Defense traits in ponderosa pine and potential tradeoffs with growth
Plant defense responses to herbivory and pathogen attack have major physiological, ecological and evolutionary implications. Plants face a fundamental dilemma; they must grow fast enough to acquire resources and compete, yet maintain the defenses necessary to survive in the presence of herbivory. Under limiting resources when plants cannot maximize both growth and defense simultaneously, tradeoffs may arise. Tradeoffs have received much attention in the literature because they maintain trait diversity within populations. However, evidence for growth-defense trade-offs, particularly in coniferous species, remains fairly ambiguous. A primary physical and chemical defense system of pines is the production of oleoresin from a network of resin ducts located in the xylem. Here, we used a ponderosa pine (Pinus ponderosa) genetic trial planted in 1974 in western Montana that underwent a recent mountain pine beetle outbreak to address the following questions: (1) does survival to mountain pine beetle attack relate to resin duct traits?; (2) do families (genotypes) with higher survival rates to mountain pine beetle attack have increased resin duct defenses compared to families with lower survival rates?; and (3) Do faster growing genotypes invest relatively less in resin duct defenses than slower growing families? (i.e. is there a trade-off between growth and defense?). Individual trees were selected based on mortality and genotype after using geostatistical techniques to account for spatial autocorrelation. Growth and resin duct characteristics were then measured on selected trees using standard dendrochronological methods.
The outbreak lasted from 2008 to 2013 with peak mortality rates in 2009 and 2010. Overall, 36% of the trees in the stand were killed. Beetles preferred larger trees throughout the outbreak phase, suggesting a phenotypic trade-off between growth and survival. In 2010, the second year of highest mortality, there was a strong genetic component of survival: some families survived more than others. Further, there was a significant negative genetic correlation between growth and survival in 2010 that was not caused by a depletion of large trees in slow growing families. Our preliminary results indicate that, overall, trees that survived the outbreak had higher number of resin ducts relative to trees that died. Future analyses will allow us to determine whether the genetic component of survival is driven by resin duct production and whether faster growing families exhibit relatively higher investment in resin duct defences relative to slow growing families.