Patterns of Fraxinus phenology are unique in the abnormally warm year of 2012

Background/Question/Methods

Plant dormancy during winter months in the temperate zone is much more than a pause in growth. To synchronize developmental processes with the changing of seasons requires perception and interpretation of both exogenous and endogenous signals.  The growing season is becoming longer in mid- to high latitudes with climate warming.  It has been hypothesized that phenological shifts, whereby plants have earlier spring bud burst and later autumn senescence, could have large effects on productivity.  Current evidence is mixed suggesting earlier, similar, and later bud burst in response to warming. Temperature may, therefore, not be a reliable indicator of shifts in phenology.  In this study, we investigated leaf emergence and senescence across a latitudinal gradient of 10 Fraxinus populations planted at the western edge of their distribution in a common garden in Lawrence, KS across 4 years.  Weather patterns shifted in 2012, with spring arriving earlier, coupled with the occurrence of the warmest temperatures ever recorded in North America. This provided a unique year to measure phenology shifts that may be more indicative of future responses.

Results/Conclusions

Leaf emergence began, and nearly finished, 1 month earlier during any previous year measured.  All populations had much earlier leaf emergence in 2012 than any previous year.  The weather in this anomalous year may be indicative of future conditions in climate and, therefore, suggests leaf emergence will occur much earlier in future years.  Latitude, annual precipitation, growing degree days, and cumulative chilling hours were not reliable indicators of phenology in Fraxinus, yet year-to-year variability accounted for the greatest amount of variation in the response of leaf emergence (r²=0.53). Populations showed responses to environmental cues similarly among years with northern populations showing later leaf emergence. Using leaf-level photosynthesis as a proxy, we measured senescence among populations.  Northern populations showed the earliest senescense (and earliest leaf emergence), suggesting these populations may lose time for carbon acquisition in the future.  It will be important to understand how climate change will affect phenology of plants in future years in order to understand the potential of plants as a terrestrial carbon sink.  As temperature alone does not appear to be a reliable indicator of shifts in phenology, we must begin to rethink this complex process.