The sensitivity of carbon fluxes to spring warming and summer drought depends on plant functional type in boreal forest ecosystems
Introduction
From the 1970s to 2005, surface air temperatures in arctic and boreal biomes increased by approximately 0.4 °C per decade (ACIA, 2004, Hansen et al., 2006). The consequence of these temperature increases, and further increases predicted over the next 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).
In northern forests, there are multiple competing effects of increasing air temperatures on gross primary productivity (GPP) and ecosystem respiration (Re). Warmer springs lead to an earlier onset of photosynthesis (Black et al., 2000, Tanja et al., 2003), enhance GPP during spring months (Goulden et al., 1996, Arain et al., 2002, Angert et al., 2005) and often increase annual net carbon uptake (Chen et al., 1999, Black et al., 2000, Barr et al., 2002, Chen et al., 2006). However, increased temperatures can also 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).
Low moisture conditions during drought (created by either anomalously low precipitation or anomalously high temperature that increases evapotranspiration) cause both GPP and Re to decline (Ciais et al., 2005, Kljun et al., 2006). There is still debate about whether GPP or Re is most adversely affected by drought and thus the sign of the net ecosystem exchange (NEE) response in different ecosystems. An extreme drought in Europe during 2003, for example, caused many ecosystems to lose carbon (Ciais et al., 2005). In contrast, Goulden et al. (1996) found that Re was more sensitive to reduced soil moisture availability than GPP, and this caused carbon to accumulate at a faster rate during a late-summer drought in a temperate deciduous forest. Similarly, decreases in soil respiration from limited moisture availability causes net ecosystem carbon uptake to increase during the dry season in moist tropical forest ecosystems (Saleska et al., 2003). The net effect of drought may depend on the severity of moisture limitation. Reichstein et al. (2002) hypothesize that during conditions where only the surface soil layers are affected, heterotrophic Re will be impacted more than GPP (increasing net carbon uptake) and that it is not until severe drought conditions substantially lower the water table that GPP will be affected adversely through plant water stress.
Physiological and phenological differences between deciduous and coniferous forests (e.g., Falge et al., 2002) are likely to modulate the response of these two forests to climate variability. Monson et al. (2005), for example, propose that the annual carbon balance of deciduous forests is limited by the fraction of the year that leaves are still expanding and have not reached maximum leaf area. This fraction is reduced in years with early spring leaf-out, thereby increasing GPP. In contrast, the annual carbon balance of evergreen conifers may be regulated more strongly by a reduction in GPP caused by mid-summer drought stress which typically increases in years with earlier springs as a result of earlier snowmelt and surface runoff (Monson et al., 2005).
Interannual eddy covariance measurements in the boreal zone show that warm springs increase GPP substantially in deciduous aspen forests and to a lesser degree in evergreen black spruce forests (Black et al., 2000, Arain et al., 2002). During warm summers, in the absence of drought, deciduous aspen forest Re remains largely unchanged (Arain et al., 2002, Griffis et al., 2003, Kljun et al., 2006), whereas evergreen black spruce Re increases substantially (Goulden et al., 1998, Arain et al., 2002). Therefore, warm years appear to increase annual net carbon uptake in aspen forests (Black et al., 2000, Arain et al., 2002), and may decrease annual net carbon uptake in black spruce forests (Goulden et al., 1998). Kljun et al. (2006) found that higher soil moisture contents due to inherently low soil drainage at an evergreen black spruce forest buffered the effect of drought compared with that in a nearby drier deciduous aspen forest with higher rates of soil drainage.
Here, we report measurements of NEE at a deciduous aspen forest and an evergreen black spruce forest over 3 years (2002–2004) in interior Alaska. Our objective was to determine how plant functional type (deciduous versus evergreen) modulates ecosystem carbon flux response to interannual climate variability. These two forests were part of a fire chronosequence that has been used in the past to examine the effects of post-fire stand age on the soil microbial community (Treseder et al., 2004), variability in burn severity (Kasischke and Johnstone, 2005), surface energy fluxes (Liu et al., 2005), and the seasonal cycle of atmospheric CO2 and δ18O-CO2 (Welp et al., 2006). Spring air temperatures increased progressively during 2002 through 2004. The summer of 2004 was one of the hottest and driest in Alaska (ACRC, 2006), contributing to the worst fire season on record (AGDC, 2006). Because of the close proximity of the sites to one another, it was possible to directly compare the relative effects of the same climate variability on net carbon uptake in two different forest types. We found that the deciduous aspen forest was much more sensitive to variability in climate than the evergreen black spruce forest.
Section snippets
Site description
We measured NEE using the eddy covariance technique at two forests in interior Alaska near Delta Junction (63°54′N, 145°40′W). One had an overstory primarily comprised of Picea mariana (black spruce) and is hereafter referred to as the evergreen conifer forest. Understory species at this site in 2002 included Vaccinium uliginosum, Vaccinium vitisidaea, Betula glandulosa, and Ledum palustre. Dominant mosses included Hylocomium splendens and Pleurozium schreberi. Lichens within the moss layer
Mean seasonality of ecosystem carbon fluxes
Eddy covariance NEE measurements at each of the two forests from 2002 through 2004 are presented in Fig. 1. The period of carbon accumulation was shorter and more intense at the deciduous forest than at the evergreen forest (Fig. 1). The mean interval of net carbon accumulation, defined as the continuous period when daily total NEE was negative, was 100 days at the deciduous forest (from 23 May through 31 August) and 144 days at the evergreen forest (from 21 April through 12 September) (Fig. 2).
Conclusions
Simultaneous eddy covariance NEE measurements of deciduous and evergreen forests in years experiencing different climate conditions showed that warm springs and summer drought resulted in greater variability in NEE and GPP at a deciduous forest than at an evergreen forest. This result implies that (1) the current variability in atmospheric CO2 in the northern hemisphere may have a disproportionately higher contribution from deciduous forests than from evergreen conifer forests, after
Acknowledgements
This work was supported by grants from NSF's Office of Polar Programs (NSF OPP-0097439) and NOAA's Office of Global Programs (NA03OAR4310059), the Powell Foundation and from the Davidow discovery fund at Caltech. We thank J. Lindfors, C. Dunn, and J. Henkelman for help developing eddy covariance systems, F.S. Chapin for support through University of Alaska, Fairbanks, and J. Garron for data collection. LRW received support from the NCER STAR EPA fellowship program.
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