Divergent growth-differentiation balance strategies and resource competition shape mortality patterns in ponderosa pine

Dynamic resource availability leads to trade-offs among functions in plants. The growth-differentiation balance hypothesis (GDBH) predicts greater allocation of carbon to defense than growth when resources are scarce; with optimum defense production occurring at a point between the minimum and maximum growth rates. While the GDBH has been widely tested, consideration of phenotypic variation in rates for which defense is traded for growth and what this variation means for plant resistance remains rare. For defense, pines produce and store oleoresin in "resin ducts." Retrospective comparisons of resin ducts in pines have revealed that trees with greater numbers, sizes, or areas of xylem resin ducts are more likely to avoid or survive insect attacks. We used tree ring chronologies to quantify phenotypic variation in growth and resin duct defenses in pairs of living and bark beetle-killed Pinus ponderosa trees in southern New Mexico, USA, and to test the utility of the GDBH for explaining tree mortality. We also assessed the sensitivity of annual growth to climate and competitor density in years preceding mortality in each pair.  Survivors had greater growth rates and total cross-sectional areas of resin ducts than trees killed by bark beetles. We did not observe a difference in climate-growth relationships among the groups, however, trees killed by bark beetles suffered negative effects of competition while survivors did not. Growth-defense trade-offs conformed to the GDBH's prediction of a quadratic relationship, however, the two groups significantly differed in the rate at which defense was traded for increasing levels of annual growth.  Our results demonstrate that phenotypic variation in the trade-off between growth and defense could be used to characterize trees that were killed by or survived recent natural enemy epidemics. We hypothesize that the GDBH could be integrated with the characterization of phenotypic variation in growth-differentiation strategies—along with parsing of gene versus environment influences on phenotypes—at both local and landscape scales to increase our understanding of patterns of natural enemy impacts in plant populations.