Arsonists set fire to about 1,200 trees in a park - and many of the 11-year-old conifers may not survive, police said.
One-and-a-half acres of woodland in Sence Valley Park on Ravenstone Road, Ibstock, Leicestershire were damaged.
Police want to talk to three youths seen throwing matches at the trees on Easter Sunday, 8 April. They then rode away on cycles.
A police spokesman said the fire service contained the flames which did not threaten the public.
Pc Shaun Wilson said: "Thankfully the fire was extinguished quite quickly but the damage caused was still considerable.
"The consequences of this incident could have been far worse had the fire spread further and become a risk to public safety.
"The Forestry Commission have told us they don't expect many of these trees to survive."
Police have issued descriptions of the three youths.
The first is in his early teens, about 5ft 6ins (1.67m) tall and has short, blond hair. He was wearing a red England football shirt.
The second is also in his early teens. He was wearing a blue top.
There is no clear description of the third youth but one of the group was wearing a distinctive pair of long, white boots.
Police want to talk to anyone who saw any suspicious activity in or around Ravenstone Road between 1500 and 1600 BST on Easter Sunday.