Causes and consequences of spatial aggregation for seedling establishment of a long-lived desert perennial shrub (Chrysothamnus nauseosus: Asteraceae): Interaction between intrinsic properties of individuals and fine-scale environmental heterogeneity

Describing how individuals in a population are distributed in time and space is central to understanding plant population dynamics. Where seeds and seedlings survive in the environment determines the initial spatial pattern of individuals and may ultimately determine the spatial pattern of adult plants. Seedling establishment patterns are influenced by the effect of intrinsic properties of seeds on seed and seedling performance (defined as the probability of germination, emergence, seedling size and survival) and by the immediate environment surrounding individuals. Few studies investigate how the intrinsic properties of individuals interact with fine-scale environmental heterogeneity to establish plant population structure. I present results from three studies conducted on a desert perennial plant, Chrysothamnus nauseosus spp. consimlis in the sand dune ecosystem near Mono Lake, California. Chapter one described the causes and consequences of the initial spatial distribution for seedling growth and survival. Seedlings were highly aggregated. This pattern was caused by microenvironments that varied in effectiveness for seed retention and seedling survival. Seedlings experienced facilitation (by nurse plants) and competition on different temporal and spatial scales. Seedlings that were initially larger at germination remained larger over the growing season and lived longer than seedlings that were initially smaller. Chapter two showed that the initial size differential among seedlings begins with differences in seed mass. Greenhouse and field experiments determined that larger seeds had a higher probability of germinating and successfully emerging (expansion of cotyledons) than smaller seeds. Compared to small seeds, large seeds produced initially larger seedlings that remained larger and lived longer. Chapter three explored whether seeds of different sizes responded differently to varying microsite characteristics. Environmental heterogeneity and seed mass both influenced seed and seedling performance. Larger seeds always performed better than smaller seeds, and seedlings only survived under adult shrub canopies.; These studies elucidate the mechanisms that drive seedling establishment patterns. Both the intrinsic properties of individuals and environmental heterogeneity determine the initial spatial distribution of individuals in the population. Even though seedling establishment is brief in the life history of this species, these earliest life history determine which individuals will recruit into the population and become reproductive.