As the days shorten and temperatures drop in the northern hemisphere, leaves begin to turn. We can enjoy glorious autumnal colours while the leaves are still on the trees and, later, kicking through a red, brown and gold carpet when out walking.
隨著北半球白晝縮短、氣溫下降，葉子開始變黃。當葉子還在樹上時，我們能觀賞到秋日的絢麗色彩，在外出散步時，還能踩著紅色、棕色和金色的「地毯」。
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 nutshell, is the process of photosynthesis. The more photosynthesis, the more carbon is locked away.
當春天來臨，氣溫回升，樹木重新煥發生機。在溫暖的月份，樹木從大氣中吸收二氧化碳並將其儲存在複雜分子中，氧氣作為副產物隨之被釋放出來。簡而言之，這個過程就是光合作用。光合作用越多，被鎖住的碳就越多。
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 new study has turned this theory on its head and could have profound effects on how we adapt to climate change.
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。隨著氣候變暖導致植物生長季延長，一些研究人員認為，樹木和其他植物吸收的二氧化碳將比以前多。但是一項新研究顛覆了這一理論，並可能對人類如何適應氣候變化產生深遠影響。
The researchers, led by Deborah Zani at the Swiss Federal Institute of Technology, studied the degree to which the timing of colour changes in autumn tree leaves was determined by the growth of the plant in the preceding spring and summer.
由瑞士聯邦理工學院的黛博拉·扎尼領導的團隊研究了秋天樹葉變色的時間受春夏生長情況影響的程度。
Temperature and day length were traditionally accepted as the main determinants of when leaves changed colour 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 new study recorded how much carbon each tree absorbed per season and how that ultimately affected when the leaves fell.
傳統上，氣溫和白晝長度是決定葉子何時變色和掉落的主要因素，這導致一些科學家認為氣候變暖會推遲這一過程。新研究的作者們考察了七葉樹、銀樺樹和英國橡樹等歐洲落葉樹種，記錄了每棵樹每季吸收的碳量及其最終對落葉時間的影響。
Using data from the Pan European Phenology Project, which has tracked some trees for as long as 65 years, the researchers found in their long-term observational study that as the rate of photosynthesis increased, leaves changed colour and fell earlier in the year. For every 10% increase in photosynthetic activity over the spring and summer growing season, trees shed their leaves, on average, eight days earlier.
研究人員利用泛歐洲物候學項目的數據，經過長期觀察發現，隨著光合作用速率的提高，樹葉變色和掉落的時間都提前了。泛歐洲物候學項目對一些樹木進行了長達65年的追蹤記錄。研究發現，在春夏生長季節中的光合作用活動每增加10%，樹葉掉落的平均時間就會提早8天。
Climate-controlled experiments on five-year-old European beech and Japanese meadowsweet trees suggest what could be behind this unexpected result. In these trials, the trees were exposed to full sun, half shade or full shade. The results show that there is a limit to the amount of photosynthesis that a tree can carry out over a growing season. Think of it like filling a bucket with water. It can be done slowly or quickly, but once the bucket is full, there is nowhere for any more water to go.
對五歲的歐洲山毛櫸和日本繡線菊的氣候控制試驗揭示了這一意外結果背後的原因。在這些試驗中，樹木分別被暴露在充足的日光中、半陰以及全陰環境中。結果顯示，一棵樹在生長季中能發生的光合作用總量是有限的。你可以將其想像成用水桶打水。你可以慢慢地將水桶裝滿水，也可以快速地裝滿，可是一旦水桶滿了，就再也裝不進水了。
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 colour. 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 it's often the amount of available nitrogen that limits total growth. This is why farmers and gardeners use nitrogen fertilisers, to overcome this limitation.
研究表明，落葉喬木每年只能吸收一定量的碳，一旦達到極限，便無法吸收更多的碳。從那時起，樹葉就開始變色。這一極限是由樹木可獲得的養分（尤其是氮）以及樹木本身的物理結構所決定的。氮是植物生長所需的關鍵營養素，而氮的獲得量往往會限制植物的總生長量。這就是農民和園丁使用氮肥來克服這一限制的原因。
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.
總之，這些限制意味著在生長季吸收碳是樹木和草本植物的一種自我調節機制。植物所能吸收的碳是有極限的。
In a world with increasing levels of carbon in the atmosphere, these new findings imply that warmer weather and longer growing seasons will not allow temperate deciduous trees to take up more carbon dioxide. The study's predictive model suggests that by 2100, when tree growing seasons are expected to be between 22 and 34 days longer, leaves will fall from trees between three and six days earlier than they do now.
在大氣中的二氧化碳水平不斷上升的今天，這些新研究發現意味著氣候變暖和生長季延長不會讓落葉喬木吸收更多的二氧化碳。該研究的預測模型表明，到2100年，預計樹木生長季將延長22至34天，樹葉掉落的時間比現在要提早3至6天。
This has significant implications for climate change modelling. If we accept that the amount of carbon taken up by deciduous trees will remain the same each year regardless of the growing season, carbon dioxide levels will rise more quickly than was previously expected. The only way to change this will be to increase the capacity of trees to absorb carbon.
這對氣候變化建模具有重要意義。如果我們認同無論生長季長短，落葉喬木每年吸收的碳總量不變，那麼二氧化碳水平的上升速度將比原先預期的更快。改變這一局面的唯一方法就是提高樹木吸收碳的能力。
Plants that aren't limited by the amount of nitrogen available may be able to grow for longer in the warming climate. These are the trees which can take nitrogen from the air, such as alder. But these species will still lose their leaves at roughly the same time as always, thanks to less daylight and colder temperatures.
在氣候變暖的情況下，不受氮肥限制的植物生長時間可能會更長。這種樹可以從空氣中獲取氮，比如榿木。但是入秋後由於白晝縮短、氣溫降低，這些樹仍然會在和往年差不多的時間落葉。
But on the upside, with the prospect of some trees losing their leaves earlier and others losing them at the time they do now, there might be the prospect of prolonged autumnal colours—and more time for us to kick through the leaves.
從好的方面來看，由於一些樹提早落葉，而另外一些樹和現在落葉時間一致，我們未來看到秋葉美景的時間可能會延長，這意味著我們將有更多時間可以踩著落葉走路。