Chemical defense strategies, induction timing, growth, and trade‐offs in Pinus aristata and Pinus flexilis

Trade-offs between plant defense investment and fitness traits, including growth, are often invoked to explain evolutionary strategies targeted at resisting herbivores. Many Pinus species have specialized herbivores, including the mountain pine beetle (MPB), Dendroctonus ponderosae, and have historically been a focus of defense investigations. We compared defense traits of two high-elevation Pinus species, P. aristata and P. flexilis, that are hosts to MPB and hypothesized to have different growth and defense traits and potential trade-offs. Interspecific differences were assessed by sampling trees within the same stands, and intraspecific differences were assessed by sampling stands at sites across latitudes where both species co-occurred. Constitutive defenses were measured at Day 0, and the timing, concentration, and composition of an induced resin defense response were assessed by sampling at 1, 4, and 30 days following either mechanical wounding only or a simulated MPB attack using its primary fungal symbiont Grosmannia clavigera. At Day 4, induced resin concentrations did not differ between mechanical wounding and simulated MPB attack in either species. By Day 30, resin defense concentrations in response to simulated MPB attack were greater than those in response to mechanical wounding and were >19-fold greater than constitutive levels. Results suggest that initial induced resin defense responses in the two species are likely generalized, with a delayed response that is targeted specifically at MPB and G. clavigera. At all sites, P. aristata had higher concentrations of constitutive and Day-30 induced resin defenses than P. flexilis, although P. flexilis induced proportionately more. Trade-offs in growth and defense between the species were only found at the two most climatically favorable sites where P. aristata grew slower than P. flexilis. No trade-offs were found between the two defense types at either biological scale. Overall, our findings highlight that the two pine species growing in the same stands (1) have a delayed response to a specialized native herbivore and fungal symbiont, (2) only exhibited interspecific defense-growth trade-offs at two climatically favorable sites, and showed no intraspecific defense-growth trade-offs, (3) showed no trade-offs between constitutive and induced defenses at either biological scale, and (4) have evolved different defense strategies.