Pastor Elbert Street noticed a small crack in the wall behind the pulpit at Grace Christian Baptist Church.

It widened. Then a branch appeared. A cluster of leaves unfurled.

That was when Street realized that something was growing between the walls of his East Baltimore church: an 8-foot-tall tree.

The 79-year-old pastor and others took the wall apart, hacked down the tree and hauled away bags of branches and roots. The tree came back. They cut it down again. And then again.

A contractor estimated it would cost $5,000 to remove the tree and fix the walls — money the church didn't have.

Now, three years later, leafy branches jut from the bricks at the rear of the church. The inner wall, covered in a fresh layer of plaster, remains unbroken. At least for now.

"It's only a matter of time before it returns," Street said.

They appear in long-vacant buildings and carefully tended structures. Seeds dispersed by wind and birds take root, needing only water, sun and a pinch of soil. The trees are reminders that Baltimore was once a forest, and, if the trees had their way, would become one again.

At least 13 grow from buildings in a four-block area on the west side of downtown. Among them are a pair of broad-leafed paulownia trees that lean from second-floor windows in vacant buildings in the 400 block of N. Howard St. Large and verdant, they appear almost to be mocking the scrawny saplings growing in sidewalk wells.

Nearby, on Tyson Street, a trunk muscles through bricks on the ground floor of an abandoned building. More trees poke from upper windows like nosy neighbors.

Smaller trees appear on some of the city's finest buildings. One is perched on an upper balcony of the Clarence M. Mitchell Jr. Courthouse. Another sits high on a balcony of the Belvedere in Mount Vernon.

The trees usually belong to two invasive species, the paulownia and ailanthus. They push apart walls, twist metal beams, and, in time, destroy foundations.

Julie Day, the city housing department's deputy commissioner of land resources, said trees growing in vacant properties can pose serious risks for adjacent buildings.

The roots can grow directly into brick and cause flooding by damaging gutters. They can even dislodge the foundations of houses, she said. Workers will remove trees in city-owned buildings if they trigger complaints, she said, although the city does not keep track of the number it has removed.

Removal costs have ranged from $800 to $3,500. A home with a large tree growing in it will usually need to be gutted, Day said.

Despite their destructive nature, they can also create shade, homes for birds, and, in the eyes of some, beauty.

One such tree presides over the back lot of the Current Galleryon the west side of downtown.

It shades the lot during the day, and, at night, Christmas lights twinkle in its leaves.

"If we didn't have a tree back there, it would be like a gross parking lot," said Monique Crabb, a co-director of the gallery.

Martin Kasey, the coffee director at Station North's Canteen cafe, mourns the tree that grew between the bricks of his nearby apartment. The tree shaded his apartment's back porch before it was cut down.