十万个为什么·科学阅读30篇（1）

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为什么有些树秋天落叶，有些却常青？

Deciduous trees—like maples and oaks—shed leaves each fall to conserve water during cold, dry winters.
They form a special corky layer at the base of each leaf stem, cutting off water and nutrient flow gradually.
Chlorophyll breaks down first, revealing yellow and orange pigments that were always present but hidden.
Red anthocyanins are newly made in autumn, possibly protecting leaves while nutrients are reabsorbed.
Evergreen trees—such as pines and spruces—keep needles year-round because their waxy coating and antifreeze compounds prevent freezing damage.
Their needles have fewer pores and thicker cell walls, reducing water loss even in freezing winds.
This adaptation lets evergreens photosynthesize whenever temperatures rise above freezing—even in winter.
Foresters consider leaf-shedding patterns when selecting tree species for urban planting in changing climates.