Time tells: plasticity and developmental asynchrony underlie trait variation in a dune‐building grass

Habitat‐modifying plants engineer landscapes through plant‐environment feedbacks. The strength of these feedbacks is determined by above‐ and below‐ground traits shaping landscape morphology. Besides interspecific differences, recent findings highlight that intraspecific trait variation, such as shoot density, can also influence landscape morphology. Trait expression often changes throughout a plant's life cycle and in response to environmental conditions. Traits advantageous during establishment in early successional stages may differ from those facilitating long‐term persistence in stabilized environments. Yet, it remains unknown how traits change relative to environmental stressors across a landscape successional gradient. We conducted a field experiment planting 120 juvenile Calamagrostis arenaria transplants, similar in size and appearance, across three successional habitats (beach, foredune, backdune) at four dune locations, covering its full habitat range. After two growing seasons, surviving transplants were harvested to analyze how establishment success, biomass dynamics and trait expression responded to environmental stressors. Establishment, biomass dynamics, and trait expression in C. arenaria varied across successional habitats. Initially, C. arenaria's capacity to cope with burial stress is limited, impeding establishment at the beach. Over time, however, high sediment dynamics in early successional habitats promoted growth. In contrast, stabilized foredune and backdune habitats, despite benefitting establishment, provide less favorable growing conditions, reducing biomass and plant vigor. Traits associated with growth strategy, like below‐ground stem length, increased in response to burial stress, indicating plasticity. Conversely, above‐ground traits reflecting vigor, e.g. stem diameter and leaf size, changed according to plant size, suggesting divergences in developmental. Collectively, our results demonstrate that trait expression and requirements change throughout the life cycle of a habitat‐modifying plant and in response to environmental conditions. While some traits shift with plant development, others show plastic responses to stress. Our results suggest that single‐season or large‐scale studies may overlook key features determining trait expression, essentially shaping emerging landscapes.