Four-dimensional growth response of mature Larix decidua to stem burial under natural conditions

This paper illustrates the effects of abrupt stem burial (burial depth ~0.5m) on tree growth in mature (46±8years) European larch (Larix decidua Mill.) trees. In contrast to the previous research, which was mostly carried out with saplings and on experimental sites where regular aggradation occurred through the transport of sand, this work focuses on the impact of natural, abrupt burial of mature trees with rocks contained in a sandy-silty matrix typical for debris flows in mountainous headwater catchments. The effect of burial is assessed radially and axially within the stem and over almost three decades after burial. The analysis of growth disturbances and their intensity was based on the 143 cross sections (572 growth series) taken at 10-cm intervals from 6 Larix decidua Mill. The results show quite clearly that abrupt burial causes massive suppression of radial growth as compared to pre-event conditions (mean 77%, min 38%, max 92%, SD 7.2%). The trees sampled were unable to resume pre-burial growth rates even after 25years, but recovered to reference growth conditions (as measured in undisturbed, local reference trees) after 15years (min 3years, max 25years, SD 9years). The results differ only insignificantly between different heights along the tree axis and suppression is equally well expressed at different radial positions within the ste

Possible impacts of climate change on debris-flow activity in the Swiss Alps

This study uses a long dataset of past debris flows from eight high-elevation catchments in the Swiss Alps for which triggering conditions since AD 1864 have been reconstructed. The torrents under investigation have unlimited sediment supply and the triggering of debris flows is thus mainly controlled by climatic factors. Based on point-based downscaled climate scenarios for meteorological stations located next to the catchments and for the periods 2001-2050 and 2051-2100, we study the evolution of temperature and rainfall above specific thresholds (10, 20, 30, 40 and 50mm) and durations (1, 2 or 3days). We conclude that the drier conditions in future summers and the wetting of springs, falls and early winters are likely to have significant impacts on the behavior of debris flows. Based on the current understanding of debris-flow systems and their reaction to rainfall inputs, one might expect only slight changes in the overall frequency of events by the mid-21st century, but possibly an increase in the overall magnitude of debris flows due to larger volumes of sediment delivered to the channels and an increase in extreme precipitation events. In the second half of the 21st century, the number of days with conditions favorable for the release of debris flows will likely decrease, especially in summer. The anticipated increase of rainfall during the shoulder seasons (March, April, November, December) is not expected to compensate for the decrease in future heavy summer rainfall over 2 or 3days

Traumatic resin ducts in Larix decidua stems impacted by debris flows

Following mechanical injury, stems of many conifers produce tangential rows of traumatic resin ducts (TRDs), the distribution of which has been used to date geomorphic events. However, little is known about how far TRD formation extends tangentially and axially from the point of injury or what the time course of TRD appearance is. We analyzed 28 injuries in eight Larix decidua Mill. tree stems resulting from debris flows in October 2000 and November 2004. Injuries occurred outside the period of cambial activity, and TRD formation occurred in the first layers of the growth ring formed in the year following that of injury. The axial extent of TRD formation averaged 74 cm and was greater above the injury than below it. At the height of the wound center, TRDs extended horizontally to a mean of 18% of the stem circumference excluding that portion where the cambium had been destroyed. In subsequent growth rings, TRDs, if present, were confined mainly to the height of the center of injury. Both the vertical and horizontal extent of TRD formation was related to the injury size. Within growth rings, the position of TRD formation changed with increasing distance from the wound progressing from early earlywood to later portions of the growth ring

Variations in debris-flow occurrence in an Alpine catchment — A reconstruction based on tree rings

Past debris-flow activity in the Birchbach torrent (Swiss Alps) was assessed using tree-ring data. Based on the analysis of 210 Larix decidua and Picea abies, 50 events were reconstructed for the period AD 1752–2006. On average, 3.2 events per decade occurred in the torrent during the last century with distinct differences between the decades and a considerable decrease in the number of events during the last decade. Comparison with debris-flow reconstructions in neighboring catchments provided more insight in the similarities and differences in debris-flow occurrence. The recurrent debris-flow activity and current geomorphic settings suggest that the occurrence of debris flows in the torrent is rather transport-limited than supply-limited. Therefore, future investigations on debris-flow occurrence in transport-limited basins should focus on triggering rainfall events

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%)

Dynamics in debris-flow activity on a forested cone — A case study using different dendroecological approaches

Dendrogeomorphological analyses of trees affected by debris flows have regularly been used to date past events. However, this method has always been limited to forested cones where trees registered the impact of previous events. The minimum age dating of trees growing in the debris deposits can, in contrast, provide information on the latest possible moment of past activity. In this paper, we report on results obtained from a combination of these two approaches on a forested cone in the Valais Alps (Switzerland). A detailed geomorphic map in a scale of 1:1000 served as a basis for the sampling strategy. Disturbed Larix decidua Mill. and Picea abies (L.) Karst. trees growing in the deposits allowed reconstruction of 49 events between AD 1782 and 2005 as well as the determination of the spatial extent of events. In the debris-flow channels where survivor trees are missing, we selected the oldest post-event trees and assessed their age by counting their growth rings. Missing rings due to lack of center as well as to sampling height were added so as to determine real tree age. The combination of the dendrogeomorphological event reconstruction with the assessment of germination dates of successor trees allowed realistic approximation of the minimum time elapsed since the last debris-flow activity in 23 of the 29 channels present on the current-day cone surface. In general, channels in the northern part of the cone and those close to the currently active channel generally show signs of (sub-) recent activity with one last overbank sedimentation event in the 1980s, whereas signs of debris-flow activity are absent from the channels in the outermost part since the late 19th century. As a consequence of the deeply incised channel and the stabilization measures undertaken along the banks, signs of debris flows are missing in the tree-ring record for the past two decades

How to improve dating quality and reduce noise in tree-ring based debris-flow reconstructions

Tree rings have been used to reconstruct the occurrence of debris flows and other mass movements. Identification of past activity was typically based on the presence of growth anomalies in trees, with a focus on scars, stem tilting, trunk burial or apex decapitation. Clear guidelines have been missing so far and the dating of events has only rarely been based on thresholds so as to distinguish signal from noise. In a similar way, the spatial distribution of affected trees has not normally been considered in mass movement reconstructions, and was at best used as a subjective exclusion factor. This study therefore aims at improving dating quality of and reducing noise in debris-flow time series. Based on a dataset of 803 increment cores (385 trees) affected by debris flows, we reconstruct event histories using (i) a classical experts' approach, (ii) a weighted index (Wit) of responding trees as well as (iii) Moran's I and Getis–Ord Local Gi indices. We identify similarities and differences in results and then investigate subsets of the tree-ring sample to define ideal sampling positions on debris-flow cones and guidelines for sample depth