Dominique M. Schneuwly

Formation and spread of callus tissue and tangential rows of resin ducts in Larix decidua and Picea abies following rockfall impacts.

After mechanical wounding, callus tissue and tangential rows of traumatic resin ducts (TRDs) are formed in many conifer species. This reaction can be used to date past events of geomorphic processes such as rockfall, debris flow and snow avalanches. However, only few points are known about the tangential spread or the timing of callus tissue and TRD formation after wounding. We analyzed 19 Larix decidua Mill. (European larch) and eight Picea abies (L.) Karst. (Norway spruce) trees that were severely damaged by rockfall activity, resulting in a total of 111 injuries. Callus tissue appeared sparsely on the cross sections and was detected on only 4.2% of the L. decidua samples and 3.6% of the P. abies samples. In contrast, TRDs were present on all cross sections following wounding and were visible on more than one-third (34% in L. decidua and 36.4% in P. abies) of the circumference where the cambium was not destroyed by the rockfall impact. We observe different reactions in the trees depending on the seasonal timing of wounding. The tangential spread of callus tissue and TRDs was more important if the injury occurred during the growth period than during the dormant season, with the difference between seasons being more pronounced for callus tissue formation than for TRD formation. We observed an intra-annual radial migration of TRDs with increasing tangential distance from the wound in 73.2% of the L. decidua samples and 96.6% of the P. abies samples. The persistence of TRD formation in the years following wounding showed that only L. decidua trees produced TRDs 2 years after wounding (10.5%), whereas P. abies trees produced TRDs 5 years after wounding (> 50%).

Three-dimensional analysis of the anatomical growth response of European conifers to mechanical disturbance

Studies on tree reaction after wounding were so far based on artificial wounding or chemical treatment. For the first time, type, spread and intensity of anatomical responses were analyzed and quantified in naturally disturbed Larix decidua Mill., Picea abies (L.) Karst. and Abies alba Mill. trees. The consequences of rockfall impacts on increment growth were assessed at the height of the wounds, as well as above and below the injuries. A total of 16 trees were selected on rockfall slopes, and growth responses following 54 wounding events were analyzed on 820 cross-sections. Anatomical analysis focused on the occurrence of tangential rows of traumatic resin ducts (TRD) and on the formation of reaction wood. Following mechanical disturbance, TRD production was observed in 100% of L. decidua and P. abies wounds. The radial extension of TRD was largest at wound height, and they occurred more commonly above, rather than below, the wounds. For all species, an intra-annual radial shift of TRD was observed with increasing axial distance from wounds. Reaction wood was formed in 87.5% of A. alba following wounding, but such cases occurred only in 7.7% of L. decidua. The results demonstrate that anatomical growth responses following natural mechanical disturbance differ significantly from the reactions induced by artificial stimuli or by decapitation. While the types of reactions remain comparable between the species, their intensity, spread and persistence disagree considerably. We also illustrate that the external appearance of wounds does not reflect an internal response intensity. This study reveals that disturbance induced under natural conditions triggers more intense and more widespread anatomical responses than that induced under artificial stimuli, and that experimental laboratory tests considerably underestimate tree response.

How fast do European conifers overgrow wounds inflicted by rockfall

The capacity of trees to recover from mechanical disturbance is of crucial importance for tree survival but has been primarily investigated in saplings using artificially induced wounds. In this study, mature Larix decidua Mill., Picea abies (L.) Karst. and Abies alba Mill. trees growing on alpine slopes that were wounded by naturally occurring rockfall were analyzed to determine their efficiency in overgrowing wounds. In total 43 L. decidua, P. abies and A. alba trees were sampled. First, 106 samples from 27 L. decidua and P. abies trees were analyzed to reconstruct yearly and overall overgrowth rates. Cross sections were taken at the maximum extension of the injury and overgrowth rates were determined on a yearly basis. Results clearly showed that L. decidua overgrew wounds more efficiently than P. abies with an average overgrowth rate of 19° and 11.8° per year, respectively. The higher on the stem the injury was located, the faster the wound was closed. Young and small trees overgrew wounds more efficiently than older or thicker trees. In contrast, no correlation was observed between injury size or increment before/after wounding and wound closure. Second, cross sections from 16 L. decidua, P. abies and A. alba (54 injuries) were used to assess closure rates at different heights around the injury. Overgrowth was generally smallest at the height of the maximum lateral extension of the injury and increased at the upper and lower end of the injury. The efficiency with which L. decidua closes wounds inflicted by rockfall makes this species highly adapted to sites with this type of mechanical disturbance.

Assessing Rockfall Activity in a Mountain Forest – Implications for Hazard Assessment

Rockfall represents the most intensely studied geomorphic process in mountainous areas. Nevertheless, very little information exists on how rockfall frequencies and magnitudes vary over time and how hazards and risks posed by rockfall could be reliably assessed. Former studies have mainly focused on short-term observations of contemporary rockfall activity (Luckman 1976, Douglas 1980), rendering it difficult to estimate long-term accretion rates. Long-term estimates of rockfall accumulation rates have, in contrast, been derived from accumulated talus volumes (Rapp 1960), but such rates may neither be representative of the present-day rockfall activities nor of those that prevailed in the past. On slopes composed of siliceous lithologies, lichenometry has repeatedly been used to evaluate the mean age or activity of talus surfaces (Andre 1997) or to estimate rates of rockfall accretion (Luckman and Fiske 1995).

Reconstruction and Spatial Analysis of Rockfall Frequency and Bounce Heights Derived from Tree Rings

Geomorphic processes continuously shape mountain regions, with rockfall being one of the most widespread and frequent events. Its unpredictable and sudden occurrence poses major threats to settlements, human infrastructure and can even lead to the loss of life (Porter and Orombelli 1981; Bunce et al. 1997; Guzzetti 2000). In recent years, anthropogenic activities increasingly expanded into marginal regions. This development results in the construction of new infrastructure and settlements in exposed areas, leading to increased risk of casualties. In order to avoid future accidents, an accurate hazard assessment and risk analysis become more and more inevitable. Besides a comprehensive understanding of the process, risk evaluation requires the knowledge of past rockfall activity in space and time. Areas where rockfall occurs have to be identified, and the frequency and magnitude of events determined. As a result, rockfall has become one of the most intensely studied geomorphic processes in the alpine environment.

Using Event and Minimum Age Dating for the Assessment of Hazards on a Debris-Flow Cone

Debris flows are common mass-movement processes in most mountainous regions of the world, where their unpredictable and sudden occurrence represents a major threat to transportation corridors and settlements. Increased anthropogenic activity in regions exposed to debris-flow risk renders a detailed hazard assessment inevitable. As a consequence, the understanding of the debris-flow process as well as the behavior of events in space and time is crucial for the mitigation of hazards and risks (e.g. Cardinali et al. 2002; Pasuto and Soldati 2004). For many torrents in Alpine regions, however, systematic acquisition of data on past debris flows only started after the series of catastrophic events in 1987 and 1993 (Haeberli et al. 1990; Rickenmann and Zimmermann 1993; Zimmermann et al. 1997); there is still a considerable lack of knowledge about earlier events for many regions. Thus the reconstruction of past activity is essential for the understanding of current debris-flow dynamics in mountain torrents, possible future developments due to potential climatic change (Goudie 2006) and a realistic assessment of the hazards.