原文網址：https://stri.si.edu/story/flowers
6600萬年前，一顆巨大的小行星撞上今日的猶加敦半島，造成地球陷入一片黑暗當中。但是這場撞擊事件也改變了熱帶雨林，促使花朵得以稱王。
現今的熱帶雨林是生物多樣性的熱點，同時在全球氣候系統當中也佔有舉足輕重的地位。今日發表於《科學》(Science)的新研究闡明了現代雨林的起源，或許也能讓科學家更加了解雨林會如何應對未來快速變遷的氣候。
這項史密森尼熱帶研究所(STRI)的科學家主持的研究顯示，6600萬年前終結恐龍統治的小行星撞擊事件，也讓現今哥倫比亞所在之處45%的植物消失，這使得開花植物得以佔據空出來的位置，進而主宰現今的熱帶雨林。
「我們想要知道希克蘇魯伯撞擊事件這類能對生態系造成劇烈擾動的事件發生之後，熱帶雨林會產生什麼樣的變化，因此我們透過熱帶植物的化石來尋找答案，」第一作者Mónica
Carvalho表示，她也是史密森尼熱帶研究所和哥倫比亞羅薩里奧大學的合聘博士後研究員。「我們的團隊檢視了撞擊前後超過50000筆的花粉化石紀錄以及超過6000具的葉片化石。」
現今主宰美洲熱帶地區的開花植物的演化過程當中，這起滅絕事件有什麼樣的影響？在此研究之前，科學家的所知並不多。為了得到答案，地質學家前往中南美洲，從道路開挖處與礦坑尋找重見天日的化石，他們進行的速度必須很快，否則化石就會被大雨沖走或者被叢林再次隱藏起來。
這組研究團隊的主持人為STRI的古植物學家Carlos
Jaramillo，團隊成員大多數為STRI的同僚，其中許多是哥倫比亞人。他們研究的花粉顆粒取自哥倫比亞39個地方的岩石露頭以及從油井探勘鑽取出來的岩芯，這讓他們能從更大的區域性角度來呈現出森林在撞擊前後的變化。比撞擊事件更老的岩石中取得的花粉與孢子，顯示當時的雨林裡蕨類與開花植物的勢力大致相等。當時十分常見的裸子植物，像是聖誕節超市販賣的貝殼杉與小葉南洋杉的近親(南洋杉屬，Araucariaceae)，會在恐龍行經的路徑上投下它們的陰影。撞擊事件之後，裸子植物幾乎從新世界的熱帶地區徹底消失，而由開花植物接手。植物多樣性大概在撞擊事件後的一千萬年之間都沒有完全恢復過來。
葉片化石則告訴團隊更多有關過去氣候與區域環境的資訊。Fabiany
Herrera是芝加哥植物園的尼哥尼保育科學與行動研究所的博士後研究員，她和Carvalho主持的研究分析了超過6000具葉片化石樣本。在和史密森尼美國國立自然史博物館的Scott
Wing與其他人員合作之下，團隊發現的證據顯示撞擊事件之前熱帶雨林的樹木彼此之間分隔較遠，使得陽光可以照射到森林地表。在撞擊事件過後的一千萬年之間，某些熱帶森林變得較為濃密，就和今日一樣，樹葉與藤蔓會形成深厚的陰影，遮住下方較小的樹木、灌木以及草本植物。和撞擊事件之後數百萬年所形成的森林相比，撞擊之前開花植物較少、樹冠層較為稀疏的森林也比較不會把土壤中的水運到大氣層。
「雖然白堊紀時這裡依然多雨，但是森林的運作方式卻不一樣了，」Carvalho表示。
團隊在滅絕事件之前並沒有找到豆科樹木存在的證據，但在事件之後卻找到了數量與種類皆相當豐富的豆科葉片與豆莢。豆科植物在現在的熱帶雨林當中是主要的類型之一，它們透過跟細菌的合作關係，可以把空氣裡面的氮轉變成土壤中的肥料。豆科植物的崛起勢必會對氮循環產生劇烈影響。
Carvalho也和史密森尼美國國立自然史博物館的Conrad
Labandeira一起研究了葉片化石上由昆蟲造成的損害。
「植物受到的蟲害可以具體而微地呈現在一片葉子上，或者推展成整個植物群集的狀況，後者是熱帶森林營養結構的基礎，」Labandeira表示。「儲存在植物組織內的能量會透過食物鏈一路往上傳，最終到達蟒蛇、老鷹和美洲豹，而這一切的開端來自於昆蟲啃咬、咀嚼、刺穿、吸取、挖掘、磨損、蛀入植物組織的行為。要了解這種由消費者構成的食物鏈，首先要找到的證據便是這些昆蟲利用各式各樣攝取植物的方式，所留下密密麻麻且引人入勝的痕跡。」
「在撞擊之前，我們看到不同類型的植物受到的損害也不同：也就是食物和宿主之間有專一性，」Carvalho表示。「撞擊過後，我們發現幾乎所有植物身上都有同種類型的損害，代表它們的食物變得更加廣泛。」
撞擊事件造成了什麼樣的影響，才會讓恐龍時代稀疏且富含針葉樹的熱帶森林，轉變成今日的森林是由高聳的樹木組成，其中點綴著黃色、紫色、粉紅色的花朵，且有蘭花懸掛在樹梢？根據花粉和葉子得出的證據，團隊提出了三種說法來解釋這些改變，不過它們有可能都是對的。第一種想法是撞擊之前恐龍會吃掉樹木並且穿梭在森林當中，使得林地保持開闊。第二種解釋為撞擊產生的灰燼落下之後讓熱帶地區的土壤變得更加深厚，有利於成長快速的開花植物。第三種解釋則認為針葉樹的滅亡狀況較為嚴重，使得開花植物乘機接管熱帶。
「我們的研究依循的問題十分簡單：熱帶森林是如何演變的？」Carvalho表示。「我們這裡學到的是從地質時間來看，在快速的擾動之下熱帶生態系並不會完全回復原狀，它們之中的成員會遭到取代，而整個過程需要相當漫長的時間。」
How the Chicxulub impactor gave rise
to modern rainforests
About 66 million years ago, a huge
asteroid crashed into what is now the Yucatan, plunging the Earth into darkness.
The impact transformed tropical rainforests, giving rise to the reign of
flowers.
Tropical rainforests today are biodiversity hotspots
and play an important role in the world’s climate systems. A new study
published today in Science sheds
light on the origins of modern rainforests and may help scientists understand
how rainforests will respond to a rapidly changing climate in the future.
The study led by researchers at the Smithsonian
Tropical Research Institute (STRI) shows that the asteroid impact that ended
the reign of dinosaurs 66 million years ago also caused 45% of plants in what
is now Colombia to go extinct, and it made way for the reign of flowering
plants in modern tropical rainforests.
“We wondered how tropical rainforests changed after a
drastic ecological perturbation such as the Chicxulub impact, so we looked for
tropical plant fossils,” said Mónica Carvalho, first author and joint
postdoctoral fellow at STRI and at the Universidad del Rosario in Colombia.
“Our team examined over 50,000 fossil pollen records and more than 6,000 leaf
fossils from before and after the impact.”
In Central and South America, geologists hustle to
find fossils exposed by road cuts and mines before heavy rains wash them away
and the jungle hides them again. Before this study, little was known about the
effect of this extinction on the evolution of flowering plants that now
dominate the American tropics.
Carlos Jaramillo, staff paleontologist at STRI and
his team, mostly STRI fellows—many of them from Colombia—studied pollen grains
from 39 sites that include rock outcrops and cores drilled for oil exploration
in Colombia, to paint a big, regional picture of forests before and after the
impact. Pollen and spores obtained from rocks older than the impact show that
rainforests were equally dominated by ferns and flowering plants. Conifers,
such as relatives of the of the Kauri pine and Norfolk Island pine, sold in
supermarkets at Christmas time (Araucariaceae), were common and cast their
shadows over dinosaur trails. After the impact, conifers disappeared almost
completely from the New World tropics, and flowering plants took over. Plant diversity
did not recover for around 10 million years after the impact.
Leaf fossils told the team much about the past
climate and local environment. Carvalho and Fabiany Herrera, postdoctoral
research associate at the Negaunee Institute for Conservation Science and
Action at the Chicago Botanic Garden, led the study of over 6,000 specimens.
Working with Scott Wing at the Smithsonian’s National Museum of Natural History
and others, the team found evidence that pre-impact tropical forest trees were
spaced far apart, allowing light to reach the forest floor. Within 10 million
years post-impact, some tropical forests were dense, like those of today, where
leaves of trees and vines cast deep shade on the smaller trees, bushes and
herbaceous plants below. The sparser canopies of the pre-impact forests, with
fewer flowering plants, would have moved less soil water into the atmosphere
than did those that grew up in the millions of years afterward.
“It was just as rainy back in the Cretaceous, but the
forests worked differently.” Carvalho said.
The team found no evidence of legume trees before the
extinction event, but afterward there was a great diversity and abundance of
legume leaves and pods. Today, legumes are a dominant family in tropical
rainforests, and through associations with bacteria, take nitrogen from the air
and turn it into fertilizer for the soil. The rise of legumes would have
dramatically affected the nitrogen cycle.
Carvalho also worked with Conrad Labandeira at the
Smithsonian’s National Museum of Natural History to study insect damage on the
leaf fossils.
“Insect damage on plants can reveal in the microcosm
of a single leaf or the expanse of a plant community, the base of the trophic
structure in a tropical forest,” Labandeira said. “The energy residing in the
mass of plant tissues that is transmitted up the food chain—ultimately to the boas,
eagles and jaguars—starts with the insects that skeletonize, chew, pierce and
suck, mine, gall and bore through plant tissues. The evidence for this consumer
food chain begins with all the diverse, intensive and fascinating ways that
insects consume plants.”
“Before the
impact, we see that different types of plants have different damage: feeding
was host-specific,” Carvalho said. “After the impact, we find the same kinds of
damage on almost every plant, meaning that feeding was much more generalistic.”
How did the after effects of the impact transform
sparse, conifer-rich tropical forests of the dinosaur age into the rainforests
of today—towering trees dotted with yellow, purple and pink blossoms, dripping
with orchids? Based on evidence from both pollen and leaves, the team proposes
three explanations for the change, all of which may be correct. One idea is
that dinosaurs kept pre-impact forests open by feeding and moving through the
landscape. A second explanation is that falling ash from the impact enriched
soils throughout the tropics, giving an advantage to the faster-growing
flowering plants. The third explanation is that preferential extinction of
conifer species created an opportunity for flowering plants to take over the
tropics.
“Our study follows a simple question: How do tropical
rainforests evolve?” Carvalho said. “The lesson learned here is that under
rapid disturbances—geologically speaking—tropical ecosystems do not just bounce
back; they are replaced, and the process takes a really long time.”
原始論文：Mónica R.
Carvalho, Carlos Jaramillo, Felipe de la Parra, Dayenari Caballero-Rodríguez,
Fabiany Herrera, Scott Wing, Benjamin L. Turner, Carlos D’Apolito, Millerlandy
Romero-Báez, Paula Narváez, Camila Martínez, Mauricio Gutierrez, Conrad
Labandeira, German Bayona, Milton Rueda, Manuel Paez-Reyes, Dairon Cárdenas,
Álvaro Duque, James L. Crowley, Carlos Santos, Daniele Silvestro. Extinction
at the end-Cretaceous and the origin of modern Neotropical rainforests. Science,
2021; 372 (6537): 63 DOI: 10.1126/science.abf1969
引用自：Smithsonian Tropical Research Institute.
"How the Chicxulub impactor gave rise to modern rainforests."
沒有留言:
張貼留言