Fossilized leaves provide clues about high CO₂ life

Lesley Evans Ogden

As atmospheric carbon dioxide (CO₂) levels continue to rise, scientists are increasingly turning to the past for clues about plant water‐use efficiency and carbon (C) storage under high CO₂ conditions. In a recent study (Clim Past 2020; doi.org/10.5194/cp‐16‐1509‐2020), a team of researchers used gas‐exchange modeling to calculate C assimilation rates and water‐use efficiency from fossilized tree leaves found on New Zealand's South Island.

In the sediments of Foulden Maar, a fossil bed near the city of Dunedin, mummified 23‐million‐year‐old leaves are so well preserved that they can blow away, according to paleobotanist Tammo Reichgelt of the University of Connecticut (Storrs, CT) and Columbia University's Lamont‐Doherty Earth Observatory (Palisades, NY). Reichgelt partnered with Daphne Lee of the University of Otago (Dunedin) and several other colleagues to reconstruct atmospheric CO₂, leaf‐level productivity, and intrinsic water‐use efficiency in early Miocene forests from the preserved leaves.

Plants lose water via stomatal openings in leaves as they absorb CO₂, trading these gains and losses by opening or closing stomata as they probe environmental CO₂ and moisture availability. In drier times or under low CO₂ conditions, plants draw on internal pools of C and assimilate a greater proportion of ¹³C isotopes; in contrast, if water and CO₂ are plentiful, leaves will contain a greater proportion of ¹²C relative to ¹³C, explains Reichgelt.

¹³C levels and stomatal conductance were determined through analysis of 72 mummified leaves extracted from 18 fossil tree species at the site's deposits. Entering their measured ¹³C levels into gas‐exchange equations, the researchers calculated atmospheric CO₂ concentrations in the early Miocene as ranging between 450 and 550 parts per million, a level similar to that projected for 2040. Despite growing in a world 5–6°C hotter – and in a rainforest 8°C warmer – than today, the Miocene trees appeared extremely water‐use efficient. “On a spectrum of modern‐day plants, they were about as water efficient as a plant growing in a semi‐arid environment like the chaparral of California”, says Reichgelt.

The results of this study suggest that elevated atmospheric CO₂ levels in the early Miocene coincided with globally higher temperatures and enhanced intrinsic plant water‐use efficiency, indicating a forest fertilization effect.

Microscopic image of stomata in a fossilized leaf sample from the Foulden Maar site, New Zealand.

T Reichgelt

中文翻译：

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派遣

化石的叶子提供了高CO ₂寿命的线索

莱斯利·埃文斯·奥格登

随着大气中二氧化碳（CO ₂）含量的不断增加，科学家越来越多地转向过去，以获取有关植物水分利用效率和高CO ₂条件下碳（C）储存的线索。在最近的一项研究中（Clim Past 2020; doi.org/10.5194/cp‐16‐1509‐2020），一组研究人员使用气体交换模型来计算从新发现的化石叶子中C的同化率和水分利用效率新西兰的南岛。

康涅狄格大学古植物学家坦莫·赖希格特（Tammo Reichgelt）表示，在但尼丁市附近的化石床Foulden Maar的沉积物中，保存着木乃伊的2300万年前的叶子被很好地保存，可以吹走。哥伦比亚大学的拉蒙特－多尔蒂地球观测站（纽约州帕里斯德）。Reichgelt与奥塔哥大学（Dunedin）的Daphne Lee和其他几位同事合作，从保存的叶片中重建了中新世早期森林的大气CO ₂，叶级生产力和内在用水效率。

植物在吸收CO _2时会通过叶片气孔开口失去水分，而在探查环境中的CO ₂和水分供应时会通过打开或关闭气孔来交换这些得失。在较干燥的时期或在低CO _2的条件下，植物利用C的内部库吸收更多的¹³ C同位素。相反，如果水和CO ₂含量很多，则树叶中所含的¹² C相对于¹³ C会更大。

通过分析现场沉积物中从18种化石树种中提取的72颗木乃伊叶子来确定¹³ C水平和气孔导度。将他们测得的¹³ C水平输入到气体交换方程中后，研究人员计算出中新世早期的大气CO ₂浓度在百万分之450至550之间，与2040年的预测水平相似。尽管在世界范围内5-6与今天相比，温度提高了摄氏度，在热带雨林中的温度升高了8摄氏度，中新世树木的用水效率极高。Reichgelt说：“在一系列现代植物中，它们的节水效果与在半干旱环境（如加利福尼亚州的丛林）中生长的植物一样节水。”

这项研究的结果表明，中新世初期大气CO ₂含量升高与全球温度升高和植物内在水分利用效率提高同时发生，表明了森林的施肥作用。

来自Foulden Maar站点的新西兰化石叶子样本中气孔的显微图像。

赖希特