Influence of plant terrestrialization on coal accumulation and deep time terrestrial carbon storage

Plant terrestrialization—the early evolution of terrestrial vegetation originating, establishing, and expanding during the Silurian-Carboniferous—was a critical event in the evolution of the terrestrial biosphere that shaped the way to a habitable Earth. Under favorable paleobotanical, paleoclimatic, paleogeographical, and paleotectonic conditions, large quantities of plant remains can accumulate as peat in wetlands, forming a key component of the global carbon cycle, a major terrestrial carbon sink, and eventually coal deposits. However, the direct influence of terrestrialization on coal accumulation and coal quality remains unclear. This review examines the synergistic interplay between the evolutionary characteristics of coal-forming vegetation across the Rhyniophytic, Eophytic and Paleophytic Terrestrialization Phases and global coal accumulation, focusing on coal composition and seam thickness. In the Silurian radiation prelude, plants were mostly aquatic algae and early vascular plants (e.g., Cooksonia, Rhynia) inhabiting near-water environments, with poorly developed vascular tissues and low biomass. No terrestrial coal accumulated. In the Rhyniophytic to early Eophytic Floras, early vascular plants still dominated, but body sizes and biomass increased, and organs including roots and primitive small leaves evolved. Coal-bearing sediments were sporadically distributed along equatorial tropical coastlines and in shallow marine environments, composed predominantly of detrital organic matter derived. In the late Eophytic to early Paleophytic floras, evolution of secondary xylem (wood), arborescence, and concentrated root systems increased biomass, with peat accumulating from higher plants in equatorial tropical coastal plains and forming sporadically mineable humic coals with ∼50% liptinite. The early Paleophytic Flora witnessed the evolution of deep root systems and the seed habit, enabling plants to gradually occupy drier environments. Peat accumulation intensified with humic coals dominated by liptinite (∼70%) widespread in equatorial coastal plains. In the middle Paleophytic Flora, lycopods, sphenopsids, ferns and pteridosperms further developed, and arborescence became more prevalent, with industrially significant humic coals (vitrinite content 40%) accumulating widely in low-latitude tropical and mid-latitude temperate zones. The late Paleophytic Flora witnessed proliferation of existing lineages with Cordaites emerging as dominant coal-forming gymnosperms in lycophyte dominated wetlands. Extensive workable humic coal seams developed across low-latitude tropical and mid-latitude temperate zones. These patterns indicate that terrestrial vegetation not only contributed directly to peat formation but also influenced coal maceral composition and coal seam distribution. Vegetation further shaped surface environments favorable for peat accumulation in indirect and lasting ways, thereby establishing terrestrial ecosystems as major carbon reservoirs. These changes involved: i) Differentiation of roots, stems, and leaves, together with stem elongation, enhanced photosynthetic efficiency and primary productivity, fostering thick coal seams; ii) Plant-derived organic acids accelerated rock weathering, releasing nutrients (including phosphorus and potassium), and generating fine-grained sedimentary materials including clays, changes that supported plant growth, expanded wetlands, and created aquitards that preserved peat; and iii) Expanding vegetation cover (from lowlands to highlands, from wetlands to drylands) reduced physical weathering, clastic input, runoff, and drainage efficiency within basins. Collectively, these changes lowered inorganic input during coal accumulation and enabled the formation of high-quality coal resources including low-ash coal.