Natural disturbance by bark beetle offsets climate change effects on streamflow in headwater catchments of the Bohemian Forest
In central Europe, large strictly protected areas, such as the Bavarian Forest and Sumava National Park (NP) in the Bohemian Forest, and their management have come under public pressure after adopting a "benign neglect" approach concerning natural disturbances. Here the extensive dieback of Norway spruce by bark beetle (Ips typographus L. raised concern about its regional eco hydrological effects (i.e. runoff yields) and how they interact with the effects of prevailing climate change. To address these questions, we first analysed the hydrological response of nine conterminous mostly forested catchments in the Bohemian Forest to changes in climatic factors. The catchments (39.1 333.9 km2, mean elevation 800 1134 m a.s.l. cover the Bavarian Forest NP and most parts of the Sumava NP along and across the Czech German border. From 1978 to 2013, independent of land use changes and physiographic features, regional summer runoff decreased by 70 mm 21% of long term median) despite increased summer precipitation (51 mm, 8% while winter runoff did not change (8 mm) although precipitation declined 54 mm, 9% This feature results from a timing effect in streamflow due to earlier snowmelt, which is driven by the regional warming in winter, especially in April, by about 3.3 K, irrespective of altitude. However, the overall decline in annual runoff yields 59 mm, 7% despite constant precipitation, is related to higher water vapour losses due to the increased air temperature in summer (1.5+ 0.3 K) while the long term means varied between 8.9 and 13.4degC depending on altitude. A dataset consisting of three sub catchments inside the national parks (0.7 89.7 km2) was analysed for disturbance effects (58 62% of catchment area) on precipitation runoff behaviour. The larger ones, Upper Vydra and Upper Grosse Ohe, strictly followed the overall trends in runoff and high flows in winter but did not show annual trends. An analysis of runoff precipitation ratio revealed a significant step change in the Bavarian Forest NP sub catchments once cumulative disturbance exceeded 30% area (1998/1999) After this step change, catchment evapotranspiration significantly decreased by 62 120 mm and runoff increased to the same extent. The sub catchment in the Sumava NP did not respond probably due to timing and/or scale effects. Overall, the observed declining trends in runoff yields were not caused by precipitation changes but were due to warming only. However, in small embedded catchments of the national parks, reduced evapotranspiration losses after bark beetle outbreaks and windthrow currently but temporarily compensate for climate change effects. Shifting streamflow from early summer to late winter is the common hydrological response of all catchments to warming, which in the longer term may negatively affect the water supply to vegetation and people in autumn.