Changes in carbon uptake and release cause by insect outbreaks in the Colorado Rocky Mountains from 2000 through 2010

A large scale insect outbreak has progressively infected North American Forests in the Rocky Mountains over the last 8 years causing the death of millions of trees. Loss of mature trees on this scale is likely to compromise the ability of these ecosystems to sequester carbon. While a reduction of live leaf area likely leads to reduced carbon uptake gross primary productivity (GPP) the impact of the outbreak on ecosystem respiration (RE) is not clear. We investigated the response of both GPP (2000 through 2010) and RE (2005-2010) to insect out break by contrasting two locations in the Rocky Mountains, the Fraser Experiment Forest (FEF; 39.91 N, 105.88 W) which has been heavily impacted by insects and Niwot Ridge (NWR; 40.05 N, 105.58 W) where the outbreak has not yet occurred. We used a modified estimate of GPP based on enhanced vegetation index (EVI) calibrated using eddy covariance measured at NWR to examine the impact of the beetle outbreak across the region. We found that while GPP decreased significantly at Fraser after the insect outbreak did not show a time dependent decline at NWR. Since 2005 near continuous atmospheric CO2 has been measured at the bottom of FEF. We used the diurnal variation in the CO2 concentration measured at the bottom of Fraser Valley as a proximate measurement of RE. We found that from 2005 through 2009 there was a decline in apparent RE while in 2010 apparent RE increased relative to 2009. Direct measurements of soil CO2 efflux appear to bear out this trend. Barring a large shift in temperature it is possible that the increase in apparent RE in 2010 may be the result of mobilization of N or potentially recovery of GPP from regenerating vegetation. The relative changes in GPP and RE are investigated from 2005 through 2010.