Defence reactions of Norway spruce against bark beetles and the associated fungus Ceratocystis polonica in secondary pure and mixed species stands

The effects of restoration of secondary pure Norway spruce stands on defence mechanisms against bark beetle attack were tested. The test trees have been growing in forest stands with contrasting structure, stand density (thinned/unthinned) and species composition (pure/mixed). Seasonal differences in defence mechanisms were also analysed. Anatomical characteristics of the resin duct system of the stem bark, constitutive resin flow rate and preformed monoterpene content, wound reaction after low density inoculation with Ceratocystis polonica, induced resin flow and monoterpene content, were used as indicators of tree resistance. Resin flow was closely related to anatomy of radial resin ducts of the secondary phloem. Differences in constitutive resinosis between spring and summer were mainly affected by different air temperatures during resin flow measurements, Wounding and fungus inoculations induced higher resin flow rates and also an increase in monoterpene content of unaffected bark tissues. Monoterpene composition also changed considerably. Induced defence was more different between stands than constitutive defence. Trees from the mixed species stand showed higher primary resin flow, lower lesion length and lower induced resin flow compared with trees from unthinned/thinned pure spruce stands. However. only the relative increase in resin flow was significantly higher in trees from the unthinned pure spruce stand. Constitutive as well as inducible defence mechanisms were affected by tree growth. Initial monoterpene content, cross-sectional area of injured resin ducts and primary resin flow rate decreased and the length of lesions increased with increasing radial growth indices. These results support the hypothesis that there is an inverse relationship between growth and defence. Both primary defence, as well as secondary induced defence, were affected by tree-individual growth history. (C) 2002 Elsevier Science B.V. All rights reserved.