Leafing Dates of Oak (1746–present)

This graph tracks changes in the dates that oak trees in southeastern England first show their leaves—one measure of the start of spring. Data recorded between 1746 and 1958 by Norfolk’s Marsham family are in blue. Data from Jean Combes, who recorded leafing dates in Surrey from 1947 to the present, are in red.

For oaks in England, it is generally true that the warmer the temperature is, the earlier the trees leaf. On this graph, the overall trend shows average leafing dates (represented by the smooth line) advancing by several days over the period shown.

Phenology is the study of the timing of natural events, like bird migrations or the blooming of flowers. To find trends in climate change, researchers often turn to past records of phenological events—like these records of oak tree leafing. But researchers must be cautious about the conclusions they draw. How reliable is data collected hundreds of years ago? And how might factors like changes in air pollution and precipitation offset the trends researchers see?