The colonization of drylands by early vascular plants: Evidence from Early Devonian fossil soils and in situ plant traces from South China.

The mid-Paleozoic colonization of land by plants induced profound changes in the Earth's surface environments. However, it remains poorly understood how the earliest vascular plants, arising during the Silurian and Devonian periods, adapted to and functioned in ancient drylands. Today, drylands occupy ~45% of the Earth's land surface and support diverse ecosystems forming the Earth's most extensive biome. By contrast, only sparse records of dryland systems have been documented from Silurian- and Devonian-aged deposits, and furthermore, in most cases, the evidence for recognizing major plant types is compromised by poor preservation potential of organic matter in such dry environments. Here, we recognize an Early Devonian dryland river system represented by the Guijiatun Formation, Qujing, Yunnan, South China, based on an integrative study combining paleopedology, geochemistry, and paleobotany. This formation contains diverse and abundant redbed calcareous paleosols, most being classified as Calcisols. Such paleosols are interpreted as developing within well-drained landscapes that experienced a semiarid climate with seasonal wet-dry cycles, and are geochemically characterized by the subsurface accumulation and reorganization of pedogenic carbonates. Extensive plant traces are preserved in most of the paleosols observed in the Guijiatun Formation and show K- or H-shaped branching structures that are morphometrically comparable to the belowground rhizomes of zosterophyllopsids and early lycopsids. These plant-bearing paleosols provide direct evidence for the development of primitive vegetation on drylands. The soil inorganic carbon (SIC) contents stored as Ca-Mg carbonates in the Guijiatun paleosols are estimated at 38 ± 8 g C kg−1, comparable to the highest levels of present-day dryland ecosystems. We argue that dryland floras could have been a significant component of the Early Devonian landscapes, controlling the retention of both mobile elements (Ca and Mg) and fine-grained sediment on land, and mantling and protecting buried soil carbon against erosion. [ABSTRACT FROM AUTHOR]