|Tijdschrift||Biologisch Jaarboek (Dodonaea)|
|Status||Gepubliceerd - 1998|
Publication Authorstring : Hoffmann, M.; Vanhecke, L.; Dekoninck, W.; Meire, P.
Publication RefStringPartII : <i>Biologisch Jaarboek (Dodonaea) 65</i>: 154-156
Although several club-rush species are known from the Schelde-estuary, colonisation of free mud flats by these characteristic estuarine perennials was not observed for a long time. Six taxa have been observed until now, i.e. Schoenoplectus triqueter, S. pungens, S. lacustris, S. tabernaemontani, S. x scheuchzeri and S. x carinatus. Spontaneous native populations are restricted to artificial boulder slopes in a narrow zone between 0.6 m and 1.6 m beneath MHW. These robust, rhizomatous species are nonetheless known as good mud flat fixators, which might help stabilise river banks with large mud flats. Therefore Grey club-rushes (S. tabernaemontani) were planted on an experimental scale in 1993 at two sites in the fresh water tidal zone. At several other sites in the brackish tidal part commercial plantations ( chair-bottoming) of the same species were realised in the eighties. In both cases rhizome material was introduced from Dutch plantations (IJsselmeer, Oude Maas). To test whether these planted-alien "populations" differ morphologica1ly and/or ecologica1ly from native-spontaneous populations we compared S. triqueter, S. x scheuchzeri/ S. x carinatus and native S. tabernaemontani with planted-alien S. tabernaemontani. Significant differences (p <0,05) were found for almost a1l morphological characters (convexity (triangularity) of the stem, mid-stem section area, mid stem perimeter, fertile stem length, # spikelets/inflorescence, # seeds/spikelet, # fertile stems/m2, total # stems/m2) between S. triqueter and hybrids (S. x scheuchzeri), S. triqueter and S. tabernaemontani and between hybrids and S. tabernaemontani. Therefore S. triqueter is readily distinguishable from hybrids (S. x scheuchzeri) and from S. tabernaemontani. Hybrids (S. x scheuchzeri) are also rather easily distinguished from S. tabernaemontani. Planted versus spontaneous S. tabernaemontani show no marked morphological differences, i.e. when both are considered as homogeneous groups (as if belonging to one population). When we compare spontaneous S. tabernaemontani with one particular Grey club-rush plantation, planted at Appels in May 1993, some differences occur (planted material is generally larger and has thicker stems then spontaneous populations). The stem base of all three tested native spontaneous club-rush taxa is inundated with almost every high tide (> 98 % ). The stem base of planted S. tabernaemontani on the other hand is far less frequently inundated. This is not due to selective planting at higher altitudes, since all plantations were initially along a wide altitudinal gradient. AII plantations withdrew during their development to higher altitudes, not surviving at lower altitudes, at which native club-rushes normally appear. This is a remarkable feature indicating that the alien plant material is less capable to withstand frequent and high inundation then native club-rushes. Inflorescences of spontaneous S. tabernaemontani populations are only inundated by ca. 15 % of the high tides, inflorescences of planted Grey club-rush on the other hand are never inundated, while inflorescences of S. triqueter and S. x scheuchzeri are very frequently inundated, what might (partially) explain the lower number of seeds/spikelet compared to S. tabernaemontani. This might have important dispersal consequences, favouring generative dispersal by fruits of planted S. tabernaemontani above native S. tabernaemontani, and certainly above S. triqueter and S. x scheuchzeri. Field observations proof nevertheless that hybrids are far more successful then S. tabernaemontani. Until now we assume that vegetative dispersal is far more important then dispersal by fruits. Club-rushes almost exclusively grow between boulders on artificial dike slopes ; space limitations for rhizomes to attach between these boulders might be more limiting for S. tabernaemontani then for hybrids, which form thinner rhizomes then S. tabernaemontani. The rareness of S. triqueter (only 21 individual populations counted in 1995) on the other hand can not be explained in this way. Although morphological differences are relatively small, important ecological differences between p1anted and spontaneous club-rushes were detected. This indicates that introduction of a1ien plant material should only be done with greatest care. Use of native plant material has strong preference over introduction. Until now there are no reasons to believe though that the introduction of alien material of S. tabernaemontani threatens native populations of S. tabernaemontani, S. x scheuchzeri or S. triqueter: There are no field indications that competition for potentially suitable habitats threatens the dispersal possibilities of native club-rush populations. Hybridisation between alien Grey club-rush and native club-rushes can not be excluded though and might cause genetic pollution.