Devonian forests could contribute greatly to CO 2 decline and coastal consolidation

Guangdedendron may be monocarpic and dioecious, showing the earliest stigmarian root

The earliest forests in the Devonian can be very large and potentially abundant

Since the Late Paleozoic, forests have become distributed worldwide and significantly changed the Earth’s climate and landscapes, but the record of forests is rare in the Devonian (419–359 Ma in age) when they first appeared. From the Upper Devonian (Famennian with the age of 372–359 Ma) of Xinhang, Anhui, China, we report a very large in situ forest, which includes locally dense stands of lycopsid plants. The Xinhang forest is monospecific with a small tree lycopsid Guangdedendron gen. nov., probably dioecious with monocarpic reproduction. The plant shows the earliest stigmarian rooting system typical of giant tree lycopsids dominating Carboniferous forests. It colonizes coastal clastic wetlands that were influenced by floods. This significantly increases the paleogeographical coverage of in situ Devonian forests, and contributes to our understanding of atmospheric CO 2 decline and coastal consolidation.

Here, we record the earliest forest of Asia, which is exceptionally large in outcrop, with small and isoetalean lycopsid trees of locally high density. This forest contributes to our understanding of the reproductive mode, growth architecture, and evolution of the rooting system in early lycopsids. We also discuss the relationship between Devonian forests and environments, including habitat, climate, and landscape.

The arborescent lycopsids (extinct trees) originated in the Late Devonian and dominated Carboniferous and Permian lowland environments, especially wetland habitats such as tropical swamps []. These plants, termed rhizomorphic lycopsids, are placed in the Isoëtales sensu lato (the most derived lycopsid clade) and represented by herbaceous Isoetes as their only extant relative []. The rhizomorph is a root-producing subterranean structure of the isoetaleans, and the Stigmaria-type or stigmarian rhizomorph extends out from the stem base, dichotomizes, and bears laterally arranged rootlets in helices []. The stigmarian rhizomorph characterizes the arborescent lycopsids, but its early evolution in the Devonian has long been obscure due to the paucity of fossil record []. Although a lycopsid forest had been documented in the Late Devonian, the plant growth is localized, and some traits including reproduction and habit are unclear [].

Isoetalean lycopsid evolution: from the Devonian to the present.

Comparative ecology and life-history biology of arborescent lycopsids in Late Carboniferous swamps of Euramerica.

During the Devonian, the upland forests with roots had greatly accelerated the atmospheric COdrawdown, leading the Earth into a major icehouse [], and the river banks were stabilized by the root development of riparian vegetation []. Trees originated in the Mid-Late Devonian when they were abundant and include three main types, i.e., archaeopteridalean progymnosperms, fernlike plants, and lycopsids (club mosses) []. However, the earliest forests are rare in the Devonian, limited to Euramerica [], relatively small in exposed surface areas, and their relationship with environments needs to be known in more detail. By contrast, the Carboniferous (Pennsylvanian Subperiod) forests yielding extensive coal seams are common (including hundreds of sites), even exposed in hectares and among the best-understood Phanerozoic terrestrial ecosystems [].

Environmental and ecological variability of Middle Devonian (Givetian) forests in Appalachian basin paleosols, New York, United States.

Belowground rhizomes in paleosols: The hidden half of an Early Devonian vascular plant.

The carbon cycle and CO 2 over Phanerozoic time: the role of land plants.

Results

0 deposits [ 18 DiMichele W.A.

Falcon-Lang H.J. Pennsylvanian ‘fossil forests’ in growth position (T0 assemblages): origin, taphonomic bias and palaeoecological insights. 19 Li X.X.

Cai C.Y.

Ouyang S. Recent advances in the study of the Wutong Formation of the Lower Yangtze Valley. 19 Li X.X.

Cai C.Y.

Ouyang S. Recent advances in the study of the Wutong Formation of the Lower Yangtze Valley. 20 Gao L.D. The Late Devonian–Early Carboniferous miospore zonation in the Lower Yangtze Valley, China and the Devonian–Carboniferous boundary. Figure 1 Location, Distribution, and Stratigraphy of the Fossil Forest Show full caption (A) Locality at Xinhang, Guangde of Anhui. (B) Distribution of fossils at two mines near Jianchuan village. The arrow indicates “isolated” in situ stems at west corner of Jianchuan mine. The rectangle indicates part enlarged in (C). (C) Occurrence of lycopsid rhizomorphs or erect stems in Wutong Formation (Leigutai Member) at west and east quarries of Jianchuan mine. The quarry boundaries were extended from 2009 to 2018, when the plant fossils were recorded in their exact positions by mapping. Symbols for plants indicate only their positions, but the plant sizes are not to scale. (D) Stratigraphic column of parts of the Leigutai Member, showing horizons of stems and/or rhizomorphs of in situ lycopsid. Abbreviations are as follows: WQ, west quarry; EQ, east quarry; T (m), thickness (meters). See also Figures S1–S7 and Data S1 The fossil forest occurs mainly in the Jianchuan clay mine near Jianchuan village, Xinhang Town, Guangde County, southeastern Anhui Province, China ( Figures 1 A and 1B ). The mine includes a west quarry (WQ) and east quarry (EQ), where most in situ lycopsid plants were buried in growth position ( Figures 1 B and 1C) and thus indicate Tdeposits []. The forest was also found in Yongchuan clay mine, which is ca. 1 km northwest of Jianchuan mine and preserves some in situ lycopsids ( Figure 1 B). The outcrops in these two mines represent the Wutong Formation. This formation is widespread in the Lower Yangtze Valley, including Anhui Province, and comprises two parts [], i.e., Leigutai Member characterized by interbedded quartz sandstone and mudstone ( Figure 1 D) and underlying Guanshan Member mainly with quartz sandstone. Paleontological evidence, including spore zonation, indicates that the Wutong Formation (perhaps excluding the uppermost part of Leigutai Member) is Famennian (Late Devonian) []. At Jianchuan mine, Archaeopteris, a ubiquitous Late Devonian progymnosperm plant, was discovered in the lower part of Leigutai Member with upper part missing and 60-meter-thick strata.

2 (gray-blue part in 2 at Yongchuan mine (gray-blue part in 2. At Jianchuan and Yongchuan mines, the plant-bearing strata of the Leigutai Member have an inclination angle of ca. 7°. Based on this small angle, boundary of strata and distribution of in situ lycopsids at WQ and EQ, the minimum area of the fossil forest at Jianchuan mine is estimated as ca. 200,000 m(gray-blue part in Figures 1 B and 1C). The in situ lycopsids also occur at west corner of Jianchuan mine ( Figure 1 B, arrow). The exposed area of the fossil forest is ca. 50,000 mat Yongchuan mine (gray-blue part in Figure 1 B). Therefore, the Xinhang forest is at least 250,000 m

Vegetative leaves, 2.0–(4.5)–9.2 cm long and 1.2–(3.0)–4.5 mm wide, are linear and have entire margins ( Figures 2 G, 4 N, 5 A, 5C, 5F, and 5G). Leaf bases are helically arranged in parastichies on many stems or branches (e.g., Figures 4 K–4M, 4O, 5 A, and S6 G). The angle between the parastichy and transverse plane is 40°–65°. Leaf bases are closely disposed and narrowfusiform in shape.