为什么秋天树叶会变色?

Albert Camus once said, "Autumn is a second spring when every leaf is a flower." It's easy to agree with such a sentiment if you've ever taken a scenic autumn drive in New England or the Rockies, but what's the science behind those breathtaking fall colors?

加缪说过:“秋天是第二个春天，每一片叶子都是一朵花。”如果你曾经在新英格兰或落基山脉进行过秋景驾车旅行，你很容易就会同意这种观点，但这些令人惊叹的秋色背后的科学依据是什么呢?

A brilliant canopy of fall foliage in Bukk, Hungary. (Photo: Fesus Robert/Shutterstock)

There are several reasons why leaves change color in the fall, but the most significant contributing factors are shorter daylight hours and longer nighttime hours, and how those factors affects the chemical process inside each leaf.

叶子在秋天变色有几个原因，但最重要的原因是白天时间变短，夜晚时间变长，以及这些因素如何影响每片叶子内部的化学过程。

It all comes down to biological pigments (also known as "biochromes"), which are molecular substances that manifest in living things as specific colors by absorbing or reflecting wavelengths of lights.

这一切都归结于生物色素(也被称为“生物色素”)，这是一种分子物质，通过吸收或反射光线的波长在生物中表现为特定的颜色。

You might already know a little something about chlorophyll — it's the green pigment produced by plants during the photosynthesis process. Some of the other pigments found in plants are carotenoids, which are responsible for oranges, and anthocyanins, which yield red and purple leaves. While chlorophyll and carotenoids are present throughout the growing season, most anthocyanins are produced exclusively in late summer and early autumn.

你可能已经对叶绿素有所了解了——它是植物在光合作用过程中产生的绿色色素。在植物中发现的其他一些色素是类胡萝卜素和花青素，类胡萝卜素是橙子的主要成分，花青素产生红色和紫色的叶子。虽然叶绿素和类胡萝卜素存在于整个生长季节，但大多数花青素只在夏末秋初产生。

As the days become shorter and the nights become longer, the amount of light required for photosynthesis wanes and chlorophyll production gradually comes to a halt. Without any new chlorophyll being produced, the leaves' characteristic green color begins to break down and vanish. This mechanism essentially "unmasks" the colors of the carotenoids and anthocyanins that were lurking beneath.

随着白天变短，夜晚变长，光合作用所需的光量减少，叶绿素的产生逐渐停止。没有任何新的叶绿素产生，树叶特有的绿色开始分解和消失。这一机制从本质上“揭示”了隐藏在下面的类胡萝卜素和花青素的颜色。

The gorgeous red leaves of a Japanese maple tree in autumn. (Photo: TOMO/Shutterstock)

While the waning hours of sunlight are the most significant contributing factor to affect changing foliage colors, temperature and humidity can also play a role in the intensity of these seasonal displays. For example, warm, sunny days coupled with cool, mild nights are a particularly potent recipe for brilliance.

虽然日照时间的减少是影响树叶颜色变化的最重要因素，但温度和湿度也会影响这些季节性展示的强度。例如，温暖、阳光明媚的白天加上凉爽、温和的夜晚是创造辉煌的一个特别有效的方法。

As the National Forest Service explains: "During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions — lots of sugar and lots of light — spur production of the brilliant anthocyanin pigments, which tint reds, purples and crimson."

正如美国国家林业局所解释的那样:“在这些日子里，叶子里会产生大量的糖分，但凉爽的夜晚和进入叶子的静脉的逐渐关闭阻止了这些糖分的流出。”这些条件——大量的糖和大量的光——刺激了灿烂的花青素色素的产生，使红色、紫色和深红色变得鲜艳。”

The elevation and type of tree species are two other factors that affect the timing of fall foliage. Trees in higher mountain elevations are likely to change colors more quickly than their valley counterparts within the same latitude.

海拔和树种类型是影响落叶时间的另外两个因素。高海拔地区的树木比同一纬度地区的山谷树木更容易变色。

If you'd like to keep tabs on how the year's leaf-peeping season is shaping up, be sure to check out the Fall Foliage Prediction Map, which is a great resource and tool for timing your autumn road trips to coincide with peak foliage colors.

如果你想了解这一年的赏叶季节是如何形成的，一定要看看秋季赏叶预测图，这是一个很好的资源和工具，可以为你的秋季公路旅行安排好时间，使之与赏叶高峰的颜色一致。

Autumn foliage at Daigoji Temple in Kyoto, Japan. (Photo: cowardlion/Shutterstock)