The value of a forest in our current era of global warming is often judged by its ability to sequester carbon. However, a forest that is a large carbon sink does not necessarily imply a healthy forest. The health of a forest may also be judged by its biodiversity of tree species or mixture of stand ages. The strength of the carbon sink depends heavily on disturbance and regrowth. Generally a more biodiverse forest is more productive, yet may also have more decomposition, which would balance out the carbon sink. It is conceivable that a highly disturbed forest is depleted of its decomposing material and so becomes a large carbon sink as a result. This study will explore the strength of the carbon sink versus species diversity and stand age composition for forests in the conterminous U.S. using a modeling approach. The Terrestrial Ecosystems Model (TEM-Hydro) will be run for the conterminous U.S. from 1700 to 2011 including land use and land cover change (LULC) with constant climate and CO2 to isolate the effects of standage on Net Ecosystem Productivity (NEP). The primary LULC considered are agricultural conversion and abandonment, as well as timber harvest.
Results/Conclusions
A model run for the conterminous U.S. from 1700 to 1860 shows that the effect of standage on NEP is in the range of a reduction of 7-10 gCm-2yr-1 every decade. The peak NEP occurs in 25-50 year old forests, though many mature forests also have high NEP. Continuation after 1860 will include more young forests. An experiment with changing climate shows that the effect of climate variability on NEP obscures the standage signal. The effects of storm and fire disturbance will be explored using our current stochastic approach as well as adjusted LULC to account for proper stand age based on work by Lu et al (2015). A followup will be to include competitive interactions for light, moisture, and nitrogen to allow species competition, and to then incorporate more realistic forest compositions incorporating functional diversity. The results will highlight under what circumstances a large carbon sink does not necessarily imply a forest with high diversity and mixture of stand ages.