InvestigatingDevonian trees as geo‐engineers of past climates: linking palaeosols to palaeobotany and experimental geobiology

Abstract

We present the rationale for a cross-disciplinary investigation addressing the 'Devonian plant hypothesis' which proposes that the evolutionary appearance of trees with deep, complex rooting systems represents one of the major biotic feedbacks on geochemical carbon cycling during the Phanerozoic. According to this hypothesis, trees have dramatically enhanced mineral weathering driving an increased flux of Ca 2+ to the oceans and, ultimately, a 90% decline in atmospheric CO 2 levels through the Palaeozoic. Furthermore, experimental studies indicate a key role for arbuscular mycorrhizal fungi in soil-plant processes and especially in unlocking the limiting nutrient phosphorus in soil via Ca-phosphate dissolution mineral weathering. This suggests co-evolution of roots and symbiotic fungi since the Early Devonian could well have triggered positive feedbacks on weathering rates whereby root-fungal P release supports higher biomass forested ecosystems. Long-standing areas of uncertainty in this paradigm include the following: (1) limited fossil record documenting the origin and timeline of the evolution of tree-sized plants through the Devonian; and (2) the effects of the evolutionary advance of trees and their in situ rooting structures on palaeosol geochemistry. We are addressing these issues by integrating palaeobotanical studies with geochemical and mineralogical analyses of palaeosol sequences at selected sites across eastern North America with a particular focus on drill cores from Middle Devonian forests in Greene County, New York State.

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

“…These changes have been linked to the colonization of terrestrial landscapes by rooted plants (1)(2)(3)(4)(5). Previous studies have emphasized the effects of early trees and forests, which first appeared in the Middle Devonian (6)(7)(8), on enhanced chemical weathering rates and soil development (2,(9)(10)(11). However, the roles that vascular plants played in soil production and landscape stability before the evolution of trees and deep roots (the interval from ∼420 to 390 Ma) are poorly understood.…”

“…Some species have been recorded as bearing shallow rooting systems (or root-like structures) (16)(17)(18)(19)(20), others as having surficial or shallowly subterranean rhizomatous axes with rhizoids (21)(22)(23). Thus, the geochemical effect of these plants on the substrate (or soils) was presumably limited (11), and their capacity for landscape stabilization was trivial. However, because of a poor fossil record, it remains quite unclear how the "hidden half" ecosystem (24), with buried structures growing in soils, functioned during the early stage of vascular plant radiation.…”

The origin and evolution of mycorrhizal symbioses: from palaeomycology to phylogenomics

“…Roots and mycorrhizal symbionts are thought to have a major impact on biogeochemical cycles (Taylor et al ., ); however, estimating the magnitude of these effects in the earliest forests is challenging. Experiments using extant mycorrhizal trees as analogues of past forests have been performed under elevated CO 2 atmosphere conditions approximating those of the middle Devonian (Morris et al ., ). This has demonstrated that under elevated atmospheric CO 2 , trees support larger hyphal networks and AM fungi increase physical alteration of silicate mineral surfaces (Quirk et al ., ).…”

“…In contrast to the wealth of information from plants with rhizoid‐based rooting systems from the Rhynie chert, knowledge of fungal symbioses associated with the primitive roots remains surprisingly limited. Nothing is known about fungal mycorrhizal symbionts in the rooting systems of the trees in the earliest forests of the Mid and Late Devonian (398–359 million yr ago (Ma); Morris et al ., ). These forests were formed by Cladoxylales (an extinct group of uncertain affinities, reproducing by spores) (Xu et al ., ) and progymnosperms, (a group of plants reproducing by spores but with secondary growth of the vascular tissues similar to true gymnosperms; Figs , ).…”

“…This has demonstrated that under elevated atmospheric CO 2 , trees support larger hyphal networks and AM fungi increase physical alteration of silicate mineral surfaces (Quirk et al ., ). The rationale for a cross‐disciplinary approach integrating palaeontological, mineralogical and geochemical analyses of palaeosol sequences was recently presented (Morris et al ., ) and this type of approach needs to be developed.…”