As the climate has warmed, many plants are starting to grow leaves and bloom flowers earlier. A new study published in the journal, Nature, suggests that most field experiments may underestimate the degree to which the timing of leafing and flowering changes with global warming.

Understanding how plants are responding to climate change will help develop more accurate indicators of spring, forecast the onset of allergy season or the chances of western wildfires, manage wildlife and invasive plants, and help inform habitat restoration plans.

In this new study, scientists evaluated the sensitivity of plants to changes in temperature using two sources: experimental plots versus historical observations from natural sites.

The experiments analyzed in this study were conducted by artificially inducing warming in small study plots, and then measuring plant responses. The historical observations entailed long-term monitoring of multiple species at natural ecological research sites without any manipulation. The date of leafing and flowering was synthesized for dozens of warming experiments and monitoring sites across the Northern Hemisphere.

Scientists conclude that compared to warming experiments, historical monitoring shows temperature sensitivity to be four times greater for leafing and over eight times faster for flowering.

On average, the warming experiments predicted that every degree rise in Celsius would advance plants' flowering and leafing from half a day to 1.6 days, while historical observations indicate a temperature sensitivity of about 5 to 6 days per degree Celsius. The finding was strikingly consistent across species and datasets. Conclusions from this study are based on analysis of more than 1600 plant species on four continents.

The study of how climatic variations and trends impact seasonal events in plants and animals is termed "phenology." This includes when cherry trees or lilacs blossom, when robins build their nests, when salmon swim upstream to spawn, or when leaves turn colors in the fall.

The study was funded by the National Science Foundation, the State of California and the University of California, Santa Barbara. The U.S. Geological Survey (USGS) and the USA-National Phenology Network (USA-NPN) also provided support and assisted with assembling and analyzing historical phenological observations and climate data.