Thursday, 12 March 2015

Not So Poplar!

Sound looking wood, but dead all the same.
Note the pollarded, but still living,  poplar behind.
Whenever we do any major habitat work on the Reserve people's first reaction will be "Crikey!" (or something like that.) This is especially true when you arrive back at Paxton after a break and find that your favourite tree is lying flat on the ground in handy sized take-away pieces. I feel the same myself, but there is always a reason for the work we do.

It's good when people ask why, because I get a chance to write yet another blog.

Please forgive the following lecture; I’m an ex-teacher and I love this stuff!

This week's topic for discussion is tree-felling. In particular the felling of a seemingly health poplar tree on the big bend of the Great Ouse, along the side of the Great Meadow where our windpump is situated.

The culprit:
Hornet Clearwing Moth
When these grey poplars trees reach maturity, they almost always get attacked by hornet clearwing moths that lay their eggs in cracks at the base of the tree. You can easily see the exit holes from which the moths hatch. The holes are at ground level and each hole is about the diameter of a pencil. Most trees can cope with this.

Several years ago we found that all three of the trees on the riverbank had been attacked by the moth, but they were not coping as well as the other poplars on the reserve. We think this is due to several wet winters that flooded the path and encouraged a fungal infection to develop in the base of trunk at ground level. The first sign was a die back of some branches. At this stage, the trunks and lower branches looked perfectly healthy.

The Result:
Exit holes made by the moths and rot at ground level
due to fungal infection.
(Boring bit) As we all know, the old wood at the heart of the trunk is practically dead. It’s remaining function is to hold the tree up. The outer (younger) rings of growth hold the capillaries that transport nutrients and water from the roots to the branches, drawn up as the leaves transpire. When those capillaries get blocked by fungus, the branches die off and so eventually does the tree. But a dead tree stays up for a long time because the heart-wood is still sound, and we know that standing deadwood is a good thing for wildlife.

We decided to pollard the dead branches and hoped that the trees would be able to recover. Of course, we had just treated the symptoms, not the cause, but at least no-one was killed by a falling limb. We marked the trees with metal discs and continued to monitor them on our rounds.

Health and Safety nightmare!
As it turned out, two of the trees have survived the surgery but one didn't. Our regular checks involved giving the trunk a good poke with a screwdriver which sank in for several inches. I’m afraid the other two poplars may well go the same way in year or two, especially if stressed by either flood or drought.

In a situation like this, the precautionary principle applies; if a tree is likely to come down on a path in the next year, we fell it. If we can save it by pollarding, we try that first.

Tree surgery is expensive, by the way. We have probably spent over £600 on trying to save these three poplars alone.

As a rule we leave the trunk in big pieces to benefit wildlife, but we are not allowed to do this by the river because the next flood might carry the logs away and cause no end of damage to boats and weirs.

There is good news though. These poplars grow back from succours on the roots and so we do not have to replant them. Of course, once they reach maturity, the moths attack again, and so the cycle continues. It’s not unlike the situation with Dutch Elm disease. Near the River Viewpoint you can see elms sprouting every year, but they get attacked by elm beetles as soon as they have enough girth. We don't have to fell them because they never grow big enough to be a problem.