DELAYED INDUCIBLE RESISTANCE IN MOUNTAIN BIRCH IN RESPONSE TO FERTILIZATION AND SHADE

Delayed inducible resistance (DIR) is triggered by artificial or herbi vore-caused foliar damage and is manifested as decreased performance o f herbivore generation(s) feeding on the trees subsequent to the gener ation during which the damage took place. DIR is associated with incre ase in concentrations of foliage phenolics and decrease in nitrogen. T he growth-differentiation balance hypothesis, and the carbon-nutrient balance (CNB) hypothesis contained in it, claim that DIR is caused by nutritional stress after defoliation of trees growing on nutrient-poor soils. In these environments, nutrient uptake limits plant growth mor e strongly than does photosynthesis: that is, carbon-based secondary m etabolites (e.g., phenolics) are prone to accumulate. According to the CNB hypothesis, an excess of limiting nutrient(s) or reduced photosyn thetic rate should lead to elimination of DIR. We tested this using sa me-aged Betula pubescens ssp. tortuosa trees of five open-pollinated f amilies growing in a common arboretum in northernmost Finland. In addi tion to unmanipulated control trees, we had three nutritional treatmen ts during three successive growth seasons: N-fertilization, PK-fertili zation adding all nutrients except N, and shading to decrease carbon a ssimilation. Half of the trees in each treatment were artificially def oliated (50% leaf area) in the second study year, one year before the growth trial with geometrid (Epirrita autumnata) larvae. Tree growth m easurements showed that N is the growth-limiting nutrient in our study area. N-fertilization and shading of the trees affected foliage chemi stry generally as proposed by the CNB hypothesis. For example, they re duced foliar concentrations of total phenolics and condensed tannins. The birch families differed significantly in foliage chemistry. sugges ting genetic differences, but the differences were not associated with fertilizations, shading, or defoliation of the trees. Contrary to fer tilizing-shade treatments, changes in leaf chemistry and E. autumnata performance caused by defoliation were not consistent with the CNB hyp othesis. For example, defoliation caused significant DIR irrespective of N-fertilization or shading. There were no significant differences a mong the birch families in performance of the moth larvae or in effect s of fertilization, shade, or defoliation on larval performance. These responses to defoliation contrast with those of some other studies, e specially those on the Betula resinifera-Rheumaptera hastata system in Alaska, which provide clear support for the CNB as an explanation of DIR. We find methodological differences to be an unlikely explanation for the different results but are unable to propose any single mechani sm that will explain the diverse plant responses.