Two pairs of conifer trees were deliberately set alight within half a mile of each other.
Firefighters from Fairweather Green were called to Olive Grove, off Thornton Road, Bradford, at about 9.30pm last night where they found two trees on fire.
Two hours later, they were called out again to Hedge Way, Allerton, where two more conifers were well alight.
The fires did not spread to property.
A fire service spokesman said: "They were within half a mile of each other so it looks like someone has been going around lighting them."
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