The dropping of tree leaves in the fall begins in earnest shortly after reaching peak color. In the D.C. metro area that usually occurs between around Halloween and November 10.
The triggers for the turning colors are linked to early autumn changes in temperature and the declining day length. But a recent paper summarized in Scientific American argues that another factor is important in the timing of leaves: The bursting from their buds the preceding spring.
On a year-to-year basis, the paper indicates the earlier or later spring appearance of leaves was associated with an earlier or later autumn leaf fall and peak color for individual species and at a regional scale.
This unexpected finding was based on analysis of 13 years of foliage satellite data for the eastern U.S. along with approximately 20 years of tree foliage observations from the Harvard Forest in Massachusetts and the Hubbard Brook Experimental Forest in New Hampshire.
The data showed that, for every day leaves emerged earlier or later in spring, falling leaves were on average 0.6 days earlier or later in the fall. The reasons behind this result are not clearly understood. One possibility mentioned is that the life span of leaves may be programmed in the spring buds to some relatively fixed length and thereby exercise some control of when leaves will color and drop from trees.
So, what do these results mean for autumn foliage in the D.C. area this year?
This year, spring seemed to arrive one to two weeks later than usual (this is a subjective estimate, that I discussed with colleagues). That seemed apparent in the comparable lateness of leaves on trees. Recall the Cherry Blossoms peaked about 6 days later than normal and the Capital Weather Gang did not pronounce winter over until March 30!
To the extent the premise of the study is valid; there is reason to believe peak colors will occur 4 to 8 days later than otherwise predicted in the D.C. area this fall. So maybe closer to Veterans Day than Halloween? Stay tuned.