Calculating and Presenting


The main advantage of using pollen traps for sampling modern pollen deposition is that the results can be expressed as pollen influx (grains cm-2 year-1). Influx is calculated relative to the quantity of added exotic using the formula:

(number of Lycopodium spores added/number of Lycopodium spores counted) x number of pollen grains counted

NB, however, that this gives the value for the whole trap and so the resulting figure must be divided by 19.6 (the surface area of the trap opening when the diameter of the opening is 5 cm) in order to produce influx values cm-2. The formula is the same whether one is calculating influx for individual pollen taxa or the total pollen count. The calculation can be made automatically using the Tilia program (Grimm, 1990) if 19.6 is entered under 'q.sam..', the code for 'size of sample'. Note, too, that the codes necessary for calculating influx (in the Tilia program the calculation option is 'concentrations' because this is what is most frequently calculated for fossil samples) differ between different versions of the program as follows:

Tilia 2.b.4 Tilia 1.10
Lycopodium Spores Counted ... .
Size of Sample q.sam.. =S
Number of Tablets Added q.spk.. =V
Number of Lycopodium Spores/Tablet c.spk.. =C

As is obvious from the discussion about the number of Lycopodium tablets originally added and the basic count of the pollen sum (see the relevant sections above) calculated influx values will not be exact. They should, therefore, be rounded to the nearest 10 grains, when given in numbers, and should be illustrated with error bars when shown as graphs. The range of variance in the influx calculation necessary for the error bars can be assessed using the graphs provided by Maher (1972) or by using an Excel-based program based on this publication (Huusko pers. comm.) available from Sheila Hicks (sheila.hicks@oulu.fi).

When illustrating the results of a number of traps for several years the order of the years should be with the most recent at the top and the oldest at the bottom - i.e. in the same chronological order as horizons in a sedimentary sequence. NB this is the opposite of the instructions given in the original guidelines and illustrated there (see Hicks et al. 1996 p.17, Fig. 2.)

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