Recurrent wildfires have damaged many varieties of plants in Hat Chao Mai National Park in Sikao and Kantang districts of Trang.
Blazes have been breaking out in various areas across Hat Chao Mai since last Saturday, according to Manot Wongsurirat, the national park chief.
About 846 rai (42%) of the park's mangrove forest consisting of avicennia alba (samer khao) trees had been scorched, Mr Manot said on Friday.
He suspected that some of the fires were started by people who had trespassed into the forest in the southern province for illegal purposes.
Investigators have gathered some evidence and are currently conducting further investigations.
Mr Manot said drought and heat had also caused grasses underneath the trees to dry up, making them more susceptible. The flames spread rapidly because samer khao trees are highly flammable, he added.
The situation has taken a long time to bring under control because the park is short of staff and does not have enough equipment to cover all areas, he added.
A forestry official tries to extinguish a fire at Hat Chao Mai National Park on Friday. (Photo by Methee Muangkaew)