Differing responses to extreme rainfall events in headwater areas recorded by wood anatomy in roots (Gorce Mountains, Poland)

Local, short-lasting downpour is typically observed more frequently in headwater areas than in overall catchments. Headwater systems act as buffers and serve as starting points for stream channels. Therefore, recognition of the magnitude and frequency of their transformation is important for the understanding of the functioning of entire mountain catchments. Despite numerous studies on extreme events, the headwater areas are still poorly recognised. There are a number of steep forest-covered headwater areas in the Gorce Mountains, a range of flysch-type mountains that form part of the Polish Carpathians, which have not yet been studied in relation to this issue. Therefore, the main aim of this study was to determine the nature of geomorphic activity acting within different parts of the headwater areas. In order to date extreme geomorphic events precisely, a dendrogeomorphic approach was performed based on anatomical changes in exposed roots. A total of 59 spruce Picea abies L. Karst roots were sampled. Besides the reduction in tracheid lumen area in earlywood that is traditionally used, a recently developed approach using an abrupt change in the amount of latewood as an indication of the moment when geomorphic activity takes place was also taken into consideration. Data from exposed roots were compared to rainfall data. The results showed that the headwater areas experienced a variety of geomorphic processes. The timing of processes was assessed for the years 1944 to 2001. The main difference between the roots can be observed between the upper and lower parts of the headwater area. In the upper part of the headwater area, anatomical changes within the roots were observed when heavy rainfall events occurred, i.e. in 1958, 1970, 1971, 1972, and 1985. Roots in the lower part of the headwater area had become exposed during continuous rainfalls in 1997 and 2001. This research provides a fundamental review of dendrogeomorphological methodology applied to the identification of extreme geomorphic events acting within headwater areas.