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The effect of instream wood on stream hydraulics and geomorphic processes depends on wood and channel dimensions. We investigated abundance and characteristics (i.e., wood orientation, proportion of spanned channel width, stability and decay classes) of large wood (LW; defined here as having a length ≥ 1 m and a diameter ≥ 0.1 m) and small wood (SW; including two categories with length ≥ 0.5 m and diameter ≥ 0.1 m or length ≥ 1 m and diameter ≥ 0.05 m) in three steep, confined headwater channels of medium-high mountain ranges of the Western Carpathians (Central Europe). Results show that SW is more frequent than LW, however, active-channel LW load varied between 26 and 305 m 3 ∙ha −1 , whereas SW showed much lower active-channel load (8–16 m 3 ∙ha −1 ). Differences between LW and SW active-channel volumes were considerably smaller in streams under dominant deciduous canopy. In these streams, morphological steps – created exclusively by SW – were more frequent than LW steps. This higher frequency of SW in streams surrounded by a deciduous tree canopy can be explained by the continuous supply of branches rather than entire dead trees. On the other hand, the volume and frequency of LW was related to the proportion of conifers in the valley corridor. We observe very high active-channel load in two channel reaches for which values exceeded most of those observed in similarly small streams across the globe. We also observe an unusually large proportion of instream wood (both LW and SW) oriented parallel to the flow direction, which might suggest a higher mobility of bed material in the flysch-dominated headwater channels of our study site. [ABSTRACT FROM AUTHOR]

Landslides are frequently triggered by extreme meteorological events which has led to concern and debate about their activity in a future greenhouse climate. It is also hypothesized that dry spells preceding triggering rainfall may increase slope predisposition to sliding, especially in the case of clay-rich soils. Here we combined dendrogeomorphic time series of landslides and climatic records to test the possible role of dry spells and extreme downpours on process activity in the Outer Western Carpathians (Central Europe). To this end, we tested time series of past frequencies and return periods of landslide reactivations at the regional scale with a Generalized Linear Mixed (GLM) model to explore linkages between landslide occurrences and triggering climate variables. Results show that landslide reactivations are concentrated during years in which spring and summer precipitation sums were significantly higher than usual, and that triggering mechanisms vary between different types of landslides (i.e. complex, shallow or flow-like). The GLM model also points to the susceptibility of landslide bodies to the combined occurrence of long, dry spells followed by large precipitation. Such situations are likely to increase in frequency in the future as climate models predict an enhancement of heatwaves and dry spells in future summers, that would be interrupted by less frequent, yet more intense storms, especially also in mountain regions. [ABSTRACT FROM AUTHOR]