Even if trees cannot walk，they are still on the move.

　　In

　　parts

　　of

　　the

　　Arctic，entire

　　forests

　　are

　　moving northward . Across the Arctic，temperatures are rising faster

　　than anywhere else in the world.As that happens，the tree line that marks where forests stop and the treeless tundra (冻原) starts has been shifting northward.Trees growing along the tree line must protect themselves from the cold wind.To do this(水平的) branches low to the ground.The energy it takes for trees to grow this way means they don't have enough energy to make seeds.ppens especially in places where white spruce (白云杉) grows.－blown seeds end up on the tundra beyond the tree linehey eventually can sprout (发芽) new trees.This explains how a forest can move.Of courseping the sun's rays from making the

　　white surface disappear.Instead，the trees absorb the sun's heat.This warms the surrounding air.The extra warmth encourages even more trees to produce seeds.That further boosts a forest's ability to expand.In

　　addition，more

　　trees

　　will

　　trap

　　more

　　snow，preventing much of it from being blown away. Snow can trap heat in the soil below，which encourages trees to grow.The recent rise of temperatures has helped more trees grow past the tree line.People worry about impacts on the animals that depend on frozen conditions for food and shelter.

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