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EFFECTS OF CUMULATIVE DEFOLIATIONS ON GROWTH, REPRODUCTION, AND INSECT RESISTANCE IN MOUNTAIN BIRCH
Author(s) -
Kaitaniemi Pekka,
Neuvonen Seppo,
Nyyssönen Tuija
Publication year - 1999
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(1999)080[0524:eocdog]2.0.co;2
Subject(s) - resistance (ecology) , insect , ecology , reproduction , biology
Mountain birch is a subarctic tree that faces a high risk of severe attack from herbivores. During outbreaks of its major pests, geometrid moths, a typical sequence of defoliation lasts 2–3 successive years. We simulated a defoliator outbreak by different combinations of 25% and 75% defoliations over three successive years. We studied patterns of resource allocation during and after the simulated outbreaks by quantifying the effect of different degrees of cumulative defoliation on mountain birch resistance, growth, and reproduction. Bioassays showed that the delayed inducible resistance largely waned within 2 yr, whereas the effects of defoliation on tree growth and reproduction persisted longer. Cumulative defoliations intensified birch resistance only slightly, and the efficacy of resistance remained in the range induced by a single defoliation event in other studies. This suggests that, even after cumulative defoliations, the “killing power” of induced resistance in mountain birch accounts for only a fraction of that required to end a geometrid outbreak. A low level of defoliation (25%) did not generally affect the trees, but high (75%) defoliation decreased their growth and reproduction and also decreased their palatability to insects. Sequential annual defoliations reduced the mass of female catkins, whereas seed‐specific mass and seed viability were most affected by previous‐year defoliation. The number of catkins per tree was most affected by defoliation 2 yr earlier. Thus, previous‐year defoliation affected catkin and seed properties due to the exhaustion of nutrients required for their development, but earlier defoliation limited the catkin number by affecting bud type determination. Similarly, long shoot number was affected by defoliation 2 yr earlier, but cumulative defoliations also reduced the length of long shoots. Trees maintained leaf biomass at the expense of growth and reproduction; leaf dry mass, leaf area, and specific leaf mass were only slightly and nonsignificantly reduced by previous‐year defoliation. For mountain birch, maintenance of normal leaf biomass may be a tolerance strategy for coping with herbivory that is at least as important to a long‐lived plant as is induced resistance.