Genetic and environmental determinants of silver birch growth and herbivore resistance

Plant growth and herbivore resistance is affected by both environmental and genetic factors. The aim of this study was to examine (1) whether genotype explains variation in herbivore resistance, plant growth and tolerance of feeding within natural silver birch populations, (2) whether there is significant interaction between birch genotype and environment or the study year, and (3) whether there are defence costs in field grown juvenile silver birch saplings. The 22 genotypes studied were assigned insect exposure and insect removal treatments in two different environments. Resistance to insect herbivory was measured as the amount of undamaged leaves of the saplings and tolerance of feeding as a difference in relative height increment of the saplings between insect exposure and removal treatments in relation to the amount of damage (i.e. opposite number of resistance). Our study shows that a local silver birch population has substantial genotypic variation in resistance to insect herbivores and this variation is not affected by the environment or the study year. Genotypic variation was not observed for tolerance of feeding. We also showed that, in contrast to genotypic variation in resistance, the genotypic variation in growth among silver birch genotypes strongly depended on the environment, i.e. silver birch genotypes have a high degree of phenotypic plasticity in their growth. Therefore, defence costs in terms of lower growth rate appear to be highly dependent on the environment. To conclude, our results indicate that silver birch populations have a good potential to adapt to possibly increasing herbivory in boreal forests in warmer future climates.