Predicting secondary succession in wetland mesocosms on the basis of autecological information on seeds and seedlings.
Abstract
A variety of autecological attributes of seeds and seedlings (seed weight, germination rate, germination percentage, absolute growth and RGR variations in growth form) were determined for 15 wetland species (including several grasses) using: germination tests under conditions of constant temperature and moisture and natural daylight; 3 weeks' growth in monoculture in a controlled environment; and 3 months' growth in monoculture in a greenhouse in special root chambers with fixed water tables. A mesocosm experiment was conducted in which the same collection of wetland species was sown into cylindrical soil containers of 0.14 m3 volume and grown under highly competitive conditions. Factorial combinations were applied in respect of amount of sown seed (5 or 15 g/m2), comparison of sown seed (with or without adjustment of the mixture according to C-S-R plant strategy theory (sensu Grime)), and amount of flooding (none or 4 days of flooding per 3-week period). A predictor/response investigation, employing correlation and multiple regression analysis, estimated the relative value of each autecological attribute for predicting the establishment and composition of vegetation under the various conditions used in the mesocosm experiment. Across all species, germination rate over the first 4 days of the germination test, leaf weight ratio in the growth-room experiment and total shoot length after 61 days in the root-chamber experiment were clearly correlated with biomass production in the mesocosm experiment, most strongly so at the sown density of 15 g/m2, in the seed mixture without C-S-R adjustment, and in the non-flooded treatments. In the multiple regression analysis, the combination of germination rate in the first 4 days and total shoot length after 61 days explained 83% of variance in biomass production in the treatment of the mesocosm experiment without C-S-R adjustment; and presented the most significant regression of all. Inferences are drawn regarding the importance of the contrasting autecological attributes of different species for the composition of seed mixtures used in the recreation of semi-natural wetland vegetation. A scheme is constructed to illustrate the sequence of importance of different autecological attributes during secondary succession: germination rate shortly after sowing; seed weight; and RGR. Finally, the relative importance of biomass partitioning to capture space, as opposed to biomass production per se, is emphasized in the context of competitive conditions.