Does low temperature prevent Spartina alterniflora from expanding toward the austral-most salt marshes?

Along the Atlantic coast of South America, the northern salt marshes (lower than 43A degrees S) are dominated by Spartina species while the southern salt marshes (greater than 43A degrees S) are dominated by Sarcocornia perennis. The most abundant Spartina species are Spartina densiflora which is present in most coastal marshes, and Spartina alterniflora that was never recorded above the similar to 42A degrees 25'S. It is not clear why S. alterniflora has not succeeded in the southern marshes, in which the low marsh zone remains as an extensive bared mud flat. We address the hypothesis that the absence of S. alterniflora in the south is driven by the cold temperatures inversely related with increasing latitudes along the East coast of Patagonia. To evaluate this hypothesis, we carried out an experiment in which we manipulated the temperature in combination with frost formation and photoperiod. We found that cold temperature produced a negative effect on S. alterniflora, and this effect seems accentuated by the frost but not by the reduction in the photoperiod. Our results support the hypothesis that the absence of S. alterniflora in the southernmost salt marshes of Patagonia is a consequence of the frost as an outcome of the co-occurrence of low temperature and high humidity. The importance of our results are discussed in the context of the global warming and how Spartina species enlarge their distributional range toward higher latitudes.