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 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?

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Leafing Dates of Oak

Leafing Dates of Oak (1746–present) - This graph shows how the leafing dates of oaks in southeastern England have changed over the past 256 years. Source: Woodland Trust

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Notice that the graph includes peaks and valleys that vary radically from year to year. Phenomena that are naturally inconsistent produce what scientists call “noisy” data. With so much variation, it can be hard to see any trends at all. To track long-term changes, researchers must take averages of the changes they see. At the same time, they must make sure that their own biases and expectations don’t influence their interpretation of the data.

 related sites  

The Woodland Trust - A site devoted to monitoring changes in the timing of natural events in England.

Audubon Naturalist News/“Witnesses to Global Warming?” - An article on tracking changes in leafing dates in the Washington, D.C. area with information on the effect of climate changes on animal migration.

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