Over 1,350 patches of dead pine trees have been reported in southwestern Finland since April, when researchers started collecting observations from the public.
"Every day we receive more in our mapping service," Turku University geography professor Risto Kalliola told AFP.
He described the phenomenon as a "local mass-death of patches of pine trees".
Most affected were rocky coastal areas with barren soil easily exposed to drought, he said.
Browned groups of dead pines suddenly started to appear along Finland's southern coast a few years ago, and researchers are now trying to find out the cause of the phenomenon.
"Something is happening in our nature and we have to take it seriously," Kalliola said.
Similar deaths of pine trees have also occurred in other northern European countries, including neighbouring Sweden.
"What is new in Finland is that this phenomenon has quite recently begun to be common," he said.
He believed several factors could be causing the local die-offs, such as insect pests and fungal diseases -- all exacerbated by global warming.
"During warm summers with heatwaves and weeks without rain, those trees which are growing in vulnerable places start to suffer and their ability to defend themselves against pathogens weakens," he said.
Kalliola said the exceptionally warm summer in Finland this year had stressed the trees.
"The less available water in the soil the trees can suck up with their roots, the less they can withstand heatwaves and drought," he said.
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