Tradition behind Boston Common's Christmas tree has endured decades and a pandemic: "This is a symbol of what we can do"

About 40 miles outside Halifax, Nova Scotia, on the Gourley family's fifth-generation dairy farm, a tree stands so tall it still holds the lights that were strung some 20 Christmases past. The lights remain on the tree because it outgrew the family's highest ladder long ago.

Bette Gourley, the matriarch of the family, told CBS News' Nancy Chen that the white spruce has been growing alongside her family for nearly four decades now. But, as a Christmas tree-in-waiting, it's still a few feet short of reaching the height it needs to be before it is sent more than 600 miles away to Boston, Massachusetts.

"It's been five or eight years since they said they might consider us," said Chester Gourley. "So it's something we've been waiting for. And it's a big honor."

It is a tradition dozens of Nova Scotian families have performed before them. The spruce the province sends each year becomes Boston's official Christmas tree. 

It's a gesture of appreciation more than 100 years after Boston lent a hand following the Halifax explosion — a disaster that was the largest man-made explosion until Hiroshima.

New Glasgow's town crier James Stewart recounted the story: During World War I, Halifax ported ships that were getting ready to convoy across the Atlantic. 

On December 6, 1917, the day of the explosion, the S.S. Mont Blanc was on its way to France when it collided with the S.S. Imo. The Mont Blanc was loaded with explosives. 

"When the collision took place, sparks basically flew, and one of the ships had munitions on the deck," said Stewart. "It was, in a nutshell, overloaded."

Just 20 minutes later, as people gathered to see what happened, the ship erupted in a massive fireball.

"The city was basically flattened, the north end of the city," said Stewart.

About 2,000 people were killed during the explosion and an estimated 9,000 people were injured, many by shattering glass. Nearly 600 people were treated for eye injuries. To make matters worse, rescue efforts were hampered by a blizzard blanketing the region the next day, stalling assistance from other major cities. 

But Boston was able to help.

"Boston was the best bet because it was a city that was big enough, close enough, and they dispatched the relief trains," said Stewart.

Based on a single plea for help by telegraph, medical teams from Massachusetts set out that night. Their train made its way through the snow to set up relief camps in the city. Their arrival signified hope for the people of Halifax. 

"Well, it's very much like the cavalry arriving," said Stewart. "They brought Halifax aid when Halifax needed it absolutely the most. And it's something that's never been forgotten."

A grateful Halifax delivered Boston a Christmas tree the next year. The gift was repeated in 1971 and then every year since.

Sandra Johnston, the manager of the Tree for Boston program, considers contenders from across Nova Scotia. She said she looks for a tree that is about 45 to 50 feet tall. She keeps a list of about 15 trees.

"I don't think I could sleep at night if I didn't have a list of trees coming up in the future," said Johnston.

Trees are nominated by the public as well as landowners volunteering their own. The trees have become a bond between the two communities that neither distance nor time has changed.

"Most Nova Scotians love and remember the assistance that Boston gave us 104 years ago," said Johnston. "And you don't have to go too far when talking to anybody from Nova Scotia to find that connection."

Each year as the tree makes the long trip from Nova Scotia to Boston, local residents line the truck's route with send-off parades. The journey persists through winter storms, city traffic and even the COVID-19 pandemic.

Last year, as land borders were closed to nonessential goods, the tree traveled by container ship.

Earlier this month, a 60-year-old tree from a Cape Breton nonprofit was cheered as it was lit on the Boston Common. The premier of Nova Scotia and Boston Mayor Michelle Wu attended the event. 

"This is a symbol of what we can do," said Wu, "when we step up to see how we can be there for each other. I'm incredibly grateful to Nova Scotia for continuing this important tradition."

It's a tradition the Gourley family will be proud to take part in one day.  The tree towering over their front yard is just a small token of appreciation for the help from a faraway city so many years ago.  

"Well, they helped us enormously," said Bette Gourley. "And this is just a thank you. And I think it's the — you know, I think it's a good thing to do."