How Human and Natural Disturbance Affects the U.S. Carbon Sink

Abstract:

Gridded datasets of Net Ecosystem Exchange derived from eddy covariance and remote sensing measurements (EC-MOD and FLUXNET-MTE) provide a means of validating Net Ecosystem Productivity (NEP, opposite of NEE) from terrestrial ecosystem models. While most forested regions in the U.S. are observed to be moderate to strong carbon sinks, models not including human or natural disturbances will tend to be more carbon neutral, which is expected of mature ecosystems. I have developed the Terrestrial Ecosystems Model Hydro version (TEM-Hydro) to include both human and natural disturbances to compare against gridded NEP datasets. Human disturbances are based on the Hurtt et al. land use transition dataset and include transient agricultural (crops and pasture) conversion and abandonment and timber harvest. Natural disturbances include tropical storms and hurricane and fires based on stochastic return intervals.

Model results indicate that forests are the largest carbon sink, seconded by croplands and pastures, if not accounting for decomposition of agricultural products and animal respiration. Grasslands and shrublands are both small sinks or carbon neutral. The NEP of forests in EC-MOD from 2001-2006 is 240 gCm2yr-1 and for FLUXNET-MTE from 1982-2007 is 375 gCm-²yr^-1. With potential vegetation, the respective forest sinks for those two time periods are 54 and 62 gCm^-2yr^-1, respectively. Including the effects of human disturbance increases the sinks to 154 and 147 gCm^-2yr^-1. The effect of stochastic fire and storms is to reduce the NEP to 114 and 108 gCm^-2yr^-1. While the positive carbon sink today is the result of past land use disturbance, net carbon sequestration, including product decomposition, conversion fluxes, and animal respiration, has not yet returned to predisturbance levels as seen in the potential vegetation. Differences in response to disturbance have to do with the type, frequency, and intensity of disturbance. Fire, in particular, is seen to have a net negative effect on carbon storage in forests due to decomposition of coarse woody debris and the fact that some nitrogen is lost during volatilization. Croplands become a carbon source if assuming product decomposition occurs where the crops are grown, and pasturelands become carbon neutral if accounting for animal respiration.