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Joy Clein, A. David McGuire, Eugenie S. Euskirchen, and Monika Calef

. 2007 ). In contrast, a recent remote sensing study has found that photosynthetic activity in the boreal forest of North America unaffected by fire has declined in the last two decades ( Goetz et al. 2005 ). While process-based biogeochemical models generally simulate a small net carbon sink in the recent past for the distribution of Arctic tundra ( Sitch et al. 2007 ), there is much spatial variability in estimated net carbon balance by these models (e.g., see McGuire et al. 2000a ; Euskirchen et

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J. S. Kimball, K. C. McDonald, and M. Zhao

Eurasia and North America ( Nemani et al. 2003 ; Barber et al. 2000 ; Bunn et al. 2005 ), and the short-term, negative effects of fire-related disturbance on LAI ( Hicke et al. 2003 ; Goetz et al. 2006 ). The SSM/I-derived timing of the primary thaw event in spring was inversely proportional to AVHRR PEM–derived annual anomalies in both GPP ( r = −0.851; P = 0.0002) and NPP ( r = −0.857; P = 0.0001) for the study domain (see Figure 6 ). Years with relatively early seasonal thawing showed

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M. A. Rawlins, S. Frolking, R. B. Lammers, and C. J. Vörösmarty

problem in the Arctic where gauge undercatch is often substantial. Precipitation underestimates of 20% to 25% have been determined across North America ( Karl et al. 1993 ), while biases of 80% to 120% (in winter) have been estimated for the terrestrial Arctic north of 45°N ( Yang et al. 2005 ). In regions where precipitation exceeded potential evapotranspiration (PET), uncertainty in precipitation translated to an uncertainty in simulated runoff of roughly similar magnitude ( Fekete et al. 2004 ). To

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J. S. Kimball, M. Zhao, A. D. McGuire, F. A. Heinsch, J. Clein, M. Calef, W. M. Jolly, S. Kang, S. E. Euskirchen, K. C. McDonald, and S. W. Running

likelihood of rapid terrestrial carbon losses from fire disturbance, especially in North American boreal forests dominated by frequent stand replacing fires. Recent long-term studies of North American boreal fire regimes also indicate that the extent and frequency of large fires in Alaska and Canada are increasing with global warming ( McGuire et al. 2004 ). The soil organic carbon pool represents approximately 75% of the estimated total terrestrial carbon reservoir for the region, while boreal and

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Sheldon Drobot, James Maslanik, Ute Christina Herzfeld, Charles Fowler, and Wanli Wu

. 1996 ), 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analyses (ERA-15; Gibson et al. 1997 ), and the Climatic Research Unit/University of East Anglia CRUTEM2v (CRU; Jones et al. 2001 ) datasets were analyzed. The results indicated that temperature differences between the NCEP1 and CRU datasets were largest in winter and smallest in summer, with NCEP1 being warmer over North America; comparisons for NCEP1 and ERA-15 were similar, whereas ERA-15 was noticeably warmer than CRU

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T. Scott Rupp, Xi Chen, Mark Olson, and A. David McGuire

annual burning of 5–15 Mha of boreal forest ( Stocks et al. 2002 ; Lavorel et al. 2005 ; Flannigan et al. 2006 ). Current estimates are that an average of 2.3 Mha burn annually across the North American boreal forest, with the amount of annual area burned ranging between 0.5 and 8 Mha ( Amiro et al. 2001 ; Kasischke et al. 2006 ; Csiszar et al. 2004 ), and there is a growing awareness of the importance and vulnerability of the region to forecast climatic change ( Weber and Flannigan 1997

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