When did plants start taking over drylands?

Field outcropping, paleosoil and plant fossils of the early Guijiatun Formation of the Qujing Devonian period in Yunnan (Photo provided by interviewee)

About 400 million years ago, in the Silurian period, vascular plants began to appear on land, and since then they have begun a great journey to conquer land and shape the surface.

How do vascular plants originate? How does radiation evolve? What impact do they have on the surface environment? These scientific questions have been explored for more than a hundred years. Long-term research has gradually formed a deep-rooted belief that early vascular plants are confined to moist habitats such as wetlands or water edges, and are not drought-resistant.

However, “the academic understanding of early vegetation is biased and inaccurate. Xue Jinzhuang, an associate professor at Peking University’s School of Earth and Space Sciences, questioned and led the team to carry out long-term retrospective and research to gain new insights about early dryland vegetation. Recently, the results of the relevant research were published in Earth-Science Reviews.

Previous fossils have come from wetlands

The conquest of land by plants is considered to be the third major biological evolution event after the origin of life and the Cambrian explosion, which profoundly changed the earth’s surface system and is one of the important milestones in the evolution of the earth’s habitability.

Relevant fossil evidence shows that the earliest land plants appeared in the middle Ordovician, to the Silurian and Devonian periods, vascular plants as the dominant group of land plants first radiation evolution, modern clades such as stone pines, true ferns, seed plants, etc. laid the foundation in this period, roots, large leaves, secondary xylem, seeds and other key traits appeared and evolved rapidly, forests have become an important component of land vegetation since the middle Devonian.

With the evolution of plant tissues, organs and taxa diversity, biochemical weathering and soil formation have increased, and important changes have taken place in biogeochemical cycles in terrestrial and coastal areas.

Although the origin, early evolution and global environmental effects of land plants have received extensive attention from the academic community, for a long time, the fossil evidence of early vascular plants mainly comes from wetlands and related environments. Wetland provides a good burial window for the preservation of plant residues and organic matter, and at the same time, the lush vegetation in the peat wetland in the geological history period has created the coal resources on which human society depends, so rich and exquisite wetland plant fossils have been the focus of paleobotanical research for more than 100 years.

However, data show that about 45% of the land surface on Earth today is dryland, supporting rich vegetation. So, when and how did early vascular plants adapt to drylands? This scientific question has not been well answered.

Trace plant fossils in red-layer paleosoil

In 2003, Xue Jinzhuang was a doctoral student at Peking University, and since then he has been investigating Devonian strata and collecting plant fossils in the Qujing area of Yunnan.

“Qujing in Yunnan is a classic area for the study of early vascular plants.” Xue Jinzhuang told China Science News, “Many years ago, when we were surveying the Early Devonian stratigraphic profile in the Qujing area, we were puzzled by the red layer (referring to the purple-red rock formation) in the stratigraphic units of the Xitun Formation, the Guijiatun Formation and the Xujiachong Formation. ”

In 2013, Professor Hao Shougang of Peking University published The Early Devonian Posongchong Flora of Yunnan, which proposed innovative ideas such as the outbreak of early Devonian vascular plant branches, including a chapter devoted to the Early Devonian terrestrial environment and the ecological habits of early vascular plants.

Inspired by this book, Xue Jinzhuang’s team has tried to study and reconstruct early land vegetation from multiple perspectives such as paleobotany, sedimentology, and burial in recent years. The team found abundant plant rhizome fossils in the red layer paleosoil of the Early Devonian of Qujing, which demonstrated the clonal growth habits of early vascular plants and their important role in promoting soil formation, enhancing geomorphological stability and promoting the formation of meandering river configurations, and the relevant research results were published in PNAS in 2016.

Since then, they have conducted more extensive research in the Qujing area, with a focus on red-bed paleosoils. “Previous studies have suggested that plant fossils in red-layer paleosoil have extremely limited burial potential, so not many people are willing to spend time looking for fossils here.” Xue Jinzhuang explains, “But we found that these ancient soils are a very unique fossil treasure trove, especially they record the information of ancient vegetation that grows in situ. ”

However, a great challenge in the research process is how to demonstrate the biological properties and taxonomic locations of plant fossils in red layer paleosoil.

In this regard, Xue Jinzhuang introduced: “On the one hand, we almost ran all over the outcrop of the Guijiatun Formation near Qujing, collecting as many fossil materials as possible, especially those fossils with clear characteristics, and striving to reveal the morphological characteristics of fossils in an all-round and multi-angle manner.” On the other hand, through comparison, we found that the K-type or H-type branching of early vascular plants is the most basic developmental unit, according to which the different preserved ferns and primitive stone pines can be linked. ”

Vascular plants successfully occupy dry land

Based on the study of the red layer of the Guijiatun Formation in the Qujing area (formed about 410 million years ago), Xue Jinzhuang’s team conclusively demonstrated that early Devonian plants had participated in the construction of dryland vegetation.

They pointed out that the main body of the Guijiatun Formation was a floodplain sedimentary of the dryland river system, and the paleosoil with a typical leaching-calcium layer structure was estimated to have an average annual precipitation of about 390 to 680 mm. These paleosoils and their in-situ plant fossils provide solid evidence for the existence of early dryland vegetation.

The Gui Jiatun Group’s research inspired them to reinterpret paleophyt-soil systems in other regions and other profiles. Based on this, the team put forward a new view, suggesting that in the early Devonian period, dryland herbaceous vegetation was more common in the middle and low latitudes of the world, consisting of cryptosporidium, leni ferns, industrial ferns or primitive stone pines, and representative strata include the Guijiatun Formation and Xujiachong Formation of Qujing in Yunnan, the Raglan Mudstone Formation of Wales, and the Wood Bay Formation of Svalbard.

“The successful occupation of drylands by vascular plants, no longer confined to wetlands or the edges of water bodies, is a critical step in the process of plant landing.” Xue Jinzhuang pointed out that in terms of geological record, it has been recognized that calcium-forming nodules (mainly developed in dryland soils) appeared in small quantities before the Devonian period, and increased sharply until the Devonian period, but previous studies ignored the vegetation information in calcareous paleosoils, and the link between vegetation and paleosoil development has not been empirical.

By geochemical estimation, the team found that the amount of inorganic carbon buried in the paleosoil of the Guijiatun Formation was close to the highest level of soil inorganic carbon content in the modern arid and semi-arid area.

For a long time, the academic research on dryland vegetation in geological history has been very low, and the source-sink effect of inorganic carbon in dryland paleosoil and its impact on the global carbon cycle have been ignored. Xue Jinzhuang’s team found that early dryland soils have significant inorganic carbon sequestration, so they play an important role in the global carbon cycle.

He revealed that in the future, the team will continue to study the coupling relationship between plant landing, climate change, surface environmental change and carbon cycle from the aspects of geological section observation and model simulation.

The research work was supported by the National Key Research and Development Program of China “Environmental Resource Effects of Plant Landing” and the National Natural Science Foundation of China. (Source: Shen Chunlei, China Science News)

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