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Atmospheric concentrations of carbon dioxide (CO2) are rising rapidly and are expected to be c. 40% higher in 2050 than they are today (Houghton et al., 2001). Given that CO2 is the raw material of photosynthesis, this historically unprecedented rate of increase, along with accompanying changes in global climate, is expected to have profound effects on plant physiology and growth, community dynamics, species distributions, and probabilities of extinction (Bazzaz, 1990; Davis & Shaw, 2001; Poorter & Navas, 2003; Niklaus & Körner, 2004; Reich et al., 2006). In particular, elevated CO2 (eCO2) stimulates photosynthesis and can alter light compensation points, often resulting in increased plant growth (Körner, 2006). The effects of CO2 concentration on plant physiology and growth can impact ecological interactions in several ways, including allowing plants to grow in deeper shade (Körner, 2006), altering competitive interactions (Brooker, 2006; Körner, 2006), and influencing interactions with herbivores, pathogens, and mutualists (Bazzaz, 1990; Bezemer & Jones, 1998; Coviella & Trumble, 1999; Mitchell et al., 2003; Johnson et al., 2005). As experimental evidence documenting these ecological consequences has accumulated, it has stimulated interest in the potential for elevated CO2 (eCO2) concentrations to alter the evolution of plant populations.
Rapid evolutionary responses may be important because genetic changes within species could alter predicted ecological responses to eCO2 and other types of environmental change (Geber & Dawson, 1993; Bazzaz et al., 1995; Curtis et al., 1996; Thomas & Jasienski, 1996; Yoshida et al., 2003). While evolution is often assumed to proceed slowly relative to ecological change, evolutionary responses over a few decades have been documented in response to heavy metal contamination of soils (McNeilly & Bradshaw, 1968; Wu & Bradshaw, 1972) and even over a few years in response to drought (Grant & Grant, 2002) and predation (Reznick et al., 1990; Arendt & Reznick, 2005). Evidence of rapid evolutionary change in still other contexts is accumulating steadily (e.g. global warming (Reale et al., 2003) and biological invasions (Strauss et al., 2006)). Understanding how the CO2 environment affects evolutionary dynamics is necessary for a full understanding of the biological impacts of increasing CO2 concentrations, as well as for evaluating the robustness of ecological predictions.
Several lines of evidence suggest that atmospheric CO2 concentrations influence the evolution of vascular plant populations, although the importance of elevated CO2 as a selective agent remains an open question. First, several studies have documented that the effects of CO2 concentrations on plant growth or fitness are genetically variable within species (Table 1), indicating either that genotypes with highest fitness in an eCO2 environment will be different from those today or that patterns of selection will differ with CO2 environment. Several studies, however, have failed to detect genetic variation in responses to eCO2 (Table 1). Second, surveys of herbaria specimens reveal correlated changes in CO2 concentrations and traits putatively involved in CO2 uptake (e.g. stomatal densities) over the past 150–300 yr (Woodward, 1987; Penuelas & Matamala, 1990; Radoglou & Jarvis, 1990, but see Körner, 1988). The magnitude of change in herbaria specimens is similar, however, to plastic responses to eCO2; therefore, genetic changes need not be invoked to explain the observed changes (Woodward, 1987, 1993). Third, plants from populations growing near geothermal vents where concentrations of CO2 are naturally elevated have, in some instances, expressed higher fitness when grown in eCO2 than those from populations that grow in more typical conditions (Woodward et al., 1991; Woodward, 1993). These experiments, however, have been conducted with limited replication, making it difficult to disentangle the effects of CO2 from other environmental variables, such as temperature and soil type, that also differ among locations. Moreover, other studies fail to detect adaptation to elevated CO2 (Collins & Bell, 2006) or only demonstrate differences in growth between populations at subambient CO2 concentrations (Ward & Strain, 1997).
Table 1. Studies detecting or not detecting statistically significant genotype × CO2 environment interactions on plant biomass or fitness
|Species||Trait||Method||No. of genotypes||Referencesb|
|Studies detecting genotype × CO2 environment interactions|
|Abutilon theophrasti||Biomass, fruit biomass||GC|| 3|| 1|
|Arabidopsis thaliana||Biomass, fruit no., seed no.||GC|| 3–5|| 2–4|
|Betula alleghaniensis||Biomassa||GH|| 3|| 5|
|Bromus erectus||Biomass||GC|| 7|| 6|
|Gentianella germanica||Survival||OC||30|| 7|
|Pinus ponderosa||RGR||GC|| 4 pop.|| 8|
|Plantago lanceolata||Seed weight||GC|| 4|| 9|
|Populus tremuloides||Biomass, RGR||GH|| 6||10|
|Studies not detecting Genotype × CO2 environment interactions|
|Arabidopsis thaliana||Biomass||GC|| 2||12|
|Arrhenatherum elatius||Biomass||F|| 9–14||13|
|Carex flacca||Biomass||GH|| 9||15|
|Dactylis glomerata||Biomass||F, GH|| 9–14||13,14|
|Festuca ovina||Biomass||GC, OC|| 5,18|| 6|
|Festuca pratensis||Biomass||F|| 9–14||13|
|Holcus lanatus||Biomass||F|| 9–14||13|
|Lolium multiflorum||Biomass||F|| 9–14||13|
|Lolium perenne||Biomass||F|| 9–14||13|
|Phlox drummondii||Biomass, seed no.||GC|| 4 pop.||16|
|Pinus ponderosa||Biomass||GC|| 4 pop.|| 8|
|Plantago lanceolata||Biomass||GC, OC|| 6,18||17,18|
|Populus tremuloides||Biomass||OC|| 6||19|
|Ranunculus friesianus||Biomass||F|| 9–14||13|
|Rhaphanus raphanistrum||Flower no., fruit no.||OC|| 5,36||20,21|
|Rumex acetosa||Biomass||F|| 9–14||13|
|Rumex obtusifolius||Biomass||F|| 9–14||13|
|Salix myrsinifolia||Biomass||GC|| 3,4||22,23|
|Sanguisorba minor||Biomass, fruit no.||GH||77||24|
|Trifolium pratense||Biomass||F|| 9–14||13|
|Trifolium repens||Biomass||F|| 9–14||13|
|Trisetum flavescens||Biomass||F|| 9–14||13|
Despite suggestive evidence that evolutionary responses could occur, experiments that have artificially selected for increased fitness in eCO2 environments have found no evidence that plant populations will adapt to eCO2 (Maxon Smith, 1977; Potvin & Tousignant, 1996; Ward et al., 2000; Collins & Bell, 2004). That is, experimental populations selected under eCO2 conditions do not have higher fitness than populations selected under ambient CO2 (aCO2) conditions when reared in eCO2 environments. Nevertheless, some of these selection experiments have found that physiological and phenological traits have evolved in response to artificial selection in eCO2 environments; after 1000 generations of growth under eCO2, the unicellar alga, Chlamydomonas reinhardtii, showed changes suggestive of relaxed selection on photosynthetic efficiency (Collins & Bell, 2004), and five generations of selection on Arabidopsis thaliana seed production in eCO2 vs subambient CO2 environments resulted in differences in flowering time (Ward et al., 2000). Because such experiments may impose stronger selection than populations typically experience in nature and focus primarily on the outcome of the evolutionary process, questions about the mechanisms underlying adaptive responses to environmental change remain. In the examples above, adaptation to eCO2 environments could fail as a result of lack of genetic variation in CO2 responsiveness, similarity of the intensity and direction of selection in aCO2 and eCO2 environments, or genetic constraints.
Here we report on the results of a large and statistically powerful experiment designed to predict evolutionary changes resulting from increased concentrations of atmospheric CO2. We focus on ecologically important traits whose genetic basis is complex. We therefore use a quantitative genetic approach that allows us to predict the short-term evolutionary trajectory of populations grown in aCO2 and eCO2 environments. We consider all three components of evolution and use an experimental population of the model annual plant A. thaliana to estimate patterns of selection on growth, morphological, and phenological traits; heritabilities, which influence the rate of response to selection; and genetic covariances between traits, which may constrain the rate and direction of responses to selection. The advantage of this approach is that it allows for explicitly examining the mechanisms underlying evolutionary change and provides a basis for explaining why rising CO2 concentrations may or may not affect evolution. Further, we compare the genetic relationship between fitness in aCO2 vs eCO2 treatments to assess directly differences in expected response to natural selection in the two CO2 environments (Antonovics et al., 1988). To accomplish these objectives, we collected data on traits of individual A. thaliana plants growing outdoors in a free-air CO2 enrichment (FACE) facility. Making use of FACE allowed us to examine the effects of increased CO2 in relatively natural field conditions, including natural amounts of light, rain, wind, and airborne pathogens.
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Increasing atmospheric CO2 concentrations and related changes in global temperature and precipitation patterns are expected to impact plant growth, community dynamics, and ecosystem function. If increasing CO2 concentrations also alter patterns of natural selection or other components of the evolutionary process, then the effects of eCO2 on plant communities may be ameliorated or exacerbated by genetic changes that occur within plant populations (Geber & Dawson, 1993; Bazzaz et al., 1995; Curtis et al., 1996; Thomas & Jasienski, 1996; Yoshida et al., 2003). In a statistically powerful experiment using the model vascular plant A. thaliana grown in a relatively natural environment, little evidence was detected that increasing CO2 concentrations will alter the short-term evolutionary trajectories of ecologically important traits. In particular, we detected no significant differences between aCO2 and eCO2 treatments in the magnitude or direction of selection gradients, heritabilities, or genetic covariances between traits. Selection differentials were also very similar across CO2 treatments and did not differ significantly, with two exceptions: both the phenotypic selection analyses and the REML analyses indicated that eCO2 may affect selection on leaf number and May rosette size. Although these results may be indicative of changes in selection regimes between CO2 environments, selection on both of these traits was very weak and the differences in the magnitude of selection were slight (0.02); therefore, the change in selection with increasing CO2 concentration would result in only minor differences in plant phenotypes. For example, the smaller selection differential for May rosette size under eCO2 would result in only a 0.1 mm difference in rosette size, after 10 generations of selection. These results reinforce those obtained in other studies measuring intensities of selection under aCO2 and eCO2 environments: both Steinger et al. (2007) and Bazzaz et al. (1995) show only minor differences in selection on biomass between eCO2 and aCO2 treatments.
We also detected little evidence for genetic variation in plastic responses to CO2. Considering fitness, in particular, we found that the same genotypes favored under current CO2 concentrations were favored under eCO2 conditions, as indicated by the cross-environment genetic correlation for fitness approaching 1 (r = 0.98). The G matrix also remained remarkably constant across environments, indicating that trade-offs that may contribute to genotypic differences in fitness will persist with rising CO2 concentrations. In short, evidence for eCO2 to alter predicted evolutionary trajectories was lacking despite highly significant estimates of selection, heritability, and genetic covariance within each of the separate CO2 environments.
While our results suggest that eCO2 will have little impact on the evolution of a variety of ecologically important traits, we did not measure selection on all traits thought to be important to CO2 responsiveness (e.g. stomatal density or photosynthetic rates). However, the genotype × CO2 environment interaction for fitness, the most direct assessment of difference in selection between environments, was not detectable, despite the large scale of the experiment. Thus it does not support the inference that rising CO2 concentrations will alter which genotypes are favored by natural selection. Therefore, it is not expected that selection on unmeasured traits will differ across CO2 conditions, unless under the unlikely scenario where genotypes differ in plasticity and patterns of selection differ between CO2 environments in a manner that exactly counteracts these differences so as not to result in a genotype × environment interaction on fitness.
The lack of genotype × CO2 interaction in our study contrasts with results from four of the five other studies investigating G × CO2 interactions in A. thaliana (Table 1). While four studies detected significant G × CO2 interactions on fitness components, in one case, the interaction resulted entirely from a strong response of only one accession (Norton et al., 1995), and in a second example, the G × E interaction appeared to be driven primarily by a subambient CO2 treatment rather than the elevated CO2 treatment (Ward & Strain, 1997). Additionally, most studies were performed in growth chambers, often with limited replication. In the field, increased environmental variation may overwhelm any genotypic effects that are minor in magnitude.
Finding similar patterns of selection, genetic variance, and genetic covariance in aCO2 and eCO2 environments is surprising for at least two reasons. First, several previous studies have suggested that evolutionary responses to rising CO2 concentrations are likely (reviewed in Ward & Kelly, 2004). However, only 11 out of 39 experiments testing for genotypic effects of eCO2 on growth or fitness have detected genotypic variation in response to eCO2 (Table 1). Therefore, the preponderance of evidence appears consistent with the results from this study in suggesting that eCO2 will not directly alter which genotypes are favored by natural selection. The second reason that the negligible effect of eCO2 on plant evolution is surprising is that eCO2 had large phenotypic effects. Elevated CO2 increased biomass by 40%, increased fruit production by 20%, and reduced specific leaf area by 15%. Even if CO2 per se does not alter patterns of selection, these large phenotypic effects might be expected to influence resource allocation and plant development, potentially changing patterns of selection, genetic variation, or evolutionary constraints. Instead, our data suggest that selection acting on a multitude of growth traits is linear across a wide range of phenotypic variation and that the genetic constraints that influence evolutionary responses to selection appear to be little affected by either CO2 or the growth differences that occur when plants are reared under eCO2 vs aCO2. Together these results suggest that selective surfaces may be constrained across a large range of phenotypic trait values and demonstrate that environmental changes that have dramatic impacts on plant growth and morphology, community dynamics, and ecosystem functioning will not necessarily influence evolutionary trajectories.
Because our study population was composed of RILs generated from crosses between genetically diverged natural populations, we expected to maximize the opportunity to detect genetic variation in response to CO2. Yet, we detected genetic variation in all traits measured, with the notable exception of CO2 responsiveness. The low amount of genetic variation in CO2 responsiveness may reflect historically low amounts of variation in atmospheric CO2 concentrations across natural environments. There is little spatial variation in CO2 concentrations at fine or coarse scales, and atmospheric CO2 concentrations fluctuated temporally only over very long timescales before the industrial age. Temporal and spatial variation in selection, combined with genotype × environment interactions (i.e. different genotypes favored in different environments), may contribute to the maintenance of genetic variation in natural populations (Gillespie & Turelli, 1989; Turelli & Barton, 2004). Although few other environmental variables are either as spatially uniform or as temporally predictable as atmospheric CO2 concentrations, genetic variation in fitness responses to other entirely novel environmental conditions, such as insecticide or heavy metal contamination (Bradshaw, 1991; Alhiyaly et al., 1993; Macnair, 1997), is present in some populations and lacking in others (reviewed in Blows & Hoffmann, 2005).
While we employed FACE technology to grow plants under more natural environmental conditions than most previous studies investigating the potential for evolutionary responses to eCO2, this experiment was conducted in a less complex environment than plants experience in nature. If many of the effects of eCO2 on plant evolution are indirect (Thomas & Jasienski, 1996), increased concentrations of atmospheric CO2 may impact evolutionary trajectories when plants experience competition, greater herbivore damage, natural soil environments, or abiotic stress (e.g. drought or heat stress). For example, Bazzaz et al. (1995) showed that genetic variation, and thus the predicted evolutionary response, of Abutilon theophrasti biomass production was threefold higher under eCO2 than under aCO2, but only when plants were grown in competitive environments. Similarly, other studies have documented significant shifts in genotypic ranks in growth or fitness only when plants were grown at high density (Bazzaz et al., 1995); however, other studies have demonstrated the opposite pattern, only observing genetic variation in responsiveness to CO2 in the absence of competition (Steinger et al., 1997). Interestingly, more pronounced evolutionary impacts of eCO2 in complex than in simple ecological environments would be the opposite of the phenotypic effects of eCO2 on plant growth and fitness, which tend to be greater in simple environments (reviewed in Ainsworth & Long, 2005).
Regardless of environmental complexity, the results of this study indicate that patterns of natural selection and quantitative genetic parameters are robust to large increases in CO2 concentration and that eCO2 itself will have minimal impact on the evolutionary trajectory of this A. thaliana population. Our study therefore suggests that the biotic changes that occur in response to eCO2 will be primarily, if not entirely, ecological. It remains to be determined, however, whether this finding generalizes to other plant populations growing in biotically more realistic environments.