在大約40年前的Dartmouth大學，有位名為Herb Bormann的年輕生物學教授，取了一株蕃茄，小心地將其根部，分成兩個部分，並將兩部份的根分別種植在兩個盆子裡。他持續對兩個盆子澆水，直到這株植物長大。接著，他只對其中一個盆子澆水。然而，蕃茄依舊生長良好。

　照片由David Parsons提供，NREL/PIX.

　　接下來，他重複多次實驗，每次都加入了另一株植物，但其根部只種植在沒有澆水的盆子裡。這些新加入的植物，有些仍然維持生長，有些雖然枯萎了，但其枯萎速度比控制組(沒有任何植物「donor」相伴)遲緩許多。隨後的實驗結果中，更顯示植物除了會供給水分給另一株植物，還會提供放射性同位素。

　　接著，Bormann在Dartmouth校區附近的白松林進行試驗，在根部注射染劑、殺草劑，以及放射性追蹤劑。他發現這些樹木不僅將這些物質四處散播，且它們的根部也生長成複雜的網狀「嫁接」(grafts)系統。經由這些連接的根系，大樹可以將養分傳送給小樹，弱勢的樹木。即使有些樹頭並未重新長出枝幹、或是葉片，也可以持續生長十年以上。

　　Bormann根據研究結果敘述：「這個研究指出，我們高估了樹木為了生存，而互相競爭的行為。在許多方面，樹木結合成為一個有機共同體。這個共同體的成員，都會失去了部分的個體特色，而成為互相影響的同伴的一部份。」

　　即使Bormann發表了這項研究，但是樹木在地表下的互相連結，並未受到重視。他的師長之一，已告訴他橡木會由這些連結的根部而傳染疾病。早在1920年代，松木、道格拉斯樅木，與香脂冷杉，其根部連結現象已被發現，這個現象被稱為「有生命的樹頭」(living stumps)。

　　在1966年時，Bormann與其同事B.F. Graham寫了一篇總回顧，列舉出超過180篇科學文獻，顯現至少有150種樹木，會經由根部傳送物質，包括了橡木、楓樹、菩提樹、赤松、樅木、落葉松、樺樹、棕樹與松樹。即使在不同的樹種間，根部也會有連結的現象。如楓樹與樺樹，樺樹與榆木，檀香與熱帶樹木Eugenia。(檀香經由連結的根部，將其香味傳導給Eugenia)

　　有時候，這些傳輸是經由根部的連結。有時候，是經由土壤傳送的，如Bormann所種植的蕃茄。有時候，會經由樹木間的傳遞者，一種稱為mycorrhizae(根瘤菌)的共生菌。這種真菌生長在樹木的根上，吸收樹葉所生產的蔗糖，並回報以土壤中的混合養分。最常被運送的為蔗糖、生長荷爾蒙、殺草劑，與病原體。較少被傳送的為水分(以總量來計算)，或是微量元素。

　　生物學者對於樹木在地底下的傳導早已非常熟悉，但並未重視之。我修習過許多生物相關課程，但是，在幾個月前，在森林生態學家Scot Zens的演講中，聽到關於根部連結的現象時，感到非常驚訝，彷彿我早已知道此事；但是，我並沒有。這個觀念打破了我以前的觀念，改變了我對森林的看法。為什麼根部要共同生長？樹木會四處散佈物質嗎？這其中有什麼意義呢？

　　這對於森林的經營管理有什麼意義呢?是選擇性的砍伐，還是全部砍除？這對於酸雨、旱災、噴灑農藥，以及當我們試著對抗樹木的疾病時，有什麼提示呢？這對於我們互相競爭、而非協助的文化與觀念，有什麼意義呢？我們是否只偏執的重視個體，大於重視互相連結的群體呢？我們對森林與樹木的觀念，是否正確呢？

　　致力於研究森林傳導現象的學者，開始寫作關於森林的根部連結現象，來取代沿襲前人研究的競爭理論。當砍去一株樹木，而留下樹頭時，可能對防止疾病散播並無助益。當選擇性的注射殺草劑到某些樹木上，想要將其殺死時，可能會將毒素散佈到非目標的樹木上。對有強健連結根系的樹木進行環狀剝皮，不一定能夠將之殺死，至少被剝皮的部位以下的部分不會死亡。互相連結的樹木，對於狂風可能有較強的耐受性，但是，對於疾病的抵抗力則較弱。選擇性的砍伐，可能會導致非預期的結果。你可能不會殺死目標物，反而殺死與它連結的其它樹木。

　　我非常樂意設計一個深入的課程，來取代那些應用的。我對於教導我科學的教授存有偏見，雖然他們很努力地灌輸我一些觀念。但是，我情願相信互助合作的原則，甚於相互競爭。我很樂意去思考那複雜的連結系統。我的直覺告訴我(有些人會認為這是不理性的)，當一株樹木倒下時，整個森林都會得到訊息，並為之哀悼。我很喜愛樹木之間，擁有秘密的連結的這個想法。

　　就像在科學上與我持有同樣看法的人士一樣，我並未盲目的相信自然現象給予人類社會的暗示。啊，我可以這麼說。大樹會支持小樹。樹木們會一起成長，並非互相爭鬥。當我們帶走其中一株，這個訊息將會傳遞到整個群落。如果這些現象真的存在於樹木上，那麼，為什麼不能存在於人類社會呢？

　　當然，將森林當作樹木的集合之想法，太過於單純。在森林中，有著許多競爭。樹木間的傳遞，不僅可以傳導養分與荷爾蒙，也會傳導疾病與毒素。沒有人知道為什麼樹木要與其它個體分享所有。相對於我們簡單的世界觀，自然界實在是太複雜了。無論我們對樹木間的相互連結有多少認識，我們所不知道也許還有更多。

　　如果有任何關於森林群落的無偏看法，也許就是這些了。

Donella H. Meadows，永續學會會長，Dartmouth大學之環境研究副教授

原文與圖片詳見: http://www.gristmagazine.com/grist/citizen/citizen071299.stm

版權歸屬 Earth Day Network，環境信託協會 (黃曉菊 譯　鄭先祐　審校)

About 40 years ago, a young Dartmouth biology professor named Herb Bormann took a tomato plant, gently pulled its roots apart into two bunches, and planted it in two pots, one clump of roots in each pot. He watered both pots until the plant got established. Then he watered only one pot. The tomato did fine.

Photo by David Parsons, NREL/PIX.

Then he repeated the experiment many times with the addition of a second plant rooted only in the unwatered pot. Some of those second plants also did fine. Others wilted, but more slowly than did control plants in unwatered pots without a "donor." Later experiments showed that one plant could pass not only water but also radioisotopes to another.

Bormann began digging around in a white pine forest near the Dartmouth campus and injecting tree roots with dyes, herbicides, and radioactive tracers. He found that the trees not only passed these markers around, but that their roots had actually grown together into complex networks of "grafts." Through the grafts, large trees tended to pass nutrients to smaller, weaker trees. They even kept alive stumps, which went on growing, sometimes for decades, though they never regenerated branches or leaves.

Bormann later wrote: "This research indicated that the role of ... competition in determining which trees in a ... stand will survive was overrated. In many stands, many trees are linked into organic unions. ... Participants in the union have lost at least part of their individuality and are subject to the ... influence of their grafted companions."

Even when Bormann did this research, the knowledge that trees interconnect below ground was not new. One of his teachers had told him how oak trees transmit diseases through root grafts. As long ago as the 1920s, root grafts had been observed in white pine, Douglas fir, and balsam fir forests, as had the phenomenon of "living stumps."

By 1966, Bormann and his colleague B.F. Graham could write a review article listing more than 180 scientific articles on the passage of materials between the roots of at least 150 species of trees, including oak, maple, linden, spruce, ash, larch, birch, fir, and pine. There are root grafts between different species -- maple and birch, birch and elm, and sandalwood and the tropical tree Eugenia. (The grafts reportedly give the Eugenia the scent of sandalwood.)

Sometimes the transfers occur through root grafts. Sometimes they just pass through the soil, as with Bormann's tomatoes. Sometimes there are inter-tree carriers, mats of cooperating fungi called mycorrhizae (pronounced mike-o-rye-zee), which intermingle with tree roots, subsist on sugars produced by the tree leaves, and in return mobilize nutrients from the soil. The most common traffic is in sugars, growth hormones, herbicides, and disease organisms. Less common are transfers of water (in large amounts) or mineral nutrients.

This underground tree trade is apparently well known to biologists, but they haven't made a big deal of it. I have taken many biology courses, but I was surprised when forest ecologist Scot Zens mentioned root grafts in a conversation a few months ago, as if I knew about them. I didn't. The very idea stopped me in my tracks. It changed my whole view of what a forest is. What? The roots grow together? The trees pass stuff around? What does that mean?

What does it mean to forest management, to selective harvesting, to clear-cutting? What does it mean when acid rain falls, when drought comes, when herbicide is sprayed, when we try to fight off tree diseases? What does it mean to our cultural notion that the world runs on competition, not cooperation? Are we biased toward seeing only collections of individuals rather than interconnected systems? Have we literally been failing to see the forest for the trees?

The scientists who blazed research trails on tree transfers, instead of following the well-trampled professional highway investigating competition, have written mainly about what root grafts mean for forestry. Cutting a tree but leaving its stump may be no help in preventing the spread of disease. Trying to kill selected trees by injecting herbicide could spread the poison to non-targeted trees. Girdling trees with strong root grafts may not kill them, at least not below the girdle. Interlocked trees are probably more resistant to wind throw but less resistant to disease. Selective cutting may produce unexpected results -- you may remove not a competitor but a partner.

I would love to draw deeper lessons than those practical ones. I have a bias opposite to the one my science professors so assiduously tried to instill in me. I prefer to believe in cooperation as a working principle, not competition. I like to think about complex interacting systems. I have an instinctive (some would say irrational) notion that the forest knows and weeps when a single tree falls. I love the notion of secret traffic among trees.
Like my scientific brethren and sistren on the opposite side of the bias, I am not blind to the social implications of lessons purportedly drawn from nature. Aha! I can say. Big trees support small trees. Trees stand together, they don't struggle against each other. When we take one away, we send waves through an interconnected community. If this is true for trees, might it not work for people?

Which is, of course, as oversimplified a picture as is the notion that the forest is nothing but the sum of its trees. There is plenty of competition in a forest. Inter-tree networks spread disease and poisons as well as nutrients and hormones. No one understands the reason -- if there is one -- why any tree shares anything with another. Nature is too complex to ratify our simple worldviews. Whatever we understand about the interconnections among trees, there is probably much more that we don't understand.

If there's any unbiased lesson to be drawn from the community of trees, that's probably it.

Donella H. Meadows is director of the Sustainability Institute and an adjunct professor of environmental studies at Dartmouth College. 

http://www.gristmagazine.com/grist/citizen/citizen071299.stm