In Professor Richard Primack's Lab at Boston University, we study the effects of climate change on the timing of seasonal biological events and species diversity. By teaming up with Henry David Thoreau and other local naturalists, we investigate how climate change is affecting plants, insects and birds right here in the Boston area.
"This pond …. indicates
better than any water hereabouts the absolute progress of the season, being
least affected by transient changes in temperature." -Thoreau, Walden, p. 324
On April 4, ice still covered almost all of Walden Pond,
except for an open strip of water along the edge. The ice continued to melt
over the next five days, still covering most of the pond.
By April 11, the ice covered only a small percentage of the
pond. After two more days, the pond was completely ice-free.
Ice-out has long been a favorite sign of spring, a way to
mark the transition from winter ice skating and ice fishing to summer swimming
and boating. Concord is home of one of the most iconic of these sentinels of
spring— the date of ice-out on Walden Pond.
Henry David Thoreau recorded the ice-out times at Walden
Pond from 1846 to 1860, and modern residents have continued this tradition.
Walden Pond is a sensitive indicator of seasonal temperature; ice-out occurs
earlier in springs following warm winters and later in springs following cold
winters. According to the Blue Hill Observatory, the country’s longest-running
meteorological observatory, the average winter temperature of 2015 was the
coldest in the Boston area since 1934, and this winter broke the record for the
This year ice-out at Walden occurred on April 11—that is, on
that date the pond was 90% ice free—the latest ice-out date in the past 20
years. According to observations made by naturalist Steve Carlin, and posted on
the Journey North website, from 1995 to 2014 ice-out at Walden Pond has varied
from as early as January 29 in the record-breaking warm winter of 2012 to as
late as April 5 in 2005 and April 4 in 2014. In Thoreau’s time, ice-out
averaged about two weeks later than in recent decades, varying from March 15 to
April 18 and reflecting the colder climate of his time. So although Thoreau
might not have been surprised by this year’s ice-out date of April 11, present
day Concordians should regard this as an unusually late year for our modern era.
Concord’s cold winter of 2015 was made even more exceptional
by how sharply it contrasted with the warm winter throughout the rest of the
world this year. The National Oceanic and Atmospheric Association announced last
month that the global average winter
temperature in 2015 was the warmest since records began in 1880. But as we
experienced first hand in eastern Massachusetts, a warm globe does not mean it
is warm everywhere. The northeastern United States was among the only places in
the world to experience a colder-than-average winter in 2015. The rest of the Northern Hemisphere got an exceptionally early
spring this year, as expected in our warming world. Meanwhile, we have a chance
to enjoy our much rarer event, a spring reminiscent of the colder springs of
Thoreau’s Walden and Concord.
Climate change biologists want to understand the factors that determine when trees, shrubs, and vines leaf out in the spring. However, it is difficult to conduct leaf-out experiments on these plants, in part because individuals can be very large and difficult to maneuver. The Primack Lab and other researchers have recently employed a
new method using dormant twigs as representatives of wild plants in order to overcome
this problem and predict the future impacts of climate change.
Along with Annette Menzel and Julia Laube from the
Technical University of Munich, we recently published a Viewpoint article in Annals of Botany describing this new experimental method. The method involves cutting dormant twigs from wild trees, shrubs, and
vines, at a single date or multiple dates, putting the twigs in water in
controlled environments and monitoring leaf-out, flowering, or
Dormant twigs in water in controlled laboratory conditions, photographed by Julia Laube
Experiments using this method have already shown that
trees and shrubs rely mainly on a mixture of measuring the length of the winter
and spring warming to determine their leaf-out times. Also, non-native invasive
shrubs are much faster to leaf out in a very early spring than native trees and
shrubs, which could give them the advantage of an extended growing season in
Using twig experiments, the Primack lab demonstrated that non-native invasive species will leaf out faster than native species in a very early spring
Read the full article, “From observations to experiments in
phenology research: investigating climate change impacts on trees and shrubs
using dormant twigs” HERE.
The flowering of
cherry trees is one of the most beautiful sights in the spring. Cherry trees
are also extremely sensitive to temperature, flowering earlier in warm years
and later in cold years. The Japanese people greatly enjoy their annual cherry
blossom festival, taking time off from work, walking under the trees, and
meeting friends for outdoor parties.In
Tokyo, a warming climate has caused the blooming of the cherry trees to
gradually change over the last century from mid-April to late March.
Figure 1. People enjoying the cherry blossom festival in Japan
The oldest records
of cherry blossom flowering come from Kyoto where the festival has been
celebrated for over 1200 years. There have been long periods of earlier
flowering and later flowering associated with global changes in temperature. Starting
about 150 years ago, the cherry trees have been flowering ever earlier due to a
combination of urbanization in Kyoto (more buildings, more pavement and
streets, and fewer trees) and warming temperatures. Over recent decades, cherry
trees in Kyoto have been flowering earlier than they ever have in the past.
Figure 2. Cherry blossoms in Japan are getting earlier over time, and have been particularly early in recent years
The effect of
urbanization on cherry trees is illustrated by the city of Osaka. Using 80
observation points around the city, researchers show that trees flower earliest
in the center of the city where it is warmest due to urbanization.
Figure 3. This map
of Osaka shows that trees flower earliest in the center of the city
Last month, Caitlin McDonough and I had the pleasure of presenting the Primack Lab's research to the 6th graders of the Pierce School in Brookline, MA! As part of their interdisciplinary climate change unit, we shared with Pierce students how Henry David Thoreau's field notes help us to see that climate change causes plants to flower and leaf out earlier now than in the 1800's. We also had a lively discussion about how to separate the effects of habitat loss and climate change on species loss in Concord, MA and Acadia, ME.
The 6th graders read the lab blog prior to our visit, and came prepared with great questions. Thank you for having us, Pierce school, and we hope to see you again next year!