THE EFFECT OF FLOODING ON THE RECRUITMENT OF REED MARSH AND TALL FORBPLANT-SPECIES

Recruitment of plant species in wetlands dominated by Phragmites austr alis often results in a zonation of two vegetation types. A developmen t of reed marshes takes place in the shallow flooded parts where the d ominant P. australis becomes accompanied by interstitial marsh species . The vegetation on higher elevations becomes dominated by tall forbs. We investigated whether this zonation is related to the effects of fl ooding on the recruitment of component species of both vegetation type s. All but one species had maximum germination at field capacity and g ermination was strongly reduced in drier soils and flooded soils. Near ly all seedlings of reed marsh species, irrespective of whether they i nitially supported two or six true leaves, survived eight weeks of tot al inundation. Survival of tall forbs was however significantly decrea sed by inundation. At both seedling stages a considerable number of Ci rsium arvense lost their leaves when submerged, but all plants were ab le to recover when drained. Eupatorium cannabinum suffered extra morta lity when transferred from flooded to drained conditions, whereas only young seedlings of Epilobium hirsutum died. Of all investigated speci es E. hirsutum was the only one with a significantly increased stem le ngth after one week of submergence. Root porosities did not differ bet ween reed marsh species and tall forbs but stem porosities of the reed marsh species Mentha aquatica and Lycopus enropaeus were significantl y higher than those of the tall forbs C. arvense and E. cannabinum. It was concluded that both reed marsh species and tall forbs require low ered water levels for germination. Flooding during the seedling stage will assign species to their position on the elevation gradient. Reed marsh species establish at lower elevations because they are best able to survive flooding and because their higher stem porosities allows t hem to withstand higher levels of partial submergence.