ハワイ山火事、被害拡大の原因は「侵略的外来植物」

【ワシントンＡＦＰ＝時事】米ハワイ州マウイ島で１００人以上の死者を出す大惨事となった山火事。大火となった原因として注目されているのが、過去数十年でハワイ諸島にはびこるようになった侵略的外来種の植物だ。（写真は米アリゾナ州トゥーソンで枯れたまま残る侵略的外来種ヒゲクリノイガ〈資料写真〉）
　外来種の植物は干ばつに強く、複雑な地形にも入り込み、固有種を徐々に駆逐していく。同じく大規模な山火事が多い米国西部でも脅威を増している。
　「侵略的外来種の植物は景観を一変させてしまう。そして燃えやすい」と指摘するのは、カリフォルニア大学サンタバーバラ校の生態学者、カーラ・ダントニオ教授だ。「火が発生しやすい状況を作り出し、そして突然、火災が多発する」
　ヒゲクリノイガ、ギネアキビ（ギニアグラス）、トミツグラス（トウミツソウ）といった外来種の草はほとんどがアフリカ原産。牧草として導入された当時は、数十年後に及ぼす危険性など知るよしもなかった。
　こうした外来種は枯れても腐らず、「骨のように乾燥した状態で長期間、立ち枯れたままだ」とダントニオ氏は説明する。また固有種よりもたくましく、火災にもよく耐え、徐々に固有種と置き換わっていく。
　ハワイではグローバル化の影響で１９９０年代にサトウキビ・プランテーションが廃止され、土地が放棄されたために、外来種の侵入を許す広大なスペースが生まれた。
　ハワイ大学マノア校の火災生態学者クレイ・トラオアーニヒト氏は「ハワイでも乾燥化は進んでいるが、火災問題のほとんどは『ポスト・プランテーションの時代』に入って、土地所有者が放置した土地が外来種の草原になってしまったことに起因する」と指摘する。
　同氏によれば、ハワイの年間焼失面積はここ数十年で４倍になっている。
　マウイ郡は報告書で「危険な燃料源」となっている外来種をハワイ固有の植物に再び置き換え、「発火の可能性を減らすと同時に土地の保水力を高める」計画を推奨している。
　■米本土でも問題の外来種
　問題はハワイに限ったことではない。米国本土でも「西部の砂漠や針葉樹林、沿岸部の低木林などに侵略的外来種のイネ科植物が生い茂り、生態系の一部と化している」とダントニオ教授は指摘する。
　教授自身、毎週土曜の夕方にはカリフォルニア州サンタバーバラ近郊の山間部で、近所の住民と道端の除草活動をしている。たばこの吸い殻や車のオーバーヒートによる火災を防ぐためだ。２０１８年に８０人以上の死者を出した山火事は、送電線が乾燥した草に接触し発火した。
　アリゾナ州のサワロ国立公園を象徴するサボテンも、外来植物のヒゲグリノイガに脅かされている。この草がびっしりと生えてしまい、サボテンの芽が育たない上、山火事が助長されているのだ。
　２０１９年の研究によると米国では、侵略的外来種６種のせいで山火事の頻度が最大２．５倍となった。
　ダントニオ氏は外来種の侵入に加え、人為的な景観の改変、気候変動によって悪化した干ばつ、さらに災害に対する準備不足がハワイの山火事のような悲劇を引き起こしているという。「平均的な火災ではなく、極度の火災にどう備えるかが大事だ」【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2023/08/21-16:31）

After a catastrophic wildfire that killed more than 100 people in Hawaii, eyes have turned toward an unexpected culprit: invasive grass species that have spread massively over the archipelago for decades, serving as the perfect fuel.
Drought-resistant, capable of invading difficult terrain, and gradually muscling out local species, they are also a growing threat in the western United States, where devastating fires are increasing.
Invasive grasses are very ignitable. They change the landscape, Carla D'Antonio, a professor of ecology at the University of California, Santa Barbara told AFP.
They make conditions that are more conducive to more fire, and all of a sudden, we just have a lot more fire.
Rather than decomposing when they die, they stay standing there for a long time, dry as a bone, said D'Antonio, who has been studying these species for more than 30 years. They're also hardy, surviving fires better than native species and gradually replacing them.
Most of these grasses -- buffelgrass, Guinea grass, molasses grass -- came from Africa, and were introduced as pasture for cattle, without knowing the danger they would come to represent decades later.
In Hawaii, the demise of sugar cane plantations in the 1990s as a result of globalization had disastrous consequences: huge tracts of land were abandoned, allowing the invasive species an opening.
Yes, many parts of Hawaii are trending towards dryer conditions, but the fire problem is mostly attributable to the vast extents of non-native grasslands left unmanaged by large landowners as we've entered a 'post-plantation era,' said Clay Trauernicht, a fire ecologist at the University of Hawaii at Manoa.
Trauernicht said the annual area burned in Hawaii has increased by 300 percent in recent decades.
A 2021 fire prevention report by Maui County described fires as a growing threat due to increasing temperatures and prolonged periods of drought as a result of climate change, and the growing menace of intrusive grasses.
Hawaii, despite its tropical reputation, is getting drier: a 2016 study found 90 percent of the state received less rain compared to a century earlier.
The Maui County report recommended an aggressive plan to replace these hazardous fuel sources with native plants to reduce combustible fuel while increasing water retention.
- 'Nothing natural about it' -
The problem isn't confined to Hawaii. Over in the mainland United States, the deserts of the West and the conifer forests, and then the shrub lands in the coastal zone, invasive grasses are here to stay, they're now part of the ecosystem, said D'Antonio.
She herself spends some Saturday evenings weeding roadsides with neighbors in a mountainous area near Santa Barbara, California. Their goal: to prevent a fire from starting from a cigarette butt or an overheating vehicle.
Most of the major fires of the Mojave and Great Basin have been fueled by invasive grasses, she says, while also citing the Camp Fire of 2018, which destroyed the small California town of Paradise, killing more than 80 people. It was started by a power line igniting dry grass.
(I'm) not making the mistake of calling it a natural disaster because there's almost nothing natural about it, emphasizes the scientist.
One of the invaders, buffelgrass, also threatens the emblematic cactus of the Saguaro National Park in Arizona, by smothering young saguaros and fueling fires in the region. Organizations regularly organize clearing operations. The same species is spreading in Mexico and in Australia.
According to a 2019 study, six invasive grass species caused fire frequency to increase by up to 150 percent in US ecosystems.
For D'Antonio of UC Santa Barbara, tragedies like that of Hawaii are linked to many factors: the alteration of the landscape by humans, the invasion of alien species, droughts made worse by climate change, but also a lack of preparation.
In the American West, widespread logging of conifer forests in the 19th century and a long history of excessive fire suppression in the 20th century contributed to accumulation of tinder on the forest floor.
The potential for disaster is huge, said D'Antonio, leaving society with daunting questions to address. How do we plan for the extreme? Not for the average fire, but the extreme fire?