Biotic and abiotic control of the dynamics of gray dogwood (Cornus racemosa Lam.) shrub thickets

1. Gray dogwood is the most abundant shrub in old-fields and utility rights-of-way in the Hudson Valley, New York, USA. It is present across an extremely broad range of environments, but most abundant on relatively wet sites. Once a site has been colonized, it spreads primarily through vegetative production of new shoots from extensive networks of horizontal roots (runners). 2. Remeasurement of permanent quadrats revealed that gray dogwood cover increased, on average, in all community types between 1987 and 1990, but most in communities on moist soils. The few quadrats with declining dogwood cover in this period were in communities on drier sites. 3. Analyses of stem densities, recruitment, growth, and mortality along transects through dogwood thickets showed that the thickets expand by gradual infiltration of a wide zone around the margin of the closed shrub canopy. The closed shrub margin expands through slow lateral development of the crowns of widely scattered stems outside the current margin. 4. Matrix projections of the transitions between understorey, subcanopy and canopy stems between 1989 and 1990 in three different positions within thickets showed that the observed stage distributions differed from expected stable distributions, except in the centres of thickets in dry sites. Stem population growth rates in wet and mesic sites declined with increasing thicket size as individual stem diameters and crown areas increased. Elasticity analysis showed that changes in stem density in all three positions and all three environments were most sensitive to canopy and subcanopy stem survivorship. 5. A field experiment demonstrated that surrounding grass and herb-dominated communities provided little inhibition of the rate of spread of dogwood thickets except on wet sites. In contrast, the experiment confirmed results from our field censuses that thicket expansion was very sensitive to soil moisture regimes. On dry sites, the rate of spread of dogwood was slow because of slow growth and high mortality of stems in the periphery of the thicket, regardless of the presence or absence of surrounding vegetation. Nonetheless, the presence of extensive dogwood thickets on even very dry sites indicates that gray dogwood is ultimately capable of dominating such sites under selective management regimes that protect it from major disturbance and prevent it from being overtopped by trees.