Demography of a dormancy-prone geophyte: influence of spatial scale on interpretation of dynamics

Both temporal and spatial scales are important in the evaluation of population dynamics, but the latter often receives less attention in demographic analyses. We used a 5-year demographic data set for a long-lived geophyte, Calochortus lyallii, to investigate the pattern and components of spatial variation at two scales (population and microsite). We found that neither the projected population structure nor asymptotic population growth rate (lambda) varied greatly across either scale, although the underlying contributors to the variation in lambda, V(lambda), did differ between scales. Life table response experiment analyses showed that V(lambda) among populations came primarily from variation in seedling survival and progression of non-reproductive plants, whereas V(lambda) among microsites was primarily due to the variable fertility of large adults. Prolonged dormancy was important in reducing V(lambda) among quadrats at both the scales, partly countering fluctuations in other transitions such as recruitment. This result represents some of the first evidence that underground bulb banks could function to offset the effects of a spatially heterogeneous environment in a manner analogous to seed banks. Future work is needed to isolate the specific, sometimes idiosyncratic, life history phenomena acting to modulate plant population dynamics in a spatial context.