Debris flow; Dendrogeomorphology; Wood anatomy; Dendrochemistry; Post-mined area

The degradation of forest ecosystems represents a severe environmental issue in anthropogenically modified landscapes worldwide. We demonstrated the negative impact of contaminated debris flow deposits on riparian forest in the Romanian Carpathians through the innovative combination of several dendroecological approaches. Reconstruction of debris flow activity using 95 individuals of Picea abies (L.) Karst. was performed to identify 11 events during the period 1952–2018. Recent events (e.g., 2007, 2014, 2016–2017) filled the valley floor after the stream was dammed in the 1980s, which deteriorated tree growth. This state was worsened by site acidity (pH = 3.8), low organic carbon, high sulfur content, and high concentrations of several metals in debris flow deposits. Wood anatomical analyses confirmed that trees producing tangential rows of traumatic resin ducts as a possible response to burial by toxic debris material subsequently produced smaller and flatter earlywood tracheids. In particular, the most obvious changes (average decrease in tracheid lumen area by 62.9% over two years) occurred after the 2016 debris flow event. Using inductively coupled plasma optical emission and/or mass spectrometry, we identified peak or increased concentrations of thallium (max. 0.7 mg kg−1), caesium (max. 0.4 mg kg−1), and aluminium (max. 434 mg kg−1) in tree biomass that could be related to debris flow activity in 1996, 2007, and 2016. Regarding the individual-specific responses of sampled trees, we conclude that the forest decline is caused by complex interaction between the depth of tree burial by debris flow deposits, the mechanical weight and chemistry of deposits, and the rate of nutrient and toxic element uptake.