Population Growth, Structure, and Seed Dispersal in the Understory herb Cynoglossum virginianum: a Population and Patch Dynamics Model

Abstract

Abstract We use a combined model of forest canopy dynamics and patch‐specific demography to examine the response of a forest understory herb, Cynoglossum virginianum, to changes in rates of forest disturbance, rates of long distance seed dispersal, and patterns of seed dormancy. The matrix modelling approach followed that of Horvitz and Schemske (1986), in which empirically‐determined demographic parameters of a population experiencing open canopy conditions are modified depending upon demographic decay functions describing plant demography in a series of patch‐types undergoing succession toward fully closed canopy. In our analysis, demographic transition elements were estimated from 14 years of long‐term census data on three subpopulations at the Smithsonian Environmental Research Center, Anne Arundel County, Maryland. One of the three subpopulations was subjected to full canopy exposure in the second year of data collection, while the other two subpopulations remained in full understory conditions for the entire census period. Our results suggest small negative effects of a doubling of the rate of canopy disturbance (defined as the rate at which new treefall gaps appear in closed canopy) and only small positive effects of long distance dispersal on population growth rate, which predicts tittle selection for long distance dispersal. However, our analyses predict positive benefits of enhanced seed dormancy in this species, particularly if dormancy is higher in new gaps relative to understory conditions. These results are primarily a product of enhanced reproductive output in the years immediately following gap opening which is counterbalanced by increased seedling mortality in those years and a near complete suppression of reproductive output in later years (closing gaps). The suppression of reproduction in closing gaps may be a manifestation of a delayed effect of high costs of reproduction imposed in prior years. We discuss our results in terms of observed patterns of seed dispersal and in terms of the overall potential influence of forest gap dynamics on the demography of understory herbs similar to C. virginianum, in which demographic responses to changes in canopy conditions differ in strength and direction among life history stages.