There are parts of the world, such as the Gaspe Peninsula (Quebec), northern Scandinavia, and northernmost eastern North America, where Betula pollen grains represent up to 80% of the total terrestrial pollen percentage. In many cases, the species differentiation, commonly based on the size of the pollen grains, becomes interesting for the data interpretation. Usually, the size distribution curves are drawn for each sample. We propose a different graphic representation displaying the percentages curves of the individual diameter values along the entire stratigraphic column. To our knowledge, this display has not been used previously. Lac J'Arrive pollen diagram, which shows a complete postglacial sequence from the northern Gaspe peninsula, will serve as an example.
In Quebec, two groups of species are present: dwarf birches (B. glandulosa, B. nana, and B. pumila) and tree birches (B. populifolia, B. borealis, B. papyrifera, B. lenta, and B. alleghaniensis). We do not intend to suggest that all the species may be identified by measurements, since no exhaustive pollen morphological study has been done yet on Betula in Quebec. However, three groups have been recognized according to the size of their pollen: dwarf birches around 20 µm, the first tree species of birches (the B. papyrifera group) around 26 µm, and B. alleghaniensis around 31 µm.
The measurements and data preparation. The diameters of 100 triporate, non-folded Betula pollen grains are measured in polar view, at the same magnification routinely used for pollen counting (400X). The slides are mounted in glycerin; consequently, pollen identification and measurements of the Betula grains are done soon after the chemical preparation in order to minimize the effect of the progressive swelling of the grains. Up to now, the measured values were directly rounded to the nearest micrometer unit by the pollen analyst. For higher precision, the results will further be noted as graticule units. The counts of the individual sizes is estimated from the total Betula count. The individual percentages for each size are then calculated (counts per size divided by the total pollen sum) in an Excel® spreadsheet, .WK1 saved, and transferred to Tilia·graph for display.
The interpretation for Lac J'Arrive. At lac J'Arrive, the Betula grains are initially a mix of dwarf and tree birches (zone B1), representing long distance transport (see the pollen concentration curve). In zone B2, the tree birches from extra-regional origin are still present but the smallest sizes become regularly represented, probably reflecting the approach of the dwarf birches (B. glandulosa) to the site, the next zone B3 being the principal B. glandulosa period. Zone B4 is a transition zone during which B. glandulosa is gradually replaced by tree birches. In zones B5, greater sizes are well represented, showing the local and regional development of principally B. papyrifera (paper birch). Zone B5b, with the biggest pollen grains diameters, probably corresponds to the development of B. alleghaniensis (yellow birch) in the area.
The potential. The routine measurement of thousands of Betula grains in samples from over 150 sites (lakes and bogs) all over Quebec produced results that are internally coherent. The sequence from lac J'Arrive is typical in this regard. In addition, when we find macroremains of Betula, we find the sequential replacement of B. glandulosa by B. papyrifera with a B. alleghaniensis intrusion in some areas of southern Quebec.
This original display opens some new perspectives. First, quantitative zoning on Betula sizes can be done to get an objective assessment of the changes. Second, there are possibilities for quantitative assessment of the timing and duration of the replacement of various Betula species based on the size of the grains. In this regard, the identification of the transition zone between the development of successive Betula populations is promising. Third, comparison of the display between diagrams for large regions could reveal geographic patterns in population dynamics within the genus. Even in regions where it is expected that only one Betula specie is involved, the display could be useful in graphing the contribution of each size-unit to the total population of Betula grains.
The future. We will probably create a set of Betula-size variables (B15µm, B16µm, ...B40µm) and will enter the counts directly in TILIA. The data will then be stored in the P_Counts table of Paradox, the database manager we currently use (as part of the NAPD_EPD venture). A script in Paradox® will handle the appropriate calculations to replace the initial counts by the share of each Betula-size from the total Betula counts. An appropriate designation of the Betula-size will ensure that these counts will be kept outside the total pollen sum.
References.
Andersen, S.T., 1960. Silicon Oil as a Mounting Medium for Pollen Grains. Danmarks Geologiske Undersøgelser, IV Series, 4 (1): 1-24.
Andrews, J.T., Mode, W.N., Webber, P.J., Miller, G.H. and Jacobs, J.J., 1980. Report on the distribution of dwarf birches and present pollen rain, Baffin Island, N.W.T., Canada. Arctic, 33: 50-58.
Birks, H.J.B., 1968. The identification of Betula nana pollen. New Phytologist, 67 (2): 309-314 + 3 fig., 4 tab.
Cherevko, M.V., 1969. On pollen morphology in some species of Betula L. and Alnus Gaertn. Ukrajin, Botanisheskii Zhurnal, 26 (6): 39-45.
Claugher, D. and Rowley, J.R., 1987. Betula pollen grain substructure revealed by fast atom etching. Pollen et Spores, 29 (1): 5-20.
Clausen, K. E., 1960. A survey of variation in pollen size within individual plants and catkins of three taxa of Betula. Pollen et Spores, 2 (2): 299-304.
Comtois, P., 1981. Diamètre pollinique de quelques espèces tourbicoles de Betula. Le Naturaliste canadien, 108 (4): 471-474.
Eneroth, O., 1951. Investigations of the possibility of differentiating the pollen of different species of Betula in fossil material. Geologiska Föreningens, Stockholm. Förhandlingar., 73 (3): 343-405.
Erdtman, G. and Terasmæ, J., 1951. On the pollen morphology of Betula nana. Svensk Botanisk Tidskrift, 45 (2): 358-361.
Gaillard, M.-J., 1983. On the occurrence of Betula nana L. pollen grains in the Late-glacial deposits of Lobsigensee (Swiss plateau). Studies in the Late Quaternary of Lobsigensee 2. Revue de Paleobiologie, 2 (2): 181-188.
Ives, J.W., 1977. Pollen separation of three North American birches. Arctic and Alpine Research, 9 (1): 73-80.
Jentys-Szaferowa, J., 1928. La structure des membranes du pollen de Corylus et Myrica et des espèces europeennes de Betula et leur determination à l'etat fossile. Bulletin de l'Academie polonaise des Sciences et des Lettres, serie Biologie, Botanique.
Johansen, J., 1968. A pollen diagram from the Faroe Islands showing former presence of Betula nana. Annal. Soc. Scient. Faeroensis, 16: 119-128.
Leopold, E.B., 1956. Pollen size-frequency in New England species of the genus Betula. Grana, 1 (2): 140-147.
Lieux, M.H., 1980. An atlas of pollen of trees, shrubs, and woody vines of Louisiana and other southeastern states. Part II. Platanaceae to Betulaceae. Pollen et Spores, 22 (2): 191-243.
Oschurkova, M.V., 1959. The description of pollen of different species of Betula occuring in the USSR (en russe, resume en anglais). Akademiia nauk SSSR, Moscov; Problemy Botaniki, 4: 68-91.
Prentice, I.C., 1981. Quantitative birch (Betula L.) pollen separation by analysis of size frequency data. New Phytologist, 89: 145-157.
Richard, P.J.H., 1970. Atlas pollinique des arbres et de quelques arbustes indigènes du Québec. III. Angiospermes (Salicacées, Myricacées, Juglan- dacées, Corylacées, Fagacées, Ulmacées). Le Naturaliste canadien, 97: 97-161 + 29 pl.
Terasmæ, J., 1951. On the pollen morphology of Betula nana. Svensk Botaniska Tidskrift, 45: 358-361.
Usinger, H., 1975. Pollenanalytische und stratigraphische Untersuchungen an zwei Spätglazial-Vorkommen in Schleswig-Holstein. Mitteilungen der Arbeitsgemeinschaft Geobotanik in Schleswig-Holstein und Hamburg, 25: 183 p.
Vasari, A. and Vasari, Y., 1985. Relic occurence of Betula nana L. in Massif Central, France. Ecologia Mediterranea,11(1): 65-68.
Wenner, C.G., 1953. Investigation into the possibilities of distinguishing pollen of the various species of Betula in fossil material. Geologiska Föreningens i Stockholm Förhandlingar, 75 (3): 367-380.