Spring phenological timing modulates intra-annual growth: evidence of a seasonal advantage for early-flushing trees

The timing of budburst is a key determinant of a tree’s fitness within its environment. While numerous studies have investigated the environmental drivers of phenology, less attention has been paid to how phenological traits influence individual tree performance within the forest. Here, we assessed at the individual-tree scale whether earlier budburst is associated with enhanced growth and how intra-annual growth dynamics differ between early- and late-flushing trees. We analyzed annual and intra-annual growth together with budburst phenology across the Spanish plots of the ICP Forests intensive monitoring network (Level II) representative of major forest ecosystems in Spain, which include Mediterranean and temperate forests. Annual growth data were obtained from 296 trees monitored over 28 years, while phenological observations covered 13 years and intra-annual growth 11 years for a subset of 127 trees. On average, trees with earlier budburst exhibited higher annual growth, and the relationship was positive in seven of the nine species, although it was individually significant only in Pinus sylvestris and P. pinaster , and the differences among species were not statistically conclusive. No significant relationship was found in P. halepensis , P. pinea , P. nigra , Quercus ilex , Q. suber , Q. petraea , or Fagus sylvatica ; in F. sylvatica the tendency was negative, possibly owing to its high sensitivity to late frosts. Neither leaf habit nor any single climatic or edaphic variable significantly modified the relationship, although the plots of the two species with a significant relationship all belonged to the group of cooler sites, and none of the species from the warmer and drier sites showed it. The seasonal course of growth differed between early- and late-flushing trees: the higher growth of early-flushing trees was not restricted to the onset of the growing season but extended over the whole growth period.