Landscape-level spread of beetle infestations from windthrown- and beetle-killed trees in the non-intervention zone of the Tatra National Park, Slovakia (Central Europe)

The European spruce bark beetle (Ips typographus) causes widespread Norway spruce (Picea abies) mortality in European forests. The pattern of landscape-level tree mortality varies over the course of beetle outbreak and by the presence and location of active breeding sites. Increased understanding of rules governing the unmanaged spread of beetle-induced tree mortality over the landscape would help to optimise management control strategies on the borderline between highly valuable protected areas and surrounding managed forests. Our study aimed to quantify the dynamics of standing tree infestation patterns from two infestation sources: windthrow and previous-year beetle infestations. Specifically, we (i) evaluated dispersal distances between the nearest infestation source and new infestations, (ii) quantified size and shape of infestation spots, (iii) modelled an infestation gradient and (iv) probability of new infestation during the incipient, peak and decline phases of beetle outbreak. Based on one- and two-year records of colour-infrared aerial photography, taken between 2005 and 2015, we identified windthrown and beetle-killed trees in the non-intervention zone of Tatra National Park, Slovakia (Central Europe). The size and compactness of infestation spots evolved from small and simple to more extended and complex shapes during beetle epidemics. In total, 40% of infestations were smaller than 100 m(2) and 79% smaller than 500 m(2). Spot growth dominated over spot initiation, with the mean spot growth extent during peak epidemic (54.8 m). Beetle infestations reached the upper tree line (1605 m a.s.l.). In total, 71% of new infestations emerged within 100 m and 97% within 500 m from an infestations source. Emergence of new infestations varied between infestation sources and phases of beetle outbreak. New beetle infestations emerged near windthrown locations during incipient and peak phases and near source beetle infestations during peak and decline phases following inverse power-law function. We found that the forest within 100 m from the active infestation compared to more distant buffers had the highest risk of infestations. The distance to previous-year infestations should be considered as one of the main factors in determining the risk of subsequent tree mortality, especially if other predictors are absent. If the active wind disturbances or beetle infestations neighbour the border of the unmanaged protected area, the search and sanitation felling of active breeding trees should be concentrated within 500 m from the border of unmanaged protected areas in order to restrain the spread of beetle infestations from nature reserve to surrounding managed forests.