Consistent responses of forest carbon dioxide fluxes to short term weather anomalies

The growth rate of atmospheric CO2 varies annually and most of this variability has been attributed to CO2-fluxes between terrestrial ecosystems and the atmosphere. Biomass burning and climatic variability influence CO2-fluxes and thus the growth rate of atmospheric CO2. However, these top- down insights, mainly derived from inverse and global modeling, provides relatively little insight into the mechanisms driving the relationships between climatic variability and terrestrial ecosystem processes that influence net ecosystem exchange. Although our understanding of carbon cycling in terrestrial ecosystems improved considerably since the development of the eddy covariance technique, conflicting results have been reported regarding the relationships between climatic drivers and the variability in NEE. The apparently conflicting results may be partly related to scaling i.e. differences in the temporal resolution across analyses. In addition, simple relations between CO2-fluxes and single climatic variables are not particularly helpful and do not represent the more complex interactions of multiple climatic factors and ecosystem processes. This study uses 26 long-term (> 2000 days) NEE time series from boreal, temperate and tropical forests as a first step in understanding the short-term mechanisms underlying anomalies in ecosystem CO2- fluxes. To this aim, anomalous climatic events and anomalous CO2-fluxes were identified. Subsequently, four different states exist: both climate and CO2-fluxes are anomalous, climate is anomalous when the fluxes are regular, climate is regular where the fluxes are anomalous and both climate and fluxes are regular for a given period of time. For all four states we identified the climatic conditions, time of the year and time they occur and describe the similarities and differences across sites.