Seasonal Cycles: Budburst

There are perhaps no signals of spring’s arrival more welcome than the blooming of flowers and leafing of trees. Thoreau spent countless hours recording when these events happened for individual species. The timing of when plants flower and leaf out in the spring is closely tied to temperature. In years when the average temperature in March, April, and May is warmer, flowers bloom and leaves come out earlier than in colder years. Over time, as the average temperature in Concord has increased, spring has been arriving earlier.

Plants in Concord are responding to warming temperatures by flowering two days earlier for each increase in temperature of 1°F. When Thoreau was recording flowering dates, the average spring temperature was 42°F and the average date of first flowering of 32 selected species was May 15. In recent years (2004-2012), the average spring temperature has been six degrees warmer at 48°F and the average date of first flowering of those same 32 species is eleven days earlier on May 4. A similar trend is seen with leaves now emerging earlier in the spring than in Thoreau’s time.