This fall, volunteer panelists on opposite sides of the country popped open small containers and inhaled the sometimes spicy, often nostalgic odor of six different Christmas tree species.

The sensory panels, conducted by scientists at the Washington State University School of Food Science, and simultaneously at the University of Florida, inform research for improved holiday firs that can grow fast, appeal to consumers, and stand up to a month or more of display.

"We're asking these trees to do a lot of different things," said WSU Regents Professor Carolyn Ross. "They must not only retain their aroma, but have a nice shape, keep their needles, and meet growers' and consumers' needs."

She led the WSU aroma test, in which panelists were asked to smell and examine small cuttings of Fraser, noble, balsam, concolor, Nordmann, and Trojan firs, then rate their perceptions and preferences. The results, still being compiled, could help reveal regional differences in preferred tree varieties and drive future commercial breeding and planting decisions.

Surveys by industry groups have found that aroma is an important consideration among buyers of real trees.

"The question is, what aroma?" said Gary Chastagner, a WSU emeritus plant pathologist known nationally as "Dr. Christmas Tree." "What's contributing to that aroma, and is there a way to improve on it?"

Both Ross and Chastagner are part of a $7.4 million, USDA National Institute of Food and Agriculture-funded multistate research project led by Justin Whitehill, assistant professor and director of the Christmas tree genetics program at North Carolina State University.

In the four-year project, scientists are identifying and helping breed firs that appeal to consumers, are more efficient to grow, and have enhanced resilience against a warming climate, pests, and pathogens. Aroma is one of many factors at play.

The characteristic odor of Christmas trees comes from terpenes: volatile organic compounds that plants use to resist diseases, attract pollinators, and defend against browsing herbivores. It's the same odor encountered in living forests.

Chastagner, who grew up in California, has never forgotten the memory of childhood trips to Lake Tahoe.

"You'd open the car door and there was that aroma!" he said. "You knew you were in the mountains. It was imprinted on you!"

Different varieties of Christmas trees may have different terpenes and differing mixes of the compounds.

"There are hundreds of these compounds," Ross said. "They can contribute to all kinds of aromas, such as citrus or kerosene. It's tricky to identify which ones contribute to a smell you like or don't like. It could be a balance of compounds, or a single problematic one. That's part of what we're trying to learn."

Chastagner's laboratory is studying how Christmas firs stand up to drought as well as a fungal disease called phytophthora root rot, a major limiting factor in where varieties can be planted. He has taken the lead in testing new species from mountainous areas of Russia, the Republic of Georgia, and Turkey - the Nordmann, Turkish, and Trojan firs - that resist the rot. Two of those were part of Ross' aroma panel.

Volunteer panelists expressed surprise at the different aromas of each variety, Ross said.

"Some were stronger smelling, and some were weaker," she said. "Some were more piney; others were more citrusy."

As a sensory scientist, Ross was intrigued to explore the relationship between aroma, emotion, nostalgia, and behavior.

"I like solving the puzzle of extending results from a sample to a real, full-size Christmas tree," she said.

For Chastagner, the project is a continuation of a more than 40-year interest in the ornamental conifers.

"I want to find ways to help growers produce high-quality trees that consumers want to display in their homes," he said. "If you can better understand your consumer, you can make better decisions. This is a tremendous opportunity to understand those drivers."