Is flowering phenology driving rapid changes in forest trees reproductive synchrony?

Seed production in many forest tree species is characterized by mast seeding, a reproductive strategy involving large interannual fluctuations in seed production that are highly synchronized among individuals. These fluctuations underpin forest regeneration processes and have cascading effects on forest dynamics. Over the past few decades, a decline in the synchronization of seed production has been observed in many masting species across several regions, raising concerns about the future of forest regeneration and ecosystem dynamics.

Pollen limitation plays a central role in masting processes, driving both the magnitude of interannual variation in seed production and the synchronization of fruiting among trees. This limitation is primarily governed by two key processes: floral phenology and interannual resource investment in flowering. In the context of climate change, a central question is whether the observed changes in the synchronization of fruit production result from shifts in resource allocation to flowering between trees or from changes in flowering phenology. This distinction is critical, as each mechanism implies different trajectories for forest ecosystems under ongoing climate change.

Flowering phenology, which is strongly controlled by meteorological conditions, is a key determinant of pollen limitation because it influences the weather conditions experienced during pollen maturation and dispersal. Early flowering is often associated with unfavorable weather conditions, leading to increased flower abortion and reduced pollen dispersal. The degree of synchronization in flowering phenology among individuals is also a critical component of pollen limitation, as strong synchrony enhances pollen exchange and increases fruit production. Resource investment in flowering, by contrast, is largely driven by summer weather conditions, with higher temperatures generally promoting greater allocation to flowering. Climate change induces multiple environmental changes and can notably affect the homogeneity of microlocal conditions experienced by individual trees. For instance, warm spring conditions have been associated with more homogeneous microlocal environments, resulting in increased synchronization of floral phenology among trees. In this context, it is urgent to determine how the influence of microclimatic conditions on tree reproduction and flowering phenology is evolving under climate change.

This work combines more than 30 years of annual monitoring of individual fruit production across over 200 trees spanning the entire distribution range of Q. lobata, together with pollen surveys and tree-level meteorological measurements. It aims to disentangle the respective roles of floral phenology and resource investment in shaping masting dynamics, and to assess how masting in Q. lobata responds to the constraints imposed by climate change across its distribution range.