Turbulent energy and carbon dioxide exchange along an early‐successional windthrow chronosequence in the European Alps

Abstract

Intensifying natural disturbance regimes are driving changes in turbulent forest‐atmosphere exchange of energy and carbon dioxide (CO2), which are affecting the atmosphere at local to global scales. In the European Alps, windthrow and bark beetle infestations have been increasing in severity and intensity; however, studies employing eddy covariance to investigate net ecosystem‐atmosphere exchange of sensible (H) and latent heat (LE), and CO2 (NEE) along forest disturbance chronosequences in this ecoregion have yet to emerge. This paper presents the first eddy covariance measurements at windthrow sites in the European Alps. Despite the complex terrain, turbulent flux data quality at the sites was high, as indicated by the near‐100% energy balance closures and the large proportion of data satisfying strict quality criteria. Seasonal datasets (15th May–15th October) from the two sites (disturbed in 2009 and 2007, respectively) were subsequently combined in a time series‐chronosequence approach to investigate H, LE, and NEE over the 2nd to 8th growing seasons post‐windthrow. While varying weather conditions strongly influenced energy partitioning, post-disturbance vegetation recovery coincided with a general decrease in median summer Bowen ratios (H/LE), although the decline was most pronounced between the 2nd and 3rd summers. Net CO2 emissions on the other hand demonstrated a clear and consistent post‐disturbance decline, though the seasonal fluxes point toward large losses of ecosystem carbon (C) after disturbance. In the 3rd season post‐windthrow, a net total of 405±15gCm−2 was lost to the atmosphere, while the seasonal net ecosystem production of −4±16gCm−2 observed at the 2007 windthrow site indicates that 8 years after disturbance the site remains an annual net C source. This study therefore demonstrates that eddy covariance can provide robust and valuable insights into forest disturbance impacts on H, LE, and NEE in the European Alps.