Why Do Leaves Change Color in the Fall?为什么秋天 树叶会变色？

Within temperate and boreal forests， trees and shrubs that drop their leaves are called deciduous plants; they include groups such as aspens， cottonwoods， maples and oaks. On the other end of the spectrum are plants that don’t shed their leaves—for example， the pines， spruces， cedars and firs that make up the conifers， or evergreens. Both types of trees produce less energy in the winter， but conifers have adaptations， such as a waxy coating to minimize water loss， that help them keep their needles year-round.

In early fall， the weather conditions start to change： It gets cooler—with crisp， but not freezing， nights—and the duration of daylight is reduced. Collectively， there’s less sunlight for plants to harvest using the chlorophyll in their leaves， so deciduous trees begin to pull back their investment in keeping them alive.

By the time the first freezes hit， the trees are well on their way to dormancy.

While chlorophyll is the most common pigment that plants use to harvest light—green leaves look that way because chlorophyll absorbs red and blue light and reflects green light—plants often have a variety of secondary pigments， too. As the chlorophyll in a tree’s leaves begins to wane， these underlying pigments become visible. This is what we’re seeing when fall foliage peaks， Kristina Bezanson， an arboriculturist at the University of Massachusetts Amherst， told Live Science in an email.

在温带和北方森林中，会落叶的乔木和灌木被称为落叶植物，包括杨树、棉白杨、枫树和橡树等。而不会落叶的植物包括松树、云杉、雪松和冷杉等针叶树或常绿树。这两种类型的树木在冬季产生的能量都较少，但针叶树适应性强，树叶的蜡质涂层可以减少水分流失，这有助于它们全年不落叶。

初秋时节，气候开始变化：天气变凉爽，夜晚温度下降，但尚未结冰，日照时间变短。总的来说，植物通过叶绿素从阳光中获取能量的机会减少了，因此落叶树木开始减少叶绿素的生成，以保持其生命力。

当气温降至零摄氏度以下时，树木陆续进入休眠状态。

绿叶之所以是绿色的，是因为叶绿素吸收红光和蓝光，反射绿光。虽然叶绿素是植物用来获取光能最常见的色素，但植物通常还有各种辅助色素。随着树叶中的叶绿素开始减少，这些潜在的色素就变得可见。美国马萨诸塞大学阿默斯特分校的树木学家克里斯蒂娜·贝赞森在写给《生活科学》杂志的邮件中提到，这就是我们在秋季落叶季看到的景象。

Red and purple hues stem from secondary pigments called anthocyanins， while carotenoids and xanthophylls yield oranges and yellows， respectively. Within a single group， such as the maples， different species have evolved their own suite of pigments. Red maples， for example， turn a brilliant scarlet in the fall， while black maples become yellow.

And while these pigments make the trees beautiful， they also serve an important purpose.

“Remember that trees are autotrophs; they make their own food through photosynthesis， and the leaves are often called the ‘food factories’ for the tree，” Bezanson said. Having a range of pigments that can target different wavelengths of light allows plants to harvest more energy during photosynthesis. It can be expensive to invest in so many pigments， so not all trees pursue this strategy. But those that do are often relatively fast growers.

红色和紫色色调来自辅助色素花青素，而类胡萝卜素和叶黄素分别产生橙色和黄色色调。在某个植物类群中，例如枫树，不同的品种进化出了自己的色素组合。例如，在秋天，红枫的树叶变成鲜艳的猩红色，而黑枫的树叶则变成黄色。

这些色素在使树木变得美丽的同时，还有重要的用途。

“请记住，树木是自养生物，它们通过光合作用制造养分，树叶通常被称为树木的‘食物工厂’。”贝赞森说。拥有一系列可以吸收不同波长光线的色素可以让植物在光合作用中获取更多的能量。而产生如此多色素会很费力，所以并不是所有的树木都会采用这种策略。但那些这样做的树木，它们的生长速度往往相对较快。

Word Bank

deciduous /dɪ'sɪdʒuəs/ adj. 落叶的

pigment /'pɪɡmənt/ n. 色素

yield /jiːld/ v. 出产，产生 n. 产量

scarlet /'skɑːlət/ adj. 鲜红的；猩红的