Life Histories of Tillers of Eriophorum Vaginatum in Relation to Tundra Disturbance

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Abstract
(1) Life-tables were constructed for vegetative tillers of Eriophorum vaginatum from undisturbed and disturbed tussock tundra in Alaska. Life-tables were also constructed for tillers from tundra that had been fertilized with N, P and K. The life-tables were used to estimate population parameters and to construct constant coefficient matrix models of tiller demography. (2) Tiller survival was lower in disturbed than it was in undisturbed tundra, and daughter tillers were produced sooner. Addition of nutrients (as fertilizer) produced no change in tiller survival and in the age distribution of daughter tiller production in the first year following fertilization. The shapes of the survival curves were intermediate between Deevey Type I and Deevey Type II curves. (3) The generation time for tillers in undisturbed tundra was 5.3 years--about twice as long as in disturbed tundra--while the annual rate of increase of tillers did not differ greatly. Fertilization produced large increases in the annual rate of increase, but the generation time was shortened only slightly. (4) Reproductive values, defined as the expected number of daughter tillers from tillers of a given age relative to the expected number from a tiller aged 0, fell into one of three patterns: (i), decline from maximum value at age 0; (ii), increase from age 0 followed by a monotonic decline; (iii), increase from age 0 followed by a gradual, irregular decline and a plateau in the older age classes. The peak in reproductive value for patterns (ii) and (iii) occurred 2-4 years after the onset of tillering at age 1 or 2 years; this result was attributed to increases in age-specific tillering rate with tiller age. (5) Sensitivity analysis of the matrix models revealed that sensitivity of the annual rate of increase to changes in tillering rate and survival declines with age. The annual rate of increase was equally sensitive to changes in tillering rate and survival from age 0 to age 3. In the older age classes the annual rate of increase was more sensitive to changes in tillering rate than to changes in survival.