Eucalyptus is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with other genera in the tribe Eucalypteae, they are commonly known as eucalypts. Plants in the genus Eucalyptus have bark that is either smooth, fibrous, hard or stringy, leaves with oil glands, and sepals and petals that are fused to form a “cap” or operculum over the stamens. The fruit is a woody capsule commonly referred to as a “gumnut”.
Most species of Eucalyptus are native to Australia, and every state and territory has representative species. About three-quarters of Australian forests are eucalypt forests. Wildfire is a feature of the Australian landscape and many eucalypt species are adapted to fire, and resprout after fire or have seeds which survive fire.
A few species are native to islands north of Australia and a smaller number are only found outside the continent. Eucalypts have been grown in plantations in many other countries because they are fast growing and have valuable timber, or can be used for pulpwood, for honey production or essential oils. In some countries, however, they have been removed because they are highly flammable.
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Adaptation to fire
Eucalypts originated between 35 and 50 million years ago, not long after Australia-New Guinea separated from Gondwana, their rise coinciding with an increase in fossil charcoal deposits (suggesting that fire was a factor even then), but they remained a minor component of the Tertiary rainforest until about 20 million years ago, when the gradual drying of the continent and depletion of soil nutrients led to the development of a more open forest type, predominantly Casuarina and Acacia species.
The two valuable timber trees, alpine ash E. delegatensis and Australian mountain ash E. regnans, are killed by fire and only regenerate from seed. The same 2003 bushfire that had little impact on forests around Canberra resulted in thousands of hectares of dead ash forests. However, a small amount of ash survived and put out new ash trees as well.
Fire hazard
Eucalyptus oil is highly flammable; ignited trees have been known to explode. Bushfires can travel easily through the oil-rich air of the tree crowns. Eucalypts obtain long-term fire survivability from their ability to regenerate from epicormic buds situated deep within their thick bark, or from lignotubers, or by producing serotinous fruits.
In seasonally dry climates oaks are often fire-resistant, particularly in open grasslands, as a grass fire is insufficient to ignite the scattered trees. In contrast, a eucalyptus forest tends to promote fire because of the volatile and highly combustible oils produced by the leaves, as well as the production of large amounts of litter high in phenolics, preventing its breakdown by fungi and thus accumulating as large amounts of dry, combustible fuel. Consequently, dense eucalypt plantings may be subject to catastrophic firestorms. In fact, almost thirty years before the Oakland firestorm of 1991, a study of eucalyptus in the area warned that the litter beneath the trees builds up very rapidly and should be regularly monitored and removed. It has been estimated that 70% of the energy released through the combustion of vegetation in the Oakland fire was due to eucalyptus. In a National Park Service study, it was found that the fuel load (in tons per acre) of non-native eucalyptus woods is almost three times as great as native oak woodland.
Falling branches
Some species of eucalyptus drop branches unexpectedly. In Australia, Parks Victoria warns campers not to camp under river red gums. Some councils in Australia such as Gosnells, Western Australia, have removed eucalypts after reports of damage from dropped branches, even in the face of lengthy, well publicised protests to protect particular trees. A former Australian National Botanic Gardens director and consulting arborist, Robert Boden, has been quoted referring to “summer branch drop”. Dropping of branches is recognised in Australia literature through the fictional death of Judy in Seven Little Australians. Although all large trees can drop branches, the density of eucalyptus wood is high due to its high resin content, increasing the hazard.
ユーカリ
https://ja.wikipedia.org/wiki/ユーカリ
Eucalyptus
https://en.wikipedia.org/wiki/Eucalyptus
Eucalyptus is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with other genera in the tribe Eucalypteae, they are commonly known as eucalypts. Plants in the genus Eucalyptus have bark that is either smooth, fibrous, hard or stringy, leaves with oil glands, and sepals and petals that are fused to form a “cap” or operculum over the stamens. The fruit is a woody capsule commonly referred to as a “gumnut”.
Most species of Eucalyptus are native to Australia, and every state and territory has representative species. About three-quarters of Australian forests are eucalypt forests. Wildfire is a feature of the Australian landscape and many eucalypt species are adapted to fire, and resprout after fire or have seeds which survive fire.
A few species are native to islands north of Australia and a smaller number are only found outside the continent. Eucalypts have been grown in plantations in many other countries because they are fast growing and have valuable timber, or can be used for pulpwood, for honey production or essential oils. In some countries, however, they have been removed because they are highly flammable.
**
Adaptation to fire
Eucalypts originated between 35 and 50 million years ago, not long after Australia-New Guinea separated from Gondwana, their rise coinciding with an increase in fossil charcoal deposits (suggesting that fire was a factor even then), but they remained a minor component of the Tertiary rainforest until about 20 million years ago, when the gradual drying of the continent and depletion of soil nutrients led to the development of a more open forest type, predominantly Casuarina and Acacia species.
The two valuable timber trees, alpine ash E. delegatensis and Australian mountain ash E. regnans, are killed by fire and only regenerate from seed. The same 2003 bushfire that had little impact on forests around Canberra resulted in thousands of hectares of dead ash forests. However, a small amount of ash survived and put out new ash trees as well.
Fire hazard
Eucalyptus oil is highly flammable; ignited trees have been known to explode. Bushfires can travel easily through the oil-rich air of the tree crowns. Eucalypts obtain long-term fire survivability from their ability to regenerate from epicormic buds situated deep within their thick bark, or from lignotubers, or by producing serotinous fruits.
In seasonally dry climates oaks are often fire-resistant, particularly in open grasslands, as a grass fire is insufficient to ignite the scattered trees. In contrast, a eucalyptus forest tends to promote fire because of the volatile and highly combustible oils produced by the leaves, as well as the production of large amounts of litter high in phenolics, preventing its breakdown by fungi and thus accumulating as large amounts of dry, combustible fuel. Consequently, dense eucalypt plantings may be subject to catastrophic firestorms. In fact, almost thirty years before the Oakland firestorm of 1991, a study of eucalyptus in the area warned that the litter beneath the trees builds up very rapidly and should be regularly monitored and removed. It has been estimated that 70% of the energy released through the combustion of vegetation in the Oakland fire was due to eucalyptus. In a National Park Service study, it was found that the fuel load (in tons per acre) of non-native eucalyptus woods is almost three times as great as native oak woodland.
Falling branches
Some species of eucalyptus drop branches unexpectedly. In Australia, Parks Victoria warns campers not to camp under river red gums. Some councils in Australia such as Gosnells, Western Australia, have removed eucalypts after reports of damage from dropped branches, even in the face of lengthy, well publicised protests to protect particular trees. A former Australian National Botanic Gardens director and consulting arborist, Robert Boden, has been quoted referring to “summer branch drop”. Dropping of branches is recognised in Australia literature through the fictional death of Judy in Seven Little Australians. Although all large trees can drop branches, the density of eucalyptus wood is high due to its high resin content, increasing the hazard.
みんなのひろば
植物Q&A
自然発火の仕組み
日本植物生理学会
https://jspp.org/hiroba/q_and_a/detail.html?id=1920
JSPPサイエンス・アドバイザー
柴岡 弘郎
オーストラリアにはブルーマウンテンと呼ばれる遠くから見ると青く見える山があります。
この青色はユーカリの葉から空気中に放出されたテルペンによるものです。
テルペンの放出量は高温で多くなるので、夏期にはユーカリの林の中のテルペン濃度はかなり上昇します。テルペンは引火性の物質なので、何かの原因で発火した場合山火事を引き起こす元になります。
しかし、故意に自発的発火させている訳ではありません。植物は自分で動くことが出来ませんから、故意に発火させようとしても、させることは出来ないのです。
ユーカリの林の中に生えているヤマモガシ科のバンクシアという植物の種子は山火事に会わないと発芽しないことで有名ですが、この植物の種子を発芽させるために、ユーカリが自発的に山火事を起して自分の林を破壊するとはとても考えられません。
マツの仲間の葉もテルペンを放出します。カナダの平原を覆っているマツ林でも自然発火と見られる山火事があり、成熟したマツを枯らしていますが、その後に生えて来るのは、マツではなくてポプラの仲間とかシラカバの仲間のような広葉樹です。マツは広葉樹の林の林床で発芽し、やがて広葉樹を追いこして再びマツ林を再生させますが、それにはかなりの年月が必要です。故意に自発発火して山火事をおこしているとはとても考えられないでしょう。意識を持たない植物が故意に何かをすることは無いと思って下さい。
林の下の土の中で、発芽せずにじっとしていて、山火事の高温に会うと発芽してくる植物の身近な例はウルシ科のヌルデです。
すごい自然のショールーム 火事と共に生きるユーカリ
ネイチャーテック研究会, 東北大学大学院 環境科学研究科
http://nature-sr.com/index.php?Page=11&Item=93
火事と共に生きるユーカリ
なにがすごいの?
オーストラリアの森の4分の3を占めるのがユーカリの樹です。
ユーカリの葉を食べているコアラの姿は、とても愛くるしいものです。
オーストラリアは1年を通してとても乾燥した地域で、山火事が頻繁に起こります。
ユーカリのつぼみは、葉などから作られる成長を抑制する物質により発芽が抑えられています。
山火事の炎によって、葉が焼け落ちることにより、成長を抑制する物質がなくなり、ユーカリのつぼみは発芽します。
火災直後のユーカリの緑の葉は、この新しい後生芽から生産されています。
ユーカリの樹が、生長していく過程でも山火事は発生します。
他の樹木と違って、ユーカリは火事のときも自分を守ることができます。
幹の燃えやすさが、幹の外側の皮(樹皮)と内側にある層(形成層)では異なるのです。
樹皮が非常に燃えやすい性質を持ち、樹皮に火がつくと幹から剥がれ落ちるので、幹の内側まで火が燃え移ることはありません。
樹皮が犠牲となり、形成層とよばれる、樹木にとって大切な部分を守られるのです。
また、ユーカリの根っこの部分には栄養源や、芽の原料を蓄えています。
こうして、火災の後も蓄えられた栄養源により成長し続けることができ、また新しい芽をつけることもできます。
どうやって役立てるの?
ユーカリは火を使って、自分を発芽させる一方、成長した段階では火から体を守るという複数の火に対するメカニズムを体内に持っています。
その戦略は、火災との関わりあい方を私たちに教えてくれます。
どんな研究をしているの?
ユーカリには、普段は葉や枝から作られる物質(成長抑制物質)によって発芽が抑えられているつぼみが存在することがわかりました。
炎によって、葉や枝が焼け落ちてしまうと、発芽を抑えられていたつぼみが発芽します。
こうして火災直後にも、生きるのに必要なエネルギーを作りだす光合成を行うことができることが分かりました。
どんな技術開発ができるの?
ユーカリの木がうまく火を利用し、また火によってさらに生命力を高めるというシステムを私たちの生活の中に取り入れることができるかもしれません。
ユーカリは、火に対して1つ以上の反応システムを持っていて、それを様々な場面で使い分けています。
火災の色々な段階で、反応できるシステムをもった新しい火災報知機や消火器を開発できるかもしれません。
またユーカリのように火を利用できるような機能も備えた機器が開発できれば、火災はただの敵ではなくなりますね。
例えば火災が起きてしまったときに、火のエネルギーで消火器が動けばユーカリ的戦略と言えるかもしれません。