The Dakota County Sheriff's Office is offering a $5,000 reward for information leading to the arrest of a serial tree killer whose victim list now includes 20 black cherry trees at Lebanon Hills Regional Park in Eagan.

Authorities launched an investigation last week after someone stripped rings of bark — a procedure known as "girdling" — from seven trees near a mountain bike trail. A larger search of the park found 13 other girdled trees. This type of damage deprives the roots of nutrients that flow through the bark, causing trees to starve and die.

It remains unclear whether the perpetrator, toting some sort of sharp tool, damaged all the trees at once or returned on a mischievous spree, said Sheriff Tim Leslie.

"This is a very deliberate act," said Leslie, who noted that cuts were made in a very narrow band around the trunks. "I don't know anyone who would do that. It's a poor defenseless tree, for cryin' out loud."

Because the damage exceeds $1,000, the crime would be charged as a felony, he said. Many, if not all, the trees may have to be removed. Girdling is almost always fatal.

Authorities hope the unprecedented reward — the first ever offered for property damage in Dakota County — will act as an incentive for someone to squeal on the individual responsible.

'This is unheard of'

Park officials first learned of the vandalism through the Minnesota Off-Road Cyclists, said Steve Sullivan, Dakota County parks director. The group also posted a picture to Facebook, drawing emotional responses.

Sullivan suspects that the vandal used a hatchet or chisel and hammer.

"This is unheard of," Sullivan said. "[It is] inconsistent with the values of the people in Dakota County."

The 2,000-acre park spans Eagan, Apple Valley and Rosemount. So far, officials have found girdled trees just in the Eagan area.

The girdling comes as Dakota County is investing $1.3 million in the park, which includes planting 1,500 trees and removing buckthorn.

Anyone with information about the vandalism should contact the Dakota County sheriff's office at (651) 438-4720.

Tipsters may remain anonymous and still be eligible for the reward. Police ask passersby who witness someone removing bark from trees to call 911.

Liz Sawyer 612-673-4648