# Mathematical model reveals commonality within the diversity of leaf decay

##### Oct 03, 2012 by Jennifer Chu

The colorful leaves piling up in your backyard this fall can be thought of as natural stores of carbon. In the springtime, leaves soak up carbon dioxide from the atmosphere, converting the gas into organic carbon compounds. Come autumn, trees shed their leaves, leaving them to decompose in the soil as they are eaten by microbes. Over time, decaying leaves release carbon back into the atmosphere as carbon dioxide.

In fact, the natural decay of contributes more than 90 percent of the yearly carbon dioxide released into Earth's atmosphere and oceans. Understanding the rate at which leaves decay can help scientists predict this global flux of carbon dioxide, and develop better models for climate change. But this is a thorny problem: A single leaf may undergo different rates of decay depending on a number of variables: local climate, soil, microbes and a leaf's composition. Differentiating the decay rates among various species, let alone forests, is a monumental task.

Instead, MIT researchers have analyzed data from a variety of forests and ecosystems across North America, and discovered general trends in decay rates among all leaves. The scientists devised a mathematical procedure to transform observations of decay into distributions of rates. They found that the shape of the resulting curve is independent of climate, location and leaf composition. However, the details of that shape—the range of rates that it spans, and the mean rate—vary with and . In general, the scientists found that plant composition determines the range of rates, and that as temperatures increase, all plant matter decays faster.

"There is a debate in the literature: If the climate warms, do all rates become faster by the same factor, or will some become much faster while some are not affected?" says Daniel Rothman, a co-founder of MIT's Lorenz Center, and professor of in the Department of Earth, Atmospheric and Planetary Sciences. "The conclusion is that all rates scale uniformly as the temperature increases."

Rothman and co-author David Forney, a PhD graduate in the Department of Mechanical Engineering, have published the results of their study, based largely on Forney's PhD thesis, in the Journal of the Royal Society Interface.

Litter delivery

The team obtained data from an independent 10-year analysis of North American forests called the Long-term Intersite Decomposition Experiment Team (LIDET) study. For this study, researchers collected leaf litter—including grass, roots, leaves and needles—from 27 locations throughout North and Central America, ranging from Alaskan tundra to Panamanian rainforests.

The LIDET researchers separated and weighed each litter type, and identified litter composition and nutrient content. They then stored the samples in porous bags and buried the bags, each filled with a different litter type, in each of the 27 geographic locations; the samples were then dug up annually and reweighed. The data collected represented the mass of litter, of different composition, remaining over time in different environments.

Forney and Rothman accessed the LIDET study's publicly available data online, and analyzed each dataset: the litter originating at one location, subsequently divided and distributed at 27 different locations, and weighed over 10 years.

The team developed a mathematical model to convert each dataset's hundreds of mass measurements into rates of decay—a "numerically delicate" task, Rothman says. They then plotted the converted data points on a graph, yielding a surprising result: The distribution of decay rates for each dataset looked roughly the same, forming a bell curve when plotted as a function of the order of magnitude of the rates—a surprisingly tidy pattern, given the complexity of parameters affecting decay rates.

"Not only are there different environments like grasslands and tundra and rainforest, there are different environments at the microscale too," Forney says. "Each plant is made up of different tissues … and these all have different degradation pathways. So there's heterogeneity at many different scales … and we're trying to figure out if there's some sort of commonality."

Common curves

Going a step further, Forney and Rothman looked for parameters that affect leaf decay rates. While each dataset resembled a bell curve, there were slight variations among them. For example, some curves had higher peaks, while others were flatter; some curves shifted to the left of a graph, while others lay more to the right. The team looked for explanations for these slight variations, and discovered the two parameters that most affected the details of a dataset's curve: climate and leaf composition.

In general, the researchers observed, warmer climates tended to speed the decay of all plants, whereas colder climates slowed plant decay uniformly. The implication is that as temperatures increase, all plant matter, regardless of composition, will decay more quickly, with the same relative speedup in rate.

The team also found that such as needles that contain more lignin—a sturdy building block—have a smaller range or than leafier plants that contain less lignin and more nutrients that attract microbes. "This is an interesting ecological finding," Forney says. "Lignin tends to shield organic compounds, which may otherwise degrade at a faster rate."

Rothman adds that in the future, the team may use the model to predict the turnover times of various ecosystems—a finding that may improve models, and help scientists understand the flux of around the globe.

"It's a really messy problem," Rothman says. "It's as messy as the pile of leaves in your backyard. You would think that each pile of leaves is different, depending on which tree it's from, where the pile is in your backyard and what the climate is like. What we're showing is that there's a mathematical sense in which all of these piles of leaves behave in the same way."

Explore further: Researchers help Boston Marathon organizers plan for 2014 race

## Related Stories

#### European Arctic forests expansion could result in carbon dioxide release: study

Jun 17, 2012

Carbon stored in Arctic tundra could be released into the atmosphere by new trees growing in the warmer region, exacerbating climate change, scientists have revealed.

#### Increased tropical forest growth could release carbon from the soil

Aug 14, 2011

A new study shows that as climate change enhances tree growth in tropical forests, the resulting increase in litterfall could stimulate soil micro-organisms leading to a release of stored soil carbon.

#### Global warming may affect the capacity of trees to store carbon, study finds

May 25, 2011

One helpful action anyone can take in response to global warming is to plant trees and preserve forests. Trees and plants capture carbon dioxide during photosynthesis, thereby removing the most abundant greenhouse ...

#### Predators have outsized influence over habitats, research says

Jun 14, 2012

A grasshopper's change in diet to high-energy carbohydrates while being hunted by spiders may affect the way soil releases carbon dioxide into the atmosphere, according to Yale and Hebrew University researchers ...

#### Increased carbon dioxide in atmosphere linked to decreased soil organic matter

Mar 11, 2008

A recent study at the University of Illinois created a bit of a mystery for soil scientist Michelle Wander – increased carbon dioxide in the atmosphere was expected to increase plant growth, increase plant biomass and ultimately ...

#### Drought-exposed leaves adversely affect soil nutrients, study shows

Apr 05, 2011

Chemical changes in tree leaves subjected to warmer, drier conditions that could result from climate change may reduce the availability of soil nutrients, according to a Purdue University study.

## Recommended for you

#### Egypt archaeologists find ancient writer's tomb

19 hours ago

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...

#### Researchers create methylation maps of Neanderthals and Denisovans, compare them to modern humans

Apr 18, 2014

(Phys.org) —A team of Israeli, Spanish and German researchers has for the first time created a map of gene expression in Neanderthals and Denisovans and has compared them with modern humans. In their paper ...

#### 3 Qs: Economist makes the case for new quasi-experiments as a way of studying environmental issues

Apr 18, 2014

How can scholars get traction on environmental problems, particularly those relating to pollution? In an essay appearing in this week's issue of the journal Science, MIT economist Michael Greenstone, along ...

#### New specimens of Yanornis indicate a digestive system of living birds

Apr 18, 2014

In a recent paper describing ten new specimens of Yanornis martini identified by the director of the Shandong Tianyu Museum of Natural History Mr. Xiaoting Zheng, an international team of scientists lead ...

#### Study finds law dramatically curbing need for speed

Apr 18, 2014

Almost seven years have passed since Ontario's street-racing legislation hit the books and, according to one Western researcher, it has succeeded in putting the brakes on the number of convictions and, more importantly, injuries ...

#### Changing dinosaur tracks spurs novel approach

Apr 17, 2014

Paleontologists are using a range of old and new techniques to map the Broome Sandstone dinosaur trackways.

## More news stories

#### Researchers create methylation maps of Neanderthals and Denisovans, compare them to modern humans

(Phys.org) —A team of Israeli, Spanish and German researchers has for the first time created a map of gene expression in Neanderthals and Denisovans and has compared them with modern humans. In their paper ...

#### Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...

#### Last Week's Best—Quantum mechanics breakthrough, 3-D printed human heart, and paraplegia therapy

(Phys.org) —Hello readers—we'd like to try something new here at Phys.org and Medical Xpress: offer a weekly summary every Monday highlighting what we feel are the most important stories of the past ...

#### Earliest ancestor of land herbivores discovered

New research from the University of Toronto Mississauga demonstrates how carnivores transitioned into herbivores for the first time on land.

#### Neanderthals and Cro-magnons did not coincide on the Iberian Peninsula

The meeting between a Neanderthal and one of the first humans, which we used to picture in our minds, did not happen on the Iberian Peninsula. That is the conclusion reached by an international team of researchers ...

#### Study casts doubt on climate benefit of biofuels from corn residue

Using corn crop residue to make ethanol and other biofuels reduces soil carbon and can generate more greenhouse gases than gasoline, according to a study published today in the journal Nature Climate Change.

#### Bulletproof nuclei? Stem cells exhibit unusual absorption property

Stem cells – the body's master cells – demonstrate a bizarre property never before seen at a cellular level, according to a study published today from scientists at the University of Cambridge. The property ...

#### 'Chaperone' compounds offer new approach to Alzheimer's treatment

A team of researchers from Columbia University Medical Center (CUMC), Weill Cornell Medical College, and Brandeis University has devised a wholly new approach to the treatment of Alzheimer's disease involving ...

#### Team identifies source of most cases of invasive bladder cancer

A single type of cell in the lining of the bladder is responsible for most cases of invasive bladder cancer, according to researchers at the Stanford University School of Medicine.

#### Researchers uncover link between Down syndrome and leukemia

Although doctors have long known that people with Down syndrome have a heightened risk of developing acute lymphoblastic leukemia (ALL) during childhood, they haven't been able to explain why. Now, a team ...