Understanding ancient trees
Trees….. we love them, and enjoy our time walking amongst them. Many of us have learnt their names and can recognise them. It’s a sort of ‘common knowledge’ we have and accept.
But there’s something which trees do once they become old which is very different, which goes beyond that ‘common knowledge’. People who know about it, know about it. For those who don’t, well, it’s not typically found in a tree ID book, so here’s a little step to help people understand it.
Let’s start with a typical tree…. which starts as an acorn, seed or nut. It becomes a small sapling, then a small tree, getting slightly larger and slightly wider each year until it becomes a fully mature and splendid tree. Its roots are strong, its trunk solid and sturdy, its branches strong and graceful, its crown full, and all being well it thrives and lives like this for some time.
There comes a point when it can’t keep increasing in girth and height, and it’s at this stage when different things start to happen. The tree starts to develop a lower crown, awakening dormant buds in its trunk and branches. This is called retrenchment and the photo on the right shows an oak tree doing this. It’s not instant – it takes a while. From the dormant buds grow small twigs, which become branches. The points of growth are called epicormic buds, and the twigs and branches are called epicormic growth. Branches that grow in this way look different, compared to the original growth pattern of the young or mature tree.
As the lower crown becomes established, the upper branches start to die off, and sometimes the tree will look ‘stag headed’ with dead branches sticking out of the top of a nice compact tree. The photo on the left is of a large stag headed chestnut tree.
At the same time as all this is happening, maybe a branch will fall off or break. There are areas on a tree’s trunk or branch, around the junctions of branches, which contain a collar of specialised cells that can slowly heal over. The edges around a break start to become rounded over rather than stay torn, and have strength and stability. This rounding over process can take years, decades or centuries, depending on how large the area of exposed trunk or branch is, and on the vitality of the tree.
Where there’s an opening in the bark like this, the wood is exposed to the air. This means that fungus and invertebrates can enter the tree. These soften the wood and burrow into and through the wood, causing the branch or trunk, over time, to become hollow. This hollowing area is rich habitat for a variety of very specialised creatures, and is not necessarily a bad thing. Actually the tree only needs the outer part of the trunk for its ‘life stuff’. The inner trunk is more of a structural support, and not crucial to the living day to day processes that the tree gets on with. All of these sorts of behaviours are connected with the tree being in a Veteran phase of life. The sequence of events is not strictly linear, and can be affected by many external events.
As the tree hollows and the substances within the trunk are broken down, they’re effectively recycled and are made available to the tree again. The hollowing trunk, often with holes and openings, is strong and more stable in harsh winds. By the time the tree is completely hollow it usually has a squat form, with a retrenched crown and lots of epicormic branches or growth. This sort of behaviour is connected with the tree being in an Ancient phase of its life.
The terms Veteran and Ancient are not about how old a tree is, but relate more to behaviour and habitat. Sometimes young trees become veterans before they even reach full maturity, just through circumstances. Or vast old trees can continue in their veteran phase for centuries before becoming hollow and ancient. Trees have been doing this for all of their time here on this earth, so all of this is not new to them.
It’s also worth remembering that each tree’s lifespan is different. Oaks, for example, can live for perhaps 1000 years in the right circumstances, whereas birches or beeches have much shorter lives. Yew trees behave differently again, but that’s another story!