Current maladaptation increases with age in valley oak (Quercus lobata): Implications for future populations

Abstract

Adaptation and maladaptation are two fundamental evolutionary processes shaping the success of tree populations. Local adaptation has been studied extensively, but maladaptation, which not simply be the inverse, is particularly important for long-lived species since human impact is changing the environment on a very short time scale. Because maladaptation may be observed at different stages of an organisms life cycle, temporal variation may reduce or enhance the vulnerability of a species to a new climate environment. In this paper, we use findings from an ongoing range-wide provenance study comprising two common gardens of 3,674 half-sib juvenile valley oaks (Quercus lobata) from 658 maternal families to address two objectives. First, we test whether maladaptation (a) persists as trees age and/or (b) varies in magnitude over time. We model growth rates and performance measures (height * survival; "performance") across families sourced from sites with differing climates. Second, we assess the extent to which evidence-based seed transfer can mitigate the impact of maladaptation. We find that the twelve-year-old trees are currently adapted to temperatures not only cooler than their home site, but also beyond the coolest limit of the current species range. Moreover, this maladaptive pattern is exacerbated in hotter years of our study. Despite a general pattern of maladaptation, we demonstrate the feasibility of using phenotype-informed seed transfer guidelines for replanting trees in current populations by identified high-performing individuals from varied environmental origins. Overall, our results indicate that valley oak is imperiled by current and future climate conditions, but high-performing trees well suited to sites across the species range can be identified as seed donors. We conclude that current maladaptation in a long-lived tree species is a major threat that is first apparent within a few years and worsens as trees age, but careful management of vulnerable oak populations can improve their resiliency.