Ringporigkeit des Holzes und haeufige Entlaubung durch Insekten als spezifische Risikofaktoren der Eichen

1997

S. 235-242

44 Lit. Ang.

Unselbständiges Werk

200039646

Pedunculate and sessile oaks (Quercus robur L., Quercus petaea (Matt.) Liebl.) in Central Europe are sensitive to cumulated stress caused by some biotic and abiotic factors, which lead eventually to decline and increased mortality. A hypothesis is presented to explain this specific sensitivity of aoks on the basis of lonk known facts and recent results in dendroecology, tree physiology and results of recent studies on the causes of oak decline in Northern Germany. Oaks are the only European broadleaf trees which at the same time - have a ring-porous wood structure with its typical risks in water transport, - are frequently defoliated by insects in spring and - thereafter regularly suffer secondary borer attack (Agrilus biguttatus Fabr.) . About 75% of water transport in oaks are effected by the earlywood vessels of the youngest tree-ring, which normally are highly efficient but function only for one vegetation period. Even during this period these vessels may be less efficient due to reduction in number or in diameters after insect defoliation, or become functionless by drought and frost embolisation or by damage from borer attack. In these cases water transport ist largely taken over by latewood vesseslc which are less effective but also less sensitive to these hazards, and stay functional for several years. Thus latewood vessels act normally as a security reserve of water transport capacity in oaks. However repeated insect defoliation causes a shortage of carbohydrates, which, among other effects, may result in reduced earlywood with and in complete fallure of latewood formation for several years. Such oakd become progressively inefficient in water transport and lack their security reserve of water transport capacity. During this period they are highly sensitive to any additional stress which affects the remaining earlywood vessels. Stressors may be long lasting severe drought, causing partial embolisation, especially in pedunculate oak, or deep late winter or spring frost, causing embolisation of newly forming earlywood vessels. In later stages of decline borer attack causes changes in wood structure and additional dysfunctions of adjacent xylem vessels. It is assumed, that the last oak decline episode in Northern Germany was triggered by such a cumulative effect of repeated insect defoliation followed by exceprionally late and deep frost in march and april 1985-87. Such decline events are considered to be typical risks of the oak species, causing limited mortality from time during the life time of oak stands. They do not threaten the existence of the species becaused the specific letal combination of stressors occurs rarely, and only the most stressed individuals are killed, while the remaining recover. In intensively managed forests major losses, which threaten the existance of stands, may not be tolerable. Concepts to prevent such damage include control of defoliating insects being the major inciting factor. For ecological, technical and economical reasons chemical control must be retricted to periods of high risk due to preceding extreme frost or drought and to selected high risk stands. Risk rating could probably be based on site and stand properties, which remain to define more precisely by future research. Against borer attack, which is the main aggravating factor of oak decline, silvicultural prevention by underplanting oak stands and in stands with declining trees sanitary felling are recommended.