The role of border areas for dendrochronological investigations on catastrophic snow avalanches : a case study from the Italian Alps

Snow avalanches are common in mountain environments, sometimes affecting inhabited areas and infrastructures (e.g. roads, bridges, and ski slopes). Their study is widespread and involves the use of a variety of different techniques, including dendrochronological methods. The aim of such investigations is principally to date past events, but also to detect variation in avalanche frequency as well as their spatial distribution. Trees affected by snow avalanches generally have scars, tilted trunks and broken branches, which allow past events to be dated to within a year. In this work tree rings were used to investigate a disruptive snow avalanche which occurred in 2001 in Val Mala in the Italian Alps. The event almost completely removed forest along the flow path, while the snow powder component of the avalanche also impacted on the adjacent forest. Comparison of tree reaction in surviving plants along the flow path and vegetation on the border showed i) the production of reaction wood even in apparently undisturbed trees and ii) the usefulness of border plants for dating past events. Different dendroecological indicators were investigated (i.e. reaction wood, scars, traumatic resin ducts, and variations in stem eccentricity) and reaction wood is evidently concentrated from 2001 not only in damaged trees but also in adjacent plants. The potential to investigate past snow avalanches by collecting samples from trees growing in border areas is presented: in the specific case of Val Mala, vegetation bordering the flow paths reacted to produce reaction wood in 2001, in a similar manner to plants along the flow track, demonstrating their usefulness in such investigations. Their response to the 2001 event confirms the possibility of applying dendrochronological techniques to trees adjacent to the flow path, even if they appear morphologically undisturbed, and proposes to use only border areas in cases of an absence of sufficient trees along the avalanche track.