Demographic variation in a threatened plant on soils with different serpentine chemistries
Local adaptation is common in plants growing on serpentine soils and previous experimental work has shown strong local adaption in populations of Calochortus tiburonensis (Liliaceae) growing on particularly stressful serpentine soils but, somewhat paradoxically, inbreeding depression among plants on less stressful serpentine soils. In this study, I explore whether differences in fitness measured at the level of the individual are also expressed at the population level. C. tiburonensis is listed as Threatened and is known from only one hilltop in Marin Co., CA where it is confined to disjunct patches of serpentine soils that vary in their nutrient and heavy metal concentrations. I use transition matrix models to estimate asymptotic population growth rates (lambda) over two full transitions. I compare demographic parameters and population dynamics between the locally adapted and inbred populations and discuss the conservation implications.
Results from the two populations were strikingly different. For the locally adapted population on more stressful soils, the population was stable in both years (lambda = 1.01, 1.02). No one transition had a large elasticity value (range 0.01 – 0.19) and the population was near its stable stage distribution. The population suffering from inbreeding depression declined in both years (lambda = 0.86, 0.91). In this population, the elasticity of lambda to the persistence of large vegetative plants and their advancement to flowering had a composite value of 0.52. Population growth rate was substantially more elastic to this transition than to any other. Additionally, this population deviated from its stable stage distribution, with an excess of plants entering dormancy following flowering and a scarcity of vegetative plants. I am currently conducting a field experiment to test whether alleviating inbreeding depression (by generating offspring from crosses between the two populations) can reverse population decline. Results indicate that one the populations is stable and thus not in need of aggressive management intervention. For the population in decline, management that reduced the currently heavy deer herbivory on the largest plants ought to result in some degree of population recovery.