Elevated spring temperatures induce abnormal late-season phenology in New England Viburnum

Background/Question/Methods

Phenology is a powerful indicator of plant responses to climate change. Earlier spring flowering and leafing have been widely reported as spring temperatures increase, but studies of late season phenological changes are still uncommon.  We conducted a six-year phenological monitoring program for 35 species of Viburnum in a common garden setting at the Arnold Arboretum, Jamaica Plain, Massachusetts.  This monitoring period included normal weather years and two of the warmest years on record for the area (2010 and 2012). In order to document fall phenological changes in addition to spring advancement, we monitored plants from bud break to senescence. We noted phenological abnormalities in the fall in addition to an extension of the growing season: certain species initiated a second flush of growth late in the season. We tracked the fate of these prematurely-opened buds to assess the consequences for reproductive potential of this abnormal phenological behavior. 

Results/Conclusions

All species shifted their spring phenology earlier in warmer years as compared with normal weather years. A subset of species shifted fall senescence later. A surprising 76% of species also exhibited at least a limited second phenological flush late in the growing season. Among these closely related species, there is a great deal of variability in degree of responsiveness to elevated temperatures. Some of this variability is explained by the morphological character of naked buds, present in three lineages representing independent transitions from the tropics to the temperate zone. Species with naked buds were significantly more likely than scale-bud species to initiate a late-season second flush of growth.  Their late-opened buds were also disproportionately likely to survive through the winter and provide limited spring reproductive potential. Number of buds opened in fall ranged from zero or very few (eg, V. dentatum) to 22% (V. cassinoides). Across all species in which reproductive buds opened in fall, spring flower number was significantly reduced in fall-opened inflorescences when compared to normal spring inflorescences.  This reduction in reproductive potential in the year following climate-driven abnormal fall phenology introduces a new concern regarding negative consequences of species responses to warming climates.