Habitat
Gray dogwood occurs in thickets and moist soil in riparian zones, roadsides, on sandy slopes and limestone ridges (Soper and Heimburger 1982).
Ecology
Populations: Dogwood invasion of grasslands from swales, ravines, and woodland edges of floodplains is accelerated by vegetative reproduction and tolerance to wind, full exposure or partial shade, and dry soils (Pound and Clements 1900, Costello 1931, Steyermark 1940, Albertson and Weaver 1945, Weaver 1965, Duxbury 1982).
As density within a dogwood thicket increases, groundcover vegetation decreases and may become entirely absent (Aikman 1928, Weaver 1965). Annual weeds sometimes grow beneath dogwood (Duxbury 1982, Nyboer pers. comm. 1983), and bur oak (Quercus macrocarpa) may invade dogwood thickets (Albertson and Weaver 1945, Aikman 1928). Dogwood may persist and sometimes dominate the understory of woods (Duxbury 1982).
Reproduction
Sexual reproduction: These dogwoods probably reach sexual maturity in three to four years. There is one viable seed per drupe in all four species (Stephens 1973). A complex of hybrids exists between C. drummondii, C. racemosa (C. foemina subsp. racemosa) and C. foemina (subsp. foemina). The hybrids have high pollen viability, robust growth, and fruit sometimes larger and more plentiful than that of the parent (Wilson 1965).
Seed dispersal: Seeds are dispersed by a variety of birds, including crows, vireos, redheaded woodpeckers and bluebirds (Ridley 1930), autumn through winter (Stephens 1973). Availability of perching sites may be important in dispersal. Smith (1975) included C. racemosa in his study of re-vegetation of forest openings, and found that most seeds were deposited by birds within 25 meters of the seed source, often in the shade near perching sites. About 25% of the dispersed seeds left the study area after consumption by long-distance flying birds.
Germination: Germination usually occurs in the spring following seed production and dispersal to a favorable site, but may be delayed a year due to a dormant embryo, hard pericarp (Brinkman 1974), and possible chemical inhibition by the pulp (Goodwin 1948). Mechanical and chemical scarification and stratification techniques are used commercially to stimulate germination in dogwood (Brinkman 1974). C. racemosa and C. stolonifera are described by Krefting and Roe (1949) as having "double dormancy", or requiring two periods of stratification for germination. C. stolonifera seeds that were treated first with acids then with cold stratification experienced almost 100% germination, whereas germination was much lower for those seeds receiving cold treatment only. However, seeds of both species that were twice stratified by passage through quail or pheasant gut plus cold treatment also gave relatively low percent germination. The authors suggested that this was due to a large amount of variability in the extent of scarification from the bird gizzards. Some seeds are injured or overstratified in the bird gut and some are left unscathed or understratified (Krefting and Roe 1949). Smith (1975) described C. racemosa as fruiting abundantly but having very low germinability, depending instead on vegetative reproduction to enhance its propagation. Germination tests of scarified and stratified C. drummondii seeds have shown a 25% germination in three samples after 50 days (Brinkman 1974).
Seedling establishment: Some Cornus spp. shrub seedlings are tolerant of variable light intensities, and may become established in woodland edges, within woods, or in open areas (Gatherum et al. 1963, Smith 1975). Seedlings may invade grasslands alone or with other woody plants (McClain pers. comm.).
Asexual reproduction: C. drummondii, C. racemosa, C. stolonifera and C. obliqua reproduce most successfully by vegetative growth following seedling establishment. Thickets may expand by adventitious underground shoot growth or rhizomatous growth (Stephens 1973, Wilson 1965, Smith 1975).