Roots, rock, and regolith: Biomechanical and biochemical weathering by trees and its impact on hillslopes—A critical literature review

Abstract

The role of trees and forests as a critical component of the biosphere and critical zone, and of the Earth system more generally, is widely appreciated. Less known and acknowledged are the geomorphological functions of tree roots, though their importance has been widely referred to in soil studies, paleopedology and palaeobotany. Tree roots and their impact on weathering processes and soil production were incorporated in the Devonian plant hypothesis and tree root casts served as a key evidence of recognition of past soils in geology, sedimentology and palaeopedology. However, knowledge of biomechanical and biochemical weathering induced by vascular plant roots (mainly trees) has been rarely utilized in geomorphic studies. Biogeomorphic and pedologic studies in recent decades have highlighted the importance of tree uprooting, in which roots play a primary role, in soil development, regolith disturbance and bedrock mining. Other important functions of roots were also recognized, e.g.: soil displacement by growing roots, infilling of stump holes and root cavities, root groove development,

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…A diverse array of observations implies that trees play a significant role in building and plumbing the CZ architecture (Johnson and Lehmann, 2006;Pawlik et al, 2016). For example, paleosols and sedimentary deposits have been used to argue that clay enrichment and chemical weathering was promoted by the proliferation of forest ecosystems during the Devonian, prompting the decline of atmospheric carbon dioxide and global cooling (e.g., Retallack, 1997).…”

“…It is also well known that trees use many mechanisms that modulate CZ processes and development (Amundson, 2004;Brantley et al, 2012). To be specific, trees have the ability to alter bedrock chemically and physically as well as influence the style and pace of transport (Kelly et al, 1998;Gabet et al, 2003;Pawlik et al, 2016). Also, as mentioned above, trees limit the amount of water that flows to depth by taking up water in the rooting zone and transpiring it back to the atmosphere before it has a chance to interact with deeper material (Pavich et al, 1989;Moulton et al, 2000;Keller et al, 2006).…”

“…We therefore hypothesize, along with previous researchers (Zwieniecki and Newton, 1994), that tree roots can only grow into rock and promote weathering when fractures are already present or when the rock has already been weathered to some extent. A large array of chemical and physical processes occur at the root-rock-regolith interface and some of these processes were recently reviewed, with an emphasis on the less direct (or obvious) process linkages (Pawlik et al, 2016). Although such processes have been studied to some extent, testing hypothesis 1 will require measuring root pressures for relevant species in natural settings in comparison to the strength of rocky material.…”

“…Of particular interest with respect to this hypothesis is soil associated with the rhizosphere (Hiltner, 1904;Hartmann et al, 2008). The rhizosphere is the most biologically and chemically active frontier of the soil (McNear, 2013) because this is where compounds are released which directly and indirectly affect soil minerals (Philippot et al, 2013). Specifically, roots provide carbon for the microbial and fungal communities (Berner et al, 2003;Calvaruso et al, 2009Calvaruso et al, , 2014McGahan et al, 2014).…”

“…1), they also enrich parts of the soil with these nutrients. As biotic engines, trees thus strongly impact the energy, water, and element cycles in forested and savannah ecosystems, shaping and sculpting landscapes and soils over long timescales (Reneau and Dietrich, 1991;van Breemen et al, 2000;Balogh-Brunstad et al, 2008a;Pawlik et al, 2016). Soils and landscapes in turn affect plant species composition and size as well as above-and below-ground productivity and rooting depth (Bennie, 1991;Clark et al, 2003;Hahm et al, 2014;Marshall and Roering, 2014).…”