What was done
Pseudovivipary, in the words of the authors of this study, "is an asexual reproductive strategy exhibited by some arctic/alpine grasses in which leafy plantlets are produced in place of seeds."  In this paper, they describe the photosynthetic and reproductive responses of one such plant -- the pseudoviviparous alpine meadow grass (Poa alpina var. vivipara L. -- to a near doubling of the air's CO2 concentration under two nutrient regimes [one-fifth strength (low) or full-strength (high) Long Ashton nutrient solution] when grown in sand culture in one-liter pots in the Institute of Terrestrial Ecology Solardome facility at Abergwygregyn, Gwynedd, Wales, UK.

What was learned
The authors determined that "parent leaf blade tissues experienced acclimatory loss of photosynthetic capacity after long-term growth at elevated CO2 (particularly so when nutrient availability was low)."  With respect to the acclimation index (IA), which they defined as IA = (A'e - Aa)/(Ae - Aa), for example [where Aa is the net photosynthetic rate of plants grown and measured in ambient air, A'e is the net photosynthetic rate of plants grown and measured in CO2-enriched air, and Ae is the net photosynthetic rate of plants grown in ambient air but measured in CO2-enriched air], they obtained IA values of 0.7 for plants in the high nutrient regime and 0.3 for plants in the low nutrient regime.

It should be noted, however, that the acclimation response was not complete, even in the low nutrient regime; for as long as IA is positive (which it was for the plants of both nutrient regimes), the net photosynthetic rate of the plants grown and measured in CO2-enriched air had to have been greater than that of the plants grown and measured in ambient air, which suggests that the plants grown in CO2-enriched air should surely have not experienced less growth than the plants grown in ambient air.  And they didn't, for the authors report that CO2 treatments did not affect whole plant dry weight in their experiment; while in an earlier study, Baxter et al. (1997) report that for plants of the same species and variety, "elevated CO2 markedly increased growth" in a high nutrient regime.

With respect to reproductive growth, there is also some good news to report.  For one thing, more plants flowered in CO2-enriched air than in ambient air (70.2% vs. 62.5% in the high nutrient treatment and 28.6% vs. 27.3% in the low nutrient treatment).  Also, more spikelets or plantlets (the propagules of the grass) were produced in CO2-enriched air than in ambient air (72.0 vs. 64.5 per plant in the high nutrient treatment and 60.7 vs. 58.8 per plant in the low nutrient treatment).  In addition, the dry weights of the plantlets were increased by approximately 25% in CO2-enriched air in the high nutrient treatment; but in the low nutrient treatment they were reduced by about 15%.  Nevertheless, the greatest effect was due to nutrient status alone; for in the words of the authors, "total reproductive dry weight per plant was approximately 350% higher in the high nutrient treatment compared with the low nutrient treatment."

What it means
The authors conclude that "pseudoviviparous P. alpina growing in nutrient-poor habitats will be at both a vegetative and reproductive disadvantage compared with species that experience less loss of photosynthetic capacity during acclimation to future atmospheric CO2 concentrations."  This could well be the case, although it is by no means assured, as nearly all of the CO2 effects documented in this study were not statistically significant.  There was a slight reduction in the dry weights of plantlets produced in CO2-enriched air under low nutrient conditions, but an even larger increase when nutrients were plentiful.  Also, Tissue and Oechel (1987) and Baxter et al. (1994) found that elevated levels of atmospheric CO2 tended to increase tillering in P. alpina.

Since the effects of nutrients far outweighed those of CO2 in this study, it is important to know where the nutrient regimes of current and potential P. alpina sites fall with respect to the high and low extremes investigated before firm conclusions about the plant's real-world response to the ongoing rise in the air's CO2 content are reached.  Also, it is important to know how potential competitors of P. alpina fare in terms of photosynthetic acclimation.  Are they truly less responsive than P. alpina in this regard?

Finally, the experiment of Pierce et al. may not have been run long enough to obtain totally definitive results.  Daughter tillers, for example, as the authors admit, "may not have reached a threshold dry mass (developmental age) above which the apical meristem could be induced into reproductive development, which may also explain the general lack of flowering in low nutrient treatments."

All things considered, it may be a bit premature to forecast the hardships envisioned by the authors of this study for pseudoviviparous arctic and alpine grasses in a higher-CO2 world of the future.

References
Baxter, R., Ashenden, T.W. and Farrar, J.  1997.  Effect of elevated CO2 and nutrient status on growth, dry matter partitioning and nutrient content of Poa alpina var. vivipara L.  Journal of Experimental Botany 48: 1477-1486.

Baxter, R., Ashenden, T.W., Sparks, T.H. and Farrar, J.F.  1994.  Effects of elevated carbon dioxide on three montane grass species.  I. Growth and dry matter partitioning.  Journal of Experimental Botany 45: 305-315.

Tissue, D.T. and Oechel, W.C.  1987.  Response of Eriophorum vaginatum to elevated CO2 and temperature in the Alaskan tussock tundra.  Ecology 68: 401-410.