Technology changes how we see the world: think of Antonie van Leeuwenhoek’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 (Ulmus) 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 experience. 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 the maturation of samaras, the elms’ winged fruits. Leaf out of the elms, along with the Common’s red maples, lindens (Tilia), oaks (Quercus), and scholar trees (Styphnolobium japonicum), follows over the next few weeks. As trees on the Boston Common respond to climate change in the future, ongoing photography may reveal that years like this become less anomalous.
At the other end of the growing season, the deciduous trees of the Boston Common start to prepare for winter by breaking down their photosynthetic machinery during the second half of October. The timing of those changes has not varied much over the last ten years. In the set of photos from 2018, for instance, we can see the visual transformation of the landscape that occurs each fall, with the faded greens of early autumn giving way to patches of gorgeous color, including yellow elms and reddish-brown oaks. Then, by the last week of November, the leaves have all fallen, exposing the scaffolding of branches that held them aloft all summer long. And at the tips of those branches are buds, poised to burst open in spring and start this cycle anew.
Oswald, W. W. and Richardson, A. D. 2015. Tracking the seasonal rhythms of Boston Common trees. Arnoldia, 73: 36–39.
Primack, D., Imbres, C., Primack, R. B., Miller-Rushing, A. J., and Del Tredici, P. 2004. Herbarium specimens demonstrate earlier flowering times in response to warming in Boston. American Journal of Botany, 91: 1260–1264.
Richardson, A. D. 2019. Tracking seasonal rhythms of plants in diverse ecosystems with digital camera imagery. New Phytologist, 222: 1742–1750.
Kelsey Allen is a student at Emerson College, studying literature and environmental science. W. Wyatt Oswald is a professor in the Marlboro Institute of Liberal Arts and Interdisciplinary Studies at Emerson College. He is a research associate at Harvard Forest.
Citation: Allen, K. And Oswald, W. W. 2021. A New Look at Boston Common Trees. Arnoldia, 78(3): 38–41.