Imagine a million people treading on your toes.
That's the lot of the cherry trees around the Tidal Basin and East Potomac Park now thronged by blossom watchers from near and far.
Horticulturists know that feet and root zones don't mix: The cumulative effect of thousands of feet is like that of a steamroller, especially on wet clay soil. Yet the visitors aren't thinking about soil compaction. They have their heads in the creamy clouds of Yoshino flowers.
And even if they were mindful of where they're walking, there isn't much they could do in some of the most popular viewing spots where paths are not wide enough to contain the cherry fans.
Soil compaction is slowly killing many of the trees on parade this month, though the effects are not apparent. "You might compare it to smoking or drinking or not eating right," said Robert DeFeo, the National Park Service horticulturist whose job it is to care for the trees. "Eventually it gets you."
Each spring, DeFeo is in the spotlight as the guy who predicts when the blossoms will appear. But most of his work, and that of his crews, involves keeping the trees alive, along with shade trees, grass and other plantings on the ever-expanding monumental core of the capital.
The National Cherry Blossom Festival draws as many as 1 million visitors. An estimated 20 million people a year visit the National Mall and its growing number of museums and memorials.
For the Yoshino, Akebono, Kwanzan and other cherry varieties blooming this month, "our goal is to keep [the loss] at 1 to 3 percent a year," said DeFeo. "Sometimes it goes up, sometimes it goes down."
About 125 of the original 3,000 plantings of 1912 remain, said DeFeo, most of them in areas that aren't trampled much.
On a recent two-hour tour of the Tidal Basin, DeFeo pointed out the problems: "Where you see older trees, those are areas that receive the least impact from visitors."
What's wrong with walking under trees? Do it enough, and the resulting soil compaction literally squeezes the life out of the plant, robbing the roots of air, nutrients and moisture. In our native clay soils, it also forms a physical barrier to new root growth and causes the trees to grow roots even closer to the surface to breathe. This make them more prone to drought stress come summer. Stressed trees draw pests and diseases.
Visitors who break branches to pick flowers or to climb in trees -- both illegal -- cause obvious harm to the trees, but treading in the root zone is not as clear an assault. If it killed a tree immediately, "people would understand it," said DeFeo. "It's a slow death."
In parts of the Tidal Basin, the damage is stark. In the grounds of the Jefferson Memorial, where temporary fencing keeps feet off the grass, the turf is green and the trees look healthy. On the other side of the barrier, the grass has been pounded into mud as people have made a beeline from the sidewalk to the Tidal Basin promenade. There are few old trees here, and those with any age have been pruned into misshapen specimens as DeFeo and his crews seek to slow the decline by pruning out dieback. Flooding compounds the compaction damage.
In and around the FDR Memorial, DeFeo has installed post-and-chain railings along some paths to shepherd crowds away from vegetation, though the area of cherry plantings between the memorial and the basin is well trodden. At the main entrance to the memorial, where tour buses stop to drop off visitors, shortcuts through turf and tree beds had denuded the area. Newly installed post-and-chain and makeshift rod-and-rope railings have kept visitors on the path. The worn grass has been replaced with sod. On a shade tree that was trampled, shoots have erupted from the bases of branches -- water sprouts that, to arborists, are signals of stress.
On the cherry trees, stress is measured in a shortening of the stem length between leaf buds, and in smaller leaves, yellow leaves and branch dieback.
Removing the deadwood prevents its spread. Pruning is the primary method of care, most of it done in the winter months. In some areas, the trees' root zones have been aerated -- holes are punched with compressed air and then backfilled with a soil mix less prone than clay to compaction. DeFeo says this technique, although useful for preserving trees in one-time construction zones, is of limited value to the cherry trees because the compaction resumes and is continuous.
He notes that archaeologists can trace the presence of early humans by the soil compaction they leave behind. "Compaction is forever, unless you do something about it," he said.
New paths, he said, are now 10 feet wide, instead of the old eight-foot width, to accommodate the many fans of the cherry trees. He knows that placing a layer of woodchips in the root zones would alleviate the soil compression, but that would mean ridding the trees of their grassy setting and be unacceptable.
DeFeo said he would like to extend areas of post and chain along pathways -- this has been installed around the Korean War Veterans Memorial to the benefit of trees and turf -- but doesn't have the funds to do it.
He thinks longingly of Disney World in Orlando, where the designers have separated the millions of annual visitors from the plantings using curbs and railings. This makes for a better experience, said Disney horticulturist Heather Will-Browne. "By protecting it, you're protecting the whole integrity" of the display and thus its beauty.
But in Washington, the idea of separating people from the trees "hasn't even been a consideration," said Diana Mayhew, executive director of the National Cherry Blossom Festival. "There are still 3,700 trees, and [the Park Service] takes care of them" and replaces the ones that die, she said.
The problems of compaction aren't limited to the cherry trees. The shade trees and grass along the Mall are underfoot, too. As new memorials are added to the Mall, the number of visitors has increased, making the problem worse.
"I think we are doing well" in caring for the trees, said DeFeo. "But it's becoming increasingly difficult."