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Some aspects of the demography of Iridaea splendens Dyck, Leonard James

Abstract

A population of lridaea splendens at Brockton Point, Vancouver, British Columbia was examined. The persistent alternation between diploid dominance in winter and haploid dominance in summer (DeWreede and Green, 1990) at this site was produced by differential rates of increase and decrease in density of the alternate phases. Maximum density of both phases occurred in June and minimum density in February. Density increase began earlier in spring for gametophytes and proceeded at a higher rate than for tetrasporophytes resulting in gametophyte dominance in summer. The rate of decreasing density in fall was also higher for gametophytes resulting in equal densities of the alternate phases in November and tetrasporophyte dominance in winter. The reproductively mature stages were also temporally segregated, with cystocarpic plants predominating in summer and tetrasporangial plants in winter. The alternate reproductively mature stages appeared in the population at the same time (June). Increase in density of cystocarpic plants began immediately while increase in density of tetrasporangial plants began two months later. Survivorship of tagged blades of lridaea splendens was type II (Deevey, 1947) for three separate size classes (2 - 5 cm, 5 - 15 cm, 15+ cm) of both gametophytes and tetrasporophytes, indicating blades in all categories had an equal chance of death over time. Calculation of rates of addition of new blades to the population indicated that in fall tetrasporophytes produced new blades at a higher rate than gametophytes (the reverse of the situation in spring), thereby decreasing in density at a lesser rate.

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