Predicting Stump Sprout Dynamics

Estimated Sprouting Probabilities

Stump sprouts originate from dormant buds at or near the base of stumps of harvested overstory trees. In the silviculture of central hardwoods, overstory trees are defined as those 2 in. d.b.h. and larger (Roach and Gingrich 1968). However, that definition has not been universally adopted. The biological distinction between a stump sprout and a seedling sprout is nevertheless arbitrary because all oaks, from small seedlings to large standing trees, have some potential to produce basal sprouts when the parent stem is cut. When wind, fire, or other factors destroy an oak stand, sprouts also may develop from the bases of trees that have broken off or from standing trees with dead tops (Johnson, 1993).

For several species of oaks, the percentage of stumps expected to produce sprouts after timber harvesting can be estimated from tree diameter and tree age (Johnson 1977). In general, the frequency of sprouting decreases with increasing tree diameter, age, and site quality (see figure). But other factors, such as season of cutting and shading, also can affect stump sprouting in hardwoods. For some species of oaks, there is evidence that stumps sprout more frequently when trees are cut or killed during the dormant season than during the growing season (Clark and Liming 1953, Kays and others 1988). However, some of the live oaks of the Western United States sprout prolifically regardless of season of cutting (Longhurst 1956). Although sugar maple stumps exposed to full light sprouted more frequently than shaded stumps (Church 1960), similar responses of oak stumps to shading have not been reported. McGee and Bivens (1984) observed that numbers of stems in white oak sprout clumps were about the same for stumps that had been released from directly overtopping trees and stumps that were not released. Regardless of treatment, nearly 100 percent of the stumps of trees between 2 and 8 in. d.b.h. sprouted. Larger trees or those older than 60 years produced few or no sprouts (Johnson 1993). Stump sprouts originating from pole-size and larger parent trees are, in effect, mature root systems connected to juvenile shoots. This root:shoot combination results in rapid height growth. During their first decade, open-grown stump sprouts can produce four or more flushes of shoot growth per year totaling 3 ft. or more even under droughty conditions (Cobb and others 1985, Johnson 1979, Reich and others 1980). The large root mass of stump sprouts and their large carbohydrate storage and absorptive capacity, together with other factors, facilitate multiple flushing in oaks. In contrast, multiple flushes are not produced in mature oaks, shaded seedlings and seedling sprouts, and small seedlings and seedling sprouts under water stress (Borchert 1976, Buech 1976, Cook 1941, Johnston 1941, Kienholz 1941, Longman and Coutts 1974; Johnson 1993).

Frequency of flushing and total shoot elongation in oaks usually decline as stems increase in size and age and as root systems approach their maximum size. The number of flushes in scarlet oak stump sprouts decreased from an average of about two per growing season the first year to one by the fourth growing season (Cobb and others 1985). Thus, by the fifth year, the pattern of shoot growth approached that of the single flush of a mature tree. The progression from multiple to single flushes may be attributable to a declining root:shoot ratio that results in increasingly longer periods for roots and shoot to restore "functional balance" after shoot elongation and leaf expansion (Borchert 1975; Johnson 1993).

The number and spatial distribution of sprouts around the stump also influence sprout growth. The importance of sprout distribution around the stump may be related to the pattern of vascular connections that develop between sprouts and the parent tree root system, with each stem helping to sustain a portion of the root system (Kharitonovich 1937, Kramer and Kozlowski 1979, Roth and Sleeth 1939, Wilson 1968). In the Missouri Ozarks, the number of sprouts per stump was positively correlated with the early height growth of five oak species (Johnson 1977). The same relation also was observed for various oak species in other regions (P. S. Johnson 1975, Ross and others 1986, Schwarz 1907). Collectively, the observed spatial distribution and clump density effects suggest that numerous well-distributed sprouts maintain the parent tree root system and thus an efficient root-shoot feedback system that promotes rapid early height growth (Johnson 1993).

However, the apparent benefits of a balanced distribution of stems and high clump density are short lived. In Wisconsin, rapid growth of the dominant stem in unthinned clumps of 4- to 23-year-old northern red oak stump sprouts was associated with high clump density (P. S. Johnson 1975). Similarly, northern red oak clumps thinned to one stem as early as age 4 subsequently grew faster than stems in unthinned clumps (Johnson and Rogers 1984). This would seem to indicate that competition between stems in the same clump begins very early. However, 12-year-old northern red oak stump sprouts in Appalachian forests did not respond to clump thinning (Lamson 1988). Although northern red oak commonly initiates several dozen sprouts, stem crowding soon induces rapid stem mortality so that by the end of the first decade only four or fewer stems per clump typically remain. But this natural clump thinning process varies greatly among and within species (cf. P. S. Johnson 1975, Roth and Hepting 1969, Schwarz 1907, Johnson 1993).

The diameter of the parent tree and the correlated size of the root system also affect the growth of oak stump sprouts. For five species of oaks in the Missouri Ozarks, the correlation between stump diameter and height growth of the dominant stem within a sprout clump was consistently negative for 5-year-old sprouts of all species (Johnson 1977). However, the opposite was true of oak sprouts in Virginia (Ross and others 1986). Such discrepancies might be explained by the range of stump diameters observed in any given study. For example, data from a study of black oak and white oak sprouting that included trees ranging from less than 1 in. to more than 12 in. in basal diameter showed that the most rapid height growth occurred in clumps originating from 6-in. stumps (Johnson 1979). Sprouts from stumps larger or smaller than that grew less. The height growth of oak reproduction thus changes continuously, but not unidirectionally, from small advance reproduction to large-diameter overstory trees (see figure). (Johnson, 1993)

Height Growth and Parent Stem Diameter

Because of the large variation in number of stems per clump within a stump diameter class, we might expect roots and shoots of many young sprout clumps to be physiologically imbalanced. Accordingly, among the imbalanced clumps, rapid changes in clump structure would be expected as the clumps move toward functional balance. This view is supported by the initially large but rapidly decreasing variation in shoot growth among dominant shoots within black oak and white oak sprout clumps during the first 4 years (Johnson 1979). The large amount of unexplained variation in the relation between shoot growth and stump diameter also reflects rapidly changing root-shoot relations. Other factors that may be significant sources of variation in the shoot growth of oak sprouts include site quality, genetic variation, competition, parent tree age, and season of cutting. The significance of these factors may vary among oak species and regions (Johnson, 1993).