Encroachers set fire to clear land for plantations, Kupin Simbolon, the head of Tesso Nilo National Park, said here on Tuesday.
Last June, when Riau Province were hit by massive forest fires, 18 hotspots were detected in the national park, he said.
The areas which are prone to encroachments and thus man-made forest fires, are Bukit Kusuma, Toro dan Dolit.
"If there are no rains within three days, many hotspots are detected," he said.
The TNTN officers have intensified patrol to prevent further encroachments, he said, adding that so far no one was arrested for encroachment on forest land.
The 83,068-ha Tesso Nilo National Park is located in Pelalawan and Indragiri Hulu Districts, Riau Province.
Based on an investigation by WWF and TNTN in 2011, a total of 52,266.50 ha land in the national park has been encroached and around 36,353.50 ha forest land has been converted into oil palm plantations.
There were 1,613 families who encroached the national park until 2009. They claimed that they have lived in the forest area far before the government designated the area as a national park.