Background
Noting that "the progamic phase of a flower (i.e. the period between pollination and fertilization) covers a sequence of processes that must be highly coordinated for fertilization to occur," the authors write that "it is one of the most critical phases in the reproductive cycle of a plant and is extremely vulnerable to the prevailing environmental conditions, including temperature," citing Stephenson et al. (1992), Graaf et al. (2001) and Hedhly et al. (2009). Yet in spite of its great importance, they say that "little is known about the effect of temperature on the progamic phase in mountain plants," which plants NASA's James Hansen has claimed will be "pushed off the planet" in response to further global warming, as per his 26 April 2007 testimony before the Select Committee of Energy Independence and Global Warming of the U.S. House of Representatives. So was Hansen correct on this point?

What was done
In a laboratory experiment that has relevance to this question, Steinacher and Wagner assessed the efficacy of "pollen adhesion to the stigma, in vivo pollen germination, pollen tube growth and fertilization - which reflects the efficiency of the whole reproductive system - at temperatures ranging from -2 to 40°C." This they did in six herbaceous species having different altitudinal distributions in the European Alps: Gentianella germanica, which occurs from the subalpine to the alpine zone (Zopfi, 1991), Ranunculus alpestris and Saxifraga caesia, which are frequent species in the alpine zone (Zimmermann, 1975; Kaplan, 1995), and Cerastium uniflorum, Ranunculus glacialis and Saxifraga bryoides, which are typical of plant assemblages from the subnival to the nival zone.

What was learned
Prior to their experiment, the two researchers say they had "expected nival species to perform better in the cold and alpine species to do better in the warmth." However, they found that "this is not always true," reporting that (1) "the alpine species G. germanica was markedly less sensitive to low temperatures than the nival species S. bryoides and C. uniflorum," and that (2) "the species from the alpine belt did not cope better with heat than the species from the nival belt."

What it means
In light of their unexpected findings, Steinacher and Wagner conclude that (1) "higher minimum temperatures might promote the reproduction of species from lower elevations and facilitate their upward migration, as already documented for a number of alpine plant species (Hofer, 1992; Grabherr et al., 1995)," and that (2) "under climate warming, which has been on-going in the Alps since the last century (Beniston et al., 1997), nival species would not be discriminated from alpine species because of a poorer pollen performance in the warmth."

And thus it would appear that there may indeed be room for all at the mountaintop.

References
Beniston, M., Diaz, H.F. and Bradley, R.S. 1997. Climatic change at high elevation sites: an overview. Climatic Change 36: 233-251.

Graaf, B.H.J., Derksen, J.W.M. and Mariani, C. 2001. Pollen and pistil in the progamic phase. Sexual Plant Reproduction 14: 41-55.

Grabherr, G., Gottfried, M., Gruber, A. and Pauli, H. 1995. Patterns and current changes in alpine plant diversity. In: Chapin III, F.S. and Korner, C. (Eds.). Arctic and Alpine Biodiversity: Ecological Studies 113. Springer, Berlin, Germany, pp. 167-181.

Hedhly, A., Hormaza, J.I. and Herrero, M. 2009. Global warming and sexual plant reproduction. Trends in Plant Science 14: 30-36.

Hofer, H.R. 1992. Veranderungen in der Vegetation von 14 Gipfeln des Berninagebiets zwischen 1905 und 1985. Berichte des Geobotanischen Instituts ETH, Stiftung Rubel 58: 39-54.

Kaplan, K. 1995. Saxifragaceae. In: Weber, H.E. (Ed.). Gustav Hegi - Illustrierte Flora von Mitteleuropa Vol 4/2A, Blackwell, Berlin, Germany, pp. 130-229.

Stephenson, A.G., Lau, T.-C., Quesada, M. and Winsor, J.A. 1992. Factors that affect pollen performance. In: Wyatt, R. (Ed.). Ecology and Evolution of Plant Reproduction. Chapman and Hall, New York, New York, USA, pp. 119-134.

Zimmermann, W. 1975. Ranunculaceae. In: Rechinger, K.H. and Damboldt, J. (Eds.) Gustav Hegi - Illustriete Flora von Mitteleuropa, Vol. 3/3, Paul Parey, Berlin, Germany, pp. 53-341.

Zopfi, H.J. 1991. Aestival and autumnal vicariads of Gentianella (Gentianaceae): a myth? Plant Systematics and Evolution 174: 139-158.