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.

在溫帶和北方森林中，會(huì)落葉的喬木和灌木被稱為落葉植物，包括楊樹、棉白楊、楓樹和橡樹等。而不會(huì)落葉的植物包括松樹、云杉、雪松和冷杉等針葉樹或常綠樹。這兩種類型的樹木在冬季產(chǎn)生的能量都較少，但針葉樹適應(yīng)性強(qiáng)，樹葉的蠟質(zhì)涂層可以減少水分流失，這有助于它們?nèi)瓴宦淙~。

初秋時(shí)節(jié)，氣候開始變化：天氣變涼爽，夜晚溫度下降，但尚未結(jié)冰，日照時(shí)間變短?？偟膩碚f，植物通過葉綠素從陽光中獲取能量的機(jī)會(huì)減少了，因此落葉樹木開始減少葉綠素的生成，以保持其生命力。

當(dāng)氣溫降至零攝氏度以下時(shí)，樹木陸續(xù)進(jìn)入休眠狀態(tài)。

綠葉之所以是綠色的，是因?yàn)槿~綠素吸收紅光和藍(lán)光，反射綠光。雖然葉綠素是植物用來獲取光能最常見的色素，但植物通常還有各種輔助色素。隨著樹葉中的葉綠素開始減少，這些潛在的色素就變得可見。美國(guó)馬薩諸塞大學(xué)阿默斯特分校的樹木學(xué)家克里斯蒂娜·貝贊森在寫給《生活科學(xué)》雜志的郵件中提到，這就是我們?cè)谇锛韭淙~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.

紅色和紫色色調(diào)來自輔助色素花青素，而類胡蘿卜素和葉黃素分別產(chǎn)生橙色和黃色色調(diào)。在某個(gè)植物類群中，例如楓樹，不同的品種進(jìn)化出了自己的色素組合。例如，在秋天，紅楓的樹葉變成鮮艷的猩紅色，而黑楓的樹葉則變成黃色。

這些色素在使樹木變得美麗的同時(shí)，還有重要的用途。

“請(qǐng)記住，樹木是自養(yǎng)生物，它們通過光合作用制造養(yǎng)分，樹葉通常被稱為樹木的‘食物工廠’。”貝贊森說。擁有一系列可以吸收不同波長(zhǎng)光線的色素可以讓植物在光合作用中獲取更多的能量。而產(chǎn)生如此多色素會(huì)很費(fèi)力，所以并不是所有的樹木都會(huì)采用這種策略。但那些這樣做的樹木，它們的生長(zhǎng)速度往往相對(duì)較快。

Word Bank

deciduous /d?'s?d?u?s/ adj. 落葉的

pigment /'p?ɡm(xù)?nt/ n. 色素

yield /ji?ld/ v. 出產(chǎn)，產(chǎn)生 n. 產(chǎn)量

scarlet /'skɑ?l?t/ adj. 鮮紅的；猩紅的