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The Projected Response of a Swiss Grass-Clover Sward to Increasing CO2 and Climate Change Over the 21st Century
Lazzarotto, P., Calanca, P., Semenov, M. and Fuhrer, J. 2010. Transient responses to increasing CO2 and climate change in an unfertilized grass-clover sward. Climate Research 41: 221-232.

The authors write that "white clover (Trifolium repens L.) is the most important pasture legume grown in temperate climates in association with a variety of grasses, notably perennial ryegrass (Lolium perenne L.)," adding by way of explanation that "white clover improves the nutritional quality and digestibility of the herbage," and that it "contributes substantially to the nitrogen status of the sward through biological nitrogen fixation." They say, however, that there is some concern that future drought, such as is predicted by climate alarmists to occur in tandem with CO2-induced global warming, will hurt clover more than the grass with which it is intermingled, thereby degrading the nutritional quality and digestibility of pasture swards.

What was done
In light of this mix of facts and presumptions, Lazzarotto et al. planned and conducted a study wherein, as they describe it, "mechanisms controlling transient responses to elevated CO2 concentration and climate change in an unfertilized grassland on the Swiss Plateau were examined in light of simulations with PROGRASS," a process-based model of grass-clover interactions developed by Lazzarotto et al. (2009), where "daily weather for a series of transient climate scenarios spanning the 21st century were developed for the study site with the help of the LARS-WG weather generator," which is described by Semenov and Barrow (1997) and Semenov et al. (1998), and where "changes in the length of dry and wet spells, temperature, precipitation and solar radiation defining the scenarios were obtained from regional climate simulations carried out in the framework of the PRUDENCE project," which is described by Christensen and Christensen (2007).

What was learned
"Compared to 1961-1990," in the words of the Swiss and UK scientists, the climate scenarios they developed for a CO2 increase from 370 to 860 ppm "indicated that for 2071-2100 there would be a noticeable increase in temperature (roughly 3°C in winter and 5°C in summer), a significant drop in summer precipitation (of the order of -30%) and a nearly 2-fold increase in the length of dry spells." So how strongly were these significant changes in climate calculated to affect the grass-clover swards?

The four researchers report that "clover abundance did not decline even in the absence of CO2 stimulation [italics added]." And when the atmospheric CO2 concentration was programmed to gradually rise from an initial value of 370 ppm to a final value of 860 ppm, they report that "clover development benefited from the overall positive effects of CO2 on nitrogen acquisition," which they say was also "the reason for increasing productivity of the [entire] sward."

What it means
For Swiss grass-clover swards, it would appear that the rather large increases in temperature and decreases in precipitation that are predicted for the remainder of the 21st century, even if they come to pass, will not have much of an effect on them, but that the concomitant increase in the air's CO2 content will benefit them considerably. In addition, Lazzarotto et al. say that it is likely that "technical progress in the management of grasslands and pastures," which will surely occur, will help such pastures even more. All things considered, therefore, the future of Switzerland's (and many other countries') clover-grass associations would appear to be bright indeed.

Christensen, J.H. and Christensen, O.B. 2007. A summary of the PRUDENCE model projections of changes in European climate by the end of this century. Climatic Change 81: 7-30.

Lazzarotto, P., Calanca, P. and Fuhrer, J. 2009. Dynamics of grass-clover mixtures -- an analysis of the response to management with the PROductive GRASsland Simulator (PROGRASS). Ecological Modeling 220: 703-724.

Semenov, M.A. and Barrow, E.M. 1997. Use of a stochastic weather generator in the development of climate change scenarios. Climatic Change 35: 397-414.

Semenov, M.A., Books, R.J., Barrow, E.M. and Richardson, C.W. 1998. Comparison of the WGEN and LARS-WG stochastic weather generators for diverse climates. Climate Research 10: 95-107.

Reviewed 18 August 2010