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Less Frequent but More Extreme Rainfall Events in Semi-Arid Grasslands
Reference
Heisler,-White, J.L., Knapp, A.K. and Kelly, E.F. 2008. Increasing precipitation event size increases aboveground net primary productivity in a semi-arid grassland. Oecologia 158: 129-140.

Background
General circulation model projections of changes in rainfall characteristics in response to global warming, according to Knapp et al. (2008), "have been consistent for intensified intra-annual precipitation regimes (through larger individual precipitation events) with longer intervening dry periods than at present (Easterling et al., 2000, IPCC, 2007)," for which projections they add there is "emerging empirical support from global climate data sets (Karl et al., 1995; Kunkel et al., 1999; Groisman et al., 2005)."

What was done
Working at the Central Plains Experimental Range in northeastern Colorado within the Shortgrass Steppe Long-Term Ecological Research site of this semi-arid grassland, Heisler-White et al. investigated the effect of less frequent but more extreme rainfall events on aboveground net primary productivity (ANPP) via the use of rainout shelters, which allowed them to supply the long-term (30-year mean) growing season (May-September) precipitation quantity as 12, 6 or 4 events, applied manually according to typical seasonal patterns.

What was learned
The three researchers report that "soil moisture data indicated that larger events led to greater soil water content and likely permitted moisture penetration to deeper in the soil profile," with the result that "plots receiving fewer, but larger rain events had the highest rates of ANPP (184 38 g m-2), compared to plots receiving more frequent rainfall (105 24 g m-2)."

What it means
Heisler-White et al. say their results indicate that "semi-arid grasslands are capable of responding immediately and substantially to forecast shifts to more extreme precipitation patterns," and, we might add, they appear to be capable of doing it in a very positive way, i.e., by posting a 75% increase in aboveground net primary productivity for the same amount of rainfall received in fewer but more extreme precipitation events.

References
Easterling, D.R., Meehl, G.A., Parmesan, C., Changnon, S.A., Karl, T.R. and Mearns, L.O. 2000. Climate extremes: Observations, modeling, and impacts. Science 289: 2068-2074.

Groisman, P.Y., Knight, R.W., Easterling, D.R., Karl, T.R., Hegerl, G.C. and Razuvaev, V.A.N. 2005. Trends in intense precipitation in the climate record. Journal of Climate 18: 1326-1350.

IPCC [Intergovernmental Panel on Climate Change]. 2007. Climate Change 2007: The Physical Science Basis. Summary for Policymakers. Cambridge University Press, New York, New York, USA.

Karl, T.R., Knight, R.W. and Plummer, N. 1995. Trends in high-frequency climate variability in the twentieth century. Nature 377: 217-220.

Knapp, A.K., Beier, C., Briske, D.D., Classen, A.T., Luo, Y., Reichstein, M., Smith, M.D., Smith, S.D., Bell, J.E., Fay, P.A., Heisler, J.L., Leavitt, S.W., Sherry, R., Smith, B. and Weng, E. 2008. Consequences of more extreme precipitation regimes for terrestrial ecosystems. BioScience 58: 811-821.

Kunkel, K.E., Andsager, K. and Easterling, D.R. 1999. Long-term trends in extreme precipitation events over the conterminous United States and Canada. Journal of Climate 12: 2515-2527.

Reviewed 17 December 2008