Net ecosystem productivity during the first two years of woody biomass plantation establishment at two grassland sites in northern MI and WI, USA

Background/Question/Methods

There is potential for large-scale production of short-rotation woody bioenergy crops (SRWC) in the Northern Lakes States as part of a new bioenergy economy.  Land-use change could result in environmental impacts such as greenhouse gas (GHG) emissions or changes to soil carbon (C) stocks and nitrogen (N) availability. We established research plantations with willow, hybrid-poplar, and control (unplowed mixed-species grassland) plots in spring 2010 at two sites in Northern MI (ES) and WI (RH).  To address the short-term effects of plantation establishment on GHG balance, we compared sites in terms of biomass productivity and GHG emissions over two years.  We hypothesized high initial C and N availability would result in high productivity and high GHG emissions, and that overall net ecosystem productivity (NEP) would be similar between sites.  NEP was calculated as the difference between total NPP and heterotrophic soil carbon dioxide (CO₂) emissions (R_h) in each year.  GHG emissions (CO_2-eq) include nitrous oxide (N₂O), methand (CH₄), and heterotrophic CO₂ flux.  Total NPP is the C percentage of above and belowground NPP each year. 

Results/Conclusions

After two years of growth, poplar and willow plots are a C source while control plots are a slight C sink at RH but a C source at ES, with -19.4 and -9.6 Mg-C ha^-1 for poplar, -16.8 and -2.8 Mg-C ha^-1 for willow, and -1.1 and 6.8 Mg-C ha^-1 for control at ES and RH, respectively.  RH had greater productivity in willow and control plots and lower emissions in poplar and willow plots.  More years of monitoring are necessary to separate annual variation from the underlying site conditions which result in high GHG emissions and/or productivity.