An introduction to Lichens
Lichens are a self supporting association between a fungus and an alga. They come in a variety of forms and colours, as well as being of interest for their aesthetic beauty; they are a key part of small scale ecosystems supporting and sheltering a range of invertebrates and are good indicators of the health of the environment. Lichens obtain nearly all of their requirements from the atmosphere but are often specific to the type of substrate they colonise. Often slow growing, many however respond rapidly to environmental change while others require there to be a long continuity of habitat. These factors make them good indicators, useful in a lowland county such as Hertfordshire to measure man-made changes in the environment. In particularly they chart the change from high levels of atmospheric sulphur dioxide - which reached a peak in the 19th and early 20th century when the burning of coal was widespread - to more recent enrichment of the environment caused by the deposition of nitrogen in the form of oxides and ammonia from car exhausts and fertilisers.
Habitats important for lichens in Hertfordshire
Your own garden
If you are new to lichens, gardens are good places in which to study them. At first the vast assortment of different species, many seeming rather non-descript, can be off-putting, but in your garden you can get to know your local species at leisure. Fruit trees make a good place to begin as they usually have range of large foliose (?leafy?) lichens. Garden ornaments and paving stones have a more bewildering arrange of crusts, but are an interesting example of the colonisation of man-made substrates and the succession of different species. The hidden charms of these crustose species start to be revealed when you look at them under a hand lens and you start to see your garden in a new light. You never know what you may be walking over. Timber structures, as they weather, also display different associations of species. Because of the variety of features some gardens may have more that 50 different lichens.
The importance of lichens in churchyards has long been recognised. Churches are constructed of a range of different building materials and their yards have a vast variety of headstones. This is important for lichens in a county such as Hertfordshire which doesn?t have any natural exposures of hard rocks. Surfaces incline at different angles and face different aspects. Added to that the antiquity of many churchyards offers a long continuity of habitat for lichens to develop. It is not surprising that many churchyards ? including over a dozen in Hertfordshire ? have more than 100 species.
Churchyards are good places to study lichens because many species are found in a small place. Churchyard recording gives a better understanding of lichen distribution and has turned up some interesting species including Lecania coeruleorubella at Great Wymondley Church which had not been recorded since 1860 and presumed extinct listed by English Nature as one of England?s lost species. For enquiries about churchyard surveys please contact Judith Evans by email.
Species on tree bark are especially vulnerable to pollution and most were wiped out by high levels of sulphur dioxide often leaving Lecanora conizaeoides as the sole species on bark. With cleaner air, there has now been a secondary recolonisation of bark. Some species more indicative of ecological continuity, such as the Barnacle Lichen Thelotrema lepadinum, have persisted in some of our larger woodlands such as Sherrardspark and Wormley Woods. Tring Park in the relatively unpolluted west of the county, gives a glimpse of what must have been the original lichen communities of Hertfordshire in the past. Here again is an example of the importance and interest of lichen recording - documenting the survival and continuing return of lichens to our county.
Other habitats in Hertfordshire
Some individual wayside trees can have a remarkable number of species on them. The highest count made on one tree is 27 on a cherry tree in Hertford Heath (see photo, right). Perhaps you know of other candidates. A study of the colonisation of weathered fences can be fascinating. They often support species of acidic substrates which have declined as bark has become enriched.
Some of our commons support a few terricolous lichens (growing on soil) where there is acid grassland/heath and the turf is short and open. These lichen communities are also vulnerable to nutrient enrichment which encourages more vigorous growth of the vegetation swamping them out, but several colonise old gravel pits which are interesting for their early successional phases.