The Influence of Root Geometry on Soil Cohesion and Anchoring Ability through Geologic Time

Abstract

Vegetation has become ubiquitous among most modern landscapes. However, for much of the Earth’s history, land plants were absent. Their rapid diversification throughout the Devonian and Carboniferous brought about a massive shift in geomorphology and landscape evolution. Complex rooting structures were the principal agents of change, mechanically reinforcing their substrates and generating cohesive sediments through weathering. This work examines the root systems of three major tree genera from these periods: Calamophyton, Lepidodendron, and Calamites. Simplified reconstructions were designed, 3D printed, and uprooted from a sand testbed to explore the effects of root geometry on anchoring ability. Force and displacement data were gathered for each model and used to calculate anchoring strength and uprooting work. Force laws were then derived to approximate the anchoring contributions of root weight, sediment weight, static friction, and shear strength. This analysis revealed a strong dependence on the span, surface area, and volume of the root system, which were used to normalize values across different geometries. The Calamophyton model required the greatest uprooting force per unit length, whereas the Lepidodendron model required the greatest uprooting force per unit area and volume. These results were interpreted within the environmental context of each genus alongside particular features of root geometry. Calamophyton contributed less to soil cohesion due to its simple unbranched architecture, however it likely increased wetland habitability for subsequent species. Meanwhile, Lepidodendron would have bolstered cohesion on account of its densely-packed dichotomous rootlets. Calamites is unique in its clonal reproductive habit and nodal branching architecture, which could have helped it colonize particularly unstable environments. We maintain that the earliest trees played a key role in surface stabilization within their ecosystems and likely paved the way for species that followed.