Growth and Hare Resistance of Birches: Testing Defense Theories

We tested resistance to the hare (Lepus timidus) of one-year-old seedlings of ten birch (Betula) species showing different life history strategics. Also, we tested the effect of fertilization and shading on the resistance and growth of eight species, including several geographic origins of some. Resistance varied substantially and consistently among the ten birch species. both in field and in cafeteria experiments. Three species exhibited high resistance (B. ermanii. B. platyphylla, and B. resinifera), and six high susceptibility (B. alleghaniensis, B. grossa, B. maximowicziana. B. papyrifera, B. pubescens and B. schmidtii). B. pendula exhibited substantial variation in resistance among individuals and origins. Inherently fast-growing species showed a strong response to fertilization. Growth rate was not related to resistance/susceptibility at the species level. Fertilization significantly lowered the apparent resistance of two of the three resistant species (B. resinifera was invariant), but did not change the susceptible species. Shading substantially decreased seedling mass at high, but not at low nutrient levels, and generally had only a minor effect on resistance. The resistance of B. pendula and B. platyphylla seedlings from different geographic origins varied significantly, but growth and resistance of these seedlings were not related. There was no significant difference in resistance between the easternmost of the western Siberian and the Finnish origins of B. pendula, nor was there any east-west (55°-78°E longitude) clinal trend in its resistance. The resistance of progeny from fast growing Finnish B. pendula families (plus trees) was similar to that of randomly selected forest origins from Finland and western Siberia. Resistance and growth were not related in this comparison either. These results emphasize the great genetic and phenotypic variabiiity both within and among birch species but lend little support for the hypothesis predicting a trade-off between growth and hare defense investments in one-year-old birch scedlings.