Mapping the vegetation around the pollen traps
It is obvious that for the pollen influx values to have any meaning they must be objectively related to the vegetation situation in which they are being deposited. This requires that the vegetation around each pollen trap should be mapped. The question is to what level of accuracy and for how big an area? To a certain extent this will depend upon what data are already available. Where a high resolution (ground accuracy less than 1 m2) digitized map of vegetation based on air photos, coupled with phytosociologically mapped ground control exists there is no problem because computer experiments can be made to see which size and shape of catchment best fits the average pollen influx (once values are available for several years).
Where there is no, or only scanty, existing data some minimum guidelines are needed. Since it is known that AP can be dispersed over great distances but that NAP is usually only dispersed over very short distances (it seems that in the majority of cases the closer to the ground that flowering takes place the shorter the distance over which pollen is dispersed) a case can be made for mapping the field and ground flora within a circle of 30 m diameter around the trap. For this whichever vegetation mapping system is normally in use in the country in question, is acceptable.
For the arboreal vegetation the minimum requirement is to assess the degree of openness or closedness of the tree cover and the approximate percentage presence of each tree species represented within an area of at least 100 m radius around each trap, or, if circumstances allow, within an area of 300 m radius around the trap. It is realised that these are arbitrary values. Since it is evident that the size of the catchment area varies with the size of the opening in the canopy, participants are encouraged to look at the theoretical calculations of Sugita (1994, in press) in order to calculate whether their trap locations require a larger area of vegetation to be mapped or whether the necessary information can be obtained from a smaller area. Again discretion based on local knowledge should be used. It is also significant to note the age of the tree stands in respect of their flowering capabilities.