Ecophysiology of tidal and non-tidal populations of the invading cordgrass Spartina densiflora: seasonal and diurnal patterns in a Mediterranean climate

The invasion of salt marshes by alien species poses interesting questions concerning the mechanisms that determine their distribution. Spartina densiflora is one of the species that is invading the Atlantic marshes of SW Iberian Peninsula, where it tolerates a broad range of environmental conditions and it competes with the indigenous vegetation. The high ecological versatility of S. densiflora provides a good tool to study the ecophysiological responses of marsh plants. This article aims to compare the ecophysiological responses of S. densiflora in populations from two contrasting habitats: tidal (middle marsh) and non-tidal (drainage intercepted marsh). The higher values in A (net photosynthesis rate), A/Ci (apparent carboxylation efficiency), water use efficiency (intrinsic WUE) and FV/FP (potential photochemical efficiency) recorded at the non-tidal population, except in summertime, agree with previous studies that found better physiological state at lower salinity and higher redox potential. A clear reduction of A at higher irradiance was recorded in spring and summer in the tidal population, coinciding with the lowest w (leaf water potential) values. These results help to explain the high primary productivity recorded in S. densiflora populations at brackish marshes in Mediterranean areas. Different responses in gas exchange and chlorophyll fluorescence during summer were recorded for each population. The tidal population showed the maximum values of A in summer. In contrast, the non-tidal population suffered reductions in A, A/Ci and FV/FP during summer, when salinity was higher. Thus, salinity limits carbon fixation in S. densiflora nontidal populations during Mediterranean summer drought. In tidal populations, photosynthesis seems to be more influenced by anoxic conditions. High levels of photoinhibition and low A were recorded on the coldest and less cloudy day, which provoked permanent damages to the photosynthetic apparatus of S. densiflora. This may limit its winter production, as well as its invasion of marshes at higher latitudes in Europe. Finally, discussion about the possible effects of Global Climatic Change on S. densiflora invasion is undertaken.