How quickly does wood fragment in rivers? Methodological challenges, preliminary findings, and perspectives

Abstract

Large wood plays a significant role in fluvial ecosystems, influencing river geomorphology and ecology. However, it poses both benefits to river systems and risks, making it essential to understand its dynamics for effective management. A better understanding the wood breakdown process is required to evaluate the flood risk of wood in rivers. This paper aims to evaluate early‐stage fragmentation of wood in rivers after being recruited through bank erosion, taking into account its mobility and residence time. Two methods for characterising and monitoring wood fragmentation are suggested and compared: 1) photo‐interpretation based on ground and drone photo and 2) terrestrial LiDAR. We used Quantitative Structure Modelling (QSM) of point clouds to accurately simulate the full branching structure of trees. Close relationships exist between photo‐interpreted and LiDAR‐derived complexity metrics, but a scaled parameter (i.e., diameter) is needed to correlate branching complexity with volume. The debranching process occurs quickly, with a median reduction in branching complexity of over 80% within the initial 2 years. Further research with a larger sample size is necessary to investigate the impact of context – including transportation, submersion, accumulation, and isolation of wood pieces – on the fragmentation process. Field observations indicate that immobile wood pieces can experience a rapid reduction in their branching complexity, similar to the ones that are transported. Partial fine branching structure can be maintained on transported pieces. Both photo‐interpretation and terrestrial LiDAR offer complimentary approaches to monitoring wood fragmentation. Photo‐interpretation is easily implementable and may be used as a proxy for mechanical fragmentation, while terrestrial LiDAR may be used to monitor 3D wood fragmentation, volume and length evolution, following QSM modelling.