How do different CO2 fluxes respond to changes in temperature and S level?
Gross photosynthesis was decreased slightly under simulated climate warming. This finding is supported by ecophysiological measurements of these (heated) communities, which revealed increased midday stress in summer with subsequent downregulation of photosystem 2 (B. Gielen et al. unpublished data), and decreased stomatal conductivity (Lemmens et al., 2006) in response to the drier conditions (De Boeck et al., 2006). When expressed on a per unit biomass basis, photosynthesis actually increased in heated communities, apparently in contrast with the above-mentioned ecophysiological measurements. However, because above-ground biomass production declined under warming, and considering that biomass and leaf area index in grasslands are related (Spehn et al., 2000), canopies were less dense under warmer conditions. This probably stimulated the photosynthetic rates per unit biomass as fewer leaves were shaded (Larcher, 2003), while the total photosynthesis was lower because less green mass was present.
The ecophysiological measurements showed that photosynthesis was probably limited under high temperatures and dry soils. Adverse effects of warming on Pgross would therefore have occurred especially during the summer season. The yearly course of gross photosynthesis in the current experiment indeed showed that the largest Pgross decreases in heated communities were recorded in midsummer. Likewise, in a pan-European study on carbon fluxes during the warm 2003 summer, Reichstein et al. (2007) concluded that such a heat wave resulted in reduced photosynthesis. Furthermore, Knapp et al. (2002) showed in a 4-yr experiment on grasslands that CO2 uptake and productivity were negatively related to variability of SWC. As drying of the upper soil layer is faster in warmer conditions, and therefore in heated communities, and as this increases SWC variability, this effect may have further reduced photosynthesis and productivity under warming, especially considering that an important portion of roots is located in this top soil layer (Jones & Donnelly, 2004). Such an effect would again have been most pronounced in summer, when high temperatures stimulate rapid soil drying.
Above-ground respiration was not lowered under warmer conditions, while the respiratory flux per unit biomass was increased by a substantial amount. This was probably associated both with a direct temperature-induced stimulation (Barnes et al., 1998), and possibly with higher levels of abiotic stress resulting in increased maintenance respiration (Larcher, 2003). Below-ground respiration seemed largely unresponsive to warming. It is possible that any direct, temperature-driven respiration increases through improved metabolic efficiency of heterotrophs and higher root turnover (Edwards et al., 2004) were counteracted by secondary responses. The warming-induced drought (De Boeck et al., 2006) could, for example, have limited heterotrophic activity, in accordance with other studies (Gorissen et al., 2004; Harper et al., 2005). Furthermore, the decreased plant productivity observed both above- and below-ground (H.J.D.B. and co-workers, unpublished data), probably also counteracted direct warming-induced stimulation of soil respiration, with several studies highlighting the importance of productivity on below-ground respiration (Saleska et al., 2002; Zhang et al., 2005).
The CO2 fluxes in heated communities were possibly dampened by some degree of acclimation, as Vicca et al. (2007) in another study performed on the same communities observed thermal acclimation of both photosynthesis and total ecosystem respiration. Finally, while we found that SWC did not affect the relationship between the environmental drivers and the CO2 fluxes, this does not signal the absence of drought effects on the fluxes, but merely that these effects were uniform across the entire SWC range, and that no drought thresholds that would disproportionately affect fluxes were exceeded. Meteorological data from the nearby Lint meteo station show that precipitation amounts in both the first (808 mm) and second (733 mm) measurement year were close to average, with slightly wetter-than-average summers, while the average temperature was 11.0°C in both years. The fact that both temperatures and precipitation were very similar in both experimental years makes important drought stress differences between these years unlikely.
Both photosynthesis and above-ground respiration were higher in multispecies communities. This corresponds with our observations on above-ground biomass production, and is also in line with other studies on the relationship between species richness and productivity (Roscher et al., 2005; van Ruijven & Berendse, 2005). In another study, we found that complementarity (including facilitation) caused most of the productivity differences between S levels (H.J.D.B. and co-workers, unpublished data), while net selection effects were small or absent, even though shifts in species success were observed (De Boeck et al., 2006, 2007). Multispecies communities were probably able to capture more of the available light because of less-uniform and hence better-filled canopies (Cernusca, 1976; Spehn et al., 2000; Middelboe & Binzer, 2004), directly resulting in higher photosynthesis. The increased space filling also seemed reflected in the fact that Pgross in mixtures was lower per g biomass in such dense canopies.
Below-ground respiration was similar in communities of different S. Several studies have examined links between species richness and soil respiration, but most of the effects appear to be caused by increased above-ground productivity and subsequently increased litter production in multispecies systems (Zak et al., 2003; Dijkstra et al., 2005). These effects were probably suppressed in our experiment because of the half-yearly mowing and removal of above-ground biomass. Nevertheless, below-ground production was also increased in mixtures (H.J.D.B. and co-workers, unpublished data), and could have stimulated soil respiration. However, the lower SWC in mixtures (De Boeck et al., 2006) may have counteracted any such production-induced stimulation of below-ground respiration.
Do warming and S level affect the CO2 sink or source capacity of grasslands?
Despite the probably overly expanded confidence intervals, we can conclude that the grasslands in this experiment acted as net sinks for CO2. This sink capacity decreased, rather than increasing, under simulated climate warming, especially when also considering the harvested biomass, which was significantly lower in heated communities. Carbon flux studies have reported very varying responses to climate warming, depending mainly on which climatological factor is most limiting to plant growth in the ecosystem studied. While warming could mitigate constraints of low temperatures on metabolic activity in polar regions (Marchand et al., 2004), it could increase heat and drought stress in warmer regions (Llorens et al., 2003). In our temperate grasslands, warming seemed to benefit plant activity in autumn and winter, with increased photosynthesis and above-ground respiration. However, in late spring and summer, when soil moisture limits plant growth more than temperature, we observed a predominantly negative effect of warming, especially on photosynthesis. Studies on currently exceptionally warm summers in temperate regions have shown decreased productivity (Ciais et al., 2005) and inhibited CO2 fluxes (Reichstein et al., 2007) under such conditions. Our research shows that this is likely to occur during many summer seasons in a future warmer world. Even if warming prolongs the growing season (Myneni et al., 1997; Walther, 2003), or increases plant activity in winter, negative effects of warming during summer will probably be dominant in the yearly C balance, as the summer is the main growing season in temperate regions. The effects of species richness on the net CO2 flux were largely unsubstantial, although if taking the harvested biomass into account, multispecies communities were probably larger CO2 sinks than monocultures.
Are there interactive effects on CO2 fluxes of the two global changes studied?
In previous studies on these communities, we have observed consistent interactions between warming and species richness, regarding both water relations (De Boeck et al., 2006) and primary production (H.J.D.B. and co-workers, unpublished data). The differences between the two temperature treatments were most pronounced in the nine-species communities. A similar trend was observed regarding the net CO2 fluxes, although only in year 1 and probably reflecting the photosynthesis fluxes. It is clear that the ‘insurance hypothesis’ (Naeem & Li, 1997) did not apply to these communities, as the negative effect of warming was not dampened at higher species-richness levels. Tilman et al. (2006) argue that the insurance hypothesis may not apply in the short term, after an experiment showed that following an 8-wk drought, the absolute biomass loss was greater at higher species richness (Pfisterer & Schmid, 2002). Our results suggest that the insurance effect may not apply in the longer term if abiotic stress is increased. We have put forward several possible explanations for this (De Boeck et al., 2006; H.J.D.B. and co-workers, unpublished). It is, for example, possible that interspecific competition increased substantially under the higher levels of abiotic stress experienced in heated communities (Callaway et al., 2002; Michalet et al., 2006). This may also explain why the effects of warming were least pronounced, or even absent, at the monoculture level. These findings emphasize the need to study global changes simultaneously, as the responses to single changes are not necessarily additive.
We would argue that careful extrapolation of the current experiment is not unrealistic, for several reasons. First, although drought effects may have been stronger in this experiment compared with natural conditions caused by the absence of a water table, this effect was probably not defining, because in grasslands up to 80% of the roots can be found in the top 30 cm (Jones & Donnelly, 2004). This suggests that grasslands depend mostly on precipitation, and not so much on the water table. Second, although the communities were small in size, both experimental and modelling studies have shown that positive relationships between plant species richness and biomass production are robust and independent of spatial scale or species pools (Cardinale et al., 2004; Roscher et al., 2005). Individual plants interact mainly with close neighbours, making the small-scale distribution of plants of particular importance (Pacala, 1997). Third, in natural grasslands more species are often present, but the objective was to examine the course of S effects, and the strongest changes usually occur at low S levels (Spehn et al., 2005). Finally, we are confident that the direction of the responses will also be consistent in the longer term. We used a variety of grassland species from different functional groups, and opted to include species that also occur in warmer climates, such as F. arundinacea and M. sativa, as well as species that occur in humid grasslands, such as R. acetosa and L. perenne. This ensured that the competitive success of the species could differ depending on which (sets of) traits were beneficial under warmer conditions. Although we did observe shifts in the competitive success of the different species under warming, with species from warmer regions apparently gaining in importance (De Boeck et al., 2006, 2007), the community response was a decrease in production. This suggests that, regardless of compositional changes of temperate grassland communities in the future, warming will probably have a negative effect on community productivity. However, it is imperative that further studies are carried out, also in natural communities, to investigate whether the general effects of warming and species richness that we observed here are consistent.