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Insect outbreak shifts the direction of selection from fast to slow growth rates in the long-lived conifer Pinus ponderosa

Significance Understanding selection dynamics over time is fundamental for predicting evolutionary potential. This is critical in long-lived organisms, particularly trees, which are at increased risk of mortality because of climate change-related drought and associated stresses. For ponderosa pine, we show rare direct empirical evidence of strong context-dependent fluctuating selection on growth rates over time: fast growth was selected for before a mountain pine beetle outbreak but against after the outbreak, when a growth–survival trade-off emerged. Our results provide insight into the mechanisms contributing to the maintenance of genetic diversity in a long-lived organism, and show that strong and fluctuating selection can contribute to the evolutionary potential of trees under climate change. Long generation times limit species’ rapid evolution to changing environments. Trees provide critical global ecosystem services, but are under increasing risk of mortality because of climate change-mediated disturbances, such as insect outbreaks. The extent to which disturbance changes the dynamics and strength of selection is unknown, but has important implications on the evolutionary potential of tree populations. Using a 40-y-old Pinus ponderosa genetic experiment, we provide rare evidence of context-dependent fluctuating selection on growth rates over time in a long-lived species. Fast growth was selected at juvenile stages, whereas slow growth was selected at mature stages under strong herbivory caused by a mountain pine beetle (Dendroctonus ponderosae) outbreak. Such opposing forces led to no net evolutionary response over time, thus providing a mechanism for the maintenance of genetic diversity on growth rates. Greater survival to mountain pine beetle attack in slow-growing families reflected, in part, a host-based life-history trade-off. Contrary to expectations, genetic effects on tree survival were greatest at the peak of the outbreak and pointed to complex defense responses. Our results suggest that selection forces in tree populations may be more relevant than previously thought, and have implications for tree population responses to future environments and for tree breeding programs.