Climate change affects autumn phenology

Posted by Richard B. Primack and Amanda Gallinat

“October is the month of painted leaves. Their rich glow now flashes round the world.” 

–Henry David Thoreau

Walden Pond in Concord MA, autumn 2013 (photo by Richard Primack)

Our research over the past 12 years has emphasized the effects of a warming climate on spring phenomena. As spring temperatures rise, flowering dates, leaf out times of trees, arrivals of migratory birds, and first flight times of butterflies all happen earlier. However, in order to understand the effects of climate change on temperate breeding and growing season length, we must also understand how climate affects autumn phenology. Autumn events like leaf senescence, bird departures, and insect diapause have been neglected in climate change research due to their perceived complexity, as species respond to a combination of variables including temperature, soil moisture, shortening day length, and even spring phenology. In coming years we plan to devote more time to investigating the effects of climate change on autumn phenology.

Trees are already responding to a warming autumn climate, delaying when they change color and drop their leaves. Our work on ginkgo, cherry and mulberry trees in Japan demonstrates that their delay in autumn leaf senescence over the past 60 years is greater in number of days than their shift toward earlier flowering and leaf out times. We are continuing this work in the Boston area through our investigations of the timing of leaf color change and drop using more than 1000 tree and shrub species at the Arnold Arboretum.

A Veery captured during autumn migration at Manomet Bird Observatory in Plymouth, MA (photo by Sam Roberts)

We are also pursuing the question of how milder autumn weather affects the timing of autumn migration, when birds depart New England and fly south to their wintering grounds. Some bird species in North America and Europe are advancing their arrival dates, shifting their breeding season forward, and departing earlier in autumn. In contrast, other species are taking advantage of arriving earlier in spring by producing more broods, expanding the breeding season and departing later in autumn. We are currently investigating if this pattern holds up for New England. Stay tuned!