How do bark beetle outbreaks impact water quality in temperate forested catchments?

Abstract

Temperate forests provide ecosystem services such as protecting water environment, timber and fuel production, carbon sequestration, and reduction of nutrient loss. During the last decades, large forest areas were decimated world-wide by bark beetle attacks. Under climate change, drought and higher temperatures increase the risk of infestation. Future forest ecosystem services are therefore at risk of deterioration and it is essential to understand how bark beetle attacks and following management practices influence nutrient cycling, nitrate (NO3) and dissolved organic carbon (DOC) fluxes in seepage water. After dieback, accelerated mobilization of nutrients can be expected due to an increase in mineralization rates and the lack of plant nutrient uptake, whereas the lack of litter input may reduce nutrient leaching. The significantly reduced interception and evapotranspiration might furthermore increase soil water contents and seepage fluxes. Regeneration strategies (e.g. site clearance vs. keeping the dead trees, natural vs. artificial regeneration, regeneration with nurse crops) are decisive for the extent and persistence of the impact of calamities on water quality and quantity. We use a meta-data-analysis to gather knowledge out of approx. 60 studies around the world, to assess the expected behaviours of DOC and NO3 concentrations in seepage water and streams after bark beetle outbreaks in temperate forests and to identify gap of knowledge. Most studies focussed on nitrate leaching and only few on DOC. Overall, DOC concentrations increase in seepage water and streams directly after dieback, reaching a peak 2 to 3 years after disturbance. In the opposite, the first evidences of increased NO3 concentrations are visible approximately one year after disturbance and peak is reached within 3 to 10 years (on average after 5 years), when DOC decreases. NO3 maxima never exceeded drinking water limit. In all studies, DOC and NO3 concentrations recovered to pre-event or, in some cases, were even below the pre-dieback conditions only few years after the peak. Forest ecosystems seem therefore to be resilient to disturbances showing overall rapid recovery of ecosystem functions. However, the timing and duration of the concentration peaks largely differed among the studies, which might be explained by the extent and velocity of tree dieback in the studied areas, the harvest management practices and the type of vegetation re-growth after disturbance, but also by the local climatic conditions and the catchment size. Only few studies specifically analysed these effects on nutrient fluxes and their results differ considerably. More research is needed for assessing the influence of different regeneration strategies after calamities on water quality risks in forested catchments. A bark beetle attack currently decimating the Norway Spruce forest in the well-monitored Lange Bramke catchment (Harz, Germany), offers a unique opportunity to answer this question. With long-term datasets of NO3 and DOC concentrations in stream and the recent installation of a network of lysimeters at three soil depths in a) a healthy forest area, and infested areas b) with dead trees standing, and c) with site clearance, we will be able to better understand the effect of regeneration strategies on nutrient fluxes.