十万个为什么·科学阅读30篇（1）
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Why Do Some Trees Lose Leaves in Autumn While Others Stay Green?
为什么有些树秋天落叶，有些却常青？
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Deciduous trees—like maples and oaks—shed leaves each fall to conserve water during cold, dry winters.
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They form a special corky layer at the base of each leaf stem, cutting off water and nutrient flow gradually.
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Chlorophyll breaks down first, revealing yellow and orange pigments that were always present but hidden.
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Red anthocyanins are newly made in autumn, possibly protecting leaves while nutrients are reabsorbed.
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Evergreen trees—such as pines and spruces—keep needles year-round because their waxy coating and antifreeze compounds prevent freezing damage.
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Their needles have fewer pores and thicker cell walls, reducing water loss even in freezing winds.
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This adaptation lets evergreens photosynthesize whenever temperatures rise above freezing—even in winter.
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Foresters consider leaf-shedding patterns when selecting tree species for urban planting in changing climates.