Can Trees Help Put Water Back Into the Soil?

摘要 ABSTRACT

在亞馬遜河以南，有一片廣闊的、生物多樣的稀樹(shù)草原，科學(xué)家們認(rèn)為它正在面臨更大的威脅，這片稀樹(shù)草原叫塞拉多（Cerrado）。科學(xué)家想了解生存在塞拉多的植物是如何影響那些進(jìn)入地下補(bǔ)給蓄水層的雨水的。這些植物中有許多是樹(shù)木，它們可以捕獲雨水，并將這些雨水排到覆蓋著樹(shù)皮的樹(shù)枝和樹(shù)干上。這些水流被稱(chēng)為“樹(shù)干莖流”，如果樹(shù)枝樹(shù)皮的大小和形狀合適，它可能是作為補(bǔ)給蓄水層的一種方式。本文將介紹塞拉多的常見(jiàn)植物種類(lèi)，以及描述它們是如何影響降水進(jìn)入地面的方式，并討論樹(shù)皮和樹(shù)枝是如何有助于儲(chǔ)存水源的。

Just to the south of the Amazon, there is a vast and biodiverse savanna that scientists believe is under even greater threat, called the Cerrado. Scientists want to understand how the plants that live in the Cerrado affect the rainfall that enters the ground to recharge the aquifer. Many of these plants are trees that capture rainwater and drain it down their bark-covered branches and trunks. This water, called stemflow, may be one way to recharge the aquifer, if the branches and bark of the trees are the right size and shape. This article will introduce the common plant species of the Cerrado, describe how they affect the way rainfall enters the ground, and discuss how bark and branches may help conserve water.

你知道塞拉多嗎？

DO YOU KNOW THE CERRADO?

塞拉多起源于至少4000萬(wàn)年前，是地球上最古老的生物群落之一。塞拉多十分龐大，起初覆蓋了巴西22%的領(lǐng)土面積，涵蓋了巴西所有生物群落中最高的生物多樣性。塞拉多在其整個(gè)區(qū)域并不都一樣；相反，它是由不同類(lèi)型的土地覆蓋組成的拼湊而成，包括森林熱帶草原、多樹(shù)熱帶草原，公園熱帶草原和草木熱帶草原。森林熱帶草原是塞拉多地區(qū)唯一一片擁有大片森林的土地類(lèi)型。塞拉多的植物種類(lèi)繁多，包括棕櫚樹(shù)、果樹(shù)和木材樹(shù)，還有許多各種各樣的草、鳳梨（像迷你菠蘿）、蘭花和其他小型植物。一些物種，如木材樹(shù)，樹(shù)干蜿蜒曲折且十分粗大，它的高度可達(dá)20米。其他木本植物的樹(shù)干則較短，更像灌木叢，如草莓番石榴?！咀g者注：cerrad?o (“big cerrado”), typical cerrado, campo cerrado, dirty cerrado field, and clean cerrado field.這個(gè)真的不知道怎么翻譯好了，就在維基百科上找到了下資料，勉強(qiáng)翻譯成“森林熱帶草原、多樹(shù)熱帶草原，公園熱帶草原和草木熱帶草原。森林熱帶草原”，如果各位有什么建議的話(huà)可以公眾號(hào)留言哈！】

The Cerrado originated at least 40 million years ago, making it one of the oldest?biomes?on Earth. The Cerrado is huge, originally covering 22% of Brazil, and it contains the highest biodiversity of any biome in Brazil. The Cerrado isn’t the same over its entire area; instead, it is a patchwork made of different types of land cover, including cerrad?o (“big cerrado”), typical cerrado, campo cerrado, dirty cerrado field, and clean cerrado field. The cerrad?o is the only type of land in the Cerrado with large forests. There is a great diversity of plants in the Cerrado, including palm, fruit, and timber trees, plus a wide variety of grasses, bromeliads (like mini pineapples), orchids, and other smaller plants. Some species, like timber trees, have twisty and thick trunks reaching up to 20 meters in height. Other?woody plantsare shorter and more like bushes, like the strawberry guava.

這種植物的多樣性令人感到驚訝，因?yàn)槿嗝磕甓加幸粋€(gè)旱季，這個(gè)旱季從4月持續(xù)到9月，在這個(gè)時(shí)間段內(nèi)的土壤養(yǎng)分普遍較低。科學(xué)家們認(rèn)為許多樹(shù)木可以在貧瘠的土壤中和漫長(zhǎng)的旱季中存活要?dú)w功于它們發(fā)達(dá)的根部系統(tǒng)：有些樹(shù)根可以深達(dá)15米。這就讓它們能夠從地表深處被稱(chēng)為蓄水層的天然儲(chǔ)水區(qū)獲取水。流經(jīng)這片龐大稀樹(shù)草原的雨水為瓜拉尼（Guarani）蓄水層提供了水源，而瓜拉尼含水層對(duì)南美洲的許多國(guó)家都至關(guān)重要。許多測(cè)量和估算表明，樹(shù)冠在地表上的高度僅僅是根部延伸到土壤中距離的三分之一。由于其根系可能比樹(shù)頂更加廣闊，塞拉多也被稱(chēng)為“本末倒置的森林（upside-down forest）”，因此，塞拉多本末倒置的森林強(qiáng)有力地扎根于它下面的蓄水層。

This diversity of plant life is surprising because the Cerrado has a dry season every year that can last from April to September and the soils are generally low in nutrients. Scientists believe that many trees survive the long dry seasons in poor soils due to their deep root systems: some roots can reach up to 15 meters in depth. This allows them to capture water from natural water storage areas called?aquifers, deep below the surface. The rainwater that flows through this huge savanna contributes water to the Guarani aquifer, which is important to many countries in South America. Many measurements and estimates suggest that the height of the tree canopy above the ground is only a third of distance that the roots extend into the soil. Because its root system can be more extensive than its treetops, the Cerrado is also called the “upside-down forest.” So, the Cerrado’s upside-down forest is strongly rooted into the aquifers below it (Figure 1).

圖1-塞拉多森林也被稱(chēng)為“本末倒置”的森林

注意樹(shù)根深入土壤的程度比樹(shù)冠伸向天空的程度要深得多

Figure 1 - The Cerrado forest is also called the “upside-down” forest.

Notice how the tree roots go much deeper into the soil than their canopies reach to the sky! Figure credit: Dr. John Toland Van Stan, II.

樹(shù)木是如何輔助補(bǔ)給蓄水層的？

HOW TREES HELP TO RECHARGE AQUIFERS

在旱季，植物根部會(huì)從塞拉多的蓄水層中汲取大量水分，但它們也有助于之后將大量的水運(yùn)回蓄水層中。你瞧，塞拉多的氣候中也存在雨季，一般發(fā)生在十月和次年二月之間。蓄水層被這些雨水所補(bǔ)給，但它們需要依靠植物將雨水下滲到其中。在旱季從蓄水層中排向地下水的植物根系，為雨水在雨季中進(jìn)入土壤提供了通道。這些通道可以幫助雨水流入蓄水層并補(bǔ)給水分。事實(shí)上，如果我們對(duì)這些植物中的其中一種植物進(jìn)行細(xì)致觀察，我們可以數(shù)出數(shù)以千計(jì)甚至數(shù)以百萬(wàn)計(jì)條樹(shù)根——那是供雨水沿途補(bǔ)給蓄水層的上千百萬(wàn)條的通道！如果我們用顯微鏡尋找細(xì)小的樹(shù)根，我們會(huì)看到無(wú)數(shù)個(gè)與其他植物的根系交織在一起的迷你根系通道。因此，塞拉多錯(cuò)綜復(fù)雜的根系可以在雨季促進(jìn)水從土壤表層流入蓄水層，同時(shí)也在旱季滋養(yǎng)植物。

Plant roots take up lots of water from the Cerrado’s aquifers during the dry season. But they also help to put lots of water?back into?these aquifers later. You see, the climate of the Cerrado also has a rainy season, which generally occurs between October and February. The aquifers are recharged by this rain, but they depend on plants to get the rainwater down into them. The same plant roots that remove groundwater from the aquifers during the dry season provide drainage pathways for rainwater into and through soils during the rainy season. These root pathways can help rainwater to drain into and recharge the aquifers. In fact, if we looked closely at one of these plants, we could count thousands or even millions of roots—that’s thousands to millions of pathways for rainwater to travel along to recharge the aquifers! And, if we look for?tiny?roots with our microscopes, we will see countless other mini-root water pathways that intertwine with the roots of other plants [1]. So, the Cerrado’s complex root system can promote water movement from the soil surface to the aquifers in the wet season, while also nourishing the plants in the dry season.

雨水必須克服樹(shù)葉和樹(shù)皮的障礙。

RAIN MUST OVERCOME LEAFY AND BARKY OBSTACLES

不過(guò)樹(shù)根和蓄水層的故事會(huì)復(fù)雜一些，因?yàn)槿嗟臉?shù)梢和土壤表面之間的雨水發(fā)生了許多變化。當(dāng)雨水流到森林的樹(shù)葉和樹(shù)皮時(shí)，它會(huì)被迫沿著三個(gè)通道的其中一個(gè)流向土壤表面...并不是所有通道都會(huì)通向土壤表面！首先，一些水分會(huì)儲(chǔ)存在樹(shù)葉和樹(shù)皮中，最終會(huì)蒸發(fā)回大氣中。其次，一些水分會(huì)滴到表面，就像一個(gè)樹(shù)枝變成了一個(gè)漏水的水龍頭。最后，一些雨水會(huì)粘在樹(shù)皮、樹(shù)枝和樹(shù)莖上，一直流到樹(shù)干底部的土壤表面。我們把最后一個(gè)過(guò)程稱(chēng)為 "莖流"。許多枝條上的莖流一起流下來(lái)，匯聚在樹(shù)干表面上的某一點(diǎn)上，就可以讓水流非常集中地留到該位置的土壤中。莖流不僅可從雨水中獲得。事實(shí)上，在一些干旱和半干旱地區(qū)，那里降水稀少，空氣中的水蒸氣在接觸到植物時(shí)會(huì)液化成液態(tài)水，并通過(guò)莖流流到土壤中。

The story of roots and aquifers is a little more complicated though, because a lot happens to rainwater between the Cerrado’s treetops and the soil surface. When the rain reaches the forest’s leaves and bark, it is forced along one of three paths… and not all of them lead to the soil surface! First, some water will be stored on leaves and bark and will eventually evaporate back into the air. Second, some water will drip to the surface, as if a branch has become a leaky faucet. Finally, some rain will stick to the bark, branches, and stems and will drain all the way down to the soil surface at the bottom of the tree trunk. This last process is called?stemflow. Stemflow comes from many branches all draining down to one spot at the surface right next to the trunk, so it can result in a very concentrated flow of water to the soil in that location. Stemflow does not just come from rain. In fact, in some?arid and semi-arid regions?where there is little rainfall, water vapor from the air condenses into liquid water when it contacts the plant and drains to the soil via stemflow.

如今許多研究者正試圖了解哪些因素會(huì)影響塞拉多和世界許多其他地區(qū)莖流的產(chǎn)生。莖流取決于森林的結(jié)構(gòu)，如在特定區(qū)域內(nèi)樹(shù)木的數(shù)量以及現(xiàn)存的植物種類(lèi)。例如，樹(shù)木密度較大、樹(shù)葉和樹(shù)皮面積較大的森林往往會(huì)吸收更多的降水，這些降雨水變成莖流。有的時(shí)候，一片植物葉子的結(jié)構(gòu)可以減少莖流量，就如那些毛茸茸的樹(shù)葉。在這些情況下，葉子可能會(huì)保留水分，最終這些水分會(huì)蒸發(fā)而不流入土壤。一棵樹(shù)樹(shù)皮的形狀也很重要。塞拉多有許多各種各樣的樹(shù)皮。厚一些的樹(shù)皮（圖 2A-C）可以像海綿一樣吸收所有從樹(shù)干中排出的水，從而減少莖流。薄一些的樹(shù)皮（圖2D-F）則不會(huì)吸收太多水分，這樣就能使莖流流達(dá)地面。

Many researchers are now trying to understand what factors affect the production of stemflow in the Cerrado and in many other regions around the world. Stemflow depends on how the forest is structured, such as the number of trees in a certain area and the types of plant species present. For example, forests with a larger density of trees and a greater area of leaves and bark tend to catch a greater amount of rainfall that becomes stemflow. Sometimes, the structure of a plant’s leaves can reduce the amount of stemflow, for example if they are hairy. In those cases, the leaves may retain the water, which will eventually evaporate and not make it to the soil. The way a tree species’ bark is shaped is also important. There are many types of bark in the Cerrado. A thicker bark (Figures 2A–C) could reduce stemflow by acting like a sponge and sucking up all the water draining down the trunk. A thinner bark (Figures 2D–F) would not absorb as much water, allowing more stemflow to reach the ground.

圖2-來(lái)自塞拉多樹(shù)皮類(lèi)型的實(shí)例

注意最上面一排的樹(shù)木有厚厚的樹(shù)皮，可以吸收一些莖流，而最下面一排的樹(shù)木有較薄的樹(shù)皮，可以讓更多的莖流到達(dá)土壤中。(A) candeia, (B) 雪松, (C) angico, (D) 棕櫚, (E) 牛掌, (F) 黃巖??(這葡萄牙語(yǔ)我放棄了....)

Figure 2 - Examples of bark types from the Cerrado.

Notice how the top row of trees has thick bark that can absorb some of the stemflow, while those in the bottom row have thinner bark that would allow more stemflow to reach the soil.?(A)?candeia,?(B)?cedro,?(C)?angico,?(D)?palm,?(E)?pata de vaca,?(F)?ipê amarelo.

最后的障礙：越過(guò)枯枝落葉

THE FINAL HURDLE: CROSSING THE LITTER

在到達(dá)樹(shù)干底部時(shí)，莖流可能還沒(méi)有遇到土壤......它可能會(huì)遇到枯枝落葉，即厚厚的落葉、樹(shù)枝、昆蟲(chóng)、花朵、果實(shí)，甚至還有掉落下來(lái)的樹(shù)皮。這些垃圾可以保護(hù)流入土壤的水分，阻擋可能會(huì)導(dǎo)致土壤水分蒸發(fā)的風(fēng)和太陽(yáng)。一些研究者發(fā)現(xiàn)莖流可以直接穿過(guò)枯枝落葉層進(jìn)入土壤。如果這十分常見(jiàn)，那么莖流可能會(huì)通過(guò)依靠著著樹(shù)皮直接進(jìn)入土壤和具有保護(hù)作用的枯枝落葉層。在塞拉多枯枝落葉層下面的土壤中發(fā)現(xiàn)了更多的水。因此，也許樹(shù)皮有雙重作用！首先它有助于將雨水沿著莖部排到土壤中。其次， 枯枝落葉中的樹(shù)皮也許有助于保護(hù)滲入土壤的水分，使這些水分能夠滲入土壤并補(bǔ)給蓄水層!

Upon reaching the bottom of the trunk, stemflow may still not meet the soil quite yet…it might instead meet the?litter, a thick layer of fallen leaves, branches, insects, flowers, fruits, and even fallen tree bark, too! This litter can protect the water that makes it into the soils below, blocking soil water from the wind and sun that could cause it to evaporate. Some researchers have found that stemflow can pass right by the litter layer and into the soils. If this is common, then stemflow may make it to the soils and the protection of the litter layer by riding the bark straight into the soils. In the Cerrado, more water has been found in soils that lie beneath the forest litter. So, maybe the bark does a double duty! First, it helps to drain rainwater along stems and into the soils. Then, perhaps the fallen bark in the litter helps protect the water that makes it to soils, allowing it to infiltrate and recharge the aquifers!

宏偉壯麗的本末倒置森林

THE MAGNIFICENT UPSIDE-DOWN FOREST

塞拉多的本末倒置森林，有著龐大的根系，厚而粗獷的樹(shù)皮，因此被恰當(dāng)?shù)胤Q(chēng)為 "水的搖籃"。我們希望您現(xiàn)在對(duì)森林和水的關(guān)系有了更好、更深入的了解。研究人員對(duì)森林水文學(xué)的繼續(xù)研究是非常重要的，因?yàn)槲覀兣c生物群落、其結(jié)構(gòu)、土壤特性和水文學(xué)之間有很多具體的相互作用。更長(zhǎng)遠(yuǎn)的研究將幫助我們了解和保護(hù)位于南美洲的塞拉多森林。所以，我們希望在某一天我們可以依靠您的幫助來(lái)研究和保護(hù)這個(gè)宏偉壯麗的生物群落。

The upside-down forests of the Cerrado, with their large roots and trees with thick and rugged bark, have been fittingly called a “cradle of waters.” We hope that you now have a better understanding of forest and water relation. It is important for researchers to continue to study forest hydrology because we have a lot of specific interactions among biomes, their structure, soil properties and hydrology. Further research will help us to understand and protect Cerrado forests in South America. So, we wish someday we can count on your help to study and protect this magnificent biome!

Glossary?術(shù)語(yǔ)表

Biome:?↑?They are types of ecosystems, habitats or biological communities with a certain level of homogeneity.

生物群落：↑ 它們是具有一定同質(zhì)性的生態(tài)系統(tǒng)、棲息地或生物群落。

Woody Plants:?↑?Forest species providing products such as wood.

木本植物：↑ 提供木材等產(chǎn)品的森林物種。

Aquifer:?↑?An area that can store or transmit water underneath the ground.

蓄水層:?↑ 在地表下能儲(chǔ)存或輸送水的區(qū)域。

Stemflow:?↑?Water that is captured by plant leaves and branches and drained down their stems.

莖流：↑ 由植物的葉子和枝條捕獲的水，并順著它們的莖部流動(dòng)。

Arid and Semi-arid Regions:?↑?Regions where the rain is usually short-lived but very intense.

干旱和半干旱地區(qū)：↑ 雨水通常持續(xù)時(shí)間短但非常密集的地區(qū)。

Litter:?↑?Is the layer composed by the deposition of plant remains (leaves, branches) and the accumulation of organic material in different stages of decomposition that superficially covers the soil.

枯枝落葉（垃圾）：↑ 是由植物殘余物（葉子、樹(shù)枝）的沉積和處于不同分解階段的有機(jī)物的堆積組成的土層，表面上覆蓋著土壤。

附上作者信息：