggering the onsct of allergic processes. e1·en up 10 t"o months after maximum pollen conccntmtions are recorded (EKEBOM e1 al., 1996; RANTlO· LEHTI-MÁKI e1 al., 1996).

In vicw of the above (mainly the abun· dance and allergenic nature of this palien), severa! researchcrs havc carricd oul acro­palynological studic> on this ta'on, in order to determine thc model of seasonal and daily bchavior of birch poli en and the innuencc uf the different mcteorological paramctcrs on pollen conccn1rarions (SPIEKSMA el al., 1989; ATKINSON e1 al .. 1990. GALÁN eral

1991: NORRIS·HlLL &EMBERUf\. 1991~· SPIEKSMA e1 al., l995). ln th is way, models

Pole11

can be cstablished in nrder to predict both thc beginning and severity of the pollen smon (RUFFALDI & GREI'FIER, 1991; ANDER EN, 199l;LARSSON, 1993:CARA· MIELLOcral., I994;BRICClllelal., 1995).

In Spain. pollen birch data for rcccm ycars from diffcrent localnics forming thc Spanish Aerobiology 1\etwork (REA) were published in the REA 13ullelin. Neverthelc>s. very few studies have focused cxclusively on the acropalynological aspects of Bewltr; in Spain. this has only been studied by AIRA et al. (1998) in Santiago de Compostela. The aim of thc present study was 10 determine thc bchavior of this polleo, which is of such great allcrgenic import:mce. in severa! parts uf Spain with different bioclimalic and plant charactcristics. :md tn delimll thc arcas in which signir,cant concentrntions are reached.

MATERL<\LAND METI IODS

The periods studied diffcred according to thc individual s:tmpling stattons: Ourense, Santiago, Cordoba and Granada have the most extensi\'c records (1993-1998); Lcéln, llarcelona, Mi laga and Madrid (1994- 1998): Vigo (1995-1998); Estepona ( 1995-1997); and Lleida, Tarragona and Girona ( 1996-1998).

AJI ofthe stattons u>cd Hirst poli en trnps, siluated at approximately 25 metcrs abovc thc ground. The methodology recomrncnded by the Spantsh Network of Aerobiology (REA) was used to procc,s and interprctthe samplcs (DOMÍNGUEZ, 1995). ln orderto cstablish the beginning of the pollen season, the critcrion proposed by ~ULLENDERS et al. (1972) "~s adoptcd, according to which the scason begins when the running five-day average rcachcs or cxcecds 1% oft he annuar total. In tcrms of the applicatton of this

\lo/ 10 ( 1999)

Aerobio/og\' oj !Jemla m Spai11

criterion. although thts study does not cover a Iarge numbcr of ycars. thc authors followcd the satnc criterion as that u sed by SPIEKSMA er al. (!998). \\ hereby the annual tota l is decmed to be the average value of the ycars >tudied. The advantage of this ct iterion is that it enables the bcginning of thc 'cason to be predictcd with thc samc threshold. lt can therefore becalculatcd without having to wait for the total results rccorded tn cach specific year. In thosc cities and/or ycars in which the annual total Be1ula pollcn cuulll failcd lo re:~ch 100 po llcn grains. thc date of the bcginning of the pollcn sea son was taken as the r,rsl oftwo eunsecutive d:tys on which al lca.t 1 pullen gramfm> was recordcd.

RESULTS

Table 1 shows, for each s~mpling station, the annual daily rnaximum and the date on which this was recurdcd. the date o f the bcginning of the pollinalion period and the conccntration of birch pollcn rccorded on thnt day. It also shuws lhe average annual toml for cach city and the numbcr of days on which more than 50 pollen grainslm' wcre recorded.

13y basing the d:tte of the bcginning of thc palien sc.~wtl on a diffcrent average value for each city. the number of pullen grains/m' marking lhe begmning of the period varied. Thus, in thc case of Santiago, this bcg ins whcn the running fi\'e-day average reaches 23 graimJm'; in Vigo, this value is 9 grains/m1;

and in Ourcnsc, 11 gramsfml. /\fter applying this criterion, virtually no differcnces wcre observed bctween thc dates of thc bcginning of the period calculated using othcr criteria (LEJOLY GABRI:EL & LEUCHSNER. 1983; PATH IRANE. 1975; MULLENDEHS el al., 1972: DRTESSEN el al., 1989: NILSSON &

41

Jaro et al.

n"~w~·~~Fo~r J,'lrlt di '' SC:l~On Mrunuf Onysnumhcr Sil e~ Yc11n~: l't~k (•In te) ronrrn"triii'P.n tnnuml total >SO

SU SO I\ (J:n lnilm }

1993 66 119 /J ) 91' S 1 1 99~ 2S.! C!9t·n l'/.1 39 1 ~

Ourtnse 1995 1 ; (u'~ f·h J I/) 17 112 1 IJ 19% 1 ~3<171<1 11 /~ 14 l 19n 15'J.i29/31 IW 29 S 1998 37t271)¡ 21/l 32 o 1993 160 1151"' 27/J 4.~ 8 1 99~ 368 (1)141 2813 16 12 ard 13

S nnti3&0 d~ 1995 3'11 ( 1';2/-11 3013 62 1332 19 Com¡, ostel:l 1996 1?:2 (2CJ/4l 16/4 57 5

1997 56 1 llSill 171J l l 22 1998 ¡ ; 1 12 7131 l lil l9 6

199S 204 f l.t/4'¡ 291J 12 IJ

VIgo 1996 7l lll1•' IM4 Jl 876 l 1991 172 l2~131 14/J Jl 7 1998 29 (27/~ ) Z.&/3 19 o 1 99~ JR t:U~1 :! Li/~ 8 o 1?95 !3 r3/4t j/-1 23 o

Lt6 rt 1996 27 116/JI 12/J 3 209 o 1997 S?IISil\ 1 1/J 19 1 1998 5 (J.1/,)) H /J 1 o 190J 1CJ f:!f51 2613 9 o 1 99~ .'2 ( J/.&l I)J! 11 o

BArcelona 1996 1 ti5 V :!5/-H 11/J 1 165 o 1991 J8 ('1/J} 18/l 1 o 1998 J 18 v ~M.!I 1/J 1 o 1996 1 l l /2 1 o

T2rragou" 1997 21 •29/.H ~J/J J 110 o 1!>98 211 ~,_,, 1115 1 o 19% 18 126/JI IJ/4 13 o

Giron;t 1997 ~8 (2/.il ~J/j ~ 231 l I ??S 61161"1 Jll! 1 o 1996 4 (26/J, 2ol3 1 o

lld d:l 1997 2s tJt.n 1913 l 91 o 1998 J 119/SI 814 1 o 19'JJ 11 llO/J1 11/J 2 o 1995 1 t; CI JIJ 1 )/! 1 o

1\1;utrid 1996 \ ( J.C(J ) 13/J 1 43 o 11)97 1 r 1.~1.n 1 ~ll ! o 1 ~9S 6 18/51 :!61-' 2 o 1905 11 11 4/ J l 814 1 o

Kstr1>ona 1?96 2 t29 V 1CIT.\I 2713 1 21 o 1997 ~ t!VJ l 21f2 1 o 1994 l2 130/J 1 l6JJ 1 o 199S 1' ( 1 ~1.!' 11/.! 1 o

~Uiag :t 19% 3 126/J I 16J~ ' 29 o 1997 6 1)0/JI 2112 2 o 1998 2 119 ... ::!0/51 17/J 1 o

TABLE 1. Maximum birch oollen ~PflltRiinne ~:uvi..tt.. ... t.nr.nr.v.~ ... J~ "-CUI' h:":} &U."fi;.'\1 r1 eacn tocallty ano year, dates of the be;¡inning of polf91l seasons, f1rst day season concenuations, means of aMual lotal birch polleo counls and number of U>e days v. ~h ooncenlrafons of ove1 SO polen grainslm'

42

PERSSON. 1981) and the criterion obtained in this case (in most years and citics the differences ranged between 1 and 4 days), with the exception of 1993 in Ourcnsc, whcn the start date was put back up to 13 days ;~ccording 10 thc critcrion employed.

Figure la shows the evolution of annual birch palien grain counts during the study period in Santiago, Ourensc and Vigo. Thcsc localities presented thehighest values. 1n all the ycars studicd, Samiago was the station with the highest pollen concentra tions, with valucs ranging between 4,608 palien grains in 1997 and 966 in 1996. Thc cvnlution in the other localities is shown in Figure lb.

Figures 2a and 2b show, in addition 10

the averagedaily means for the ycars studied, the trend line obtained with the running five­d;~y avcragcs. In al! of the stations Studied, the pollination periods last from the middlc nrcndofMarch tothe end of i\prilorbeginning ofMay.

DISCUSSIO'l

In vicw of thesc results. it can observed that. in Spain. Bewla palien only ncachcs high concentrations in the Non h. the hi ghest concentralions bcing recorded in the 'lonh West. Within the Euro-Siberian rcginn, thcse arcas correspond to those with the greatest distribution ofbirch.

The sampling stat1ons can be dividcd into 3 groups. The first group includes the cities in which the annual counts cxceeded 1,000 pollen grains, with daily average occasionally cxcccding 100 palien grains/m1.

The cities in thts group -Samiago de Compostela, Ourense and Vigo- are located in the northwcstcrn cerner of the lberian Peninsula. The highesl valucs wcre rccorded

Vol JO ( /999)

Aero&iology of Bcwla in Spa i11

in Santiago de Compostela. with a maxtmum :mnualtotal of 4,608 polk n graius in l997 . Jn Ourense. the maximum annualtotal was 1.979 pollcn grains inl994 and in Vigo the maximum annual total of 1,540 pollcn grains was recorded in i995.Daily maximumcounts were 56 1 palien grain,.;/m3 in S:mliago in 1997, 254 palien grains/m3 in Ourense in 199-1 and 204 palien grainsfm' in Vigo in 1995. This is therefore the area of gre:uest risk in S pain; thepopulation is sensllive to this pollen sincc concentrations abovc 80 pollen grainslm3

,

considered to be suffi cient to produce symptoms in al! clinicallysensitive individuals (DETANDT & NOLARD, 1996), are normnlly reached every year.

lOOO

' 000

JO OO

lOOO

!000

Santlngo de Composte l:l

1993 199-< 1995 1996 1 ~ 91 t99S

<000 ¡_.::o"::'.::• n:::•:.:•~--3000 , ________ _

2000 ---

1000

-¡ o~-~~~-2~L-~i-~~~

!99J 1994 1995 l?% 1997 1998

I IJ\)3 !994 1995 1996 1997 199S

FIGURE 1a. Euolulion of tha annual quanlily of birch pollan grains in Vigo, Santiago and Ourense.

43

falo er al.

Thc sccond group includes the cilies m which total annual 8eut!a pollcn coums "ere gre~ter than 100 poli en grains in sorne years and daily averag.:s rarely exceedcd 50 polleo grai nsfml. This group includcs Barcelona (maximum annual total uf 522 poli en grains). Girona {5 10 poli en grains). León (436 polleo grams), Tarragona (303 pollen grains) ~nd Llcida {246¡mllcn grains). In all thesccitics, the maximum annual tota ls werc rccurded in

GOO ~00

1?93 1994 199$ 1996 1997 1998

León

400 1 lOO

:~ ; ----.......-..-.-1~-'-----:

6GO

soo 1

100

1993 11)9.i 1995 1996 J99i '"'

l~l 1994 199S 1996 1997 1991

Estepona

~=¡ o .!-----~-·---...... -----~

1993 1994 1995 19?6 1997 1?98

thc samc y~r (1997). Nevc11hclcss. it is worth h1ghlighling that thc number of years for which data are available diffcrcd; although 5 yenrs(J994 to 1998)werestudied in Lcónand Bllrcelona, in thc othcr three stations data were only available for the last threc years.

Thc 1h1rd group includes Madrid togcth· er W1th the 1wo southernmost stauons: Malaga and Estepona. lnthc~e three cities thc

600

'OD

•oo lOO

lOO

100

Glron•

o~--~--------~~--~~c_,

600 l )00

<00

199J 199• 199l 1996 1m 1991

~ ¡¡_. ___ :1...._...-~.__ ___ .,.....Jr~~ ...• ._

600

!00

•oo JOD

lOO

1991 1 ~9~ 11)95 1996 1997 1991

1~ .¡_]_....,.__.'--_.._,_~....,.__.r::~~•t_,.

FIGURE 1 b. Evolulion or annual birch pollen grain counts in the other cities.

44 Polen

-¡x>llen levcls wcrc vcry low, ncvcr cxceeding an annual total of 100 ¡x>llen grnms and with maximum dnily a'ernges of 15 palien grains/m1.

Lastly, in Cordoba and Granada birch ¡x>llen was only det.:cted on isolatcd days ~nd ~ lways with testimonial values.

In the other aerobiologtcal srarions included i11 the Spanish Network of Aero­biology, Bctula palien was only rccorded in Zaragoza. wilh annual IOlal values of220pollen grains in 1995 and 101 palien grains m 1996 (BERlv!EJO el al .. 1998). These resulls would place this city in thc sccond group.

Signtfi cant differences exist in tenns of the total birch pollen grain counts r.:corded in thc ycar> studicd in each city. ln those for which more y cm of data are nvnilnblc, 1995 and especially 1997 werc gencrally dccmcd to be the years with the highest poi len production, as comparcd with 1996 and 1998 when conccntrations wcre vcry low. There is no absolute agreement in tcrms uf thc ycar in which maximum total annual quantities wcrc recordcd. Thus, while in Santiago, León and l.larcelona rhts was 1997. in Ourcnse and Madrid ir was 1994 and in Vigo and Málaga ir was 1995. i\part from possi blc factors inhcrcntto the species that cause vanations in palien procluction in specifi c years. the prcsence of abundan! and frcqucnt rainfall during the pollination periods in 1993 and 1998 undoobtcdly helped lO e lean the atmosphere and thus reduce pollen concentration. An overall tendency rowards maintaining an al:ernate rhythm ofhigh and low conccntrations was observcd. This alternation generally occurred in allthccities in the fi nal three years ofthe study pcriod ( 199&- 1998).

ln the years in which the annual concen­trations exceeded 100 pollen grains. no significan! differences were observed in terms

Vol 10 (1999)

Aerobiolu8Y of Bcu.ta in Spain

of thc pollination start dates in thc different cities. The most significam wa~ in 1997, with a 12·day differcnce between thc st:ut of pall ination in thc ..:arliesr statiun (León) and the latest (Licida). The nonhwcstcrn starion~

(Santiago, Vigo and Ourense) wcre generally theoncs with thc e:trliest onsct uf pollination, with small diflercnces between them (from l to 5 days). whcrca s thc Cata lan s ta tio ns presentcd thc latest onset of polli natiun.

In ordcr ro suudy the differences in the onscr ofpoll ination in a city during the years studied, the authors took into accoum thc stations includcd in group 1, sincc thcse pre­sented the highest concentrarions and mure ycars of data for analysis. The differences in

150

12S

100

21

110.

125

01 llpl 01 1111)'

Ouren'•

100 -- l :- --=1 oL---~~~--~~~~'----~ o:~

100

121

100

71

VIgo

!<) '

': j~ {11·1':' • 0 1-~pr O lan~

FIGURE 2a. Avarage of tna daily mean concentrations of blrth pot en in VIIJO, Santiago and Oo.IRnse.

45

Jato et al.

JO ~------------------------~ Barcelona

15 ------IS

10

o l------=~--~~~~~~~~ Ol·mJr 01-arr Ol·m3\'

j0T-----------------------------, l eón

lS

20 1--------

30 Llelda -

20

IS -10

-o

Ol·mar

30 M &l laga

20 -

10 1--

o Ol·m:u

~ 'li ~

~ Ol·:wr

-

01-mnv

-1

JO Giro na

2l

20

15

ID

lOr-------------------~

Tarragon.;~~

lll--~-----------------1

lO

ll~-------------------------1

30 MiKir ld

lS

lO

15 -------10

o 01-nw

30

25 Estepona

20

15 ---10

-o 01 -mar

~ 01 -klt

. _A 01-;aor

-----A

----

FIGURE 2b . Average of tho daily mean concentrat ons of birch palien In the othets local ties.

46 Pole11

--this case are tmportant, since between 1996 (lhc ycar with the la test pollination} and 1997 (the year wi th thc carliest pollinauon}. di fferences of28 days in Ourcnse, 30 in San­tiagoand 31 in Vigo wererecordcd. In Danish ci:ies. ANDERSEN (1991} rcports differences ofup to 38days bctwecn the beginningofpolli­nation from otiC yeJr to the nex1. Tempcrature and photopenod havc bccn highlighted by severa! authors as factorsof gre:ll importance in the onset of birch pollination (SGTRA, 1992; SPlEKSMAet aL, 1989; ANDERSEN. 1991}. The bcginning of pollinauon is al>o controlled by the cooling rcqui red by the plant bcforc cnding the latency pcriod :md the to­tal of accumulatcd temperatures abovc a threshold. whtch EMBERLIN el al. (1997) reporteO to be 5.5"C. The study of longer sc­riesof data will undoubtedly enableastatistical analysis of the causes of thesc variations. However, in thceaseof 1997, acombinationof two factors was decisivc in the early onset of pollination. namely the very low mínimum temperatures rccorded in January (total lcmpcraturcs in that month wcrc much lower than in othcr ycars} and m:ucimum tempamur.;s higher than thosc in other years dunng Fcbruary. This would. on the onc hand, fa\'OUr an early nccumulation of ooldness and, on tite other, and after th is pcriod, ~1c hcm required for producing the dehiscenceof !he anthcrs 1~ould beaccumulated carl icr.

CONCLUSIONS

Btrch pollen is present m Spain's atmos­¡>herefromMarch to the endof M ay. with thc highest values being recordcd bet"een mid­March :rnd the end of Apri l. Daily values of over 100 palien grainsJml are only found in thc northwestern cerner of the lberran Península {Santiago de Compostela, Ourcnse

Vol JO (1999)

Aerobiology of Betflla i11 Spai11

and Vtgo}. In the;c ~tations. the presencc of rainy days (common in spring) ha ve~ great inOuence on the palien levels achicvcd. A second zonc. cxcluding thc aforementioncd arca . oovers tllc nort hcrn half of the country. in which daily values of 50 palien grains/m3

are r;rrcly cxcccdcd. Lastly, in the rest of thc country, B~utla pollcn lcvcb are vcry low or practically non-existent. The northwcstcrn sta tions are l\Cncral ly the first to begi n pollination, and the Catalan stations present the latest onset of pollrnation.

Forthe samc city, notable differences are obscrved in total annual Bet11/a poli en coums. and there is a general tendcncy towards maintaining an ahernate rhythm of high and low conccntration~ .

Although the clrffcrcnccs between the onset of poll ination in the samc city for diffc rent years are generally not vcry significan!, differcnt mctcorological condi­tions reoordcd from one year to thc ncxl m ay givc rise la differcnccs of up to 30 days.

ACKNOWLEDGEMEm'S

Thc authors are gratcful to the CICYT fo r the fi nanc ia) support of thc projcct AMB97-1457.C07-03.

HEFERENCFS

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