Germination of Ten Shoreline Plants in Relation to Seed Size, Soil Particle Size and Water Level: An Experimental Study

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Abstract
(1) Lakeshore plants are distributed along a gradient of exposure to waves, from sheltered bays to exposed shorelines. Soil particle sizes vary along this gradient, and may influence germination and early establishment. We therefore tested whether species with different-sized seeds germinate at different positions along a particle-sized gradient. In particular, we tested whether there was a shared preference over all species for one position on this gradient. (2) Seeds of ten wetland plants (Acorus calamus, Alisma plantago-aquatica, Bidens cernua, B. vulgata, Cyperus aristatus, Lythrum salicaria, Polygonum punctatum, Sagittaria latifolia, Scirpus americanus, Typha angustifolia) were vernalized and then sown along a particle-sized gradient with seven stages ranging from 0.125-0.250 to 8-16 mm. Two water levels, 1 cm and 4 cm below the soil surface, were provided. The proportion of seeds producing established seedlings was determined for each particle size and each species (n = 5 replicates). (3) In the drier treatment, nine out of ten species germinated differentially (P < 0.05) along the gradient. In the wetter treatment, only three out of ten species so responded. Thus, soil particle size had most influence during drier conditions. (4) In both wet and dry treatments, those species which did respond significantly (P < 0.05) to the gradient had a shared preference for the fine soil (P < 0.01). The single exception was Acorus calamus in the dry treatment. (5) The species with the smallest seeds generally showed the greatest response to the gradient. Large-seeded species therefore had the broadest tolerance for variation in soil particle size. (6) On lakeshores, the fine particles associated with sheltered bays would allow the higher recruitment irrespective of seed size. These effects would be most pronounced during periods of low water. The zonation of adult plants is apparently not produced by species with different-sized seeds requiring different soil particle sizes for maximum germination.