Historical forest baselines reveal potential for continued carbon sequestration

Abstract

One-third of net CO(2) emissions to the atmosphere since 1850 are the result of land-use change, primarily from the clearing of forests for timber and agriculture, but quantifying these changes is complicated by the lack of historical data on both former ecosystem conditions and the extent and spatial configuration of subsequent land use. Using fine-resolution historical survey records, we reconstruct pre-EuroAmerican settlement (1850s) forest carbon in the state of Wisconsin, examine changes in carbon after logging and agricultural conversion, and assess the potential for future sequestration through forest recovery. Results suggest that total above-ground live forest carbon (AGC) fell from 434 TgC before settlement to 120 TgC at the peak of agricultural clearing in the 1930s and has since recovered to approximately 276 TgC. The spatial distribution of AGC, however, has shifted significantly. Former savanna ecosystems in the south now store more AGC because of fire suppression and forest ingrowth, despite the fact that most of the region remains in agriculture, whereas northern forests still store much less carbon than before settlement. Across the state, continued sequestration in existing forests has the potential to contribute an additional 69 TgC. Reforestation of agricultural lands, in particular, the formerly high C-density forests in the north-central region that are now agricultural lands less optimal than those in the south, could contribute 150 TgC. Restoring historical carbon stocks across the landscape will therefore require reassessing overall land-use choices, but a range of options can be ranked and considered under changing needs for ecosystem services.

Fig. 1.

Dominant land cover/use (A), above-ground live forest carbon (MgC per ha of total land area) (B), and above-ground live forest carbon density (MgC per ha of forest area) (C) in Wisconsin in the mid-1800s (before EuroAmerican settlement), the 1930s (peak agricultural clearing), and the present. Carbon estimates are median values calculated from the Monte Carlo simulation results (to account for uncertainties in the historical data sources).

Fig. 2.

Total above-ground live forest carbon in Wisconsin (TgC) (A) and by forest type (MgC/ha) in northern (B) and southern (C) Wisconsin from the mid-1800s to the present. Data are medians and 95% confidence intervals.

Fig. 3.

Potential for carbon sequestration (TgC) in Wisconsin given full forest recovery and reforestation of current agricultural lands. Historical baseline is total above-ground live forest carbon in the mid-1800s, present carbon includes both forest regrowth and ingrowth into areas that historically contained less carbon. Forest potential assumes that all existing forests recover to baseline carbon stocks, whereas agricultural potential assumes reforestation of agricultural lands to historical forest carbon content.

Fig. 4.

Carbon sequestration potential (MgC/ha) in existing forests (A) and agricultural lands (B). Forest potential assumes that all existing forests recover to baseline carbon stocks, whereas agricultural potential assumes reforestation of agricultural lands to historical forest carbon content.