Wind and Topographic Impacts on Forest Biogeomorphic Disturbances

ABSTRACT Soil transport through uprooted trees represents a crucial soil disturbance agent in many forest ecosystems. In addition to tree species and forest structure, tree throw dynamics strongly depend on the character of wind flow and specifically on extreme wind events. The tracks of extra‐tropical cyclonic storms are typically west–east in Europe and the disturbances cause major pulses in forest slope dynamics. In the regime of rare but powerful winds alternating with long periods of low‐intensity wind flows, slope exposure can be a significant geomorphological driver. Slope orientation can hypothetically affect the frequency of uprooting events and the direction of tree falls. Tree orientation determines how much of the uprooted soil will return to the tree throw 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 Žofínský Primeval Forest Reserve in Novohradské Mountains, Czech Republic. We analyzed differences between 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 observations (direction and speed) were used to define the relationship between uprooting intensity and spatiotemporal linkages of uprooted tree 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. The differences were found for upslope and downslope uprooting categories, and two forest developmental trajectories—the first one dominated by gap‐dynamic treefall (associated with selective tree mortality), and the second one dominated by treefall caused by extreme storms such as 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. Upslope uprooting was more common, potentially involving higher soil mass volume being subjected to mixing than erosion. Our results show that slope aspect is a terrain feature which modulates the developmental trajectory of an old‐growth forest and soil. The changes in the forest development happen during extreme wind events which act occasionally but with great force over the European land. It is important to underline the synergistic critical impact of such geomorphic features as slope aspect and strong wind events on soils and forests, especially in the context of the expected increase in the extremity of various climate parameters, including wind speed and frequency of strong winds. The study brings new results on soil mixing/transport and forest damage evaluation using high‐resolution and long‐term monitoring data with implications for changes in soils, and shifts in forest development trajectories.