Abstract
Warming during late winter and spring in recent decades has been credited with increasing high northern latitude CO 2 uptake, but it is unclear how different species and plant functional types contribute to this response. To address this, we measured net ecosystem exchange (NEE) at a deciduous broadleaf (aspen and willow) forest and an evergreen conifer (black spruce) forest in interior Alaska over a 3-year period. We partitioned NEE into gross primary production (GPP) and ecosystem respiration ( R e) components, assessing the impact of interannual climate variability on these fluxes during spring and summer. We found that interannual variability in both spring and summer NEE was greatest at the deciduous forest. Increases in spring air temperatures between 2002 and 2004 caused GPP to increase during the early part of the growing season (April, May, and June), with a 74% increase at the deciduous forest and a 16% increase at the evergreen forest. R e increased in parallel, by 61% and 15%, respectively. In contrast, a summer drought during 2004 caused GPP during August to decrease by 12% at the deciduous forest and by 9% at the evergreen forest. Concurrent increases in R e, by 21% and 2% for the two forests, further contributed to a reduction in net carbon uptake during the drought. Over the growing season (April–September) net carbon uptake increased by 40% at the deciduous forest and 3% at the evergreen forest in 2004 as compared with 2002. These results suggest that deciduous forests may contribute disproportionately to variability in atmospheric CO 2 concentrations within the northern hemisphere and that the carbon balance of deciduous forests may have a greater sensitivity to future changes in climate.
Highlights
From the 1970s to 2005, surface air temperatures in arctic and boreal biomes increased by approximately 0.4 8C per decade (ACIA, 2004; Hansen et al, 2006)
During June and July, mean daily net ecosystem exchange (NEE) at the deciduous forest, averaged over the 3 years of measurements, was À2.7 Æ 0.4 g C mÀ2 dayÀ1 and was two to three times larger than that measured at the evergreen forest, À1.0 Æ 0.1 g C mÀ2 dayÀ1
Over 95% of annual gross primary productivity (GPP) at the deciduous forest occurred over the 119-day interval between 14 May and 9 September
Summary
From the 1970s to 2005, surface air temperatures in arctic and boreal biomes increased by approximately 0.4 8C per decade (ACIA, 2004; Hansen et al, 2006). The consequence of these temperature increases, and further increases predicted over the several decades (IPCC, 2001), for carbon stores in northern ecosystems remains uncertain because temperature changes may trigger both positive and negative feedbacks with the carbon cycle (Braswell et al, 1997; McGuire et al, 2006). Increased temperatures can increase the depth of soil thaw (Euskirchen et al, 2006), exposing more soil organic matter to decomposition (Goulden et al, 1998; Hirsch et al, 2002) and causing a net loss of carbon from ecosystems (Goulden et al, 1998; Lindroth et al, 1998; Valentini et al, 2000)
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