Eat too many brownies after dinner, and the extra calories will migrate to your middle in the form of fat. This on-board energy, though dreaded by modern humans, comes in handy during times of deprivation and sickness.
Animals aren't the only organisms that shunt their extra energy into storage caches. Plants, too, deploy a variety of methods to hoard energy for lean seasons. For example, carrots partition extra sugars into chubby taproots, while potatoes keep their reserve accounts primarily as starch.
Unable to indulge in desserts, plants must manufacture their calories from scratch. By conducting photosynthesis, plants can draw carbon dioxide gas from the atmosphere and transform it into sugars and tissues. This consumption of atmospheric carbon is known as carbon fixation.
Scientists had long assumed plants live off recently fixed, "new" carbon, the same way people burn off lunch before tapping their spare tires. Then came a surprising discovery: boreal trees were respiring "old" carbon, atoms that hadn't seen daylight for years.
Saving for a Cloudy Day
Scientists realized the trees were drawing from longstanding carbon bank accounts. After manufacturing sugars and starches, the plants must have set them aside to be metabolized at a later date. These nonstructural carbon pools are the vegetal version of cold, hard cash: easily converted into everything from metabolic energy to new roots and shoots. Only after being made into cellulose and lignin — the building blocks of stems and woody trunks — does this energy become permanent structural carbon.
Stored energy may be particularly valuable to California's native flora. "Plants that live in Mediterranean ecosystems need an insurance pool of energy to survive a catastrophic event, like drought or fire," says Claudia Czimczik, an ecosystem biogeochemist with the Department of Earth System Science at UC Irvine. These energy banks can help plants endure times when water becomes too scarce for photosynthesis or when they must resprout leaves on burned branches.
Since 2007, Czimczik has been investigating how California oaks amass and manage their nonstructural carbon pools. "We don't understand how large these pools are, when plants can make them, and when they use them," she says. By characterizing carbon pools and determining how they are metabolized, Czimczik is laying the groundwork for monitoring one of California's most important habitats.
If oaks are in fact using their nonstructural carbon as emergency savings, studying these stores could open a window onto oak forest health. Just as rapid weight changes in humans can be a sign of disease, increases or declines in carbon pool content could point to environmental stresses on oaks. "We could use the pools as a proxy to predict how stressed these trees already are and how resilient they would be to drought," she says. Such an oak checkup would be particularly helpful during the current era of accelerated climate change.
An alternate explanation for the existence of carbon pools is that they are a byproduct of modern industrialization. Fossil-fuel combustion has boosted atmospheric carbon dioxide levels by 39 percent since 1800. The ease of obtaining carbon from the air, Czimczik thinks, may have thrown plant photosynthesis into overdrive.
Czimczik and a team of fellow researchers have been studying nonstructural carbon pools in oaks across California. Her test trees reside in four UC natural reserves: Quail Ridge Reserve and Hastings Natural History Reservation in Northern California; Sedgwick and Emerson Oaks reserves in Southern California, as well as Audubon California's Starr Ranch Sanctuary in Orange County.
Obtaining samples of carbon pools from trees is no easy feat. It involves pressing a hand tool called an increment borer to the trunk of an oak and giving the instrument's wide-spaced handles some hard twists. The twists force a hollow cylindrical bit ever deeper into the wood. The resulting core resembles a skinny pencil.
Carbon Dating Sugars
Czimczik extracts carbon from the cores through a complex process that includes several chemical baths, combustion, gas purification, freezing, and reduction. The resulting graphite powder is then bombarded with cesium atoms, which knock individual carbon atoms out of the graphite. The carbon atoms are then shot through an accelerator mass spectrometer, which can distinguish the ratio of the two versions, or isotopes, of carbon present. The abundance ratio of the isotopes correlates with the date the carbon was fixed by the tree.
Thanks to the Cold War, Czimczik can calculate the exact year when a sugar molecule was fixed. Aboveground atomic bomb testing produced a spike in atmospheric levels of heavy carbon (C14) during the 1950s and 1960s. Levels of C14 have been declining ever since, due to environmental mixing and the combustion of C14-free fossil fuels. As a result, the relative amount of heavy C14 to normal C12 contained in plant tissues indicates when the carbon was fixed.
It turns out that every oak species Czimczik has tested is an assiduous saver. Evergreen live oaks, as well as deciduous valley and blue oaks, had all stashed away sizable pools of nonstructural carbon. Czimczik hasn't yet crunched the numbers for oaks, but other scientists have found that tropical trees store enough nonstructural carbon to refoliate themselves five times over.
Most of the nonstructural carbon tended to be far younger than the wood in which it was found. Czimczik determined that much of the nonstructural carbon contained within her study trees had been manufactured during the last few years or decades. Meanwhile, the rings of the trees indicated that the wood was laid down in 1950 or earlier.
The sugars, too, were as intermingled as vodka and vermouth in a shaken martini. "Sugar leftover from this year is not put into a pool that's inaccessible — it is mixing with sugars made fifty years ago," Czimczik says.
The abundance and age structure of the sugars may spell bad news for oaks. "The fact that young sugars are mixing all the way into the deeper tree seems to indicate the oaks have too much of it," Czimczik says. Unable to keep up with the sugar supply, oaks may be cramming more and more carbon into their trunks.
The characteristics of these carbon pools may be early evidence that oaks are responding to global climate change. Rising levels of atmospheric carbon dioxide could be making photosynthesis a breeze. "Now that it's more efficient to fix carbon, trees could be fixing more and more of it. They can't use it for growth, because they're limited by the availability of water or nutrients, so they just put it into sugar," Czimczik explains.
Fast Food Forests
Even more disturbing, this sugar surplus may turn oak tissues into animal junk food. Previous studies demonstrate that mineral uptake by plants tends to remain level or to fall as carbon dioxide concentrations increase, causing the nutritional content of the plant to decline. In a 2010 study published in the journal Science, UC Davis plant physiologist Arnold Bloom grew wheat and the mustard plant Arabidopsis under the carbon dioxide concentrations predicted for the next twenty to fifty years. He found these conditions caused a 10 to 20 percent decline in the protein content of these two species of plants. But wheat and mustard are only distantly related, so the results of Bloom's research suggest that many types of plants will respond in a similar manner. This work builds on previous research that shows increasing levels of carbon dioxide reduces nitrate assimilation from soil.
"If extra carbon dioxide does lead to larger sugar pools in foliage, too, it potentially has large implications for browsers. It could change the nutrient content of the plant over the long run. Animals would need to eat more to get a well-balanced diet. They would become protein deficient and be ingesting too much sugar," Czimczik says.
Czimczik is now investigating how oaks metabolize their carbon pools. "We want to see if they're actually using these old sugars that have accumulated for their daily respiration and activities," she says. "How do they allocate what goes into respiration, roots, growth, and storage? And when conditions change, does the allocation change, too? That's a big unknown."
Measuring the sugars that oaks respire requires capturing their carbon dioxide exhalations. Human physiologists do this by strapping face masks and tubes onto treadmill runners and stationary bicyclers. Greater creativity is needed to gather oak breath. For this specialized job, Czimczik's colleague Jan Muhr has devised tree respiration chambers. Making one involves scraping the outer bark from sections of oak trunk, then gluing split lengths of gutter pipe to the exposed wood. Additional sealing makes the enclosure airtight. After the tree has respired for several hours, the collected gases, including carbon dioxide, are sampled with vacuum containers. Czimczik analyzes the carbon isotope ratios in the samples to find the age of the sugars being metabolized.
So far, it looks as though different types of oaks use their carbon pools in different ways. During the peak growing season, the evergreen live oaks at Sedgwick are respiring sugars that are several years old, while the deciduous valley and blue oaks on site are respiring recently fixed carbon. Czimczik is in the process of measuring oak respiration in fall and winter. Acorn-making, however, involves drawing from different energy resources. Among all three types of oaks, these fruits are formed from fresh carbon fixed that same year.
Someday Czimczik would like to investigate whether carbon-usage patterns change across the seasons and while oaks are under drought stress. Conditions that impair photosynthesis might prod the trees to make withdrawals from their sugar cache. — KMW