| Abstract |
- Population dynamics, plant communities, and abiotic environments of three narrowly endemic, allopatric mariposa lilies (Calochortus Pursh) are described and compared. All were restricted to ultramafic soils in southwestern Oregon with high concentrations of nickel, zinc, and chromium, and low calcium to magnesium ratios. Soils inhabited by the three species differed significantly (p < 0.0001) in pH and in concentrations of nickel, cadmium, manganese, magnesium, potassium, vanadium, molybdenum, strontium, and phosphorus. During a nine-year demographic study of Calochortus howellii Watson, reproduction, recruitment, and mortality were evaluated, and possible limiting factors and causes of rarity were investigated. Reproduction fluctuates widely from year to year, with bud production correlated with spring (February to May) precipitation (r² = 0.80, n = 9, p = 0.01). Recruitment and mortality were low and episodic, averaging 3.0% and 2.0%, respectively over 7 years. Capsule production averaged 3.8% during 1987 to 1991, declining from 17.8% the previous 4 years. Growth rates, particularly of seedlings, were extremely slow. Using size-classified transition matrices, changes in population structure and stability were assessed. Three methods of classifying data for transition matrix analysis yielded similar results in equilibrium population growth rates; based on all analyses, the study population was stable ([lambda] = 1.0). Taxonomically very distinct, yet only recently discovered, C. umpquaensis Fredricks and C. coxii Godfrey & Callahan are serpentine endemics known from limited distributions. Despite its narrow edaphic restriction, C. umpquaensis occurs locally within a wide range of habitats from meadows to forests. Based on a four-year study of C. umpquaensis, bud production was highest, plants were most dense, and on average were larger in the ecotone habitat. Equilibrium population growth rates were slightly lower in the meadow habitat. Low capsule production and seedset, low recruitment, high mortality, and declining population trends ([lambda] = 0.9) indicate that C. coxii should be carefully monitored. The probability of local extinction of this taxon is high, if the years studied are typical.
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