Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s t(adjusted R 2 ) and Mallow’s CP statistics. When a minimumCP statistic and maximum R 2 are selection criteria, theresult is the best fit model with the minimum number ofparameters.ResultsNPP and ecosystem C exchangeIn both 2004 and 2006, NEE, GPP, and Reco measuredwith the closed chambers were significantly correlated withNPP measured with the point-framing method (Fig. 1a,b, c). These same ecosystem C exchange variables weresignificantly correlated with active layer thickness, but NPPbetter described overall variation (not shown). The slope andintercepts of the NPP vs. Reco and GPP relationships werenot significantly different between years, but the slope of theNEE lines did differ significantly between years.Relationship between carbon exchange and specific plantfunctional groupsWe previously reported that across this upland tussocktundra landscape, thermokarst corresponded to a relativedecrease in the NPP of graminoids while moss and deciduousshrub NPP increased (Schuur et al. 2007). Here we usemultiple regression analysis to examine whether the NPPof individual functional groups corresponded to a changein NEE, Reco or GPP. For NEE, the regression procedureselected a different combination of plant functional typesin different years. In 2004, the variance in NEE was bestexplained with evergreen shrub NPP (+ effect in model)and the coverage of dead moss (- effect in model). In 2006,the variance in NEE was best explained by moss, evergreenshrub, and graminoid NPP. Interestingly, in neither yearwere deciduous shrubs a component of the NEE modeldespite the NPP of this functional group generally increasingwith active layer thickness (Schuur et al. 2007). Indeed, forGPP deciduous shrubs were also removed from the overallmodel in both years. This suggests that additional C uptakeby deciduous shrubs was effectively replacing the C uptakeof the different functional groups that were lost.AcknowledgmentsWe thank Michelle Mack for assistance in processingvegetation samples. Terry Chapin provided laboratoryresources. Emily Tissier, Jamie Hollingsworth, and BrianCharlton provided logistical support. Tom Osterkampundertook the early research that made this project possible,and Guy Adema and Larissa Yocum of Denali National Parkand Preserve provided logistical support.GPP (g C m -2 )Reco (g C m -2 )NEE (g C m -2 )7006005004003000200-100-200-300-400-500-600-700300250200150100500-50-100200420062004 r=0.73, p