How does root biodegradation after plant felling change root reinforcement to soil?

Authors: ZHU, JINQI - Beijing Forestry University; WANG, YUNQI - Beijing Forestry University; WANG, YUJIE - Beijing Forestry University; MAO, ZHUN - Beijing Forestry University; Langendoen, Eddy; MA, CHAO - Beijing Forestry University

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2019
Publication Date: 1/1/2020
Citation: Zhu, J., Wang, Y., Wang, Y., Mao, Z., Langendoen, E.J., Ma, C. 2020. How does root biodegradation after plant felling change root reinforcement to soil? Plant and Soil. 446: 211–227. https://doi.org/10.1007/s11104-019-04345-x.
DOI: https://doi.org/10.1007/s11104-019-04345-x

Interpretive Summary: Vegetation is an environment-friendly method to stabilize hill slopes by providing additional soil strength through its root system. Timber logging and consequent root dieback may therefore destabilize hillslopes. Past research has shown that the decline in root reinforcement is slow. However, these reduction estimates are based on empirical equations and not direct measurement of soil root reinforcement. Scientists at the USDA, ARS, National Sedimentation Laboratory in collaboration with researchers at Beijing Forestry University, China have investigated the change in root reinforcement of Symplocos setchuenensis, a species used for afforestation in southwest China, over a one-year period after felling by direct measurement. Measured reduction in root reinforcement was found to be five times larger than that based on empirical relations. This discrepancy was primarily caused by a shift of root failure mode from breakage to the roots being pulled out of the soil. The findings could be used by land managers to develop new conservation measures that reduce hill slope failure risk after timber logging.

Technical Abstract: Previous studies have shown that root reinforcement provided by trees only decreases slowly over time after cutting; however, root reinforcement was not directly measured. We aimed to quantify the temporal changes in root mechanical and structural properties and root reinforcement, and describe their relationships after stem cutting. At six periods (zero, one, three, six, nine, and twelve months) after felling trees of the species Symplocos setchuenensis, we measured: shear strength for both rooted and root-free soils; root mechanical traits (pull-out force, tensile force, and Young’s modulus); and chemical components (structural sugar content). Both root mechanical and chemical characteristics significantly differed as a function of root diameter and time since felling. Additional peak and yield shear strength provided by roots decreased after plant felling. The reduction in additional yield shear strength provided by roots was much higher than the reduction in additional peak shear strength provided by roots. Our findings demonstrate that reduction in root reinforcement after tree felling is a combination of a decline in both root mechanical strength and a shift in root failure mode from tensile failure to pull-out failure, and may therefore be more rapid than previously reported in literature.