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The rising concentration of CO2 in the atmosphere (Conway et al., 1988) has prompted considerable interest in the potential impacts of elevated CO2 on natural and managed plant communities. Many studies with both crop and wild species have shown that elevated CO2 increases the growth rate and biomass of plants, although the degree of enhancement varies with the availability of other resources and the plant species (Bazzaz, 1990; Newton, 1991; Poorter, 1993). Elevated CO2 has also been reported to have a number of other impacts on plants, including changes in the mass, quality and number of seeds produced. Several studies have shown higher carbon : nitrogen ratios in seeds developed under elevated CO2 (Garbutt et al., 1990; Huxman et al., 1998; Steinger et al., 2000), while both increases and decreases in seed size in response to elevated CO2 have been detected (Garbutt & Bazzaz, 1984; Wulff & Alexander, 1985). Depending on the plant species, elevated CO2 can increase or decrease the number of seeds produced per plant (Garbutt & Bazzaz, 1984; Lawlor & Mitchell, 1991; Curtis et al., 1994; Manderscheid & Weigel, 1995; Navas et al., 1997; Leishman et al., 1999). These CO2-induced changes in reproductive parameters could have important consequences for seedling recruitment in subsequent generations. Seed number determines the number of potential colonists at establishment microsites (Turnbull et al., 2000), while seed mass and nutrient content can influence the success of germination, the rate of early seedling growth and seedling survival (Crawley & Nachapong, 1985; Parrish & Bazzaz, 1985; Bewley & Black, 1994). Species-specific changes in the mass, quality and number of seeds produced, and subsequent effects on seedling recruitment, may thus lead to changes in species composition. However, while many studies have documented the effects of elevated CO2 on the mass, quality and number of seeds produced, few have considered the impacts of changes in these reproductive parameters to seedling recruitment (Wulff & Alexander, 1985; Bazzaz et al., 1992; Farnsworth & Bazzaz, 1995; Huxman et al., 1998).
Changes in seedling recruitment might also arise from direct or indirect effects of elevated CO2 after seed dispersal on germination and seedling growth and survival. Germination, for instance, has been reported to be both increased (Ziska & Bunce, 1993) and decreased (Popay & Roberts, 1970) under elevated CO2. Furthermore, elevated CO2 may alter the size, growth rate and identity of competing plants (Bazzaz et al., 1992; Potvin & Vasseur, 1997), which in turn may affect seedling emergence and survival (Edwards & Crawley, 1999). This could potentially modify the effects of CO2-induced changes in seed mass, quality and number on seedling recruitment. For example, an increase in seed mass under elevated CO2 might be accompanied by an increase in the size of existing plants, so leading to increased competition, with little net effect on seedling recruitment.
The purpose of this study was to examine the effects of elevated CO2 on seedling recruitment of one annual and five perennial species in a sheep-grazed permanent pasture on dry, sandy soil in New Zealand. Our goal was to assess the effects of elevated CO2 during the development of seeds on the parent plant (e.g. seed quality and seed mass) and the direct and indirect effects of elevated CO2 after seed dispersal. In one series of experiments, seeds collected from parent plants grown at ambient and elevated CO2 in a free air carbon dioxide enrichment (Face) experiment were measured for germination and seedling growth in a reciprocal design at ambient and elevated CO2. As we were interested in how plant–plant interactions may affect how plants respond to elevated CO2 (Bazzaz & McConnaughay, 1992), the experiment was conducted for seedlings grown individually in compost in a pot experiment and for seedlings transplanted into undisturbed pastures. In a further experiment to assess the importance of possible changes in the number of seeds produced under elevated CO2, the emergence and survival of seedlings was measured after a fixed number of seeds collected from parent plants grown at ambient and elevated CO2 were sown into pastures maintained at ambient and elevated CO2. The implications of the changes in seedling recruitment to the species composition changes that were taking place in the pasture under elevated CO2 are discussed.
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This study was designed to evaluate the consequences of elevated atmospheric CO2 during the development of seeds on the parent plant and after seed dispersal to seedling performance for a range of pasture species. One of the most striking results was that seeds of T. repens harvested from plants grown at elevated CO2 had higher germination, produced seedlings which had greater mass when grown as individuals in compost and resulted in more seedlings when a constant number of seeds were sown into the pasture than seeds harvested from plants grown at ambient CO2. Consistent with previous studies (Scheidegger & Nösberger, 1984; Nijs et al., 1989), the mass of T. repens seedlings grown as individuals on compost was also higher when seedlings were grown at elevated CO2. The seed development effects are probably attributable to the fact that T. repens seeds developed at elevated CO2 had greater mass. When seed mass was fitted as a covariate in each experiment, the effect of elevated CO2 during seed development was replaced by a strong effect of seed mass. Previous studies have often shown that seed mass is positively correlated with germination, early seedling growth and the ability of seedlings to establish in dense vegetation (Crawley & Nachapong, 1985; Parrish & Bazzaz, 1985; Bewley & Black, 1994).
In contrast to T. repens, seeds of L. saxatilis harvested from plants grown at elevated CO2 had reduced germination and produced seedlings which had lower mass when grown as individuals on compost than seeds harvested from plants grown at ambient CO2. The reduced seedling performance of elevated CO2-developed seeds is consistent with recent studies with Arabidopsis thaliana Heynh. (Andalo et al., 1996, 1998) and Bromus rebens L. (Huxman et al., 1998). The result did not appear to reflect differences in seed mass as this was unaffected by elevated CO2 during seed development in L. saxatilis. A possible explanation is that the result reflects the effects of elevated CO2 on the nutrient content of the seeds. Although not significantly different, the C : N ratio of L. saxatilis seeds developed at elevated CO2 was 12% higher than those developed at ambient CO2. Previous studies have noted that seeds with higher N concentrations can have improved seedling performance (Parrish & Bazzaz, 1985; Huxman et al., 1998).
The root and leaf mass of A. odoratum and L. perenne seedlings grown as individual plants on compost were greater for seeds developed at elevated CO2 but only when the seedlings were grown in the elevated CO2 atmosphere. A similar interaction was found by Bezemer et al. (1998), who reported plants of Poa annua L. exhibited a much higher responsiveness to elevated CO2 when the seeds from which they were grown originated from plants reared at elevated than ambient CO2. The mechanism underlying the interaction in our study is unclear at this stage. CO2 stimulation of seedling growth has been reported to increase with increasing seed mass at the intraspecific (e.g. Miao, 1995; Steinger et al., 2000) and interspecific (Bazzaz & Miao, 1993) level. However, interactions with seed mass or seed quality are unlikely to be the case in this study as seed mass and seed C : N ratios of A. odoratum and L. perenne were unaffected by elevated CO2 during seed development.
A notable feature of the results was that none of the effects of elevated CO2 during seed development or during seedling growth detected for A. odoratum,L. perenne, L. saxatilis and T. repens seedlings grown as individual plants on compost were found when seedlings were transplanted into the pasture. The contrasting response may reflect that other factors affecting growth (e.g. herbivory, competition for light and soil nutrients) were more limiting to plant growth for seedlings grown in the pasture than for seedlings grown as individual plants with ample nutrients and water (Woodward et al., 1991; Bazzaz & McConnaughay, 1992; Leishman et al., 1999). An alternative explanation for the failure of effects of elevated CO2 during seedling growth to be evident for transplanted seedlings may be that seedlings at the base of the canopy in both ambient and elevated CO2 rings experienced high CO2 concentrations (Fuller, 1948; Bazzaz & Williams, 1991), and so were less likely to experience competition for CO2 as a resource (Bazzaz & McConnaughay, 1992). However, measurements of the CO2 concentration taken in the pasture canopy (mean pasture height = 0.08 m) at 0.015 m above ground level during the photoperiod over a 5-d period at the conclusion of our experiment in late spring gave mean values for ambient (380 µl l−1) and elevated (482 µl l−1) CO2 rings that were both similar to those found at 0.3 m above ground level (i.e. where the germination and seedling growth experiments were conducted; P.C.D. Newton, unpublished). This finding suggests it is unlikely that high CO2 concentrations at the base of the canopy for transplanted seedlings in both ambient and elevated CO2 rings contributed to the difference between experiments in our study. Regardless of the exact mechanism leading to the differing responses between experiments, the results highlight further that the response of individually grown plants to elevated CO2 can differ substantially from those within monospecific or multispecific stands (Bazzaz & McConnaughay, 1992) and stresses the need for caution in extrapolating from studies with individual plants to community responses (Navas, 1998).
There were about 20% more seedlings of L. saxatilis and T. repens in elevated than ambient CO2 rings where no seeds were sown. This result is likely to partly reflect increased seed production for these two species under elevated CO2. A previous study in the same grassland showed that L. saxatilis and T. repens both showed increased seed production under elevated CO2 (Edwards et al., 2001). Moreover, both species appeared to be seed-limited as they showed increased seedling numbers when extra seeds were sown in this study. For T. repens, the result may also be partly attributable to effects of elevated CO2 during seed development: the number of extra seedlings produced by sowing a constant number of seeds was greater when the sown seeds were developed at elevated than ambient CO2. An alternative explanation for the increased seedling recruitment, that is, differences in the extent and quality of establishment microsite conditions between CO2 treatments leading to elevated CO2 rings being more conducive to seedling recruitment, is unlikely because the number of extra seedlings that arose from the sowing of a constant number of seeds was similar between ambient and elevated CO2 rings.
Seedling survival in the pasture from the start of autumn until the end of spring was unaffected by elevated CO2 during seedling growth. Reduced seedling survival in elevated CO2 rings may have been expected had the biomass of the existing vegetation or the rate of canopy closure been enhanced under elevated CO2 (Bazzaz et al., 1992). Previous studies in permanent grasslands like the one studied here have shown that experimental treatments that cause increased above-ground biomass (e.g. herbivore exclusion, fertilizer application) often result in reduced seedling survival due to increased competition for light, water and nutrients (Howe & Snaydon, 1986; Edwards & Crawley, 1999). However, there was little effect of elevated CO2 on the biomass of the above-ground vegetation or the rate of growth between grazing events for the duration of this experiment (Edwards et al., 2001).
It is unclear at this stage whether the differences in seedling numbers of L. saxatilis and T. repens would persist in the long term and lead to differences in species composition. Seedling mortality is often high in pastures and differences in seedling numbers, even like those reported here which are for seedlings of 7–8 m of age, often disappear due to density-dependent seedling mortality or effects of competing vegetation (e.g. Edwards & Crawley, 1999). However, it is interesting to note that both L. saxatilis and T. repens showed increased abundance in the biomass in elevated CO2 rings in the first 2 yr of the experiment (Edwards et al., 2001; Newton et al., 2001). This suggests that increased seedling recruitment may be an important mechanism leading to changes in species composition, although how important they are relatively to changes in vegetative growth under elevated CO2 (e.g. tillering and branching, mortality of adult plants, see Clark et al., 1997) is unclear. However, the importance of seedling recruitment as a mechanism causing the increased abundance of L. saxatilis is highlighted by the finding that the basal diameter of mature rosettes of this species did not differ between ambient and elevated CO2 rings (Edwards et al., 2001).
This study used seeds collected from a pasture that had been exposed to elevated CO2 for at least 13 m. Despite this time period, some of the plants that seeds were collected from in the CO2-enriched rings may have been exposed to ambient CO2 in the pretreatment period; all study species except C. glomeratum were perennial. This raises the possibility of differences in storage (e.g. in roots, stolons) laid down in the pretreatment period interacting with elevated CO2 in the treatment period to influence seed quality, mass and number, and subsequent seedling performance (see Field et al., 1996). It is not clear from this study, however, whether variations in the amount of storage between species played any role in determining the differing responses between perennial species (e.g. response in T. repens but not P. pratensis). Continued monitoring of the perennial plants in this grassland with reference to the length of time that they have been enriched with CO2 will be required to ascertain the importance of these effects.
In summary, the effect of elevated CO2 during seed development on seedling performance (germination and growth) of individual spaced plants of the six study species was highly variable, ranging from no impact in C. glomeratum and P. pratensis to reduced performance in L. saxatilis and increased performance in A. odoratum, L. perenne and T. repens. Such species-specific effects on seedling performance, in addition to changes in the number of seeds produced under elevated CO2 (Edwards et al., 2001), could potentially lead to changes in species composition. However, our study also highlighted that the effects on seedling performance due to elevated CO2 during seed development were much smaller when measured within intact pasture communities. Future work aimed at predicting the effect of elevated CO2 on plant communities must not only consider whether elevated CO2 alters reproductive parameters like seed mass and quality, but also under what conditions changes in these factors will lead to changes in germination, seedling growth and seedling numbers.