Selection favors adaptive plasticity in a long-term reciprocal transplant experiment

Spatial and temporal environmental variation can favor the evolution of adaptive phenotypic plasticity, such that genotypes alter their phenotypes in response to local conditions to maintain fitness across heterogeneous landscapes. When individuals show greater fitness in one habitat than another, asymmetric migration can restrict adaptive responses to selection in the lower quality environment. In these cases, selection is predicted to favor traits that enhance fitness in the higher-quality source habitat at the expense of fitness in the marginal habitat, resulting in specialization to the high-quality environment. Here, we test whether plasticity is adaptive in a system regulated by demographic source-sink dynamics. Vaccinium elliottii (Ericaceae) occurs in dry upland and flood-prone bottomland forests throughout the southeastern United States, and shows patterns consistent with source-sink dynamics. We conducted a multi-year field experiment to evaluate whether plasticity in foliar morphology is advantageous. Both across habitats and within the high-quality upland environment, selection favored plasticity in specific leaf area and stomatal density. Stabilizing selection acted on plasticity in these traits, suggesting that extreme levels of plasticity are disadvantageous. We conclude that even in systems driven by source-sink dynamics, temporal and spatial variation in conditions can favor the evolution of plasticity.