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- Materials and Methods
Elevated [CO2] enhances photosynthesis, water-use efficiency, and biomass accumulation in many species (Nowak et al., 2004a; Ainsworth & Long, 2005). However, the dynamics of plant communities can seldom be predicted from individual plant responses, and few studies have examined how intact plant communities respond to elevated [CO2]. Prediction of community responses might be simplified if functional groups responded in predictable ways to elevated [CO2] based on their underlying physiology and phenology. For example, C3 species are expected to realize greater benefits in productivity from elevated [CO2] than C4 species (Bowes, 1993), which would, in turn, alter the composition of mixed C3–C4 communities and thus ecosystem function (Polley et al., 2012). By contrast, we have little comparative information about how evergreen and deciduous species may respond to elevated [CO2].
Whether plants respond by species or as functional groups, it is likely that elevated [CO2] will modify resource limitations and potentially alter competitive interactions. Altered competition will affect the persistence and representation of particular species in a community, with effects on relative dominance, species richness, and species diversity. Previous work measuring diversity under elevated [CO2] has not yielded a consistent response; studies have reported that diversity increased (Niklaus et al., 2001; He et al., 2002), decreased (Zavaleta et al., 2003), or remained the same (Navas et al., 1995; Morgan et al., 2007). The variability in community responses suggests that initial species composition and higher-level interactions are key elements in elevated [CO2] community responses (He et al., 2002; Davis & Ainsworth, 2012). The complex interplay among species interactions (competition, facilitation, and herbivory; Reich et al., 2001; Nowak et al., 2004a), resource limitations (water and nutrient availability), and species or functional group presence and relative representation (Curtis & Wang, 1998; Bradley & Pregitzer, 2007) make studies of intact ecosystems the best predictors of community responses to elevated [CO2]. Therefore, there is a great need for in situ long-term studies to understand how individual species and plant communities may respond to rapidly changing [CO2] in the Earth's atmosphere.
Despite the clear value of community-level experiments, few studies have assessed the effects of elevated [CO2] on diversity parameters. At many free-air CO2 enrichment (FACE) sites, the long life span of perennials and the necessity of maintaining intact, unmanaged vegetation have precluded community-level analyses (Nowak et al., 2004a). Thus, most experiments evaluating community responses to elevated [CO2] have involved short-lived species in old fields, grasslands, and annual communities (Morgan et al., 2004a; Ramseier et al., 2005; Polley et al., 2012). There is a particular dearth of studies addressing effects of elevated [CO2] on aridland ecosystems, which represent substantial amounts of global land cover. Community responses to elevated [CO2] may be stronger in arid regions because desert plants may be particularly responsive to elevated [CO2] conditions as a result of improved water-use efficiency (Melillo et al., 1993). Species richness in deserts is often low because of environmental constraints (Kier et al., 2005) and could be further reduced with increasing [CO2] as a result of competition with invasive species (Smith et al., 2000). In addition, aridland plant community responses to elevated [CO2] may be complicated by episodic droughts common to desert regions, which could limit desert plants' ability to respond to elevated [CO2] (Morgan et al., 2004b; Jasoni et al., 2005; Housman et al., 2006). Previous research has found short-term, positive effects of elevated [CO2] on above-ground or below-ground plant growth in the Mojave Desert but only in wetter years (Housman et al., 2006; Ferguson & Nowak, 2011). However, elevated [CO2] did not affect perennial plant standing biomass both above and below ground at the end of the 10 yr FACE experiment (Newingham et al., 2013). Although there was no effect of elevated [CO2] on final standing biomass, it is possible that there were changes in plant community characteristics under elevated [CO2] over the course of the experiment.
We examined plant community changes in an undisturbed Mojave Desert perennial plant community exposed to 10 yr of elevated [CO2]. Plant community shifts were assessed by examining changes in cover, species richness, and species diversity. We predicted that elevated [CO2] would increase cover through higher canopy production of the dominant species, which would decrease species richness and diversity over time. We also predicted that C3 shrub species would respond more strongly to elevated [CO2] than a C4 bunchgrass, and that the dominant evergreen shrub, Larrea tridentata, would have a stronger response to elevated [CO2] than drought-deciduous shrubs and perennial grasses.
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- Materials and Methods
During initial sampling, 30 species of perennial grasses, forbs, cacti, and shrubs were observed across all plots, while 23 species were observed in 2007 (Table 1). Total cover, species richness, and species diversity significantly decreased over the course of the experiment, but the decreases between the initial and final sampling were similar for all [CO2] treatments (Table 2; Fig. 1).
Table 1. Perennial plant species observed at the Nevada Desert Free-Air CO2 Enrichment (FACE) facility during initial (1997–2000) and final (2007) sampling
|Species||Common name||Growth form||Initial||Final|
| Achnatherum hymenoides ||Indian ricegrass||Bunchgrass||EAC||EA|
| Acamptopappus shockleyi ||Shockley's goldenhead||Subshrub||EAC||EAC|
| Ambrosia dumosa ||White burrobush||Shrub||EAC||EAC|
| Androstephium breviflorum ||Pink funnel lilly||Forb||E|| |
| Argemone corymbosa ||Prickly poppy||Forb||EA|| |
| Astragalus tidestromii ||Tidestrom's milkvetch||Forb||EAC||AC|
| Atriplex canescens ||Fourwing saltbush||Shrub||A||A|
| Baileya multiradiata ||Desert marigold||Forb||EAC||EAC|
| Delphinium parishii ||Desert larkspur||Forb||AC|| |
| Encelia virginensis ||Virgin River brittlebush||Shrub||E|| |
| Ephedra nevadensis ||Nevada jointfir||Shrub||EAC||EAC|
| Eriogonum inflatum ||Desert trumpet||Forb||EAC||E|
| Grayia spinosa ||Spiny hopsage||Shrub||AC||EAC|
| Hymenoclea salsola ||Cheeseweed||Subshrub||A||A|
| Krameria erecta ||Littleleaf ratany||Shrub||EAC||EAC|
| Krascheninnikovia lanata ||Winterfat||Subshrub/shrub||EAC||EAC|
| Larrea tridentata ||Creosotebush||Shrub||EAC||EAC|
| Lycium andersonii ||Anderson's wolfberry||Shrub||EAC||EAC|
| Lycium pallidum ||Pale wolfberry||Shrub||EAC||EAC|
| Mirabilis pudica ||Four o'clock||Forb||EAC||AC|
| Opuntia basilaris ||Beavertail pricklypear||Cactus||EAC||EAC|
| Opuntia echinocarpa ||Staghorn cholla||Cactus||EAC||EAC|
| Opuntia ramosissima ||Pencil cactus||Cactus||EC||EAC|
| Pleuraphis rigida ||Big galleta||Bunchgrass||EAC||EAC|
| Polygala subspinosa ||Spiny milkwort||Subshrub/shrub||EAC||EAC|
| Psorothamnus fremonti ||Indigo bush||Shrub||EAC||EAC|
| Sphaeralcea ambigua ||Desert globemallow||Forb/subshrub||EAC|| |
| Sphaeralcea grossulariifolia ||Gooseberry leaf globemallow||Forb/subshrub||AC|| |
| Stephanomeria pauciflora ||Wirelettuce||Forb/subshrub||EAC||EA|
| Thamnosma montana ||Turpentine-broom||Subshrub||A|| |
Table 2. ANOVA for total cover, species richness, and species diversity across treatment (nonblower control, ambient [CO2], and elevated [CO2]) and time
|Effect||df||Cover||Species richness||Species diversity|
| F || P || F || P || F || P |
|Time||1||23.8|| 0.003 ||16.9|| 0.006 ||49.3|| 0.001 |
|Treatment × time||2||1.0||0.410||1.0||0.439||0.3||0.722|
Figure 1. Percentage cover, species richness, and diversity (expressed as species number equivalents) for all species treated with nonblower controls, ambient [CO2], and elevated [CO2] at initial (closed bars) and final (open bars) sample points (1997–2007). For percentage cover, species richness, and diversity, the only significant effect was time at α = 0.05. Values are mean ± SE.
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We further explored the decrease in cover during the study by examining the five most common individual species. The relative cover of Ambrosia and Pleuraphis decreased while L. pallidum, L. andersonii, and Larrea increased over time; the ‘other species’ group did not change substantially during the study (Tables 3, 4; Fig. 2). There were treatment effects within these larger trends. Ambrosia dumosa relative cover decreased more in elevated [CO2] than in nonblower control and ambient [CO2] (Tables 3, 4; Fig. 2). Lycium andersonii and L. pallidum relative cover increased in nonblower control and ambient [CO2] plots over time but not under elevated [CO2] (Tables 3, 4; Fig. 2). By contrast, the ‘other species’ group had little change in cover for any treatment, and no differences were noted over time relative to [CO2] treatment (Tables 3, 4; Fig. 2). The absolute and relative cover of Larrea tended to increase during the study, but this increase occurred in all CO2 treatments, although it was greatest in nonblower control and elevated [CO2] plots (Tables 3, 4; Fig. 2). Pleuraphis relative cover decreased during the study (Tables 3, 4), but the relative cover of Pleuraphis decreased less in elevated [CO2] plots than in nonblower control or ambient [CO2] (Fig. 2). Overall, the patterns for the drought-deciduous shrubs and ‘other species’ grouped together suggest that elevated [CO2] negatively impacted their cover relative to Pleuraphis during the study (Fig. 2).
Table 3. ANOVA table for the effects of treatment (nonblower control, ambient [CO2], and elevated [CO2]) and time on the absolute (percentage of plot) and relative (each species relative to total plant cover) cover of the five most common species, dead plants, and other species
|Species||Effect||df||Absolute cover||Relative cover|
| F || P || F || P |
| Ambrosia dumosa ||Treatment||2||2.7||0.148||2.9||0.133|
|Time||1||25.3|| 0.002 ||12.3|| 0.013 |
|Treatment × time||2||1.6||0.276||1.2||0.369|
| Larrea tridentata ||Treatment||2||0.8||0.502||0.9||0.449|
|Time||1||6.9|| 0.040 ||56.6|| < 0.001 |
|Treatment × time||2||2.3||0.180||1.7||0.267|
| Lycium andersonii ||Treatment||2||1.3||0.350||2.5||0.165|
|Time||1||14.2|| 0.009 ||9.4|| 0.022 |
|Treatment × time||2||5.6|| 0.043 ||7.8|| 0.021 |
| Lycium pallidum ||Treatment||2||0.0||0.963||0.3||0.747|
|Time||1||0.4||0.555||15.9|| 0.007 |
|Treatment × time||2||2.2||0.188||1.7||0.265|
| Pleuraphis rigida ||Treatment||2||2.0||0.219||1.8||0.237|
|Time||1||28.4|| 0.002 ||35.5|| 0.001 |
|Treatment × time||2||3.1||0.117||2.9||0.134|
|Time||1||3.6||0.106||12.3|| 0.013 |
|Treatment × time||2||0.3||0.745||0.5||0.632|
|Time||1||10.1|| 0.019 ||0.1||0.795|
|Treatment × time||2||0.2||0.806||0.3||0.746|
Table 4. Table of initial and final absolute cover (%) and SEs of the five most common species, dead plants, and other species under nonblower control ambient [CO2], and elevated [CO2] treatments for initial and final sampling periods
|Time||Treatment|| Ambrosia dumosa ||SE|| Larrea tridentata ||SE|| Lycium andersonii ||SE|| Lycium pallidum ||SE|| Pleuraphis rigida ||SE||Other species||SE||Dead plants||SE|
Figure 2. Change in relative cover for the five most common species – Ambrosia dumosa, Larrea tridentata, Lycium andersonii, Lycium pallidum, and Pleuraphis rigida – and other species over the study period treated with nonblower controls (closed bars), ambient [CO2] (open bars), and elevated [CO2] (hatched bars). The asterisk indicates statistically significant differences from initial to final biomass (1997–2007) within species and treatment at α = 0.05. Values are means ± SE.
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We found that elevated [CO2] had no effect on total cover, species richness, and species diversity in our long-term desert FACE experiment. Other studies have found no plant community responses to elevated [CO2], although many have been shorter-term or in less natural ecosystems (Navas et al., 1995; Morgan et al., 2007; Davis & Ainsworth, 2012). Yet other studies have found that elevated [CO2] can negatively affect species richness, diversity, or both (Zavaleta et al., 2003; Dawes et al., 2011). These plant community responses to elevated [CO2] are likely mediated by precipitation and soil moisture (Niklaus & Körner, 2004). The NDFF was the most arid FACE site, representing an area with extremely low annual precipitation. Arid plant community responses to elevated [CO2] may be quite different from semiarid community responses as a result of extreme water (and potentially nutrient) limitations and highly fluctuating rates of productivity (Shaw et al., 2005) in these pulse-dominated systems. Notably, higher soil moisture has been observed under elevated [CO2] in semiarid ecosystems (Niklaus et al., 1998; Morgan et al., 2001), but there was no effect of elevated [CO2] on soil moisture at the NDFF over the long term (Nowak et al., 2004b).
Previous work during four consecutive years (1998–2001) at the NDFF revealed significant increases in photosynthetic rate (Naumburg et al., 2003) and productivity (Housman et al., 2006) with elevated [CO2] in wet years and small or no changes in normal or dry years for Larrea, Ambrosia, and Krameria erecta. Based on these short-term responses, we incorrectly predicted there would be an overall increase in long-term cover with elevated [CO2]. Long-term data from other FACE sites have exposed similar incongruities between short-term physiological data and long-term changes in cover or biomass (Navas et al., 1995). We propose that enhanced plant growth in elevated [CO2] during wet years was counteracted by plant death and substantive biomass loss through canopy dieback (Miriti et al., 2007; McAuliffe & Hamerlynck, 2010) in ensuing low rainfall years. Indeed, total cover decreased from initial to final sampling in all treatments, probably because initial sampling took place during higher than average rainfall conditions. Surprisingly, these changes were not influenced by [CO2].
Few clear-cut patterns have emerged when examining plant responses to elevated [CO2] by functional group (Poorter & Navas, 2003; Nowak et al., 2004a; Ainsworth & Long, 2005). We predicted that C3 species would respond more favorably than C4 species to elevated [CO2] based on improvements in WUE in C3 plants. However, there was no evidence that C3 species consistently benefited from elevated [CO2] to cause a shift in C3–C4 dominance. On the contrary, our results suggest that C3 deciduous shrubs may have reduced performance and the C4 bunchgrass relatively better performance in elevated [CO2] conditions in the Mojave Desert. Considering L. tridentata did not respond in the same way as the other C3 plants, this work adds to the body of literature suggesting that photosynthetic pathway is not a chief determinant of response to elevated [CO2] (Nowak et al., 2004a; Ainsworth & Long, 2005). Pleuraphis was the only C4 plant and bunchgrass that we examined, making comparative conclusions about this functional type and photosynthetic pathway untenable. Although the relative cover of Pleuraphis decreased over the study, this decrease was ameliorated under elevated [CO2], suggesting that canopy dieback and mortality were less severe. This contrasts with other studies, which have found that grasses have a competitive disadvantage under elevated [CO2] (Shaw et al., 2005).
The episodic germination and establishment of long-lived plants in the Mojave Desert limit the scope of the species diversity and species richness patterns that we report. Our initial measurements took place during a pronounced wet cycle, which temporarily increased species richness through the appearance of shrub seedlings. Housman et al. (2003) reported that neither germination nor mortality of shrub seedlings was ultimately influenced by elevated [CO2], although initial seedling survivorship of Larrea and Ambrosia was higher in elevated [CO2]. All seedlings died during the study period, which prohibited any increase in species richness. Definitive answers on the effects of elevated [CO2] on desert perennial plant recruitment must involve observations during rare periods when perennial plants establish in the Mojave Desert, which did not occur during the decade of our study.
Contrary to predictions, elevated [CO2] did not affect total cover, species richness, or species diversity, even though we found small effects of elevated [CO2] on the cover of individual species that could eventually alter the overall plant community over longer timescales. We propose that the lack of strong plant community responses to elevated [CO2] may be explained by the long-term steady state of this desert ecosystem, which has slow plant growth and rare perennial recruitment events. Indeed, perennial plant cover of c. 17% at the NDFF is a function of low precipitation and recurring drought, and elevated [CO2] may have little influence on this dynamic. It is likely that overall plant cover will remain low, particularly if precipitation in the region decreases in response to climate change (Seager et al., 2007). Our results provide the first information about the effects of elevated [CO2] on aridland perennial communities, which comprise a large and growing portion of global land cover.