Christmas tree sellers across the country are expecting an influx of new and returning customers this holiday season after experiencing a surge in sales last year.

Industry insiders say the increase in tree purchases is fueled primarily by Millennials, who are settling down and starting families of their own -- in one form or another.

Last week, the National Christmas Tree Association released findings from its latest consumer survey. The trade group, which represents thousands of Christmas tree growers nationwide, hired Nielsen/Harris researchers to interview 2,020 US adults last January, just as the holiday season ended.

Their study found a 20% spike in the purchase of real Christmas trees -- evergreen pine trees -- and a 12% rise in artificial tree purchases in 2018.

The association's study found that last year American adults bought about 5.4 million more real Christmas trees and 2.5 million more artificial trees than they did in 2017. NCTA seasonal spokesperson Doug Hundley said most of those new purchases were made by Millennials, adults born between the early 1980s and the mid 1990s.

"The Millennial generation is now of the age where they're settling down, getting married, having kids and turning to real trees to build their own family traditions," Hundley told CNN Business. "We've been looking at that demographic for some time and expecting this to happen."

The NCTA's competitor is the American Christmas Tree Association, which represents the nation's artificial tree producers, and whose members have also experienced a surge in sales in recent years. ACTA executive director Jami Warner attributes the increase to improved economic conditions.

The Christmas tree industry took a major hit in 2008, the beginning of the Great Recession, when many Americans going through financial crises of their own and opted not to buy any trees.

Now, more than a decade later, many Americans have recovered from that downturn and are even purchasing more than one tree to decorate their homes, according to Warner, who said that Millennials appear to be behind the Christmas tree revival.