16/02/2009
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Life after death for fallen trees and branches

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A major snowfall of almost three quarters of a metre in the Cairngorms has created more dead wood than during any other single natural event in the past two decades on RSPB's Abernethy forest reserve. But rather than be worried about the apparent damage done to many of the mature forest trees on what is the largest remaining expanse of the ancient Caledonian pine forest, reserve staff and scientists at the conservation charity are unconcerned with drastic change that the extreme weather has brought about to the forest.

Crested Tit
Crested Tit, Aviemore, Highland (Photo: Steve Round)

Large numbers of Scots pine trees have lost limbs, and some have even keeled over completely or have been split asunder by the sheer weight of snow that has fallen over the past week. When trees die, their biological function within the forest ecosystem is far from over, and they continue to play a critically important role in maintaining the health and biodiversity of the woodland ecosystem. As they gradually decay they become mini nature reserves in themselves — with the rotting process taking anything up to a century whilst they are gradually recycled back into the woodland ecosystem and at the same time providing niches for countless numbers of invertebrates, fungi, lichens, birds and even small mammals.

In fact, in a natural forest ecosystem free from human interference, between 20% and 30% of the existing trees will be either dead or dying. However, most of the remaining ancient or semi-natural woodlands in Scotland have been highly modified over several millennia, and this natural process is either absent or much reduced.


Abernethy (Photo: Desmond Dugan, RSPB)

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Desmond Dugan, one of the site managers at Abernethy, said: "We have had thousands of branches and limbs — some of them heavy, muckle branches — ripped off by the weight of snow. When heavy snowfall is coupled with the extreme frost — and the temperature dropped to –19°C over a couple of days — all the resin and sap in the pines gets frozen up, they become very brittle, lose their elasticity and the branches snap with extreme force. You hear a muffled crack due to the snow quietening everything. Last week it was like gunfire in the forest as limbs and branches snapped suddenly, and then thuds as they hit the soft snow beneath. The forest is just incredibly atmospheric now."

He added: "But rather than view this as damage, it is actually quite beneficial and creates opportunities for more life to flourish. All the dead wood serves as the building blocks of the higher ecosystem, because the birds feed on the invertebrates that live in the decaying wood, and then mammals feed on the birds and so on. When these trees are torn apart like this, it creates a ragged tear in the wood. This allows pathogens and bacteria to colonise and promotes decay of the fallen wood."


Cairngorms (Photo: Desmond Dugan, RSPB)

Some sixteen species of birds use dead trees at Abernethy. Woodpeckers drill nesting holes that are colonised by tree-nesting Swifts, Crested Tits or Redstarts and flycatchers. Often there is great competition to secure these scarce cosy houses, and such is the demand for this valuable real estate that nesting birds and often evicted by Pine Martens or by larger or more dominant birds including Goosanders, Goldeneye or Tawny Owls.

Tawny Owl
Tawny Owl, Ipswich, Suffolk (Photo: Bill Plumb)

The fallen branches and limbs have also had another beneficial effect. RSPB's Abernethy Forest is one of the largest registered seed stands in the UK for native species including Scots pine, holly, juniper, rowan and birch. Many of the fallen limbs are laden with this year's cone crop, making it easier for staff to collect the cones, which are sent to the Forestry Commission's research station and seed bank at Alice Holt Lodge in Farnham, Surrey. Here the seed is extracted from the cones and dried before being stored at regular temperatures in their seed bank. It is then sent out to order to nurseries, where it is germinated and grown into saplings to replenish native woodland in the northeast of Scotland.

Written by: RSPB