Resistance of Quercus rubra to general insect herbivory as affected by environmental factors and plant genotype

Poor natural regeneration of northern red oak (Quercus rubra L.) in astern US forests has been a cause for concern over recent decades. I studied the relationship among growth, defense, and storage of northern red oak seedlings under different nitrogen and light resource regimes to understand how different resource regimes might affect resistance and tolerance mechanisms of northern red oak. I also investigated genotype-environmental interactions on the resistance of northern red oak trees to the surrounding insect community.; I found, based on root/stem ratio analysis, that gypsy moth defoliated seedlings revealed trade-offs between resistance and tolerance traits under nitrogen or light-limited conditions, while the seedlings responded to defoliation by raising both resistance and tolerance traits when resources were not so limited. However, there were discrepancies between results from root/stem ratio analysis and allometric analysis, and further investigation may be needed to decide whether root/stem ratio data was misleading or not in this study. Results showed that even though loss of above-ground biomass was partially compensated after defoliation, the compensation was accompanied by a presumed cost to below-ground biomass or storage. Thus, if defoliation persists on northern red oak seedlings through several seasons, the reduced root biomass and storage may lead to long-term costs for compensatory ability and defense, and may reduce the likelihood of regeneration. It is suggested that the relationship among growth, defense, and storage of northern red oak seedlings depends upon resource availability and life-history.; Resistance of northern red oak trees to insect herbivore attack was under genetic control. However, the genotypic resistance varied over time (both year and date within season), location, nutrient environment (nitrogen fertilization level), and insect feeding guilds (chewer and galler). These may arise because of a genotype's adaptation to local environment, differences in the insect community over time and space, differential inducibility of genotypes, and differences in miscellaneous plant phenotypic traits related to resistance. For instance, genotypes has different ability to induce defense. However, the inducibility also depended on guild, location, and year. Also, genotypic variation in budbreak timing appeared to favor or disfavor different insect herbivores, at least at the level of feeding guild. (Abstract shortened by UMI.)...