Divergent phenological shifts among trees, shrubs, and grasses in an alpine forest over two decades of warming

Divergent phenological shifts among trees, shrubs, and grasses in forest ecosystems have the potential to fundamentally reshape forest structure and productivity across vertical strata. However, our understanding of the interrelationships among these plant functional types under long-term climate warming remains limited due to a lack of systematic and extended studies. Here, we present results from a 20-year phenological monitoring experiment in an alpine forest ecosystem, systematically tracking the phenology of trees, shrubs, and grasses under climate warming. Our results show that the growing season of trees lengthened by an average of 0.2 days per year, primarily driven by a significant delay in leaf discoloration and defoliation. In contrast, shrubs and grasses experienced shortening growing seasons, with average reductions of 0.43 and 2.62 days per year, respectively. While shrubs exhibited delayed budding, leafing, and flowering, grasses showed significant advancement in defoliation timing. Changes in vegetation cover corresponded to phenological shifts: tree cover increased, while shrub and grass cover declined. These findings suggest that climate warming may weaken forest hierarchical structure by disproportionately enhancing tree growth over understory vegetation. Differential phenological responses across forest strata should be a focal point in climate impact assessments. • Trees, shrubs, and grasses exhibited divergent phenological responses to climate warming. • Tree defoliation was delayed by an average of 0.8 days per year. • The growing season of shrubs and grasses declined under warming conditions. • The mechanisms behind decreased growing season differed between shrubs and grasses. • Forest hierarchical structure weakened due to these divergent phenological responses.