Strong wind patterns and slope surface exposure change the biogeomorphological component of the soil transport and development in old-growth temperate forests

Soil transport through the turning of root systems of uprooted trees represents a crucial soil disturbance agent in many forest ecosystems. In addition to tree species and forest structure, treethrow dynamics strongly depend on the character of wind flow and specifically on extreme wind events. Extreme cyclonic flow is typically west-east in Europe and causes major pulses in forest slope dynamics. In the regime of rare but powerful winds alternating with long periods of disordered flow, slope exposure can be a significant geomorphological driver. Slope orientation can hypothetically affect the frequency of uprooting events and the direction of tree fall relative to the fall line. The long-term effect is a different trajectory of soil evolution and different slope dynamics. Tree orientation determines how much of the uprooted soil will return to the treethrow pit and how much will be eroded. These aspects were explored for the first time using extensive repeated tree census data spanning from 1975 to 2007, collected in the Žofínský Primeval Forest Reserve in the Novohradské Mountains, Czech Republic. We analyzed the differences between the spatial configuration and biogeomorphic potential of uprootings under two scenarios: extreme windthrow related to a strong disturbance event and selective uprooting associated with forest gap dynamics. Wind data observations (direction and speed) were used to define the relationship between uprooting intensity and spatiotemporal linkages of uprootings' tree stems features (azimuths) with wind and terrain properties (elevation and aspect).

We found differences in tree stem azimuths of uprootings, in their spatial configuration against elevation contour lines, and terrain aspect for two classes – upslope and downslope uprootings, and two forest developmental trajectories – dominated by gap formation (selective), and excessive (extreme) damage caused by the Kyrill storm in January 2007. Azimuths of uprootings followed prevailing wind directions, suggesting this metric can be used as a bioindicator of wind properties in places without standard meteorological measurements. Norway spruce (Picea abies) was the most commonly uprooted and broken tree species, and its damage rate increased during the Kyrill windstorm. Upslope uprooting was more common, potentially involving a higher volume of soil mass being subjected to mixing than erosion.

Our results indicate that slope aspect influences the path of old-growth forest development and soil formation. It happens repetitively during extreme wind events (pulses of energy "injected" to forest ecosystems by wind currents of extreme speeds and specific directions), acting occasionally but with great force over the European land. It is essential to emphasize the synergistic and critical impact of geomorphic features such as slope aspect and strong wind events on soils and forests, particularly in the context of the anticipated increase in the extremity of various climate parameters, including wind speed and the frequency of strong winds.

The study has been supported by the Czech Science Foundation (project No. 24-11119S).