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Key messageThe fitness trajectory of long-lived forest species with mixed mating systems is shaped by a dynamic interplay between endogenous (inbreeding depression) and exogenous (environmental maladaptation) factors. Using two eucalypt species, we show that the timing and translation of inbreeding depression from growth to survival through size-dependent mortality may vary between species and may intensify under climate stress.ContextInbreeding is an important issue in evolutionary biology and breeding, as it can reduce genetic diversity and fitness and ultimately limit the adaptive response of populations to environmental stress. This is particularly relevant to forest tree species, such as eucalypts, which have a mixed mating system and long-generation intervals.AimExamine the role of inbreeding depression on the fitness trajectory of two eucalypt species, Eucalyptus globulus and E. ovata.MethodsSurvival, growth, and reproduction of controlled-crossed self and outcross, as well as open-pollinated progeny of each species grown in a common garden field trial were assessed over a 28-year period and analysed using mixed effect models.ResultsInbreeding depression resulted in the purging of inbred progeny through size-dependent mortality with the most death of inbreds occurring between 4 and 13years. After this period, differential maladaptation of the species was the dominant cause of mortality, associated with a period of drought and high temperatures, and it was evident first in the selfed populations.ConclusionThis study demonstrates the dynamic nature of the selective process in purging inbred progeny from a population, with inbreeding depression the dominant factor early in stand development, leading to older stands being dominated by outcrosses.