Browsing by Author "Ahlswede, Benjamin J."
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- Combined Carbon and Albedo Climate Forcing From Pine and Switchgrass Grown for BioenergyAhlswede, Benjamin J.; O'Halloran, Thomas L.; Thomas, R. Quinn (Frontiers, 2022-05-13)Expanding and restoring forests decreases atmospheric carbon dioxide, a natural solution for helping mitigate climate change. However, forests also have relatively low albedo compared to grass and croplands, which increases the amount of solar energy they absorb into the climate system. An alternative natural climate solution is to replace fossil fuels with bioenergy. Bioenergy crops such as switchgrass have higher albedo than forest ecosystems but absorb less total carbon over their lifetime. To evaluate trade-offs in the mitigation potential by pine and switchgrass ecosystems, we used eddy covariance net ecosystem exchange and albedo observations collected from planted pine forests and switchgrass fields in eastern North America and Canada to compare the net radiative forcing of these two ecosystems over the length of typical pine rotation (30 years). We found that pine had a net positive radiative forcing (warming) of 5.4 ± 2.8 Wm−2 when albedo and carbon were combined together (30 year mean). However the assumptions regarding the fate of harvested carbon had an important effect on the net radiative forcing. When we assumed all switchgrass carbon was emitted to the atmosphere while the harvested pine carbon was prevented from entering the atmosphere, the 30-year mean net radiative forcing reversed direction (−3.6 ± 2.8 Wm−2). Overall, while the pine ecosystem absorbed more carbon than the switchgrass, the difference in albedo was large enough to result in similar climate mitigation potential at the 30-year horizon between the two systems, whereby the direction and magnitude of radiative forcing depends on the fate of harvested carbon.
- Community Earth System Model Simulations Reveal the Relative Importance of Afforestation and Forest Management to Surface Temperature in Eastern North AmericaAhlswede, Benjamin J.; Thomas, R. Quinn (MDPI, 2017-12-13)Afforestation changes the land surface energy balance, though the effects on climate in temperate regions is uncertain, particularly the changes associated with forest management. In this study, we used idealized Community Earth System Model simulations to assess the influence of afforestation and afforestation management in eastern North America on climate via changes in the biophysics of the land surface. Afforestation using broadleaf deciduous trees maintained at high leaf area index (LAI) in the southern part of the study region provided the greatest climate benefit by cooling summer surface air temperatures (Tsa). In contrast, the greatest warming occurred in the northern extent of the study region when afforesting with needleleaf evergreen trees maintained at high LAI. Forest management had an equal or greater influence on Tsa than the overall decision to afforest land in the southern extent of the region. Afforestation had a greater influence on Tsa than forest management in the northern extent. Integrating our results, focused on biophysical processes, with other research quantifying carbon cycle sensitivity to management can help guide the use of temperate afforestation to optimize climate benefits. Further, our results highlight the potential importance of including forest management in simulations of past and future climate.
- A minimally managed switchgrass ecosystem in a humid subtropical climate is a source of carbon to the atmosphereAhlswede, Benjamin J.; O'Halloran, Thomas L.; Forsythe, Jeremy D.; Thomas, R. Quinn (Wiley, 2021-11-06)Bioenergy has been identified as a key component of climate change mitigation. Therefore, quantifying the net carbon balance of bioenergy feedstocks is crucial for accurate projections of climate mitigation benefits. Switchgrass (Panicum virgatum) has many characteristics of an ideal bioenergy crop with high yields, low maintenance, and deep roots with potential for belowground carbon sequestration. However, the assessments of net annual carbon exchange between switchgrass fields and the atmosphere are rare. Here we present observations of net carbon fluxes in a minimally managed switchgrass field in Virginia (Ameriflux site US-SB2) over 5 years (3–7 years since establishment). Average annual net ecosystem exchange (NEE) of carbon was near zero (60 g C m−2 year−1) but the net ecosystem carbon balance that includes harvested carbon (HC) was a net source of carbon to the atmosphere (313 g C m−2 year−1). The field alternated between a large and small source of carbon annually, with the interannual variability most strongly correlated with the day of the last frost and the interaction of temperature and precipitation. Overall, the consistent source of carbon to the atmosphere at US-SB2 differs substantially from other eddy covariance studies that report switchgrass fields to be either neutral or a sink of carbon when accounting for both NEE and HC. This study illustrates that predictions of net carbon climate benefits from bioenergy crops cannot assume that the ecosystem will be a net sink of carbon from the atmosphere. Background climate, management, and land-use history may determine whether widespread deployment of switchgrass as a bioenergy feedstock results in realized climate change mitigation.