Yew

Taxus baccata

Yews are one of the three native conifers in Scotland. The other two are Scots pines and juniper.

The Yew is a very unusual tree. It can live to be a great age ; the Fortingall Yew may be 2000 years old. Not only do individuals live to a great age but fossils of the species have been found from the Upper Miocene about 15 million years ago. The yew Genus, Taxus, is 140 million years old and co-existed with the dinosaurs.

Yews are often to be found in churchyards. At Rannoch they are found at West Tempar where an electric fence is used to prevent cattle from poisoning themselves by eating the leaves and along the banks surrounding the main building at Dall.

A yew tree is very slow growing it’s strategy in competition with other species is to outlive them and then exploit the gap in the canopy when they die. It may not get round to reproducing for a few hundred years but it is patient.

Yews like wet conditions but not water logged roots and in Britain they can grow between sea level and 1550 feet. Yew has a huge temperature range in which it can photosynthesise - between -8 degrees C and +39 degrees C. Yews are limited in range by very cold winters and very dry summers. It has thin bark which is not fire resistant but it lacks the resin of most conifers and tends to grow in forests where fire is less of a problem.

Yews have a fantastic root system. They put a lot of energy into growing dense and deeply penetrating roots what can store carbohydrate to get the yew through lean times. The roots provide a sound anchor and can even find their way through rock cracks to allow the yew to cling to cliffs. As with all trees the yew has a symbiotic relationship between fungi and it’s roots. In the can of the yew this is particularly close as the fungi penetrate into the actual cells of the roots - this is called endomicorrhiza.

Yew can photosynthesise in full sunlight but is also very shade tolerant. It can photosynthesise in lower light conditions than any other British tree. This allows it to grow slowly in the forest over a few hundred years waiting for one of its neighbour to die and then it can exploit the gap and the consequent sunlight. It’s shade tolerance is a good as specialised woodland herbs.

Most yew trees are dioecious which means that an individual tree is either male or it is female. It is know for some trees to have both sexes, perhaps with just one branch with the other sex and it is also known that trees can change sex through time. The pollen of yews is very light and numerous. It sinks only slowly in the relatively still air of the forest and this allows the clouds of pollen produced in the spring to spread widely and achieve 100 percent pollination of the female flowers.

The xylem vessel which carry water up from the roots are unusually thin and have extra thick spiral supports. The result is that the wood is very dense and strong. It was famously used for the English longbows that gained victory over the French at the battles of Agincourt and Crécy. One of the earliest spears, found in Essex, was made of yew.

Every part of a yew is poisonous except for the aril. Even the single seed inside the aril is poisonous although some birds such as hawfinches, great tits and greenfinches can eat them. The arils are gelatinous and sweet and are food for waxwings and thrushes amongst others.

The poison in yews causes heart attacks and respiratory failure. There is no antidote. Even dead leaves are poisonous and the poison can be absorbed through the skin so gloves should be worn when handling yew. There are 11 different poisons called taxines, of which taxine B is the most potent. Humans vomit and go into a coma before dying of heart failure in 24 hours. Horses are very susceptible and can die within minutes. Many animals know not to eat yew but horses don’t. Yews also provide chemicals called taxanes which provide medicines for treating cancer.

In Albania some shepherds have flocks of sheep that they have habituated to eating yew. They are fed a small amount at ‘dose’ is increased. They seem to survive.

A 400,000 year old spear made of Yew. Photo copyright - Natural History Museum.