CR 18:105-112 (2001)  -  doi:10.3354/cr018105

ABSTRACT: The 2 grass species that dominate the lower zones of North European saltmarshes utilise different photosynthetic pathways. Puccinellia maritima, a C₃ species, occurs at higher elevations than Spartina anglica, a C₄ species, which is a relatively recent invader and appears not to form extensive swards in latitudes north of ~54°N because of the limiting effect of low early-season temperatures and the greater competitive ability of Puccinellia. We investigate the differential responses of the 2 grasses to rising temperatures and CO₂ concentrations in the light of earlier work that predicts increases in their primary production, assuming a 3°C rise in temperature and a doubling of atmospheric CO₂. Predictions of their responses to climate change are made using evidence from present day competitive interactions in relation to marsh elevation and latitude, and from a competition experiment in which the 2 species were grown under ambient and elevated temperature (+3°C) and ambient and enriched CO₂ (+340 ppm) in all factorial combinations. Puccinellia biomass was significantly increased, as expected, by CO₂ enrichment and, probably from increasing early-season growth, by elevated temperature; the effect appeared additive, with the highest yield at the +3°C +340 ppm treatment. Spartina growth increased as predicted in response to elevated temperature, but also unexpectedly in high CO₂ treatment, possibly through increased efficiency in water-use. Competition was largely asymmetric in favour of Puccinellia, affecting Spartina height and shoot weight, with intraspecific effects on tiller production and total biomass; Spartina mainly responded to both treatments and competition by increased below-ground growth. These results are compared with other, single-factor, experiments and suggest that, depending critically on some other aspects of climate change such as rising relative sea level and nutrient dynamics, Spartina is likely to extend its range northwards as CO₂ and temperatures rise, but Puccinellia will continue to replace it in the succession, and probably at lower marsh elevations than at present.

KEY WORDS: Spartina anglica · Puccinellia maritima · C₄ and C₃ photosynthesis · Elevational niche · Competition · Elevated temperature and CO₂ enrichment · Saltmarsh range extension