Elevated CO₂ effects on semi-arid grassland plants in relation to water availability and competition

• Published in: Functional ecology Vol. 24; no. 5; pp. 1152 - 1161
• Main Authors: Dijkstra, Feike A., Blumenthal, Dana, Morgan, Jack A., LeCain, Daniel R., Follett, Ronald F.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.10.2010; Blackwell Publishing; Blackwell Publishing Ltd; Wiley-Blackwell
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Artemisia frigida; Autoecology; Biological and medical sciences; Biomass; Bouteloua gracilis; C3 and C4 grasses; C3 plants; C4 plants; Carbon dioxide; Community ecology; elevated atmospheric CO2; elevated atmospheric gases; forbs; Fundamental and applied biological sciences. Psychology; General aspects; Grasses; Grassland soils; grasslands; greenhouse experiment; Hesperostipa comata; interspecific competition; inter‐ and intraspecific competition; intraspecific competition; Linaria; Linaria dalmatica; mineralization; nitrogen; nitrogen dynamics; nutrient uptake; nutrient use efficiency; Pascopyrum smithii; Plant growth; Plants; semiarid zones; semi‐arid grassland; Soil ecology; Soil water; Species; stimulatory effect; water availability; Water treatment; USA, Colorado; Grassland; C; Carbon dioxide; C4-Type; forbs; Medium enrichment; elevated atmospheric CO; nitrogen dynamics; grasses; Dynamics; Water regime; Semi arid zone; greenhouse experiment; C3-Type; Increase; Water availability; Nitrogen; semi-arid grassland; and C; inter-and intraspecific competition; Atmosphere; Intraspecific competition; Forb; Herbaceous plant; Greenhouse
• ISSN: 0269-8463; 1365-2435
• DOI: 10.1111/j.1365-2435.2010.01717.x

1. It has been suggested that much of the elevated CO₂ effect on plant productivity and N cycling in semi-arid grasslands is related to a CO₂-induced increase in soil moisture, but the relative importance of moisture-mediated and direct effects of CO₂ remain unclear. 2. We grew five grassland species common to the semi-arid grasslands of northern Colorado, USA, as monocultures and as mixtures of all five species in pots. We examined the effects of atmospheric CO₂ concentration (ambient vs. 780 p.p.m.) and soil moisture (15 vs. 20% m/m) on plant biomass and plant N uptake. Our objective was to separate CO₂ effects not related to water from water-mediated CO₂ effects by frequently watering the pots, thereby eliminating most of the elevated CO₂ effects on soil moisture, and including a water treatment similar in magnitude to the water-savings effect of CO₂. 3. Biomass of the C₃ grasses Hesperostipa comata and Pascopyrum smithii increased under elevated CO₂, biomass of the C₄ grass Bouteloua gracilis increased with increased soil moisture, while biomass of the forbs Artemisia frigida and Linaria dalmatica had no or mixed responses. Increased plant N uptake contributed to the increase in plant biomass with increased soil moisture while the increase in plant biomass with CO₂ enrichment was mostly a result of increased N use efficiency (NUE). Species-specific responses to elevated CO₂ and increased soil moisture differed between monocultures and mixtures. Both under elevated CO₂ and with increased soil moisture, certain species gained N in mixtures at the expense of species that lost N, but elevated CO₂ led to a different set of winners and losers than did increased water. 4. Elevated CO₂ can directly increase plant productivity of semi-arid grasslands through increased NUE, while a CO₂-induced increase in soil moisture stimulating net N mineralization could further enhance plant productivity through increased N uptake. Our results further indicate that the largest positive and negative effects of elevated CO₂ and increased soil moisture on plant productivity occur with interspecific competition. Responses of this grassland community to elevated CO₂ and water may be both contingent upon and accentuated by competition.