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Biennial cycling caused by demographic delays in a fire-adapted annual plant

Abstract:

We explored models explaining population cycling in the annual Warea carteri. We modeled the life cycle of W. carteri and compared projected trajectories to independently observed trajectories (up to 16 years) of plants in 74 patches in three populations. We built matrix models with an annual time step for two populations, including four stages, (recently produced seeds, seeds in the seed bank, seedlings, and adults) and five vital rates, summarized in seven transitions. Fluctuations of both observed and modeled populations were evaluated using power spectra, autocorrelation, amplitude, and damping. Observed populations had two point cycling. Observed amplitude was higher in frequently burned populations, reached its maximum 1 year after fire, and then dampened. Asymptotic transition and vital rate elasticities showed that seedling survival was the most important factor for long-term population growth, but transient elasticities showed that recruitment from the seed bank was important during the first years post-fire. Deterministic modeling and elasticity analyses indicated that delayed germination (for 1 year) may explain biennial population cycling. Stochastic models created similar cycling with slower damping than deterministic models, but still had lower amplitudes (especially 1-3 years post-fire) than observed populations. The biennial cycle in W. carteri is likely caused by the delay in seed germination, which creates two overlapping cohorts of plants, much like a strict biennial. Fire initiates the cycle by killing aboveground individuals and promoting seedling recruitment in the first post-fire year. © 2010 The Society of Population Ecology and Springer.