The effects of population size limitation on fecundity in mosaic populations of the clonal macrophyte Scirpus maritimus (Cyperaceae)

The clonal macrophyte Scirpus maritimus (Cyperaceae) propagates locally by rhizomes and reproduces sexually by achenes. The purpose of this paper was to examine whether in size-limited habitats in patchy and discrete marshes in two Mediterranean wetlands in southern France natural populations may su ffer from a reduced maternal Fecundity due to a deficit in outcross pollen. We first verified that S. maritimus suffers from ii reduced fecundity when self-pollinated. At a site in the Camargue. mean fecundity (mean number of achenes per centimetre of spikelet) measured in 1995 and 1996 in seven and nine populations, respectively (surface area from 50 to 4500 m(2)) increas ed significantly with population surface area in 1995 but not in 1996. In t he second wetland at Roquehaute. which is composed of small ponds, fecundit y was very low in all 12 local populations studied in 1996 (1.1 achenes per spikelet. SD = 1.2) and was not correlated with the population surface are a (from 10 to 400 m(2)). We performed a pollen supplementation experiment i n five local populations at Roquehaute to determine whether this low fecund ity may be due to a pollen limitation. A significant increase in fecundity after among-pond pollinations compared to within-pond pollinations indicate d that local populations suffer from a deficit in outcross pollen, since ea ch pond appears to contain one or a few number of clones (or incompatibilit y types). In S. maritimus, clonal spread may have a cost in terms of reduce d fecundity in small habitats because each habitat is colonized by very few clones.