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Biophysical forcings of land-use changes from potential forestry activities in North America.

Zhao K, RB Jackson
Ecological Monographs
Journal Volume/Pages: 
Land-use changes through forestry and other activities alter not just carbon storage but biophysical properties, including albedo, surface roughness, and canopy conductance, all of which affect temperature. This study assesses the biophysical forcings and climatic impact of vegetation replacement across North America by comparing satellite-derived albedo, land surface temperature (LST), and evapotranspiration (ET) between adjacent vegetation types. We calculated radiative forcings (RF) for potential local conversions from croplands (CRO) or grasslands (GRA) to evergreen needleleaf (ENF) or deciduous broadleaf (DBF) forests. Forests generally had lower albedo than adjacent grasslands or croplands, particularly in locations with snow. They also had warmer nighttime LST, cooler daily and daytime LST in warm seasons, and smaller daily LST ranges. Darker forest surfaces induced positive RFs, dampening the cooling effect of carbon sequestration. The mean albedo-induced RFs for each land conversion were equivalent to carbon emissions of 2.2±0.7 kg C/m2 (GRA-ENF), 2.0±0.6 kg C/m2 (CRO-ENF), 0.90±0.50 kg C/m2 (CRO-DBF), and 0.73±0.22 kg C/m2 (GRA-DBF), suggesting that, given the same carbon sequestration potential, a larger net cooling (integrated globally) is expected for planting DBF than ENF. Both changes in LST and ET induce longwave RFs that sometimes had values comparable to or even larger than albedo-induced shortwave RFs. Sensible heat flux on average increased when replacing CRO with ENF but decreased for conversions to DBF, suggesting that DBF tends to cool near-surface air locally while ENF tends to warm it. This local temperature effect showed some seasonal variation and spatial dependence but did not differ strongly by latitude. Overall, our results show that a carbon-centric accounting is, in many cases, insufficient for climate mitigation policies. Where afforestation or reforestation occurs, however, deciduous broadleaf trees are likely to produce stronger cooling benefits than evergreen needleleaf trees provide.
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