MY notes about the Fortingall Yew (D&S, Oct 1) have prompted some enquiries as to why yew trees were planted in churchyards, or why they grow close to churches.

The truth is that many were growing on those sites long before the church was constructed. Quite often, it is the church which has been built near the yew, not the yew which has been planted near the church.

Not many years ago, people guessed the age of yews by measuring the girth of their trunks and, in a lot of cases, reckoned they were somewhere between 200 and 400 years old. This was considered very ancient in comparison with many other species.

Such trees are now thought to be of a far greater age. It is known that more than 400 yews in this country are over 1,000 years old, with a further 45 being at least 2,000. And top of the tree, in a manner of speaking, is that fabulous old yew in Fortingall, boasting at least 5,000 years of life and even rising to 9,500 according to some experts. Not many churches are so old!

It has never been easy to calculate the age of a yew because the tree is prone to a disease which destroys the interior of the trunk, making it hollow and thus impossible to count the growth rings. That is the normal means of establishing the age of a tree. Despite this, and despite pieces being cut from yews, they survive to send up new shoots some distance from their original trunk. Some believe this system of growth, which makes use of the original root system, means that a yew could be everlasting as well as evergreen, but its age can be calculated - or perhaps estimated - by measuring the girth of the trunk.

In 1796, the Fortingall yew's girth was more than 50ft - and a girth of only 40ft means a yew is 5,600 years old. A 10ft girth means 250 years of age, 15ft means 500 years, 20ft means 1,000 years, 25ft means 1,400 years and 30ft equates with 2,500 years. But these ages can only be approximate due to vagaries of climate, location and the tree's individual lifestyle.

It is highly probable, therefore, that some of our existing yew trees were growing long before Christianity came to these shores. We must also remember that many have been deliberately destroyed for various reasons, although one must wonder whether the old root systems of those lost trees have survived.

So why were churches built so close to yew trees, often with the yew on the north side of the building? The answer is that ancient peoples regarded the yew as a sacred tree, probably due to its evergreen foliage or even because it appeared to defy death.

It has been revered for thousands of years, with Druids and the Celts in particular regarding it as a holy plant. For those who respected or even worshipped the yew for religious reasons, it made sense to site their places of worship close to the tree. Many such sites were established close to yew trees.

When Pope Gregory sent Augustine to England to convert the English and become the first Archbishop of Canterbury, he was advised not to destroy the old pagan temples, but to re-use them as Christian churches.

Those early buildings and religious ceremonies would bear little resemblance to those with which we are familiar today and it is almost certain there would be a mixture of pagan and Christian rituals to ease the transition between the two faiths.

But while this turmoil was afoot, those ancient yews would stand there, almost as silent, watchful sentinels, so they then became part of the Christian tradition.

In fact, some Christians referred to the yew as the palm tree, using its leaves and branches in ceremonies on Palm Sunday and for other occasions, and it has often been used to decorate churches at festive times, such as Christmas.

This use of greenery in our churches and in our homes at Christmas (known to the pagans as Yuletide) is just one of those customs which have remained with us since pre-Christian times.

The fact that yew trees still feature in our modern places of worship is quite remarkable, for it means the tree has retained its mystical symbolism even though our faith has radically altered down the centuries.

Indeed, as a Millennium celebration, lots of yew tree cuttings were planted in our churchyards and some were blessed by the Archbishop of York at a ceremony in Northallerton parish church in October, 1999. The cuttings were taken from yews more than 2,000 years old.

For all its appeal, it must be said that all parts of a yew tree are poisonous to humans and most animals, the exception being the flesh of its bright red berries. The seed is also poisonous. This is a mysterious tree for many reasons and I like the story that there are 99 yew trees in the churchyard at Painswick in Gloucestershire, but each time a 100th is planted, it always dies.

Following the topic of trees, the route of my daily morning walk was recently changed due to nearby roadworks. Upon my new path I came across a small copse of sweet chestnut trees.

These are quite distinct from horse chestnut trees, which are so popular as small boys raid them for conkers. My local trees were heavily laden with their fruit in very spiky shells, probably another result of our damp but productive summer.

It is the fruit of this tree which many of us roast on open fires as a Christmas treat, but it seems the British specimens do not produce very large nuts. Our summers are not long enough or hot enough to swell sweet chestnuts to a useful commercial size. Those we find in fruit shops, supermarkets or on market stalls usually come from Europe, probably southern Italy, Spain and France.

It was the Romans who introduced the sweet chestnut to our islands, for it is a native of the Mediterranean regions and in some areas is known as the Spanish chestnut. Most of the trees in this country are found in parks or the gardens of large country houses where they were planted as decorative specimens. Nonetheless, some do grow in the wild and I am sure people will be tempted to harvest those prickly-coated chestnuts in the hope they may be useful at Christmas.

The tree can be identified by its spiralled bark, which can develop very deep fissures, and it produces long, oval-shaped leaves with serrated edges and rather prominent veins. In the autumn, the leaves become a beautiful golden colour, which perhaps explains the sweet chestnut's popularity as a purely ornamental tree.

Trees feature in the weather lore of this country and perhaps the most common belief is that when leaves show their undersides, rain may be expected.

This is a very widespread piece of weather wisdom and, from my own observations, it seems to be true.

Rain is also forecast if leaves curl in a wind blowing from the south. The question is why do the leaves behave like this?

The belief is not restricted to this country. The Americans believe that rain is expected if the silver maple shows the underside of its leaves and this is also the case in America if the leaves of the aspen (silver birch) are seen to be trembling.

In some areas, it is thought that certain trees, such as the sycamore, lime, plane and poplar, display this tendency and one explanation is that the moist air makes the leaf stalks more pliable and weaker. Or is there some other scientific explanation?

Another piece of lore is that when dry leaves rattle while still on the tree, we can expect snow, although if trees snap and crackle during the autumn, it indicates a spell of dry weather.

It is also believed that, if oak trees retain their acorns later than usual, it is the sign of a harsh winter. But if beech mast is plentiful in the autumn, then a mild winter can be expected.

On a more superstitious note, it used to be thought that storms were brewed by witches stirring the surface of water with the branch of an elder