XRCT images and variograms reveal 3D changes in wood density of riparian trees affected by floods

Abstract

The extension of damage and anatomical changes in riparian trees after flood impact can be quantified with X-ray computed tomography and variogram analyses. This paper combines X-ray computed tomography (XRCT) images and variogram analyses to document the response of riparian trees to mechanical damage caused by floods. Changes in wood anatomy and density have been described in the past to occur as a result of severe cambial tissue damage. However, knowledge is still fragmentary insofar as the spatial extent of responses is concerned and in terms of causative factors controlling the magnitude of response. Here, we present a novel approach combining non-destructive XRCT images with geostatistical tools to describe the extension of anatomical changes in 30 specimens of 3 Mediterranean riparian species (Alnus glutinosa (L.) Gaertn., Fraxinus angustifolia Vahl. and Salix atrocinerea Brot.) scarred by floods. We visually assess tree and wound characteristics (i.e., wound size, decayed area, callus length, callus mark) as well as the health state of trees prior to wounding. In parallel, we systematically computed 1D variograms using XRCT images so as to quantify relative tangential changes in wood density after wounding. Based on non-parametric statistical tests and Principal Component Analyses (PCA), we identify possible controls of macroscopic anatomical features on tangential affected area (TAA) and decay processes. Reactions in trees are controlled differently between species, but are driven above all by the health state of the tree prior to wounding. In view of the expected increase in the frequency of hydrogeomorphic processes and/or changes in the availability of sediments in a future greenhouse climate, wounding of trees is expected to occur more frequently in riparian forests, which could have negative effects on the sustainability of riparian vegetation.