Poleward or even approaching polar seas , the warmest month has an average temperature of less than 10 Â C ( 50 Â F ) , and the subarctic climate grades into a tundra climate even less suitable for trees . Should one go poleward or even toward a polar sea , one finds that the warmest month has an average temperature of less than 10 Â C ( 50 Â F ) , and the subarctic climate grades into a tundra climate even less suitable for trees .