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