Ecological Archives E087-126-A1

Amy Lauren Angert. 2006. Demography of central and marginal populations of monkeyflowers (Mimulus cardinalis and M. lewisii). Ecology 87:2014–2025.

Appendix A. Estimation of seed dormancy and recruitment parameters.

Methods.—In September 2002, detailed studies of seed dormancy were initiated at one central and one marginal site per species (central: M. cardinalis, Buck Meadows; M. lewisii, May Lake; marginal: both species, Carlon). At May Lake and Carlon, eight 20 × 20 cm plots per species were excavated to a depth of 15 cm to remove any previously existing seed bank and seed-free soil from above the floodplain was used to refill the excavated area. Four rings cut from sections of poly(vinyl chloride) pipe (10 cm diameter, 8 cm tall) were buried in each plot, leaving 1 cm of ring visible above ground level. Two rings per plot were randomly assigned the seed treatment, in which approximately 2160 seeds by volume were added to each ring in September 2002, and the remaining two rings served as “no-seed” controls. At Buck Meadows the design was identical except a seed shortage allowed only 1 seed treatment per plot containing approximately 1250 seeds by volume. During the seed dispersal period, rings were covered with fine mesh (“No Thrips”, 150 × 150 μ opening size, Green-Tek, Inc., Edgerton, WI, USA) to prevent new seed entry. Each year seedlings were counted and removed with forceps twice, once in early summer after most emergence had occurred and once in the autumn to capture any additional germination. Because removing germinants with forceps truncated seedling establishment, these plots could not be used to estimate recruitment from seeds to vegetative stages; these plots were used only to estimate annual seed survival in the seed bank. “No-seed” control rings had very few seedlings, but when they did have seedlings the average seedling count in the control rings was subtracted from the seedling count in each seed addition ring. Numerous plots were lost to flooding, tree fall, and animal disturbance, including all plots at Carlon. Due to small remaining sample sizes, data were pooled for species-specific estimates of seed dormancy.

Calculation of annual dormant seed survival

Mimulus lewisii, May Lake (range center, 2690 m)

a) Number of dormant seeds in 2003

            = 63 seedlings2004 × (21,600 seeds2002/2291 seedlings2003)

            = 594 seeds

b) Percentage of dormant seeds in 2003

            = 594 dormant seeds2003/21,600 seeds2002

            = 2.8%

Mimulus cardinalis, Buck Meadows (range center, 830 m)

a) Dormant seeds in 2003

            = 7 seedlings2004 × (2,500 seeds2002/38 seedlings2003)

            = 460 seeds

b) Percentage of dormant seeds in 2003

            = 460 dormant seeds/2500 seeds2002

            = 18.4%

Calculation of recruitment from seeds.— Estimates of seed germination from the seed dormancy plots could not be used for matrix calculations because seedlings were removed before establishment, causing the rate of seed germination in the seed addition plots to far exceed the rate of seedling recruitment. Better estimates of transitions from seed to vegetative classes for each site and year were obtained by calculating the ratio of recruitment to seed production. Because seeds are known to live at least one year in the seed bank, seed production was estimated as the moving sum across a two-year window, yielding the following estimate of transitions from seed to a particular stage class:

             ai1 = number of recruits in class it+1/(seed productiont + seed productiont-1)

Because the dormant seed bank is of unknown size, this calculation may underestimate the true denominator. To assess the affects of uncertainty in estimates of recruitment on population growth, I increased the size of the seed bank by as much as 50% and found that lambda decreased by 1.4 – 6.7% for M. cardinalis and 0.7 – 5.7% for M. lewisii. As for seed dormancy, lambdas at all populations responded similarly to increases or decreases in the size of the seed bank, and the magnitude of change in lambda due to variation in seed bank size was not sufficient to erase differences in lambdas between central and marginal populations.



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