Topographic effects on the initial establishment and growth of pine trees as a riparian vegetation after debris flow-induced disturbance in a mountain stream

On July 15, 2006, heavy rainstorm-triggered debris flow destroyed mature riparian forests and altered the channel geomorphology along the Hangye stream, Mt. Seorak National Park, Republic of Korea. We examined the effect of the topographic site condition on the initial establishment and growth of pine trees (Pinus densiflora) as one a dominant species of riparian vegetation along transects on new debris flow terraces formed along the Hangye stream. In the 2019 field investigation, the tree age and internode length of all P. densiflora seedlings were measured in each 25-m² plot placed at 5-m intervals for a total of 19 cross-section lines marked by a topographic survey using an auto level. For data analysis, stream channel morphology was classified into two typical types: a single channel segment with only the main channel (type A; total of 14 cross-section lines) and divergent channel segment with the main and several secondary channels (type B; total of 5 cross-section lines). The height (Hq) and distance (Dq) of each surveyed plot from the thalweg line of the main channel were considered as topographic site conditions with reference to the establishment and growth of riparian vegetation. As a result, the mean Hq and mean Dq were all greater in plots with the appearance of pine trees than in those without in both type A and type B segments, showing statistical significances at the 1% level. This study also showed that the mean age and mean annual growth rate of P. densiflora seedlings were positively correlated with Hq in type A segments but not in type B segments. Dq showed no correlation with the mean age and mean annual growth rate of P. densiflora seedlings in both type A and type B segments. These results demonstrate that the initial establishment and growth of riparian vegetation were affected by the height above the channel bed associated with the flood frequency and divergent flood flows.

 

Acknowledgement: This work was supported by the National Research Foundation (NRF) grant funded by the Korea government (MSIT) (No. NRF-2017R1C1B5076781).