Can Trees Help Put Water Back Into the Soil?
摘要 ABSTRACT
在亚马逊河以南，有一片广阔的、生物多样的稀树草原，科学家们认为它正在面临更大的威胁，这片稀树草原叫塞拉多（Cerrado）。科学家想了解生存在塞拉多的植物是如何影响那些进入地下补给蓄水层的雨水的。这些植物中有许多是树木，它们可以捕获雨水，并将这些雨水排到覆盖着树皮的树枝和树干上。这些水流被称为“树干茎流”，如果树枝树皮的大小和形状合适，它可能是作为补给蓄水层的一种方式。本文将介绍塞拉多的常见植物种类，以及描述它们是如何影响降水进入地面的方式，并讨论树皮和树枝是如何有助于储存水源的。
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.
你知道塞拉多吗？
DO YOU KNOW THE CERRADO?
塞拉多起源于至少4000万年前，是地球上最古老的生物群落之一。塞拉多十分庞大，起初覆盖了巴西22%的领土面积，涵盖了巴西所有生物群落中最高的生物多样性。塞拉多在其整个区域并不都一样；相反，它是由不同类型的土地覆盖组成的拼凑而成，包括森林热带草原、多树热带草原，公园热带草原和草木热带草原。森林热带草原是塞拉多地区唯一一片拥有大片森林的土地类型。塞拉多的植物种类繁多，包括棕榈树、果树和木材树，还有许多各种各样的草、凤梨（像迷你菠萝）、兰花和其他小型植物。一些物种，如木材树，树干蜿蜒曲折且十分粗大，它的高度可达20米。其他木本植物的树干则较短，更像灌木丛，如草莓番石榴。【译者注：cerradão (“big cerrado”), typical cerrado, campo cerrado, dirty cerrado field, and clean cerrado field.这个真的不知道怎么翻译好了，就在维基百科上找到了下资料，勉强翻译成“森林热带草原、多树热带草原，公园热带草原和草木热带草原。森林热带草原”，如果各位有什么建议的话可以公众号留言哈！】
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 cerradão (“big cerrado”), typical cerrado, campo cerrado, dirty cerrado field, and clean cerrado field. The cerradão 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.
这种植物的多样性令人感到惊讶，因为塞拉多每年都有一个旱季，这个旱季从4月持续到9月，在这个时间段内的土壤养分普遍较低。科学家们认为许多树木可以在贫瘠的土壤中和漫长的旱季中存活要归功于它们发达的根部系统：有些树根可以深达15米。这就让它们能够从地表深处被称为蓄水层的天然储水区获取水。流经这片庞大稀树草原的雨水为瓜拉尼（Guarani）蓄水层提供了水源，而瓜拉尼含水层对南美洲的许多国家都至关重要。许多测量和估算表明，树冠在地表上的高度仅仅是根部延伸到土壤中距离的三分之一。由于其根系可能比树顶更加广阔，塞拉多也被称为“本末倒置的森林（upside-down forest）”，因此，塞拉多本末倒置的森林强有力地扎根于它下面的蓄水层。
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).
图1-塞拉多森林也被称为“本末倒置”的森林
注意树根深入土壤的程度比树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.
树木是如何辅助补给蓄水层的？
HOW TREES HELP TO RECHARGE AQUIFERS
在旱季，植物根部会从塞拉多的蓄水层中汲取大量水分，但它们也有助于之后将大量的水运回蓄水层中。你瞧，塞拉多的气候中也存在雨季，一般发生在十月和次年二月之间。蓄水层被这些雨水所补给，但它们需要依靠植物将雨水下渗到其中。在旱季从蓄水层中排向地下水的植物根系，为雨水在雨季中进入土壤提供了通道。这些通道可以帮助雨水流入蓄水层并补给水分。事实上，如果我们对这些植物中的其中一种植物进行细致观察，我们可以数出数以千计甚至数以百万计条树根——那是供雨水沿途补给蓄水层的上千百万条的通道！如果我们用显微镜寻找细小的树根，我们会看到无数个与其他植物的根系交织在一起的迷你根系通道。因此，塞拉多错综复杂的根系可以在雨季促进水从土壤表层流入蓄水层，同时也在旱季滋养植物。
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.
雨水必须克服树叶和树皮的障碍。
RAIN MUST OVERCOME LEAFY AND BARKY OBSTACLES
不过树根和蓄水层的故事会复杂一些，因为塞拉多的树梢和土壤表面之间的雨水发生了许多变化。当雨水流到森林的树叶和树皮时，它会被迫沿着三个通道的其中一个流向土壤表面...并不是所有通道都会通向土壤表面！首先，一些水分会储存在树叶和树皮中，最终会蒸发回大气中。其次，一些水分会滴到表面，就像一个树枝变成了一个漏水的水龙头。最后，一些雨水会粘在树皮、树枝和树茎上，一直流到树干底部的土壤表面。我们把最后一个过程称为 "茎流"。许多枝条上的茎流一起流下来，汇聚在树干表面上的某一点上，就可以让水流非常集中地留到该位置的土壤中。茎流不仅可从雨水中获得。事实上，在一些干旱和半干旱地区，那里降水稀少，空气中的水蒸气在接触到植物时会液化成液态水，并通过茎流流到土壤中。
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.
如今许多研究者正试图了解哪些因素会影响塞拉多和世界许多其他地区茎流的产生。茎流取决于森林的结构，如在特定区域内树木的数量以及现存的植物种类。例如，树木密度较大、树叶和树皮面积较大的森林往往会吸收更多的降水，这些降雨水变成茎流。有的时候，一片植物叶子的结构可以减少茎流量，就如那些毛茸茸的树叶。在这些情况下，叶子可能会保留水分，最终这些水分会蒸发而不流入土壤。一棵树树皮的形状也很重要。塞拉多有许多各种各样的树皮。厚一些的树皮（图 2A-C）可以像海绵一样吸收所有从树干中排出的水，从而减少茎流。薄一些的树皮（图2D-F）则不会吸收太多水分，这样就能使茎流流达地面。
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.
图2-来自塞拉多树皮类型的实例
注意最上面一排的树木有厚厚的树皮，可以吸收一些茎流，而最下面一排的树木有较薄的树皮，可以让更多的茎流到达土壤中。(A) candeia, (B) 雪松, (C) angico, (D) 棕榈, (E) 牛掌, (F) 黄岩 (这葡萄牙语我放弃了....)
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.
最后的障碍：越过枯枝落叶
THE FINAL HURDLE: CROSSING THE LITTER
在到达树干底部时，茎流可能还没有遇到土壤......它可能会遇到枯枝落叶，即厚厚的落叶、树枝、昆虫、花朵、果实，甚至还有掉落下来的树皮。这些垃圾可以保护流入土壤的水分，阻挡可能会导致土壤水分蒸发的风和太阳。一些研究者发现茎流可以直接穿过枯枝落叶层进入土壤。如果这十分常见，那么茎流可能会通过依靠着着树皮直接进入土壤和具有保护作用的枯枝落叶层。在塞拉多枯枝落叶层下面的土壤中发现了更多的水。因此，也许树皮有双重作用！首先它有助于将雨水沿着茎部排到土壤中。其次， 枯枝落叶中的树皮也许有助于保护渗入土壤的水分，使这些水分能够渗入土壤并补给蓄水层!
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!
宏伟壮丽的本末倒置森林
THE MAGNIFICENT UPSIDE-DOWN FOREST
塞拉多的本末倒置森林，有着庞大的根系，厚而粗犷的树皮，因此被恰当地称为 "水的摇篮"。我们希望您现在对森林和水的关系有了更好、更深入的了解。研究人员对森林水文学的继续研究是非常重要的，因为我们与生物群落、其结构、土壤特性和水文学之间有很多具体的相互作用。更长远的研究将帮助我们了解和保护位于南美洲的塞拉多森林。所以，我们希望在某一天我们可以依靠您的帮助来研究和保护这个宏伟壮丽的生物群落。
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 术语表
Biome: ↑ They are types of ecosystems, habitats or biological communities with a certain level of homogeneity.
生物群落：↑ 它们是具有一定同质性的生态系统、栖息地或生物群落。
Woody Plants: ↑ Forest species providing products such as wood.
木本植物：↑ 提供木材等产品的森林物种。
Aquifer: ↑ An area that can store or transmit water underneath the ground.
蓄水层: ↑ 在地表下能储存或输送水的区域。
Stemflow: ↑ Water that is captured by plant leaves and branches and drained down their stems.
茎流：↑ 由植物的叶子和枝条捕获的水，并顺着它们的茎部流动。
Arid and Semi-arid Regions: ↑ Regions where the rain is usually short-lived but very intense.
干旱和半干旱地区：↑ 雨水通常持续时间短但非常密集的地区。
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.
枯枝落叶（垃圾）：↑ 是由植物残余物（叶子、树枝）的沉积和处于不同分解阶段的有机物的堆积组成的土层，表面上覆盖着土壤。
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