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Title: Impact of insect defoliation on forest carbon balance as assessed with a canopy assimilation model

item Hamerlynck, Erik

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2009
Publication Date: 1/5/2010
Citation: Schafer, K.V., Clark, K.L., Skowronski, N., Hamerlynck, E.P. 2010. Impact of insect defoliation on forest carbon balance as assessed with a canopy assimilation model. Global Change Biology. 16:546-560.

Interpretive Summary: Development and rigorous testing of process-based carbon sequestration models is a critical feature for accurate carbon accounting. In forest systems, disturbances that remove large amounts of standing biomass are extremely important in the ability of a forest to sequester carbon. However, most large-scale process-based models do not directly account for the important micrometeorological effects such disturbances have on forest/atmosphere exchange. This study parameterized and tested the sensitivity of a forest carbon assimilation model, the 4C-A model, which utilizes whole-plant water use to better constrain estimations of canopy conductance, the feature most likely to be impacted by disturbances. Comparing model results to biomass and eddy-covariance estimates of forest carbon sequestration, the parameterized 4C-A model was found to accurately assess changes in carbon source/sink activity following wide-scale defoliation by gypsy moths. Thus, the 4C-A model has the power and sensitivity to accurately assess carbon dynamics across large spatial and temporal scales, making it a powerful tool in future carbon accounting efforts.

Technical Abstract: As carbon sinks, forests are increasingly becoming important trading commodities in carbon trading markets. However, disturbances such as fire, hurricanes and herbivory can lead to forests being sources rather than sinks of carbon. Here, we investigate the carbon balance of an oak/pine forest in the New Jersey Pine Barrens using the Canopy Conductance Constrained Carbon Assimilation (4C-A) model using inputs from whole-tree sap-flow and leaf-level photosynthetic regulatory gas exchange measurements at distinct canopy levels within three oak on site species. After parameterization in 2006, evaluation and a sensitivity analysis, the model was then applied to an undisturbed year in 2005 as baseline and then to 2007 when the stand suffered extensive foliage loss by Lymantria dispars L. (gypsy moth). In 2007, woody net primary productivity (NPP) was reduced to ca 80% of previous year NPP (71 g m-2 a-1 in 2006) and canopy net assimilation (AnC), as modeled with the Canopy Conductance Constrained Carbon Assimilation model (4C-A), was reduced to ca 70% of previous year (1323 g m-2 a-1 in 2006) AnC or ca 944 g C m-2 a-1. Although the trees were defoliated for only 15% of the normal annual growing season, the stand lost ca 30% of C accumulation integrated over the year. Overall, NPP in 2007 was ca 237 g C m-2 a-1 with 60% of NPP being allocated to foliage production constituting a short-term carbon pool. On an ecosystem level, net ecosystem exchange in 2007 amounted to a release of 293 g C m-2 a-1, becoming a carbon source over the course of the year rather than being a sink for C. The overall impact of the defoliation spanned 21% of upland forests (320 km2) in the New Jersey Pine Barrens, representing a significant amount of overall C being emitted to the atmosphere from these forests rather than accumulating in the biosphere.