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Spring to Bring Earlier Leaf-Budding Times for Trees

By Nathanael Massey, E&E reporter
2/4/2013 12:15:58 PM

For those already tired of winter, some good news: Spring's colors may debut up to a half a month earlier by the end of this century, according to new research from Princeton University.

Using data from the National Phenology Network (NPN), a nationwide tree ecology network made up of citizen scientists, federal agencies and educational institutions, the researchers established a formula correlating temperature trends with the emergence of new spring leaves on deciduous trees. Applying that formula to different scenarios projected by the Intergovernmental Panel on Climate Change (IPCC), they found that the period of spring greening would likely be both earlier and narrower in the decades to come.

Photo by David Kingham

That could have important implications for climate change, explained David Medvigy, an associate professor in Princeton's Department of Geosciences and lead author of the study.

"Whether or not plants have leaves is a first-order effect in the carbon cycle," he said. "That's when they start photosynthesis, taking carbon out of the atmosphere. An earlier period of bud burst could potentially increase the terrestrial carbon sink."

Scientists often reference climate "feedback loops," in which climate-induced phenomena generate further greenhouse gas emissions and contribute to increased warming. If true, the Princeton team's findings would suggest the opposite -- a climate consequence that slows, rather than accelerates, carbon concentration in the atmosphere.

Flattening the green wave

The emergence of new spring leaves occurs first in the South, pushing steadily northward in a pattern known to researchers as the "green wave." Because climate effects are likely to be more pronounced in northern regions, the period over which this wave occurs will likely shrink in the coming decades.

As that latitudinal gradient of spring emergence shrinks -- a pattern the researchers term "flattening the green wave" -- it could give deciduous trees a competitive advantage over evergreen trees in some regions, said Medvigy.

"The eastern U.S. areas can be characterized as mixed deciduous evergreen forests," he said. "What would happen in mixed environments if, all of a sudden, bud burst occurred a week or two weeks earlier and deciduous trees had longer growing season? In regions where the trees are in balance, maybe this would tip that balance."

Competition between different deciduous species could also be affected, the study notes. The researchers found that late-budding trees, such as red maple, were more affected by changes in temperature than were early-budding trees like aspen, resulting in a shortened window of greening for the forest as a whole.

Surprisingly, the researchers found that effects to the green wave were insensitive to differences in the warming scenario. The IPCC projects these scenarios, also called regional concentration pathways, to depict likely warming under different levels of greenhouse gas emission. But even under lower emissions scenarios, the researchers found the span of the green wave to be affected.

'Probably right on the cusp'

"That was a really surprising result," said Medvigy. "It means that we're probably right on the cusp -- all we need is a little more change [in temperature], and we'll start seeing changes in bud burst. It doesn't require a huge amount of perturbation."

"Whatever happens in terms of emissions or emissions reduction, we're likely to start seeing changes in bud burst patterns soon," he added.

The team first established its predictive model of temperature-induced bud burst in 2012 and proved it accurate against actual spring greening in the northeastern United States. The limiting factor, however, was data -- while the model could be applied on a regional basis, no data set existed encompassing the entire United States.

A breakthrough came this year, when the team gained access to the NPN records for the years 2009 to 2010. The data spanned 196 sites across the United States and provided species-specific records previously unavailable to the researchers.

The NPN is made up of thousands of participants, including many citizen scientists. Its stated purpose is to monitor the influence of climate on the life cycles of plants and animals. Participants can log a wide range of observed behaviors, like plant flowering and egg laying, from their respective regions of the country.

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