David Harper & David Streeter (Life Sciences)

We have been accosted several times recently by people demanding to know whether we are responsible for the groups of students observed doing unkind things to the trees on campus.

The answer, as in all good diplomatic exchange, is 'yes' and 'no'! 'Yes' because the students were carrying out a practical project at our instigation and 'no' because what they were doing was not unkind to the trees.

They were looking at the distribution of lichens on different tree trunks as part of the first year course on the Diversity of Life. Between them, 83 students collected 441 records of 19 different species of lichens on four different tree species.

Happily, the results confirmed a familiar piece of field craft saying that more lichens grow on the well-lit south side of trees (with more moss on the north side). Ash and Elm supported the most species and Beech the least, with Sycamore intermediate. 

The commonest lichen on the campus trees was the bright orange Xanthoria parietina (found on half of the trees examined by students), a species that particularly appreciates a spot of nutrient enrichment such as bird droppings. In second place came the conspicuous yellow-green patches of Flavoparmelia caperata (found on just under half of the trees). 

One surprising observation was the complete absence of two common grey-green species (Evernia prunastri and Hypogymnia physodes) from Beech, a feature that does not seem to have been remarked upon before.

Lichens are dual organisms consisting of an intimate relationship between a fungus and a green alga or photosynthetic bacterium and are particularly sensitive to atmospheric pollution.

Thirty years ago David Hawksworth and Francis Rose devised a method whereby the levels of sulphur dioxide in the atmosphere could be deduced from the assemblage of lichens growing on tree trunks. Applying this scheme, the lichens on our trees suggest that the campus atmosphere contains an average winter sulphur dioxide level of about 60 mg/m3.

Long-tailed Tits are easy birds to identify, resembling tiny black-and-white lollipops flitting between trees, with high-pitched trilling calls. They are currently even more interested in lichens than our students.

This is because they are already building their beautiful little domed nests. These are made from moss and spider silk, lined with feathers, and then the outside is painstakingly covered with thousands of tiny flakes of lichen.

Long-tailed Tits usually select grey-green lichens, such as Evernia prunastri (found on about a quarter of trees). Some lichens, such as Parmelia species (found on nearly half of trees), have black undersides. The birds always hide these from view when adding them to their nest. A pale surface may help camouflage the nest against the sky.

Certainly, Weasels and other small mammals climbing up into trees and bushes from below seem to be the most serious threat to the eggs and chicks.