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Wood quality after ash dieback

Date: 2012.08.10

Dead water sprouts leads to clear discoloration of the wood. Photo: The Danish Nature Agency

Healthy trees are an important resource for ensuring local regeneration of ash, and for the long-term work concerning preservation and breeding of healthy ash stands. Mapping of healty ash trees has been initialized in Denmark this year, in order to build up a breeding pool.

There is only sparse knowledge of how the ash decline affects wood quality. Examples of heavy discoloration of the wood after attacks by the ash decline have been seen. Such discoloration will severely decrease wood quality. In addition, the disease weakens the tress, and often attacks by the honey fungus (Armillaria sp.) will follow.

The ash disease often kills young trees and seedlings, while Armillaria root rot kill off older trees. In order to get more knowledge and to examine and discuss experiences about wood quality in trees affected by the ash decline, the Danish Nature Agency at North Zealand organized an excursion in the fall of 2011.

Observations

Attacs by the Armillaria root rot has caused severe discoloration of the wood at the stem base. Photo: The Danish Nature Agency

The trees studied at the excursion were mainly middle-aged to old ash trees (diameter above 25 cm) that were attacked to different degrees – from almost healthy to almost dead trees. Our observations points to some external signs of disease that affect the wood quality negatively:

a. Dead water sprouts on the stem
In those cases where the water sprouts were dead, a clear discoloration of the stem wood was found in the nearby area along the trunk. If the sprouts were vital, no discoloration was found in cross-sections.

b. Attacks by Armillaria
Trees attacked by Armillaria root rot at the stem base showed clear discoloration of the wood. The closer to the stem base, the more severe was the discoloration, diminishing further up in the stem. In younger trees with smooth bark attacks by Armillaria could be seen as a brownish-purple discoloration of the bark. In older trees with furrowed bark few external signs of fungus attacks could be seen.

The ash decline and Armillaria attacks will undoubtedly affect wood quality. Especially dead water sprouts on the stem, and infection with Armillaria root rot at the stem base, seems to cause heavy discoloration of the wood. An interesting question for the future will be to study how fast the discoloration spreads in the wood in both standing and cut trees.