Predicting Seedling Cohort Dynamics

Several studies have shown that survival of northern red oak seedlings originating from a single cohort is influenced by overstory density and other stand characteristics (see survival curves). A cohort of oak seedlings are the survivors of a single acorn crop.  In general, reducing overstory density increases seedling survival and growth (Beck 1970; Crow 1992; Loftis 1988, 1990). A dense layer of lower story trees, shrubs, or ground cover also can reduce seedling survival and growth (Beck 1970, Loftis 1990, Scholz 1955). Other factors that can reduce oak seedling survival include animal browsing, insect defoliation, droughty soils, inadequate light, and frost (Crow 1992, Gottschalk 1988, Hanson and others 1987, Korstian 1927, McGee 1988; Johnson 1993).

Survival Curves for Northern Redoak

During their first 9 years, numbers of northern red oak seedlings from a single cohort in North Carolina declined exponentially beneath the parent stand (survival curve, Part A). Unfortunately, there are few detailed reports of oak seedling survival of similar duration. Short-term studies nevertheless point out the great variation in survival rate among seedlings of the same species growing in various regions representing different stand densities and different light and competition environments. They also establish the range over which we might reasonably expect the survival rates of oak advance reproduction to recur. For example, after 5 years, survival of individual cohorts of northern red oak seedlings ranged from about 0.16 to 0.86, depending on overstory density or understory competition (q.v., survival curves) (Johnson 1993).

Despite the seemingly complex problem of predicting the establishment of oak seedlings, more than half the variation in the density of black oak and white oak advance reproduction in xeric forests in northern Lower Michigan was explained by relatively simple measures of overstory density and structure (Johnson 1992). For both species, 55 percent of the variation in reproduction density was explained by total overstory basal area and the basal area of "large" trees presumed to be the primary seed producers. Large trees were defined as those at least 14 in. d.b.h. for black oak and those at least 12 in. d.b.h. for white oak (see figure, below). The related models also showed that, per unit basal area of large trees, white oak was more efficient at producing seedlings than black oak. Moreover, high densities of black oak reproduction were favored under low density stands, whereas the reverse was true for white oak. Other studies have shown that topographic factors, stand history, and site quality also influence oak reproduction density (Arend and Scholz 1969, Carvell and Tryon 1961, Nowacki and others 1990, Ross and others 1986, Walters 1990; Johnson 1993).

Density of Black and White Oak

Oak reproduction often may be absent or scarce on mesic or hydric sites in the absence of disturbance (Carvell and Tryon 1961, R. L. Johnson 1975, Will-Wolf 1991). Nevertheless, oak reproduction densities in these forests may at times exceed 50,000 seedlings per acre in mesic forests (Tryon and Carvell 1958) and 100,000 per acre in bottomland forests (R. L. Johnson 1975). When oak seedlings do occur, they may represent only one or two acorn crops. Because of low survival rates, most of the seedlings from a single cohort may die before the next good acorn crop occurs. The rapid rate of seedling disappearance results largely from the shade-intolerance of oak reproduction and the low light levels on the forest floor (Hanson and others 1987). In the absence of disturbance, these forests typically possess high overstory basal areas and multiple subcanopy layers (Braun 1967, Loftis 1990). Such vertical stratification occurs in mesic and hydric oak forests throughout the deciduous forest region (Johnson 1993).