Lynne Aldrich, who owns a farm along with her husband Lee in North Central Iowa, got a call one holiday season from a upset woman. Apparently, her husband had shown up at the Aldrich Tree Farm to pick out a Christmas tree alone. Mistake. His wife described the tree he had chosen as the ugliest one she had ever seen. Lynne Aldrich told the woman to bring the tree back and pick out a new one.

So, the couple returned and headed out into the 28-acre farm, leaving the tree leaned up against the barn. Within 10 minutes another family had driven up and claimed it. Then the complaining woman returned with a tree that, from Aldrich's perspective, was ugly, so ugly in fact that the couple hadn't even tagged it for sale.

"Beauty is in the eye of the beholder," Aldrich explained. But for American Christmas trees, there are often certain standards.

From Aldrich's perspective (and experience with customers), an ugly tree is one with gaps between its branches and a top that doesn't end in a perfect point. Usually, people arrive looking for symmetry and an absence of gaps.

"They want it perfect all the way around, forgetting that most people put it in a corner or up against a wall," Lynne Aldrich said.

We haven't always been this picky; Christmas trees were once gathered from the woods, but since that tradition waned, the aesthetic has become stricter, according to Ken Tilt, a professor of horticulture at Auburn University in Alabama. "What we would term a 'Charlie Brown' tree, a sad-looking one- or two-sided tree would now not be acceptable. Like everything we have in produce in the grocery store, we expect a tree that is perfectly shaped," he told LiveScience.

And it's not just about symmetry and flawlessness, as Aldrich points out, we want dense, cone-shaped trees. We also want trees stay fresh for weeks, even a month or more, after they are cut.

Evolution of a tradition

The tradition of bringing evergreens indoors as decoration stretches back to pagan times, but in the latter half of the 20th century, Americans developed a distinctive taste in Christmas trees.

After the German regiments hired by British during the American Revolutionary War introduced colonists to Christmas trees, Americans began harvesting from wooded areas. After World War II, an increasing number of trees were planted in plantations, plant pathologists Gary Chastagner, of Washington State University, and D. Michael Benson, of North Carolina State University, wrote in an article published in the journal Plant Health Progress in 2000.

Chastagner, 62, remembers that his family Christmas tree was set up on Christmas Eve, and presents were set out under it, after he had gone to bed. Now, people begin buying Christmas trees as soon as Thanksgiving ends, roughly a month before Christmas Day. This means the cut trees must stay presentable and tidy for a longer period of time.

"The long display periods are really only possible if you have trees that have the ability to retain moisture and good needle retention," Chastagner said.

These characteristics have driven increased demand for two popular species, the noble fir, which is native to the Pacific Northwest, and the Fraser fir, native to the mountaintops of southern Appalachia, he and Benson wrote.

While keeping trees in water is important, higher tech needle-loss prevention is on the horizon. In work published earlier this year in the journal Trees, Canadian researchers announced they had discovered that ethylene, a gaseous hormone that is responsible for ripening fruit, is behind needle loss. They currently are working on practical techniques to prevent it from decorating carpets.

To create strains of trees more likely to keep their needles, Chastagner's lab developed a detached branch test. Researchers cut limbs from promising trees in the field and keep them for about 10 days at room temperature to see how the needles respond; they repeat the process over three years to account for year-to-year variability, according to Chastagner.

"If we screen 100 trees, we can generally find maybe three trees out of a hundred that won't shed, so then those trees are propagated," he said.

All traditional Christmas trees, be they firs, pines, spruces, or cypresses, are conifers, a type of plant whose seeds are encased in cones, and which often has needle-like leaves that stay on its limbs in winter. [Christmas Trees Survival Secrets Discovered]

Buyers and growers have specific preferences. The Aldriches grow Scotch pine, White pine and Canaan fir, and have some Fraser fir on the way.

"The fir has become more popular in recent years; we sell more and more of them every year. They look like the old-fashioned Christmas tree you think of 50 years ago," Lynne Aldrich said. She described the firs – Canaan and Fraser firs are quite similar – as having a deep green color and a wonderful fragrance making no attempt to hide her own favorite.

"So, to me the beautiful trees are the firs," she said. "I would take one of those any day over a Scotch pine and a White pine."

Cartoonishly conical

In Europe, where the Christmas tree tradition began, people purchase their trees closer to Christmas and leave them up for shorter periods. Europeans are also resistant to the pruning typical of American trees. And tree researchers jokingly refer to dense American trees as "Donald Duck trees," referring to their cartoonish appearance, Chastagner said.

It's not entirely clear how the American preference for trees densely packed with limbs and leaves originated; Chastagner has heard stories that include tree-nibbling deer. It's more likely that growers, who were former foresters, began to prune branches as a way to encourage growth to fill in gaps in the tree, and consumers responded, he said.

Now sheering, or pruning a tree's sides and top (called its leader), to encourage a more dense conical shape, is standard practice.

"It's very important you trim the top, you don't want the leader too long or too short, you want the branches around the leader to be just right," Lynne Aldrich said.

>A tight market

Real trees face competition from artificial ones, which can hit the market much earlier, don't require the same care and last from year to year. And although fewer homes display real trees today than they did 60 years ago, real trees still have the edge over artificial ones.

In 2009, Americans purchased about 28 million real trees, according to the National Christmas Tree Association. This organization is not to be confused with the American Christmas Tree Association (ACTA), which describes itself as a nonprofit education association, however, its website is decidely pro-artificial tree. (Both organizations' websites tout the environmental benefits of their genre of tree, while pointing out the fire danger posed by the other.) In 2009, Americans purchased an estimated 12 million artificial trees, according to ACTA.

It's not surprising that growers are always looking for a leg up, through traits like better needle retention and insect resistance. In the southeastern United States, including Alabama, growers have turned to a surprising variety: the feathery Leyland Cypress.

These fast-growing plants are often used in landscaping as windbreaks, according to Kelly Ivors, a plant pathologist at North Carolina State University. [Image of a Fraser fir]

"It's really odd to see people are growing Leyland Cypress as a Christmas tree," Ivors said. "It doesn't have the kind of branches you would typically see on a typical Christmas tree."

At North Carolina State, where Ivors works, researchers focus on a more established Christmas tree, the Fraser fir. Its shape, scent and needle retention make this fir, "the most desirable Christmas tree," she said.

In the Pacific Northwest, researchers are working with Nordmann fir – a European Christmas tree – and the Turkish fir to develop alternatives to the trees typically grown in that region.

But at the end of the day, does it matter whether you have a sad-looking tree or a flawless one?

"When you buy a Christmas tree, you are not eating the thing. It's kind of like hanging a painting on your wall. It’s a decoration. It's a symbol of the Christmas season… Beauty is in the eye of the beholder," said Rick Fletcher, Christmas tree and forestry specialist at Oregon State University.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry.