Background
The authors write that "in many regions of the world, climate change is projected to induce higher temperatures and drier conditions (IPCC, 2007)," as well as "more frequent and more intense climate extremes such as heat waves and droughts (Meehl et al., 2000; Schar et al., 2004)," and they say that the latter phenomena "are expected to be more detrimental to plant functioning and productivity than homogenously distributed changes in temperature and precipitation," citing Karl et al. (1997), Easterling et al. (2000) and Smith (2011).

What was done
"To gain more insight into the effects of climate extremes on plants," in the words of Dreesen et al., they conducted two experiments (one in July and another in August) in an experimental field at the University of Antwerp (Belgium), in which plant mesocosms that consisted of three temperate annual or biannual herbaceous species were exposed to either a heat wave or serious drought, or both extreme stresses together, where ten-day heat waves were simulated with infrared lamps and drought (of either 20 or 17 days duration) was created by withholding water input and removing the water table, after which the plants experienced normal weather until the end of the growing season.

What was learned
The four researchers report finding, "surprisingly," that total above- and below-ground end-of-season community biomass "was stimulated in response to drought extremes in both periods," and that "effects of heat extremes varied but never reduced biomass," as was also the case with respect to combined heat and drought. "This increase in total community biomass," as they continue, "originated exclusively from stimulated root growth." They indicate that "the exact mechanism for this unexpected result could not be ascertained," but they say that "greater whole-plant nitrogen stocks clearly indicated enhanced nutrient availability," which "may have arisen from increased net mineralization or from greater root exploration under the influence of 'mid-season drought'."

What it means
Dreesen et al. conclude that plant "community responses to climate extremes are not necessarily negative." In fact, the massive worldwide analysis of Niu et al. (2012) suggests that the norm is actually positive in the case of temperature.

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.

IPCC. 2007. Climate Change 2007: The Physical Science Basis. Solomon, S., Qin, D., Manniing, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.), Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom.

Karl, T.R., Nicholls, N. and Gregory, J. 1997. The coming climate. Scientific American 276: 54-59.

Meehl, G.A., Karl, T., Easterling, D.R., Changnon, S., Pielke, R., Changnon, D., Evans, J., Groisman, P.Y., Knutson, T.R., Kunkel, K.E., Mearns, L.O., Parmesan, C., Pulwarty, R., Root, T., Sylves, R.T., Whetton, P. and Zwiers, F. 2000. An introduction to trends in extreme weather and climate events: observations, socioeconomic impacts, terrestrial ecological impacts and model projections. Bulletin of the American Meteorological Society 81: 413-416.

Niu, S., Luo, Y., Fei, S., Yuan, W., Schimel, D., Law, B.E., Ammann, C., Arain, M.A., Arneth, A., Aubinet, M., Barr, A., Beringer, J., Bernhofer, C., Black, T.A., Buchmann, N., Cescatti, A., Chen, J., Davis, K.J., Dellwik, E., Desai, A.R., Etzold, S., Francois, L., Gianelle, D., Gielen, B., Goldstein, A., Groenendijk, M., Gu, L., Hanan, N., Helfter, C., Hirano, T., Hollinger, D.Y., Jones, M.B., Kiely, G., Kolb, T.E., Kutsch, W.L., Lafleur, P., Lawrence, D.M., Li, L., Lindroth, A., Litvak, M., Loustau, D., Lund, M., Marek, M., Martin, T.A. Matteucci, G., Migliavacca, M., Montagnani, L., Moors, E., Munger, J.W., Noormets, A., Oechel, W., Olejnik, J., Paw U., K.T., Pilegaard, K., Rambal, S., Raschi, A., Scott, R.L., Seufert, G., Spano, D., Stoy, P., Sutton, M.A., Varlagin, A., Vesala, T., Weng, E., Wohlfahrt, G., Yang, B., Zhang, Z. and Zhou, X. 2012. Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms. New Phytologist 194: 775-783.

Schar, C., Vidale, P.L., Luthi, D., Frei, C., Haberli, C., Liniger, M.A. and Appenzeller, C. 2004. The role of increasing temperature variability in European summer heatwaves. Nature 427: 332-336.

Smith, M.D. 2011. The ecological role of climate extremes: current understanding and future prospects. Journal of Ecology 99: 651-655.