Drought induces oak growth synchrony at stand level but amplifies individual variability across the Romanian Carpathians

Heightened drought stress affects growth synchrony among trees in broadleaf oak forests. These effects differ by region and species but remain poorly understood and are not well connected to drought stress. This study compared synchrony that captures high-frequency variation among coexisting trees with drought-induced tree growth synchrony while controlling for age, diameter, and species. The analysis was conducted on an extensive database comprising annually resolved tree rings from 5815 individuals across 132 sites, representing two principal European oak species, Quercus robur and Quercus petraea, which were sampled across the Eastern, Southern, and Western Romanian Carpathians. Different climatic characteristics across regions are reflected by varying breakpoints in temperature trends, buffering intervals, and steeper heating gradients. The Western region experienced the highest number of months characterized by extreme drought conditions per site during the 1960s and 2000s. Throughout these decades, the growth synchrony among coexisting trees, particularly Q. petraea, rose in the West and remained comparatively low in the East. Q. robur highlighted the lowest synchrony amplitude and trend variability, with nearly uniform values across all regions. Climatic aridity increases stand synchrony while promoting tree-level growth asynchrony. This pattern is more evident in the West, where spring drought limited Q. petraea growth, while summer droughts negatively impacted Q. robur, with no statistical influence of tree age or diameter. In the South, both spring and summer droughts limited growth, but older Q. robur trees showed slow post-drought growth recovery. In the Eastern region, summer drought constrained oak growth, particularly in Q. robur. Growth gradually increased in trees with larger diameters, masking site synchrony, while individual trees remained decoupled. The impacts of increasing drought stress on tree growth synchrony partly explain the overall common heteroscedastic patterns observed across regions, species and populations.