Submitted to: Global Change Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/14/2005
Publication Date: 10/15/2005
Citation: Milchunas, D., Morgan, J.A., Mosier, A., Lecain, D.R. 2005. Root dynamics and demography in shortgrass steppe under elevated CO2, and comments on minirhizotron methodology. Global Change Biology. 11:1837-1855. Interpretive Summary: Atmospheric carbon dioxide (CO2) concentrations are rising and are predicted to lead to global warming. Since CO2 is a substrate for photosynthesis, the process whereby plants utilize energy from the sun to produce organic compounds needed for growth, there is also good reason to believe that rising CO2 will have important direct impacts on plant growth and development. This experiment focuses on the effects of a doubling of CO2 concentration over a native shortgrass prairie in Colorado on root system dynamics. Understanding the responses of roots to CO2 is important, especially in semi-arid perennial grasslands where most of the plant and all of the microbial activity occurs belowground. We found that while elevated CO2 enhanced root growth of native grasses, it also enhanced the death of roots, resulting in little change in root standing biomass. However, there was little synchrony over the 4-year course of this study between root growth and death, suggesting that the net effect of CO2 on root dynamics may change over time, potentially involving both gains and losses in root mass. Growth and elevated CO2 also increased root diameter and the branching of roots, suggesting that roots in future CO2-enriched environments may have a morphology that increases their ability to mine nutrients and water from the soil. This may be a benefit to plants obtaining additional resources to support the higher plant growth rates predicted for future CO2-enriched environments.
Technical Abstract: The dynamics and demography of roots were followed for five years that spanned very wet and drought periods in a native, semiarid shortgrass steppe grassland exposed to ambient and elevated atmospheric CO2 treatments in large open-top chambers. Elevated, compared to ambient atmospheric CO2 concentrations, resulted in much greater root-length growth (+52 %), relatively smaller but still positively greater root-length losses (+37 %), with total pool sizes intermediately greater (+41 %). The greater standing pool of roots under elevated compared to ambient CO2 was due to the greater number of roots (+35 %), not because individuals were longer. Loss rates increased relatively less than growth rates because life-spans were longer (+41 %). The average diameter of roots was larger under elevated compared to ambient CO2 treatment in the upper soil profile, but not at the deeper depth. Elevated CO2 also affected root architecture through increased branching. Growth to loss ratio regressions to time of equilibrium indicate very long turnover times of 5.8, 7.0, and 5.3 years for control, ambient, and elevated CO2 treatments, respectively. Somewhat shorter turnover times were obtained from (new-length-growth) (pool-maximum-length)-1 and qualitative differences among treatments depended upon method of calculation. Primary production was greater under elevated compared to ambient CO2 both below- and above-ground, and their ratios did not differ between treatments. However, estimates of belowground primary production differed among methods of calculation from minirhizotron data, as well as between minirhizotron and root-ingrowth methods. Users of minirhizotrons may need to consider equilibration in terms of both new growth and disappearance, rather than just growth. Very large temporal pulses of root birth and mortality rates were observed, and precipitation explained more of the variance in root growth than it did for root disappearance. There also appeared to be a dampening of the pulsing in root births and deaths under elevated CO2 during both wet and dry periods, which may be due to conservation of soil water reducing the suddenness of the wet pulses and the duration and severity of the dry pulses. However, a very low degree of synchrony was observed between growth and disappearance (production and decomposition).