Mountains are exposed to mass movements such as landslides, debris flows, and snow avalanches. These hazards affect valley slopes and endanger human settlements. They may cause severe damage to infrastructures, transportation roads and sometimes human death. Over the last century, urban sprawl in mountain areas in combination with a growing demand for mobility and recreational activities have increased avalanche risk significantly.
An accurate determination of the zones threatened by avalanches, together with their frequency, is therefore crucial for land-use planning and risk management. So far, several efforts have been deployed to build databases of past avalanche events in order to provide accurate information regarding their magnitude, spatial extent, and frequency. However, historic documentation of past avalanche activity is most often biased toward catastrophic events that caused damage to infrastructure or loss of life and remains largely nonexistent in sparsely or recently populated areas.
On forested slopes, trees represent silent witnesses of past avalanches. Using dendrogeomorphology (the science that use growth anomalies in tree-rings to reconstruct natural hazards), these natural archives have proven to compensate for the scarcity of written sources as they allow reconstruction of past avalanche activity in time and space (Stoffel et al., 2013). The approach, developed by Alestalo (1971), takes advantage of the fact that trees growing in temperate climates do not only form yearly increment rings but that they will also record the external disturbance events in their growth-ring series thus allowing accurate dating and reconstruction of natural hazards.
Yet to date, few efforts have been made to disentangle, in tree-ring series, disturbances resulting from the hazard under investigation from climatic or other exogenous disturbances related to e.g. insect and pathogen attacks, fire, windstorms or anthropogenic influences (logging, pasture).
In this context, our research aimed at showing potential interferences between climate, ecology, and geomorphology in tree-ring series in order to better isolate the snow avalanche tree-ring disturbances.
Oberwald’s avalanche paths
To perform this research, we studied avalanche paths which threaten the village of Oberwald (46°32’N, 8°20’E) in the upper Goms valley, in the Central Swiss Alps (Canton of Valais, Switzerland). Snow avalanches start spontaneously from zones located between 1680 and 2200 m asl. Once released, they pass through a forested slope mainly composed of European larch (Larix decidua Mill.) and Norway spruce (Picea abies (L.) Karst.). Historical documents report several avalanche events since AD 1720.
How to reconstruct past avalanches from tree-rings?
A total of 564 European larch (Larix decidua Mill.) (Figure 1a) and Norway spruce (Picea abies (L.) Karst.) trees were sampled and precisely positioned. We chose trees showing indication of past snow avalanche activity, such as pollarding, tilting and stem injuries. We then dated these growth disturbances (GD, Figure 1b) and we assigned intensity values to each of them to highlight those clearly associated with avalanche events.
To detect past snow avalanches in the GD series and to disentangle potential effects of snow avalanches from disturbance pulses caused by climatic or exogenous factors, such as drought years or insects/fungal outbreaks, a four-step procedure has been developed:
(i) We discriminated potential avalanche and non-avalanche years based on the number of trees showing GDs for a given year.
(ii) The grey larch budmoth (LBM, Zeiraphera diniana Gn.) is the main cause of noise in our tree-ring series. This foliage feeding moth is responsible for massive defoliation at 8-10-year intervals resulting in severe growth disturbances (Figure 2f), mainly in the interior valleys of the European Alps. Similarly, climate extremes such as cold summers and prolonged droughts can durably affect larch growth and cause growth disturbances that look similar to avalanche events. From several scientific studies, we reconstructed a chronology of LBM outbreak and climate extremes in the Swiss Alps since 1780. For potential avalanche events detected in step 1 coinciding with LBM outbreak episodes or extremely cold/dry years, we looked for specific indications of mechanical injuries to limit possible interferences between geomorphic, climatic, and LBM signals.
(iii) We then classified avalanche events into low (LLC), medium (MLC), and high (HLC) confidence categories using the intensity of detected GDs.
(iv) Finally, avalanche years and corresponding GDs were mapped (Figure 2) to visualize the minimum extent (ME) of each avalanche event.
Reconstruction snow avalanche activity and main limitations
We observed that more than half of the detected events is related to larch budmoth outbreaks or extremely cold/dry summers in the Swiss Alps. As such, these results highlight the necessity to discriminate avalanche signals from other ecological disturbances. This need is even more critical as a significant proportion of dendrogeomorphic studies have been based on larch trees which are known to be cyclically affected by larch budmoth (LBM) outbreaks.
Then, interestingly, a clear temporal trend is observed in the reconstructions for each avalanche corridor (Figure 3), with 80% of the events we identified occurred during the 20th century and 25 out of 43 avalanche events since 1950. This trend toward higher activity since the beginning of the 20th century at Oberwald cannot be attributed to climate change. It is explained by the fact that conifer trees heal wounds of past events rapidly due to the thickness of their bark. Accordingly, the detection of old scars on the stem surface is difficult. Older events can, therefore, be missed in multicentennial old L. deciduas trees.
So, what next?
Dendrogeomorphology is becoming a practical tool in the hands of policymakers. Our results demonstrate the absolute necessity to discriminate ecological from geomorphic disturbances in future works. This is all the more crucial that larch trees, a very common tree species in the alpine environment, frequently employed in dendrogeomorphic studies, is cyclically affected by larch budmoth outbreaks. The next step will be to integrate both temporal and spatial information derived from the dendrogeomorphic approach in order to compute precise return period maps of snow avalanches, a key result for policymakers and stakeholders.
This study, Disentangling the impacts of exogenous disturbances on forest stands to assess multi-centennial tree-ring reconstructions of avalanche activity in the upper Goms Valley (Canton of Valais, Switzerland) was recently published in the journal Quaternary Geochronology.