Contrasting effects of experimental warming in the initiation year and the flowering year on flower phenology of boreal understory species.

premiseHigh-latitude plants initiate flower primordia at least 1 year before flowering. While impacts of rising temperatures on phenology in the flowering year are well studied, the effects of warmer temperatures in the initiation year (IY; the year before flowering) are virtually unknown.

methodsAt 19 sites in interior Alaska, we experimentally warmed flower primordia of seven boreal species in the initiation year, flowering year, or both and tracked flowering phenology in the flowering year for two cohorts. For two species (Vaccinium uliginosum and Vaccinium vitis-idaea), we also tracked developmental stages of flower primordia.

resultsWarming in the flowering year resulted in earlier flowering for four species (V. vitis-idaea, Rhododendron groenlandicum, Viburnum edule, and Rosa acicularis), and in no change for two (V. uliginosum, and Rubus chamaemorus; not testable in Shepherdia canadensis). In contrast, warming in the IY resulted in delayed flowering for three species (S. canadensis, R. groenlandicum, and V. vitis-idaea), and no change for the others. Consistent with this pattern, V. vitis-idaea flower primordia at the end of the IY summer were less developed in warmed plots than in control plots, but for V. uliginosum there was no difference. Comparing phenology across all sites and years, early-flowering species had advanced flowering phenology following warm IY conditions, while late-flowering species had negative or nonlinear responses, with delays under the warmest IY temperatures.

conclusionsThe likely mechanism for delayed flowering in late-flowering plants following warm IY summers is early cessation of bud development at the end of the IY growing season.