Global Warming May Cause Fall Leaf Coloration To Start Later, Last Longer

Chuck Bednar for – Your Universe Online

The fall foliage transformations that so colorfully mark the start of the autumn season could soon start arriving later and lasting longer due to climate change, researchers from Princeton University report in the latest edition of the journal Global Ecology and Biogeography.

In the study, senior author David Medvigy, an assistant professor of geosciences and associated faculty member at the Princeton Environmental Institute, and his colleagues explain that global warming could cause summer temperatures to linger later into the year, thus delaying fall leaf peeping in some areas of the US.

For example, Medvigy’s team said that by the end of the century, the paper birch (New Hampshire’s state tree) could change colors one to three weeks later than usual. While some trees will be less susceptible to the ongoing heat, the more southern the region, the more likely there will be a greater overall delay in leaf coloration, they noted.

For trees to produce colored leaves, the researchers said that daily temperatures need to be low enough and daylight hours must be short enough. Their work reveals that not only can daily temperature and daylight hours be used to predict the timing of leaf coloration, but the influence of those factors also depends largely on the individual species of the tree and the specific geographical region where it is located.

“We’re really interested in understanding how these systems will change as we experience global warming or climate change,” Medvigy explained in a statement Thursday. “What these results are suggesting is that different locations will change in different ways, and that these differences are actually going to be quite interesting.”

In addition to the aesthetic aspects of the colorful fall foliage, and its economic importance to some regions of the country, the study authors said that their findings have important implications for predicting growing seasons, as well as both agricultural and ecosystem productivity, he noted. In particular, delays in when leaves change their hues could impact how much carbon ecosystems remove from the atmosphere.

“When plants have green leaves, they’re doing photosynthesis and taking carbon out of the atmosphere,” explained Medvigy. “The longer you have green leaves, the more carbon dioxide you can take out of the atmosphere. At least, that’s how the current thinking goes. So, figuring this out could potentially be important for understanding the impacts of climate change.”

Video Above: It’s the first day of autumn, and the telltale signs are here: crisp weather, pumpkin spice lattes and, most importantly, the leaves are changing colors. Ever wonder why some leaves turn red, others yellow and some just turn brown? We’ll tell you all about the chemistry behind this seasonal spectacle in the latest American Chemical Society Reactions episode.

According to University of Wisconsin-Milwaukee geography professor Mark D. Schwartz, fall leaf coloration typically signifies the end of the growing season in temperate climates, so it is essential to fully understand current and future coloration cycles to better understand what lies ahead for agriculture, water supplies and animal behavior.

The types of crops that people plant, the kinds of pests that could damage those crops, and the feeding and reproductive habits of animals are all influenced by the length of the growing season, said Schwartz, who was not involved in the research. This is especially true in the western US, where plants affect the availability of water.

While spring, the time during which the growing season begins, has been well studied, fall has been more difficult for experts to characterize, Schwartz explained. The season is more complex and more dependent on geography, and existing models are usually based on highly localized data, he said. Furthermore, the professor said that most of those models do take into account how plants respond to regional autumn conditions.

“When you get at the growing season you can relate this to a huge number of things. In order to understand how it might change in the future we have to understand how it functions now,” said Schwartz. “This research is a useful addition to what we’re trying to do in terms of improving the way that we model plants. A lot of models that we use in terms of global change are fairly simplistic.”

Medvigy came up with the idea for the study after observing that many of those models struggled to explain the timing of when leaves should change color. He teamed up with researchers at the NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL), and together they analyzed data on leaf-change dates for several different types of trees in Alaska and Massachusetts obtained through the USA National Phenology Network online database and Harvard Forest, a 3,500-acre research property managed by Harvard University.

“The species examined were American beech, aspen, black oak, northern red oak, paper birch, red maple, sugar maple and sweet birch,” the university explained. “They grouped the tree species into three categories based on their tolerance of shade. For example, birches need a great deal of sunlight; beeches can survive in a shaded environment; and oaks are somewhere in the middle. The nearly 20 species the study reviewed fell neatly into one of these three categories.”

The researchers found that US prediction modeling improved dramatically when the analyses included data from multiple sites spread throughout a large area (also known as macro-scale observations). Additionally, they also reported that both temperature and duration of sunlight are both significant factors in determining when tree leaves color in the fall. Most previous work in the field has relied on one factor or the other, but not both, said Medvigy.

“Predictions based on those studies were less effective over broader regions,” the university said. “The researchers also found that the timing of leaf change is more sensitive to temperature in warmer areas than in colder regions.” What that means is that, if fall temperatures increase, tree species in Massachusetts “will respond to a greater degree” than those in Alaska.

While Alaska’s foliage season “is in September and is unlikely to change in the next 100 years,” Massachusetts foliage season is expected to eventually be pushed back a month, from October to November, Medvigy said. In southern states, the change will take place even later. Now that he has a better grasp on the information needed to predict future changes to leaf coloration, he and his colleagues now plan to use the findings to generate more sophisticated models.

“We now have a much better understanding of how temperature, day-length and leaf color are related,” said Medvigy. “This understanding will help us make better forecasts for climate, as well as for the basic dynamics of forests. My group is now investigating these issues together with researchers from GFDL.”


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