Beschreibung:
Carbon‐nutrient balance (CNB) hypothesis predicts that improved nutrient availability increases the growth of woody plants at the expense of herbivore resistance due to carbon‐based secondary metabolites. We tested this hypothesis by growing five clones of white birch (Betula pendula Roth) in two different habitats (mineral and peat soil) for eight years. The growth parameters of the clones and their resistance to hare (Lepus timidus) browsing were measured and compared with the growth/resistance relationship of same clones tested previously at the age of 1 year. The resistance of birches was evaluated in feeding choice experiments with both captive and free‐ranging hares. Experiments using caged or free‐ranging hares gave identical results on the feeding pattern by hares indicating that both methods are reliable in measuring the relative palatability of types of woody browse. The number of resin droplets on birch twigs is a good indicator of hare resistance, and our results are partly in accordance with the CNB hypothesis: trees grown in mineral soil (optimal conditions) had fewer resin droplets than trees grown in peat soil. Two of the five clones tested were significantly more palatable to hares when grown in mineral soil than the same clones grown in peat soil. Trees grown in mineral soil were also taller and had greater biomass than trees grown in peat soil. However, the significant clone×habitat interaction in the production of resin droplets and growth/palatability relationships indicates that different tree genotypes can have individualistic responses to environmental variations, and thus the responses of all birch clones do not follow the predictions of the CNB hypothesis. Similar rank order in the palatability of the studied birch clones at the age of one and eight years indicates a strong genetic basis for resistance. However, the relationship between height and palatability of birches was negative in the 1‐year‐old plants but positive in the 8‐year‐old birches used in this experiment. This may indicate that first‐year seedlings attain both optimal growth and high resistance at high nutrient supply with no trade‐off between growth and resistance. On the whole, more experiments are needed to determine the mechanisms of resistance, especially in young seedlings and older saplings grown in different environments.