According to Recreation and Parks Department spokesman Elton Pon, 45 trees have been either felled or damaged since mid-May in five separate incidents.
The most recent vandalism was discovered Wednesday, when 15 young elms and one sycamore were cut down in the Music Concourse in Golden Gate Park, Pon said.
The saplings appeared to have been sliced clean through with some kind of saw, he said.
"We will get occasional instances of vandalism, but not a whole spate of incidents in a short amount of time like this," Pon said.
"The other night's incident lends credence to the fact that there's a pattern here," he said.
Pon said the elms were valued at $500 each.
"There's the monetary value, but also the benefits that they bring to our parks," he said.
The night before, a young magnolia in the park was sawed in half, according to Pon.
Another incident occurred in Lincoln Park in early June, when 12 young cypress and pine trees were uprooted and the bark at the bottom of four mature cypress trees was removed, according to Pon.
The reason for the vandalism is unknown.
"They're not using the trees, they're just being left," Pon said. Parks officials are now working with police to increase nighttime patrols in the area, Pon said.