As the days shorten and temperatures drop in the northern hemisphere， leaves begin to change color. We can enjoy glorious autumnal colors while the leaves are still on the trees and， later， kicking through a red， brown and gold carpet when out walking.

When temperatures rise again in spring， the growing season for trees resumes. Throughout the warmer months， trees take carbon dioxide from the atmosphere and store it in complex molecules， releasing oxygen as a byproduct. This， in a word， is the process of photosynthesis. The more photosynthesis， the more carbon is locked away.

We know that carbon dioxide is a major driver of climate change， so the more that can be taken out of the atmosphere by plants， the better. With the warmer climate leading to a longer growing season， some researchers have suggested that more carbon dioxide would be absorbed by trees and other plants than in previous times. But a study has turned this theory on its head and could have profound effects on how we adapt to climate change.

The researchers， led by Deborah Zani at the Swiss Federal Institute of Technology， studied the degree to which the timing of color changes in autumn tree leaves was determined by the growth of the plant in the preceding spring and summer.

隨著北半球白晝縮短、氣溫下降，樹(shù)葉開(kāi)始變色。當(dāng)葉子還在樹(shù)上時(shí)，我們能觀賞到秋日的絢麗色彩。樹(shù)葉掉落后，我們?cè)谕獬錾⒉綍r(shí)，還能踩著紅色、棕色和金色的“地毯”。

當(dāng)春天來(lái)臨，氣溫回升，樹(shù)木重新煥發(fā)生機(jī)。在溫暖的月份，樹(shù)木從大氣中吸收二氧化碳并將其儲(chǔ)存在復(fù)雜分子中，氧氣作為副產(chǎn)物隨之被釋放出來(lái)。簡(jiǎn)而言之，這個(gè)過(guò)程就是光合作用。光合作用越多，被鎖住的碳就越多。

我們知道二氧化碳是造成氣候變化的主要因素，所以植物從大氣中吸收的二氧化碳越多越好。隨著氣候變暖導(dǎo)致植物生長(zhǎng)季延長(zhǎng)，一些研究人員認(rèn)為，樹(shù)木和其他植物吸收的二氧化碳將比以前多。但是一項(xiàng)研究顛覆了這一理論，并可能對(duì)人類如何適應(yīng)氣候變化產(chǎn)生深遠(yuǎn)影響。

由瑞士聯(lián)邦理工學(xué)院的黛博拉·扎尼領(lǐng)導(dǎo)的團(tuán)隊(duì)研究了秋天樹(shù)葉變色的時(shí)間受之前春夏植物生長(zhǎng)情況影響的程度。

Temperature and day length were traditionally accepted as the main determinants of when leaves changed color and fell， leading some scientists to assume that warming temperatures would delay this process until later in the season. Studying deciduous European tree species， including horse chestnut， silver birch and English oak， the authors of the study recorded how much carbon each tree absorbed per season and how that ultimately affected when the leaves fell.

This research shows that deciduous trees can only absorb a set amount of carbon each year and once that limit is reached， no more can be absorbed. At that point， leaves begin to change color. This limit is set by the availability of nutrients， particularly nitrogen， and the physical structure of the plant itself. Nitrogen is a key nutrient which plants need in order to grow， and its often the amount of available nitrogen that limits total growth. This is why farmers and gardeners use nitrogen fertilizers to overcome this limitation.

Together， these constraints mean that carbon uptake during the growing season is a self-regulating mechanism in trees and herbaceous plants. Only so much carbon can be taken up.

傳統(tǒng)上，氣溫和白晝長(zhǎng)度是決定葉子何時(shí)變色和掉落的主要因素，這導(dǎo)致一些科學(xué)家認(rèn)為氣候變暖會(huì)推遲這一過(guò)程。該研究的作者們考察了七葉樹(shù)、銀樺樹(shù)和英國(guó)橡樹(shù)等歐洲落葉樹(shù)種，記錄了每棵樹(shù)每季吸收的碳量及其最終對(duì)落葉時(shí)間的影響。

研究表明，落葉樹(shù)每年只能吸收一定量的碳，一旦達(dá)到極限，便無(wú)法吸收更多的碳。從那時(shí)起，樹(shù)葉就開(kāi)始變色。這一極限是由樹(shù)木可獲得的養(yǎng)分（尤其是氮）以及樹(shù)木本身的物理結(jié)構(gòu)決定的。氮是植物生長(zhǎng)所需的關(guān)鍵營(yíng)養(yǎng)素，而氮的獲得量往往會(huì)限制植物的總生長(zhǎng)量。這就是農(nóng)民和園丁使用氮肥來(lái)克服這一限制的原因。

總之，這些限制意味著在生長(zhǎng)季吸收碳是樹(shù)木和草本植物的一種自我調(diào)節(jié)機(jī)制。植物所能吸收的碳是有極限的。

Word Bank

resume /r?'zu?m/ v. 重新開(kāi)始

The noise resumed， louder than before.

profound /pr?'fa?nd/ adj. 巨大的；深遠(yuǎn)的

delay /d?'le?/ v. 延遲；延期

species /'spi??i?z/ n. 種；物種

There are many species of dog（s）.