Wheat harvest dates in Sussex, England, from 1769 to 1910 (based on data in Russell,
1921). Considerable variation in harvest date (∼50 days) exists in a period when no rapid agricultural
advances, for example plant breeding, were taking place. Year-to-year variations in harvest date
correlate very closely with mean May–July temperatures (r = −0.710, P < 0.001).
around 1900 was early April and is currently the end of February.With a series such
as this, it is inevitable that heat and light pollution in the city will have had some
impact (R¨otzer et al., 2000) on leafing dates over and above that which would occur
in the countryside. Considerable variation in vegetation development can also be
seen in photographs of plants taken on fixed calendar dates, for example by Willis
(1944).