For Threatened Coffee Crops, Tree Rings May Offer Answers

Coffee and climate come together on the mountainous slopes of Guatemala, where fields — and the livelihoods of indigenous farmers — are threatened by changing rainfall, rising temperature and a fungus called "coffee rust." The UA's Kevin Anchukaitis seeks clues in the past lives of the firs and pines there.

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Kevin Anchukaitis went to Guatemala for the first time in 2008. Anchukaitis, associate professor in the University of Arizona's School of Geography and Development, was there to scope out trees in the Sierra de los Cuchumatanes, the highest non-volcanic peaks in Central America. Anchukaitis returns often to Guatemala to pry slivers of wood from the trees there and to meet with collaborators and community members.

This morning, Anchukaitis sits in his office in the Bryant Bannister Tree-Ring Building. A handsome blue tea kettle rests on a small table next to the door, but he never fires it up, he says, gesturing to his second cup of coffee for the day.

After raw sugar and bananas, coffee is one of the top exports in Guatemala. In 2014, Guatemala exported about $756 million worth of it. Coffee is also one of the reasons Anchukaitis has been using tree rings to peer into the country's past rainy seasons and temperature trends.

Coffee is a finicky crop. As the scientist puts it: "Quality coffee comes from a narrow range of climate." Coffee bushes thrive with about 60 to 90 inches of rainfall annually; in Guatemala, the rain comes between February and May. The bushes don't like to be too hot or too cold, too high or too low in elevation. Thus far, Guatemala's climate has been able to graciously host the crop — and its economy has depended on it.

But existing climate models, which predict decreased rainfall and rising temperatures, suggest that Guatemalan coffee bushes face an uncertain — even unfavorable — future. These models, however, are based on limited climate records from Guatemala. So Anchukaitis' work, filling gaps in the data and meeting with farmers and stakeholders, asks two questions: Are the models correct? And if they are, how can the country adapt?

Guatemalan Fir and Mountain Pine

In answering those questions, the lack of good climate records on Guatemala is a problem. "We need to expand our knowledge and reconstruct the past to really put present changes in climate into context," Anchukaitis says.

Tree rings are Anchukaitis' tool of choice for reconstructing the climate of the past — or paleoclimatology, as it is called. As a matter of fact, tree-ring science was invented at the UA by a polymath named Andrew Ellicott Douglass, who established the Laboratory of Tree-Ring Research, where Anchukaitis now works, in 1937.

Two species of trees are native to the coffee-growing regions of Guatemala that Anchukaitis uses: Guatemalan Fir, which is now threatened from human exploitation, and Mountain Pine. Both trees put on annual rings that are related to precipitation. The wider the ring, the more rain fell that year.

So far, the firs' rings are best for reconstructing past rainfall. The pine is more difficult —in some years, it doesn't put on a ring — but it has an allure of its own. The anatomy of its wood is different, and Anchukaitis says he may be able to use it to reconstruct past temperatures. Some trees in other parts of the world can be used this way, where the density of the tree ring reflects the temperature of that year. But this hasn't been applied before to tropical trees.

Although tree rings can tell scientists about the past, Anchukaitis has only been able to reconstruct climate patterns as far back as the 1600s.

"One of the interesting challenges in Guatemala is finding older trees. The trees there are pretty young compared to what we're able to find in arid sites," Anchukaitis says. By comparison, there are bristlecone pine trees in the Great Basin about 10 times as old as those he has sampled in Guatemala. Tropical trees don't tend to get that old, he says, adding, "It's not always true, but seems to be that oftentimes adversity leads to longevity."

What's more, people have been living in Guatemala for thousands of years. Throughout that time, they have needed to chop the trees for wood to burn and to build.

Even so, having a Guatemalan climate record dating to the 17th century is a feat, and it has offered insights into the future.

From Old Rings, New Knowledge

"All climate right now is an amalgamation between natural, or unforced, climate variability, and human-influenced climate trends. Anywhere you go now, that's true, because we (humans) have had such a widespread influence on the climate system," Anchukaitis says.

In other words, climate is the result of Earth's natural rhythms and human behavior. This has been true for as long as humans have been making scientific observations about climate, which presents a challenge: How do we untangle what is caused by natural rhythms, and what is caused by us?

"We don't really have a picture of climate unmodified by humans," Anchukaitis says.

To uncover this picture, scientists have to know what the natural rhythms were for centuries before humans began to influence climate. Tree rings allow that.

"Tree rings give us a long-term perspective, and rich context, on patterns of wet and dry from year to year and decade to decade," Anchukaitis says.

This way, scientists can tell what's outside of the range of natural variability and what's not. And they can use information from tree rings to help us evaluate climate models, such as those that say less and less rain is on the horizon.

The firs and pines that Anchukaitis has studied reveal that the last 20 to 30 years in Guatemala are not actually too exceptional in terms of rainfall.

"So far, there's no sign of onset of an imminent drying period in Central America. What we see so far could be explained by the natural variability that's characteristic of the last several centuries," he says. "Does that mean the models are wrong? No. But it provides context."

The tree rings also have revealed that in Guatemala, there can be up to eight consecutive years where the transition from dry to wet season is late, or dry.

"If that were to happen now, it would have big implications in terms of the harvesting of coffee crops, as you can imagine, but the paleo-record does reveal the potential for that," Anchukaitis says.

Members of Anchukaitis' research team, including Diego Pons, a Guatemalan graduate student in geography at the University of Denver, are collaborating with Guatemala's coffee stakeholders to better understand and prepare for that potential.

The Coffee Ecosystem

Aside from dry seasons, there are a number of environmental threats to Guatemala's coffee crops.

"With the tree rings, we're able to look at precipitation most directly. Temperature is a related, important issue. But even more pressing than that is 'coffee rust,'" Anchukaitis said.

Coffee rust is a fungus that infects the leaves of coffee bushes, intermittently devastating Central American crops. It covers the upper surfaces of the coffee leaves in buttery yellow blemishes, and their undersides with chalky orange lesions. According to Anchukaitis, it almost certainly likes a certain range of humidity and temperature, "so we're trying to figure how that is changing, as well."

Whether a crop is prone to coffee rust is "almost definitely tied to climate conditions, so we're just trying to figure out whether climate change is going to make it even more of a threat," Anchukaitis said.

"Knowing what's going on with the climate, in a sort of long-term context, is going to be the key to adapting to any of these changes."