2012 
2018 
OCTOBE 
RS OCTOBER 10 OCTOBER OCTOBER 20 OCTOBER 25 
fd 4 a q “aE Ht x —— Tae = =| ° : 5 
ALLEN, K. AND OSWALD, W.W. 2021. A NEW LOOK AT BOSTON COMMON TREES. ARNOLDIA, 78(3): 38-41 
A New Look at Boston Common Trees 
Kelsey Allen and W. Wyatt Oswald 
MARCH 24 MARCH 29 APRIL 3 APRIL 8 
hh. Mt cee |; ’ ih 
echnology changes how we see the world: think of Antonie van Leeu- 
wenhoek’s microscope or Jacques Cousteau diving with a video camera 
and bringing the movements of ocean life to the silver screen. For the 
past decade, a digital camera mounted on the roof of a ten-story building has 
taken photos of the Boston Common every thirty minutes. The camera is a 
simple consumer model, but the resulting set of photographs, numbering well 
over two hundred thousand, compresses time in a way that turns everyday 
changes within the tree canopy into meaningful patterns and trends. Within this 
set of images, forty seasons can be viewed as a flipbook. If you visit the Boston 
Common in April, you will see light-green leaves unfolding on elms (UJmus) and 
the warm glow of red maples (Acer rubrum) bursting into flower, yet only in an 
image set like this could you determine how these hour-by-hour moments in the 
life of a tree correspond to seasons past. Ten years can be viewed simultaneously. 
Seasonal shifts can be visualized in a way that surpasses our on-the-ground expe- 
rience. Moreover, thanks to image-analysis software, data can be extracted from 
the photographs, allowing researchers to quantify the “greenness” of the canopy 
as it changes through the growing season and from year to year. 
We know that global climate change is impacting plant phenology. Already, for 
instance, researchers have described discernable differences between flowering 
times for herbarium specimens that were collected one hundred years ago and 
those that have been collected in recent years. So far, however, the photographs 
of the Boston Common have shown relatively consistent leaf-out times in the 
spring, with the exception of 2012. The sequence of photos from that year shows 
the details of the springtime green-up, when anomalously warm temperatures in 
March triggered leaves to emerge two to four weeks earlier than other years. The 
elms turn green first, but not because of leaf emergence; in fact, we are seeing 
ae oe ame rs 
APRIL 13 
OCTOBER 30 
