Betula - Birch family
There are two species of Betula native to Ireland, Betula pendula (previously verrucosa) - Silver Birch - and Betula pubescens - Downy Birch. I have not yet found any difference in the pollen between the two. B. pendula prefers drier ground, B. pubescens is more tolerant of wet ground and so does well, and predominates, in Ireland. B. pubescens has downy stalks to the leaves, in B. pendula the leaf stalk is hairless.
A third species, Betula nana, Dwarf Birch, is mentioned by Tansley (1965) as being amongst the first plants to colonise the bleak, exposed and unvegetated lands of Great Britain - and by inference, Ireland - immediately after the recession of the ice. B. nana is now recognised as a native of Scotland but does not appear in Webb's Irish Flora. Surely it occurs in Donegal, or on the Burren? This is another one to add to the list to find. Birks (1968) did define the methods to best identify the pollen of B. nana from other Betula species, on the basis of both pollen grain size (B. nana is generally smaller, but not conclusively, than B. pubescens and B.pendula), and a ratio of grain diameter to pore depth, in which the difference between B.nana and B. pubescens and pendula is quite distinct. Being able to determine the existence of B. nana pollen grains is importaant from a climatic perspective.
Birch very readily colonises ground of a wide range of conditions,including the drier bog surfaces and margins, newly cleared woodland, and heather moorland, and heath. It is one of the first colonisers of open ground and after glacial conditions recede and abundant pollen production means it is generally very visible in the pollen record. Godwin refers to Birch woods colonising land above ice blocks in early post glacial conditions, which, when the block melts and a kettle hole results, the layer of birch wood - and pollen - is deposited and preserved on the bed of the lake.
Birch allows a reasonable amount of light through the foliage, so even in full leaf a good growth of grass and other herbage can be sustained underneath. It has been observed that sheep and cattle, and possibly deer (?), however, prefer not to graze the grass under birch trees. The standard Atlantic woodland comprises Oak, Birch and Holly, and this combination can be found across SW England and SW Ireland.
B. pendula and pubescens pollen grains are generally about 25 microns across, triporate and characteristically vestibulate. The external surface of the pollen grain, the exine, is micro rugulate.
