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.
在温带和北方森林中，会落叶的树木和灌木被称为落叶植物，包括杨树、棉白杨、枫树和橡树等。不会落叶的植物包括松树、云杉、雪松和冷杉等针叶树或常青树。
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.
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，蜡质涂层可以尽量减少水分流失，这有助于它们全年不落叶。
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.
初秋时节，天气条件开始变化：天气变凉爽，夜晚温度下降，但尚未结冰，日光时间减少。
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.
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生产叶绿素，以保持其生命力。当气温降至零下时，树木已经进入休眠状态。
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.
绿叶之所以是绿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，但植物通常也有各种辅助色素。
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.
随着树叶中的叶绿素开始减少，这些潜在的色素变得可见。马萨诸塞大学阿默斯特分校的树木学家克里斯蒂娜·贝赞森在接受“生活科学”网站采访时表示，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.
红色和紫色色调来自花青素等辅助色素，而类胡萝卜素和叶黄素分别产生橙色和黄色。在某些种类植物中（例如枫树），不同的品种进化出了自己的色素组合。
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.
例如，红枫的树叶在秋天变成鲜艳的猩红色，而黑枫变成黄色。虽然这些色素使树木变得美丽，但它们也有重要的用途。
"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.
贝赞森说：“请记住，树木是自养生物；它们通过光合作用制造养分，叶子通常被称为树木的‘食物工厂’。”
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.
拥有一系列可以瞄准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。产生如此多色素可能很费力，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往往生长速度相对较快。
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