Plant Phenology in a Warming World: Keeping Up with the Heat?
Ellwood, E.R., Temple, S.A., Primack, R.B., Bradley, N.L. and Davis, C.C. 2013. Record-breaking early flowering in the Eastern United States. PLOS ONE 8: e53788.
In an effort designed to explore - and hopefully reduce - this uncertainty, Ellwood et al. say they "used long-term flowering records initiated by Henry David Thoreau in 1852 and Aldo Leopold in 1935 to investigate this question," capitalizing on the fact that record-breaking temperatures were recorded in the spring of 2010 and 2012 in Massachusetts (USA) and the spring of 2012 in Wisconsin (USA).
In discussing their findings, the five U.S. researchers indicate that these record-breaking spring temperatures of 2010 and 2012 "resulted in the earliest flowering times in recorded history for dozens of spring-flowering plants of the eastern United States," and they also state that "these dramatic advances in spring flowering were successfully predicted by historical relationships between flowering and spring temperatures spanning up to 161 years of ecological change." In addition, they say "there is no indication that the 47 spring flowering plants we studied are delayed in their flowering by insufficient photoperiod or winter chilling requirements," noting that "these plants continue to flower earlier apparently in direct response to increasingly warmer mean spring temperatures."
Figure 1. The relationships between mean first flowering dates and mean spring (March, April and May) temperatures. Each dot represents the mean first flowering date of all sampled species for a given year in Massachusetts (left panel) and Wisconsin (right panel). Green regression lines, and 95% prediction intervals, were estimated from pre-2010 data (Massachusetts) and pre-2012 data (Wisconsin). Observed values are shown in solid red for 2012, and in green for 2010 (Massachusetts only). The 95% prediction intervals for 2010 and 2012 mean first flowering dates are indicated with vertical black lines.
Summing up their findings, Ellwood et al. conclude that "these results demonstrate that numerous temperate plant species have yet to show obvious signs of physiological constraints on phenological advancement in the face of climate change," which in this case is in the face of a global warming that has been characterized in some circles (e.g., the United Nation's Intergovernmental Panel on Climate Change) as the most dramatic of the past one to two millennia. This fact, in the words of the five scientists, "strongly suggests that most of these plants have not yet reached a physiological threshold," where such a relation might be expected to no longer hold true. And, it is quite possible that such a threshold might never be reached!