STUDIES IN ATMOSPHERIC POLLEN

Transcription

1 STUDIES IN ATMOSPHERIC POLLEN V. A DAILY CENSUS OF POLLENS AT CARDIFF FOR THE SIX YEARS BY H. A. HYDE National Museum of Wales, Cardiff [Received 24 June 1951) (With 8 figures in the text) I. INTRODUCTION The pollen census described in the first of these studies (Hyde & Williams, 1944) established the periods of occurrence and the day-to-day frequencies of the various types of pollen deposited at Llandough, Cardiff, during the year In order to determine from year to year the variations in the pollen catch at one and the same station in magnitude, composition and period of occurrence, continuous observations have been made from late summer 1942 onwards at a second Cardiff station, viz. the National Museum of Wales. The slides exposed at this station have been analysed fully for the period , and the analyses form the basis of this paper. (Slides have continued to be exposed also at Llandough; they have been analysed fully for 1943 (Hyde, 1950(2,6) but only in part since that year; so far as they go these analyses confirm the variations noted at the Museum station and about to be described.) The apparatus used and the general procedure for the exposure and subsequent examination of slides have already been described (Hyde, 1950 a). The apparatus was situated in an exposed position on the flat roof of the Museum, 60 ft. above groundlevel. This building stands on the eastern edge of an extensive system of open spaces, and the immediate neighbourhood (within i mile) contains abundant sources of pollen. The present communication is concerned only with variation in the magnitude and composition of the annual catch. Phenological variation will be dealt with later. 2. VARIATION IN THE MAGNITUDE AND COMPOSITION OF THE ANNUAL CATCH {a) The catch as a whole Of the 164,776 grains counted over the 6 years 91-6% were referred to one or other of sixty-four types. The total catches of identified grains in successive years were 1943, 25,511 (101% of the 6-year average); 1944, 26,037 (103%); 1945, 23,982 (95%); 1946, 31,658 (126%); 1947, 14,182 (56%); 1948, 29,544 (117%)- {b) Day-to-day catches of particular types These are exemplified by Figs In general the highest daily catch of a particular type during the whole of the 6 years was made during the most prolific season, and annual catches in other seasons Ml paripassu with their seasonal maxima. The following highest daily catches of particular types are greater than those published previously New Phytol. 51, 3 ^^

2 282 H. A. HYDE (Hyde, 1950^, Table 4), viz. Ulmus, 1926 (9 March 1948; the highest day's catch of any single type recorded by us. Fig. 2); Quercus, 1146 (5 May 1946, Fig. 3); Castanea, 197 (23 July 1946); Betula, 163 (14 April 1948, Fig. 5); and Tilia, 146 (19 June 1945). Table i. Synopsis of pollen catch at Cardiff Total numbers of grains counted on ^ in. square (= 5 sq.cm.) Type of pollen Total Average Acer Aescuhcs Alnus Betula Carpinus Castanea Chenopodiaceae Compositae: Artemisia Cichorieae Misc. Coniferae: Pinus Taxus Taxoid Corylus Cruciferae Cyperaceae Ericaceae Fagus Fraxinus Gramineae J Tuncaceae U.A 1 ^..-CA V'V.. C4. Vx jfuncus Luzula Ligustrum Mercurialis Plan tag0 Platanus Populus Quercus Ranunculaceae Rosaceae : ' Spiraea' Arboreal Rumex Salix Sambucus Tilia Ulmus Umbelliferae Urticaceae: Urtica Minor types Damaged or unexpanded Unidentified Total yearly catch ,228 12, n 1,123 IOI 95 1, , , , , , , , , , II 17 1,194 7 no 1, , , , ,077 12, , , , , , , " , , , , , , ,908 no 74 3, , , , , ,310 3, ,315 1, , ,638 4,057 64, , , , ,543 27, , , , , , , , , ,463 (c) Annual catches of particular types The results of analysing our 2192 slides are set out in Table i, and the annual catches of the eleven principal types concerned are summarized graphically in Fig. i. The following were caught at Cardiff in greater numbers during the years stated than at

3 Studies in atmospheric pollen 283 CQ U_ i

4 284 H. A. HYDE that or any other station in 1943 (Hyde, 19506, Table 3): Ulmus, 8247 (1945); Quercus, 7851 (1946); Plantago, 1908 (1948); Urtica, 1682 (1946); Pinus, 566 (1948); Tilia, 543 (1945); Betitla, 1327 (1948); Fagus, 750 (1948); Alnus, 869 (1944); Castanea 382, (1946); Arterttisia, 151 (1947); Ranunculaceae, 247 (1948); Acer, 138 (1948); Carpinus, 114 (1948); Rosaceae (arboreal), 93 (1948); Cyperaceae, 72 (1948); Ligustrum, 98 (1946). 3. POTENTIAL CAUSES OE ANNUAL VARIATION Variations from year to year in the catch of a particular type might be due to the fact that the actual amount of pollen formed by plants of the species concerned (the local pollen output) in the district was greater or less. In trees this might be due to a general rise or fall in productivity affecting all the individuals concerned or to variation in their average output; in herbs the numbers of plants or of fiowering shoots and/or the average output per shoot might vary. Even if the local pollen output remained constant a greater or smaller proportion might actually become airborne as the result of more or less favourable weather. Finally, other things being equal, sampling at the station concerned might be affected by weather. These three groups of potential causes and their probable effects are considered below in inverse order. {a) Sampling (i) Wind direction. Wind direction exercises a continuous infiuence on sampling, the effect of which on the catches of a particular type depends on the distribution of its source plants around the station. Thus birch (Fig. 5), ash (Fig. 4), pine, and to a less degree beech trees were scattered more or less at random around the station, and in these types the highest (usually mid season) catches came from various directions in different years; the effects of wind direction on annual catch cancelled out. Elms (Fig. 2), on the other hand, are particularly numerous around the western semi-periphery, and persistent easterly winds in 1944 and 1946 depressed the catch considerably. Again the only local aggregation of alder trees is situated between \ and -i mile to the west; high maxima occurred in 1944, 1945, 1947 and 1948, with west or south-west winds, whereas in 1943 easterly winds at the middle of the season coincided with a low maximum, and in 1946 a low trough at mid-season was similarly conditioned. Remarkably high catches of oak pollen (Fig. 3) were obtained in 1946 during a period of north-east winds; presumably this occurred in spite of the fact that oaks are in fact more numerous immediately to the west and was due to a general high release in that year, for similar catches were obtained during the same period at Llandough (2I miles away) and at two other stations situated near the Brecknock Beacons and in the middle of the Black Mountains, miles to the north of Cardiff. (ii) Rain. Rain washes the air clear of pollen, but only temporarily; the season's catches do not appear to have been infiuenced materially thereby. {b) Liberation (i) Sunshine. In grasses daily pollen liberation is related to bright sunshine (Hyde, 195 ^)' but no correlation has been traced between the total number of hours of bright sunshine experienced during the months of June and July and the total catch of grass pollen during those months. Presumably, as has been previously suggested, dull days

5 Fig. 2. Elm (Ulmus) pollen at Cardiff, (daily counts on 5 sq.cm.). Total yearly counts on right. Above each diagram of pollen frequency: daily temperatures (maxima, heavy line; minima, fine line) and directional wind 'rose' for period covering 19 days before and after peak date (observed or estimated, and indicated by a cross). Persistently low catcbes during tbe periods February and 4-10 March 1944 and 25 February-17 Marcb 1946 are to be ascribed mainly to unfavourable wind direction. Weatber data from Penylan Observatory (202 ft. O.D.), ij miles north-north-east of the Museum (Medical Officer of Health, City of Cardiff). For further explanation see text. 876

6 286 H. A. HYDE Pollen (grains per April May June Fig. 3. Oak {Quercus) pollen at Cardiff, The more or less marked annual rise in the catch after it had virtually ceased is due to Q. ilex. Total yearly catches on right. Prevailing wind direction during covering 8 days before and after peak date (observed or estimated): 1943, south-west to west; 1945, north-west to north; 1946, north-east to east; in other years winds variable during these periods. 3255

7 Studies in atmospheric pollen 287 Pollen (grains per Ssq.cm. surface) Fig. 4. Ash (Fraxinus) pollen at Cardiff, Prevailing winds during 12 days before and after observed peak date: 1943, south-west to west; 1945, south-west; 1946, north to east; 1948, northeast ; in other years variable.

8 288 H. A. HYDE Pollen (grains per 5sq. cm. surface) etu/a (Birch) 1943 focal ;atch March April May Fig. 5. Birch (Betuld) pollen at Cardiff, Prevailing winds during 15 days before and after observed peak date: 1943, south-west; 1944, variable (high catches on 20, 24 and 27 April with wind westsouth-west, south-west and south-south-east respectively); 1945, south-west; 1946, north-east (but high catch on 15 April with west wind); 1947, variable; 1948, variable (high catches on 14, 16, 19, 20 and 21 April all with winds between east-north-east and east but on 24 April with south-east and on 27 April with west-south-west winds respectively)

9 Studies in atmospheric pollen 289 occasion a temporary hold-up which is compensated by more abundant release on bright ones (cf. Hyde & Williams, 1945). (ii) Concurrent temperature. In the prevernal types frost, supervening after the opening of the season, occasioned an almost complete cessation in the catch; if the temperature remained only a little below freezing-point, the catch began again immediately it rose above freezing. Severe and prolonged frost, however, as in 1947, was followed, when the temperature rose, by a period of up to 29 days during which no grains were caught; presumably the early crops of flowers had been destroyed so that no further liberation was possible until a further crop had reached anthesis. (c) Productivity (i) Winter killing {herbs). Exceptionally severe weather was experienced during the period 25 January-12 March 1947, in the course of which the temperature fell daily to below freezing-point and on 6 days to 20 F. (-6-7 C.) or lower. In the pollen season which followed, the catches of, among others, certain herbaceous types were much lower than in any other year. The plantain catch for the year was only 51 % of average; doubtless this was due in part to the shortening of the season consequent on its lateness, but also the monthly catches for both June and July were much lower than in any other year, a fact which accords with the supposition that those catches were derived from a much smaller number of plants than usual. The count of spring Juncaceae {Luztila) was less than 10% of average, and similar considerations apply. (ii) Spring rainfall {herbs). Pollen productivity may be assumed in perennial herbs to be bound up with vegetative growth; the greater the number of actively growing shoots in the population concerned, the greater the number of flowers and therefore of pollen. Since the chief factor limiting vegetative growth in this country is lack of soil moisture, it is to be expected that pollen productivity will be related to rainfall during the vegetative period, viz. spring and early summer. This expectation is at least in part fulfilled. Thus the June and July catches of Rumex pollen (Fig. 6) seem to reflect April and May rainfall respectively. In Urtica (Fig. 8) both the July and August catches rose and fell together year after year along with the rainfall during the preceding April and May. In grasses (Fig. 7) the July catch followed the rainfall for April to June (inclusive) fairly closely, but the June catch did not accord with expectation based on rainfall in April or May. (iii) Weather during formative period {trees). Flower formation in trees is generally supposed to be favoured by warm dry weather at the time when the buds are being formed. In most forest trees this formative period is late summer (Biisgen-Munch, pp ). The annual pollen catches of such trees might be expected to vary accordingly, uring the period concerned the warmest, driest and sunniest summer (as evaluated by mmation for the period June-September of the three factors, temperature, rainfall defici and bright sunshine, expressed in terms of arbitrary scales) was 1947 followed by 1942, 1945, 1943, i944 and 1946 in that order. The joint performance of fourteen spring-flowering trees (as judged by totalling their percentages of average year by year) was best in 1948, and the other years followed in the order 1946, 1943, 1944, 1945 and 1947, i.e. according to expectation, except for the transposition of 1943 and 1946 in the second and third places. If the same types are considered singly, however, it is found that in 1948 the pollens of only six such types (or seven if Quercus ilex be

10 290 H. A. HYDE reckoned separately) were at their most abundant and one {Fraxinus) was at its least abundant. It would appear, therefore, that while dry summer weather is in general conducive to flower formation and subsequent pollen production it is outweighed in certain types by some other factor (see below). Difference from average (in.) Grains per 5sq. cm. surface Difference from average I 0 (in.) U 160 Grains per sq. cm. surface 40 May rainfall Jul/ pollen Fig. 6. Rumex pollen at Cardiff in the months of lune and July , as related to rainfall (inches above or below the 58 years' monthly average) in April and May of the same years respectively. Difference from average (in.) Rainfall Pollen 5000 per Ssq.cm. tooo surface June pollen July pollen O --o' 1000 _L J_ Flg. 7. Grass pollen at Cardiff in the months of June and July , as related to rainfall excess or deficit in the preceding months In 1947, eleven out of the fourteen types here considered were at their lowest and none at its highest. This season, in consequence of the winter severity mentioned above (p. 289), was exceptionally late, and pollen of all the spring-flowering tree types was caught only over exceptionally short periods. In the four prevernal types this was due

11 Studies in atmospheric pollen 291 in part to frost-killing of flowers; in the remainder, whose flowers were dormant during the prolonged frost, this could not have happened, and it is believed that their poor performance was due at least in part to the unfavourable summer of (d) Possible pollen cycles The wide variations in the annual catches of certain tree types still remain to be explained; in these types neither the high catches in some years nor the low ones in others appear in general to have been determined entirely by the weather experienced in either the concurrent or the preceding seasons. Bumper years recur at regular intervals, but a bumper year for one type may coincide with a low year for another and vice versa (Fig. i). It has long been known that many forest trees fruit abundantly only at intervals of 2 years or more (Busgen-Munch, p. 371). If fruiting cycles exist, closely Difference from average April May rainfall _ July and August pollen Grains per 5sq. cm. surface August - July - - August Fig. 8. Nettle pollen at Cardiff in the months of July and August , as related to rainfall excess or deficit in the preceding periods April-May. related flowering cycles might be expected. The following very tentative conclusions on the possible existence of such cycles are put forward with great reserve; they are based not only on the counts obtained at Cardiff , but also on preliminary counts from the same station for 1949 and 1950, and are conflrmed, so far as comparison is possible, by counts from Llandough, 2f miles from Cardiff (1942, Hyde & Wilhams, 1944; 1943, Hyde, 19506; , provisional counts, Hyde, unpublished). In oak 1946 was the only bumper year between 1942 and 1950 (inclusive). Pohl (1937) argued from the evidence of mast years in Germany that pollen production in oak followed a 5-year cycle. Our data are consistent with a cycle of 5 years or more. In ash {Fraxinus) maxima in 1943, 1946 and 1949 may indicate a 3-year cycle. In elm {Ulmus) the highest counts were obtained in 1942, 1945 and 1948, but in view of the apparent effect of wind on sampling the question of a 3-year cycle must remain open. In birch {Betula) and beech {Fagus) maxima occurred probably in 1942 and certainly in 1944, 1946, 1948 and 1950; both appear to follow 2-year cycles.

12 292 H. A. HYDE 4. DISCUSSION Certain applications of the observations here recorded are briefly indicated below; they will be amplified elsewhere. {a) Pollen and mast years Abundant flowering in forest trees is not necessarily followed by abundant seeding. Nevertheless, preliminary inquiries suggest that bumper pollen years and good mast years frequently coincide, and that analyses of atmospheric pollen may therefore prove to be a valuable factor in crop prediction (Hyde, 1951). (b) Pollen and allergic disease In order to diagnose a case of asthma or hay-fever as due to inhalation of a particular kind of pollen it is necessary to show inter alia that symptoms are experienced only during the period when the pollen actually is in the air in quantity. Since flowering in some kinds of trees varies greatly in intensity from year to year it seems probable that a patient who had been found to be ' skin-sensitive' to the extract of a particular pollen might show clinical symptoms in ' bumper' pollen years only and escape completely in the intervening ones. (c) Atmospheric pollen spectra as indices of local vegetation It is evident that pollen spectra based on the catches obtained at one and the same station in successive years may be very different in appearance. In order to obtain an expression of the local vegetation in terms of airborne pollen such as is likely to assist in the elucidation of pollen diagrams from the past, it will be necessary to extend pollen observations over several years. 5. SUMMARY 1. A day-to-day census of atmospheric pollen caught at Cardiff during the 6 years is described. 2. The same types of pollen were caught every year, but the magnitude of the annual catches of individual types varied considerably. 3. This variation is considered in relation to its possible causes, and it is shown to be due in the main to variation in pollen productivity. 4. The pollen productivity of the chief herbaceous types appears to be affected by rainfall during the period of vegetative growth and probably also by the occurrence or non-occurrence of killing frosts during the preceding winter. 5. The pollen productivity of prevernal and vernal trees appears to be correlated to some extent with the weather experienced during the late summer of the preceding year. 6. Certain trees, notably oak, ash, and perhaps elm, nevertheless exhibit a cyclic variation in ffowering activity largely independent of weather during the formative period. 7. The implications in various fields of the facts established by this survey are briefly indicated.

13 Studies in atmospheric pollen 293 Thanks are due to the National Museum of Wales and to the Asthma and Allergy Research Unit at Llandough Hospital (now at St David's Hospital, Cardiff) for their support. The routine pollen counts have been checked with the help of Mrs Kathleen Adams, B.Sc, who has also drawn the diagrams. The very generous assistance of the Royal Society (in the form of a Government grant) is gratefully acknowledged. RFFFRENCFS BusGEN, M., revised by E. MUNCH (1929). The Structure and Life of Forest Trees. Trans. T. Thomson, pp London. HYDE, H. A. (1950a). Studies in atmospheric pollen. IV. A survey of pollen deposition in Great Britain, Pt. I. The influence of situation and weather. Nezu Phytol. 49, HYDE, H. A. (19506). Studies in atmospheric pollen. IV. A survey of pollen deposition in Great Britain, Pt. II. The composition of the pollen catch. Neio Phytol. 49, HYDE, H. A. (1951). Pollen output and seed production in forest trees: a preliminary note. Quart. J. For. HYDE, H. A. & WILLIAMS, D. A. (1944). Studies in atmospheric pollen. I. A daily census of pollens at Cardiff. New Phytol. 43, HYDE, H. A. & WILLIAMS, D. A. (1945). Studies in atmospheric pollen. II. Diurnal variation in the incidence of grass pollen. Neio Phytol. 44, PoHL, F. (1937). Die PoUenerzeugung der Windblutler. Beih. Bot. Zbl. A, 56, REMPE, H. (1937). Untersuchungen liber die Verbreitung des Blutenstaubens durch die Luftstromungen. Planta, 27,

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