Establishing pollen trap stations
Traps in the forested part of each transect should be placed in an opening within the vegetation being monitored. It is not possible to standardize the size of this opening for the whole of Europe. Although large flat openings in the vegetation are readily available in the latitudinal forest belts of the boreal zone, comparable openings are not found further south in Europe where the natural vegetation units are much smaller in areal extent or where narrow altitudinal forest belts are characteristic. Ideally the opening should be large enough so that predominantly regional rather than local pollen is trapped. In the light of the research presented in Jacobson and Bradshaw (1981) an opening with a diameter of 30 m might be appropriate although the views expressed in Moore, Webb and Collinson (1991) indicate that a larger opening would be preferable. It is, therefore, up to the participants to assess a suitable scale for their own situation and to measure it carefully. Clearly it is not logical to compare results from a forest opening with those from beneath the canopy of a densely wooded site.
Local studies which do not involve canopy openings can still be included in the EPMP but should be clearly differentiated in the data base.
Wherever possible the trap should be sunk into the ground so that its top (i.e. the collar) is at ground level. Where this is not possible the height of the collar above the ground and in relation to the surrounding vegetation should be noted.
The trap should contain a basic mixture of glycerol/glycerin, formalin and thymol (or an equivalent poison to prevent bacterial and fungal growth) as follows:
Glycerol/glycerin: enough to cover the bottom of the trap to a depth of 3 - 5 mm.
Formalin: 10 - 20 ml, depending upon the size of the trap receptacle.
Thymol: a few crystal (northern boreal conditions) - this quantity needs to be increased as local temperature and humidity increases (4g is necessary in temperate deciduous forest zones).
The collection season is a full calendar year commencing at the end of the flowering season i.e. September/October.
Experience has shown that in many cases some form of protection for the trap is necessary. The most common is a low fence consisting of four poles joined by a rope and a notice to indicate to the public that an experiment is in progress. There are no standard rules here and the presence and design of some form of protective barrier around the trap is left to the discretion of the person setting up the trap. The question of the use of a net, sieve or some other form of protection over the trap to prevent contamination by animals or insects also arises. Parallel experimentation with different protective devices to keep the trap free of animals and insects is, encouraged but anything which inhibits the free movement of pollen (both vertically and horizontally) into the trap opening should be avoided.