COS 32-10
Phenological responses of white ash trees to the warmest year in US history
Extreme years in which temperatures are anomalously high are occurring much more frequently than in the past. Recent studies show extreme years, which occurred about 33% on average during 1951-1980, are now occurring approximately 75% of the time. Plants are expected to be increasingly more affected by extreme years as climate change progresses, likely resulting in physiological and phenological shifts and higher mortality, particularly in trees. In fact, there is already strong evidence that many tree species are responding to rising temperatures through shifts in phenology. Earlier advances in the timing of plant phenological events (e.g., leaf emergence, flowering time) have largely been attributed to increases in the average global mean temperature; however, little is known to what extent extreme years will impact phenology. Here, we report on white ash phenology across five years of study including the warmest year in U.S. history (2012) in an extensive inter-population study (43 populations), and show that warmer temperatures will advance the timing of leaf emergence for this species across its distribution.
Results/Conclusions
The timing of leaf emergence significantly varied across years with average timing of leaf emergence among all populations occurring 16, 10, 24, and 28 days earlier in 2012 compared with 2005, 2010, 2011, and 2013 respectively As compared to less extreme years, we find the unexpected result that the emergence of leaves required greater accumulated warming as measured by growing degree units in 2012 across all populations. Using a phenology model, we predict leaf emergence should have occurred one month earlier in 2012 as compared to leaf emergence observations made during the same year. As a result, in the extreme year of 2012, the accumulation of growing degree units is not a stable predictor of white ash leaf emergence. In addition, we tested population rank reliability in average timing of leaf emergence across all years of study and found that rank order is highly maintained across years. Trees originating from higher latitudes consistently show later leaf emergence, indicating trees retained phenological adaptations from their source location. We conclude that extreme years can greatly alter the phenology of trees in unpredictable ways, suggesting that modeling scenarios that assume constant growing degree units for key phenological triggers and similarity among populations may not accurately predict shifts in plant phenology during extreme years.