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Title: Elevated CO2 effects on semiarid grassland plant in relation to water availability and competition

Author
item Dijkstra, Feike
item Blumenthal, Dana
item Morgan, Jack
item Lecain, Daniel
item Follett, Ronald

Submitted to: Functional Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2010
Publication Date: 9/16/2010
Citation: Dijkstra, F.A., Blumenthal, D.M., Morgan, J.A., Lecain, D.R., Follett, R.F. 2010. Elevated CO2 effects on semiarid grassland plants in relation to water availability and competition. Functional Ecology 24:1152-1161.

Interpretive Summary: Semiarid grasslands have shown large increases in productivity, shifts in species composition, and altered carbon (C) and nitrogen (N) cycling in response to increased atmospheric CO2 concentration. Because an increase in atmospheric CO2 also resulted in increased soil moisture (because plants transpire less water through their leaves under elevated CO2), it has been suggested that much of the elevated CO2 effects on semiarid grasslands are related to this increase in soil moisture. It is unclear how important these CO2-induced soil water improvements are compared to more direct effects of elevated CO2 on plant productivity and N cycling in this system. A greenhouse experiment was conducted to test for direct effects of elevated CO2 and soil moisture effects on plant productivity and plant N uptake in five semiarid grassland species common in northern Colorado. Species were grown as monocultures and as a mixture of all five species. Species showed mostly greater productivity (particularly for C3 grasses), but not greater plant N uptake, under elevated CO2. On the other hand, both plant productivity and plant N uptake increased with increased soil moisture. This suggests that soil water improvement under elevated CO2 in field observations is a critical factor in stimulating plant productivity and plant N uptake. When grown in mixtures, species responses to elevated CO2 and increased soil moisture showed large differences compared to their responses when grown as monocultures. Thus, it is important to study the responses of these species to elevated CO2 and water in combination with other species (i.e., in the presence of competition for resources with other species). These results further suggest that the plant community in this semiarid system is highly dynamic in response to global climate change.

Technical Abstract: 1. It has been suggested that much of the elevated CO2 effect on plant productivity and N cycling in semiarid 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 semiarid 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 ppm) and soil moisture (15 vs. 20% m/m) on plant biomass and plant N uptake. Our objective was to separate direct CO2 effects 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 semiarid 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 inter-specific competition. Responses of this grassland community to elevated CO2 and water may be both contingent upon and accentuated by competition.