Variation in flowering phenology and its consequences for lupines colonizing Mount St. Helens
Bishop, John G.
Schemske, Douglas W.
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Species colonizing large-scale disturbances face heterogeneous environmental conditions that may strongly affect the relationship between phenotypic variation and reproduction. We investigated spatiotemporal variation in individual plant flowering phenology, flower and fruit predation, plant size, and fruit production in populations of Lupinus lepidus colonizing landscapes created by the 1980 eruption of Mount St. Helens. We quantified these variables in three populations in 1990, one that survived the 1980 eruption and two newly founded colonizing populations, and continued study of one newly founded population through 1992. We used structural equation modeling (SEM) to analyze the direct and indirect effects of size, phenological variables, and predation on fecundity, and to compare path coefficients among years and populations. Flowering phenologies were constant among populations and years in colonizing populations, but plants in the surviving population flowered earlier, more synchronously, and for a shorter duration. Flower and fruit predation by lepidopteran herbivores varied substantially among populations and years, and phenological variables strongly influenced herbivore damage. Although mean flowering date had a direct positive effect on fruit number in all three years in the large colonizing population, the total effect of flowering date varied among years because later flowering resulted in greater fruit predation. In the surviving population more asynchronous individuals had more fruits, but mean date had no effect. We conclude that substantial spatiotemporal variability in optimal phenology may prevent fine-scale adaptation of flowering schedules, and that phenotypic variation and herbivory may affect the demography of colonization populations.