Relative Contribution of Climate Change to Conterminous US Forest Carbon Sinks

U.S. forests, accounting for about 6% of the world forests and about 33% of U.S. landmass, contribute about 10% to the global net terrestrial carbon (C) sink in the late 20th century. This large forest C sink is usually attributed to forest regrowth after harvest and abandoned agriculture, and fire suppression. However, it may also be partially caused by forest growth enhancement due to climate change, CO2 fertilization and nitrogen (N) deposition. Recently, changing climate (e.g. temperature and precipitation) and atmospheric pollution (e.g. ozone, N deposition) have had significant direct and indirect effects on the US forest carbon cycle. Wildfires and insect attacks have increased in recent years due to climate change. The relative contribution of changing climate to the overall C dynamics of the conterminous U.S. forests remains uncertain and controversial. We used the Integrated Terrestrial Ecosystem Carbon Model (InTEC) to estimate the integrated disturbance (fire, insect, harvest) and non-disturbance (climate, CO2, nitrogen deposition) effects on the conterminous US forest carbon cycle since 1901. InTEC results show that the net biome productivity (NBP) of US conterminous forests were 210 Tg C/y on average during the period from 1951 to 2010 without CO2 and nitrogen fertilization effects. However, only 0.9% (1.9 Tg C/y) of this NBP was caused by climate change, with 3.8% (7.9 Tg C/y) contributed from the Eastern U.S. forests and -2.9% (-6.1 Tg C/y) from the Western U.S. forests. To assess the impacts of future climate, we considered the IPCC A2 and B2 climate senarios. The predicted magnitude of C dynamics for conterminous US forests under the A2 climate scenario is similar to but larger than that under the B2 climate scenario by the end of 21th century. The predicted average annual NBP over the period 2011-2100 varies between -3% (-12.7 Tg C/y) and 2% (3.9 Tg C/y) for the A2 scenario and between -2% (-8.9 Tg C/y) and 3.3% (6.6 Tg C/y) for the B2 scenario, depending on different disturbance and stand age scenarios,when the influence of CO2 fertilization is excluded. These results indicate that managing forest stand age structures in appropriate ways could influence the response of forests to climate change.