CASE STUDY Elms in England

English Elm (Ulmus procera) Thought of as a native tree but most likely brought to Britain by the Romans about 2,000 years ago. English elm naturally Propagates by root suckers Produces a clonal patch as young trees are produced in an ever increasing radius from the parent tree. English elm (Ulmus procera). Public domain image, Peter Bourne.

English elm was widely planted in new hedgerows during the Enclosures (1550–1850). – Twigs planted by foresters strike readily – Produce roots and grow quickly Why was the elm a sensible choice for hedgerows and why did this make the species especially vulnerable to disease? [3]

Answer All English elm trees are virtually genetically identical; clones of the original introduction. Ability to grow from suckers is an advantage; Regrowth is possible/ maintains hedge Genetic uniformity (identical/clones) is a problem due to susceptibility to Dutch elm disease Close proximity aids spread of disease

Dutch Elm Disease UK History There have been two waves of Dutch elm disease in Europe: – First epidemic 1910 to 1940s Killed between 10–40% of elms Caused by the fungus Ophiostoma ulmi – Second epidemic started in the late 1960s More destructive – Within 10 years it had killed 70% of elms in Britain – By the 1990s it had killed 85% – Virtually wiped out the English elm Caused by the more pathogenic Ophiostoma novo-ulmi

The fungus is transmitted to the elm by the elm bark beetle, Scolytus scolytus – The fungus spreads through the tree – Blocks xylem, restricting water transport – So that leaves wither and the tree dies Once most of the available breeding inner bark of the elm has been used and the mature trees are dead, the disease almost disappears. This allows regeneration by root suckering (basal sprouts) from the surviving roots.

Elm bark beetle gallery.Elm bark beetle (Scolytus scolytus).

Question Why has English elm not been wiped out by the disease? [2]

New suckers not susceptible OWTTE Therefore can grow once parent dies Only effected once a certain size

By the 1980s large numbers of young elm found growing. However: – Become attractive to the elm bark beetle when regenerated elms reach a certain size > 10 cm diameter and > 3 m height Dutch elm infection reappears because the newly regenerated trees are not resistant to Dutch elm disease – The regenerated trees are genetically identical to the parent plants – Have exactly the same susceptibility to the disease Ecological role of English elm is changing: – From being a dominant tree in hedgerows – To one more like a shrub or small tree

Wych elm (Ulmus glabra): tree, bark, leaves, flowers and fruit

Wych Elm All native elms are susceptible to Dutch elm disease. The Wych elm (U. glabra) is very susceptible to the fungus, but does not seem to be as affected by it as the English elm. Question: Suggest why the following might explain this. The Wych elm: – Produces fertile seed – Does not reproduce by root suckering – Produces diterpenes, which deter the bark beetles from feeding

Produces fertile seed Genetic variability from sexual reproduction May be different susceptibility levels Does not reproduce by root suckering Suckering is a means of vegetative propagation; resulting in genetically identical offspring. Produces diterpenes, which deter the bark beetles from feeding. Diterpenes put off the bark beetle They do not feed so do not infect the tree with the Dutch elm fungus