Predicting Seedling Sprout Dynamics

Because the seedling stage is usually brief, seedling sprouts are the predominant form of oak reproduction in many, if not most, oak forests. Seedlings can sprout from dormant buds anywhere along the stem between the root collar and the terminal bud cluster. Dieback and resprouting seem to be important processes in the life of oak reproduction. Although recurrent shoot dieback is common to most hardwoods, it is especially prominent and ecologically important in the xerophytic oaks, which are morphologically and physiologically adapted to survival in environments subjected to repeated fire and drought (Abrams 1990, Grimm 1984, Wuenscher and Kozlowski 1971; Johnson 1993).

The natural environment of seedling sprouts of the xerophytic oaks imposes stresses that periodically decrease shoot mass and leaf area through shoot dieback. Surviving seedling sprouts thus develop increasingly greater root:shoot ratios as roots grow incrementally larger and shoots recurrently die back. In turn, high root-shoot ratio and large root mass enable oak reproduction to opportunistically respond to favorable environmental conditions by facilitating two or more long flushes of shoot growth (multiple flushing) during one growing season (Dickson 1991, Johnson 1979).Successional replacement of oaks by oaks thus heavily depends on conditions that favor the long-term accumulation of oak reproduction with high root:shoot ratios and large root mass. Lacking those characteristics, oaks are usually at a competitive disadvantage. This is especially true of the reproduction of the xeromorphic upland species, which grow slowly even under optimal conditions until they develop the requisite root mass and root:shoot ratio. Shoot dieback thus may be an important aspect of the evolutionary development and adaptive strategy of oaks (Johnson 1993).

The accumulation of oak reproduction under a parent stand is one of the most important aspects of the regeneration ecology of oaks. Oak silviculturists call this"advance reproduction" because, in the even-aged management of oaks, it is present in advance of final harvest. Its presence and development largely determine the importance of oaks after natural or human-caused events that destroy or remove the parent stand. Oaks opportunistically capitalize on this accumulation process because it facilitates the capture of growing space when the overstory is destroyed or removed. This capacity largely depends on the characteristics of the competing vegetation and the accumulated population of oak seedling sprouts with large roots. The size distribution and age distribution of this population, in turn, depend on the balance of birth, death, and growth rates of reproduction intrinsic to each type of oak forest (Johnson 1993).