Look at old photos of New York City streets and you often notice a strangely stark, movie-set quality about them. A moment’s pondering of the black-and-white images reveals one reason:

There are no trees.

Go to the same block or neighborhood now and you may find a different scene: the blankness filled in and softened by soaring pin oaks, the camouflage-pattern trunk of a London plane, or a line of leggy young pears or maples.

The city keeps getting more built up, more crowded. New buildings block the sky or sprawl across open space. In the last century, the population has jumped by 65 percent.

Even so, over the last 50 or 75 or 100 years, sometimes without our consciously noticing, the more developed parts of the nation’s densest big city have grown greener.

Call it the leafing of New York.

There is no way to prove this assertion. Street-tree censuses were not reliable until the 1990s. The tree population fluctuates — waves of disease and storm and municipal fiscal spasm periodically thin the urban forest and undo the city’s tree-planting efforts.