Litter production and leaf-level C uptake under CO2 enrichment
Based on leaf duration data (Asshoff, Zotz & Körner 2006) and annual leaf litter production, which both remained unaffected by high CO2, we can conclude that canopy CO2 enrichment did not alter the steady leaf area index of five in this mature forest, similar to what has been reported for a deciduous plantation forest in Tenessee (Oak Ridge FACE; Norby & Zak 2011). The strong sugar signal observed in Q. petraea foliage did not feedback negatively on photosynthetic performance, which is also in line with the findings for Liquidambar styraciflua at the Oak Ridge FACE site (Sholtis et al. 2004). In the eighth and final year of CO2 enrichment, photosynthetic C uptake in the upper canopy was on average 42–48% higher in CO2-enriched trees and showed no signs of downregulation (Bader, Siegwolf & Körner 2010), consistent with earlier findings from this and other forest FACE sites (Zotz, Pepin & Körner 2005; Liberloo et al. 2007; Darbah et al. 2010; Norby et al. 2010; Ellsworth et al. 2012). However, at the Oak Ridge FACE site, progressive nitrogen limitation has eventually caused a complete loss of the photosynthetic response to elevated CO2 (Norby et al. 2010).
Radial growth and stable C isotope signatures in tree-rings under elevated CO2
Elevated atmospheric CO2 has often been reported to stimulate radial stem growth in young trees, at least temporarily, but it has also become evident that potential growth responses are highly species-specific and contingent on tree age, stand demography and the availability of resources other than CO2 (Körner 2006; Norby & Zak 2011; Smith et al. 2013a). The forest soil at our SCC FACE site is nutrient-rich and according to accepted forestry standards the currently c. 110-year-old stand grows vigorously, yet our study trees showed no persistent growth stimulation in response to CO2 enrichment, suggesting alternative pathways for the extra C assimilated under elevated CO2. By contrast, Pinus taeda growing in a temperate plantation forest (Duke FACE) and young poplar saplings in bio-energy plantations on fertile ground (POP-FACE) showed sustained enhancement of radial stem growth under CO2 enrichment (pine: 13–27% across study years, poplar: 20–29%, Moore et al. 2006; Liberloo et al. 2006). However, at the Oak Ridge FACE site, the initial stimulation in radial stem growth of Liquidambar styraciflua under elevated CO2 rapidly shifted below-ground to increased fine root production, a signal that also became insignificant towards the end of the experiment (Norby et al. 2010). Mediterranean Quercus ilex trees growing around two natural CO2-springs as well as CO2-enriched Populus tremuloides clones at the ASPEN FACE site both showed initial growth enhancement that disappeared with time (Hättenschwiler et al. 1997; Kubiske et al. 2006). Species-specific growth responses became strikingly apparent at the Swiss tree line FACE site where tree-ring increments in the late-successional Pinus uncinata remained completely unaffected by elevated CO2 over 9 years, while growth in early successional Larix decidua trees was stimulated (though diminishing with time), depending on summer temperature (Dawes et al. 2011, 2013). At the Bangor FACE site, above-ground woody biomass of CO2-enriched Fagus sylvatica saplings increased only in monoculture (+22%) but showed no response when saplings were grown in polyculture (Smith et al. 2013a). Although there was no sustained growth response in CO2-enriched trees at our SCC FACE site, the δ13C signal in newly formed tree-rings declined markedly and very rapidly following the start of CO2 enrichment and quickly returned to pre-treatment levels after the end of the experiment, highlighting the tight coupling between recent photosynthates and wood formation. Yet, the data also illustrate that C ending up in tree-rings has been assimilated over a period of 2–3 years, implying that the current year only contributes a certain percentage to the total bulk signal. The post-treatment return path shown in Fig. 4 depicts this important finding most clearly and suggests that climate correlates of seasonal isotope signals in tree-rings should be roughly halved in strength due to such mixing of photo-assimilates.
Elevated CO2 effects on tree water relations and soil moisture
Many plants reduce stomatal conductance and thus leaf transpiration in response to elevated CO2 (Medlyn et al. 2001; Holtum & Winter 2010). In our study trees, stomatal conductance was slightly diminished by CO2 enrichment on bright days, with species-specific responses between zero in Q. petraea and 22% in C. betulus (Keel et al. 2007). Similarly, whole-tree transpiration estimated from sap flow measurements was on average 10–15% reduced under elevated CO2, resulting in measurable soil water savings (Fig. 5; Leuzinger & Körner 2007; Bader, Hiltbrunner & Körner 2009). These findings are corroborated by other closed-canopy forest FACE experiments and modelling approaches (Warren et al. 2011). Water savings on a full year/all weather basis are much smaller, however. Model simulations based on historical rainfall patterns showed that rainfall distribution by far outweighs the remaining, small (< 3%) effects of CO2-related soil water savings on runoff in our study forest (Leuzinger & Körner 2010). The main effects of CO2-driven water savings are below-ground processes discussed below.
Below-ground responses to CO2 enrichment
Because of the largely lacking above-ground growth stimulation in our study trees, we assumed greater C investments below-ground. Indeed, proxy data such as higher soil CO2 concentrations suggested such a response during early experimental years (Steinmann et al. 2004; Keel, Siegwolf & Körner 2006). However, we found a 20–30% reduction in fine root biomass and new fine root growth into root-free soil cores under CO2-enriched trees in year five and six of the study and no significant treatment-related differences in soil cores taken in the following year (Bader, Hiltbrunner & Körner 2009). Similarly, elevated CO2 failed to stimulate fine root investments in larch and pine trees receiving CO2 enrichment at the tree line in the Swiss Central Alps and also in a Florida scrub oak system (Brown et al. 2007; Handa, Hagedorn & Hättenschwiler 2008). At the Bangor FACE site, CO2 enrichment merely produced a transient stimulation in fine root biomass of Fagus sylvatica saplings and even caused a decline in their coarse root biomass (Smith et al. 2013b). Fine root production dominated the CO2-driven stimulation of NPP over several years of CO2 enrichment at the Oak Ridge Liquidambar styraciflua plantation, particularly at greater soil depth. But this response ceased during the last years of the study and might therefore reflect a transitory response to a step change in CO2 supply (Norby et al. 2010).
The fossil CO2 used for canopy enrichment was consistently depleted in 13C relative to ambient atmospheric CO2 and thus allowed us to trace the flow of newly assimilated (labelled) C from the crowns to the rhizosphere of the treated trees (Steinmann et al. 2004; Keel, Siegwolf & Körner 2006). Only 11 days after the start of CO2 enrichment, the isotopic signal of the fumigation gas became detectable in soil air (Steinmann et al. 2004) and after 4 growing seasons, newly developed leaves consisted of 100%, and new tree-rings of 91% recent C (Keel, Siegwolf & Körner 2006). In fine roots (< 1 mm) formed in ingrowth cores during years five and six of the study, only 51% of the C carried the isotopic signature of the CO2 released in the canopy, implying long fine root C turnover rates of c. 12 years or utilization of old non-structural carbon reserves for root formation (Bader, Hiltbrunner & Körner 2009).
Recent photosynthates were rapidly channelled to below-ground C sinks, as evidenced by strong isotopic signals in soil air and symbiotic fungi (Steinmann et al. 2004; Keel, Siegwolf & Körner 2006). Three months after the beginning of CO2 enrichment, sporocarps of mycorrhizal fungi associated with CO2-enriched trees already consisted of 62% new C, while saprophytic fungi were devoid of 13C-depleted C after 4 years of treatment suggesting decomposition of C compounds that were formed prior to the start of the experiment (Keel, Siegwolf & Körner 2006).
Given the absence of increases in above- and below-ground growth and litter production, we assumed that the extra C assimilated under elevated CO2 might be largely respired back to the atmosphere via soil metabolism (Körner et al. 2005). Surprisingly, the stronger CO2 build-up in soil under CO2-enriched trees during growing seasons did not translate into a sustained increase in soil respiration, most likely due to reduced soil diffusivity resulting from soil water savings (Bader & Körner 2010). This contrasts with other forest FACE studies where soil CO2 efflux in closed-canopy stands increased by 12–23% (King et al. 2004; Jackson et al. 2009). Leaching of DIC and DOC was not a major loss pathway for the ‘missing C’ either. The total fluxes of dissolved C were small (c. 15 g C m−2 a−1) and remained unaffected by the CO2 enrichment (Fig. 6). In the topsoil, CO2 enrichment increased DIC fluxes by 50%, most likely reflecting the higher partial CO2 pressure in soil pores. This increase was outbalanced by an unexpected decline in DOC leaching, which we primarily attribute to reduced solubilization of soil organic matter (SOM) in the mineral soil through acidification associated with increased soil CO2 concentrations. Topsoil acidification was further enhanced by higher -N leaching as two protons are released per mole of nitrate leached, thereby suppressing DOC leaching (Evans et al. 2008). At 15 cm depth, both DOC and DIC leaching remained unaffected by CO2 enrichment, thus ruling out this ‘leak’ pathway for the additionally assimilated C, perhaps because this forest grows on shallow soil over calcareous rock and rock debris. At the Aspen and Duke FACE sites, DIC leaching increased considerably under elevated CO2 (Karberg et al. 2005; Jackson et al. 2009).
There is a growing awareness that CO2 fertilization effects will be smaller than anticipated (Leuzinger et al. 2011) and will certainly not be uniform across landscapes but rather follow availability patterns of colimiting growth resources such as water and nutrients (Oren et al. 2001; Körner 2006; McCarthy et al. 2010). Moreover, potential CO2 fertilization effects on tree growth might be offset over time by progressive N limitation in soils resulting from N sequestration in long-lived biomass or SOM pools or by reduced longevity (Luo et al. 2004; Norby et al. 2010; Bugmann & Bigler 2011). For instance, after 9 years of CO2 enrichment at the Oak Ridge site, progressive N limitation caused a decline of the strong NPP response that was previously dominated by fine root production (Norby et al. 2010). At our site however, the lacking growth responses to CO2 enrichment cannot be attributed to N deficiency since this region receives 20–25 kg N ha−1 a−1 wet nitrogen deposition, which is close to the upper threshold of critical loads for this type of ecosystem (Thimonier et al. 2010). Moreover, CO2-induced increases in soil moisture and nitrate availability resulted in enhanced nitrate leaching at this already N-rich site. Also, the increase in the natural abundance of 15N in nitrate captured by resin bags suggests accelerated net nitrification under elevated CO2 (Schleppi et al. 2012). The temporal δ15N dynamics derived from archived leaf samples of our tall study trees remained remarkably stable under elevated CO2 suggesting unaltered and ample N supply even during years when lower δ15N values of control foliage indicated declining N availability under ambient conditions (Fig. 8). This was most obvious in 2004, probably representing a carry-over effect from the preceding centennial drought year 2003 that most likely had a greater impact on soil N cycling processes under control trees compared to CO2-enriched trees simply because of the soil water savings observed under elevated CO2 (Leuzinger et al. 2005). Reductions in fine root biomass may be associated with diminished nitrate consumption under elevated CO2, however, given the largely unaltered leaf N concentrations, accelerated SOM decomposition offers a more likely explanation for the accumulation of nitrate in the soil solution under high CO2. This assumption is supported by a 14% increase in soil microbial biomass under CO2-enriched trees (Bader & Körner 2010). Based on our and other FACE data, we postulate two concurrent causes underlying the observed stimulation of soil microbes and soil nitrate accretion. CO2-induced soil water savings may have stimulated soil microbial biomass and nitrification rates, which have been shown to increase with increasing soil moisture (Stark & Firestone 1995). In addition to this water-driven effect, CO2-enriched trees can fuel microbial activity in their rhizosphere through enhanced root exudation as has been demonstrated for tropical woody species in large model communities (Körner & Arnone 1992) and more recently for Pinus taeda at the Duke FACE site (Phillips, Finzi & Bernhardt 2011). There, this priming effect caused faster rates of SOM mineralization and nitrification, which prevented (at least for the period considered) progressive nitrogen limitation in the inherently N-poor forest soil (Drake et al. 2011). Increases in soil solution nitrate at our site may have been driven by such a priming effect stimulating microbial activity and thus N mineralization. The decrease in DON that paralleled the increase in soil nitrate supports this hypothesis.
In conclusion, our data imply that tree growth in this late-successional temperate forest is not limited by current atmospheric CO2 concentrations, suggesting that these types of stands are unlikely to grow faster in a future high-CO2 world. Whether or not tree growth will be stimulated, such experiments are principally unable to infer long-term changes in ecosystem C pools (C sequestration, Körner 2006), since C storage in biomass is in essence a demographic phenomenon (Bugmann & Bigler 2011), and soil C pools are responding too slowly to allow separation of transient from steady-state responses. It remains unresolved which resources become rate-limiting under humid, C and N saturated conditions, but phosphate and key cations are likely candidates. Our findings point towards enhanced DIC and nitrate leaching from such forest soils with rising atmospheric CO2 levels. As a result, soil acidity and mineral weathering may intensify in a CO2-rich future impacting on soil biota and nutrient cycling, while at the same time, nitrate contamination of groundwater is likely to become an increasing problem, especially in already predisposed regions.
Overall, we conclude that indirect CO2 effects inducing changes in the forest N-cycle and soil water regime are far more important than first-order CO2 effects on growth in this system. With regard to future climate predictions, our findings and those from other FACE studies challenge the assumption of a sustained and universal CO2 fertilization effect on tree growth that most climate–carbon cycle models critically rely on.