FACTORS AFFECTING ACORN PRODUCTION AND GERMINATION AND EARLY GROWTH OF SEEDLINGS AND SEEDLING SPROUTS

by DAVID F. OLSON, JR., and STEPHEN G. BOYCE, respec- tively Principal Plant Ecologist, Southeastern Forest Experiment Station, Bent Creek Experimental Forest; and Director, Southeastern Forest Experiment Station, Forest Sem'ce, USDA, Asheviile, N. C.

ABSTRACT. Acorn production is extremely variable and unpre- dictable. Flowering is copious, but many climatic factors influence acorn development from initiation of flowers to acorn maturity. Acorns are consumed by birds, animals, insects, and microorganisms. The establishment of seedlings is more closely related to favorable site factors than to size of crops. A majority of oaks originate from advance reproduction that accumulates under the previous stands during several decades.

OR THOUSANDS of years our major germination of acorns, and the early growth Fspecies of oaks have become success- of seedlings and seedling sprouts. fully established from acorns. With this There are hundreds of published obser- reproductive mechanism, the oaks have vations and scientific studies on these sub- moved back and forth with the glacial jects. We have exercised our judgment to wriods of the North American continent condense existing knowledge into general %hey have survived many climatic and concepts. These concepts are presented to geologic changes and many wildfire con- provide a framework of knowledge to flagrations, including the fires of Indians guide practical decisions, as well as to iden- and the fires of Ioioneers. The oaks have also tify some research needs. We have cited occupied new sites in competition with key papers that will lead the inquiring per- many other forest species. On many of the son into the details of anatomy, physiology, blight-killed chestnut sites in the eastern and growth and development of flowers, United States, the upland oaks have re- acorns, and seedlings. Much of this litera- placed dead chestnut trees (Korstian and ture pertains either to the genus as a whole, Stickel 1927; Woods and Shanks 19J9). or to additional species of oak other than Our purpose in this paper is to discuss the five upland species that are the subject the factors that affect the production and of this symposium. Even within this group of five upland species, there are wide dif- flower production would leave larger food ferences in the processes of sexual repro- reserves for the growth of wood and other duction and early growth, and we must use vegetative organs. caution in applying generalities to guide both practice and research. FRUIT FORMATION Early ovule development begins approxi- FLOWER FORMATION mately 1 month after pollination in the sub- Acorns, the fruit of the oaks, develop genus Lepidobalanus (white oaks), and ap from fertilized flowers, never from unfcr- proximately 13 months after pollination in tilized flowers by such asexual methods as the subgenus Ery throbalanus (red oaks). parthenogenesis and apomixis. All species of Although there is a major timing difference oak produce both male and female flowers between the two groups before fertiliza- on the same tree. Both staminate (male) tion, the subsequent developmental stages and pistillate (female) flowers are formed of the embryo are relatively uniform from every year in great abundance by practi- a cytological or morphological viewpoint. cally every oak tree of flowering size and Therefore the major difference of interest age; consequently, the year-to-year vari- is the time required for seed maturation. ability in size of acorn crops is not due to The white oak group matures seed in one lack of flowering capability, nor to cyclic growing season; the red oak group re- formation of flowers. quires two growing seasons (Stairs 1964). The pistillate flowers are located in the - The loss-in fruits occurs because axils of the new leaves, whereas the stami- of premature abscission. The highest rate of nate flowers develop on long slender cat- loss occurs during the priod of pollination kins that are borne either in the axils of and fertilization (Turkel, Rebuck, and scale leaves of the current vegetative buds Grove 1955), although abscission of irnma- or in separate male buds (Stairs 1964). Pol- ture acorns continues until maturitv. We lination is by wind and air currents. do not know the reasons for these ibscis- Climatic factors are most important in sions, but we do know that trees with large controlhng flower formation and develop crowns have about the same pattern of ment, and in the pollination of flowers. abscission as trees with small crowns (Will- There are no important biotic factors that iamson 1966). Open-grown trees do not affect the formation of oak flowers. Various produce any more flowers and acorns than climatic factors, such as wind, late frost, do trees with the same crown surface area prolonged rain, relative humidity, and tem- in closed stands. However, the surface area perature have been shown to affect the of oak crowns in closed stands can be in- opening and closing of the anthers and the creased by thinning, because crown surface dissemination of pollen (Sharp and Chisman area for trees of a given age depends to a 1961). A study of pistillate flowering and large degree on stand density. Limited evi- acorn development in Pennsylvania showed dence in Russia also suggests that increas- that good crops of white oak acorns were ing the growth rate of shoots by cultural obtained when a warm 10-day period in ractices such as thinning may increase late April was followed by a cool period in Fruit-bearing in the oaks (Polozoya 1956). May (Sharp and Sprague I967). At present, The number of acorns produced by a there are no known ways to control flower tree also mav be an inherent charactellstic formation and development or the pollina- (~r~ens-~olier1955; Sharp and Sprague tion of flowers. 1967). The ability to control these processes would be a valuable tool for the forest SIZE AND PERIODICITY manager. Abundant flowering and high OF CROPS rates of pollination would insure more con- Numerous investigators have estimated sistent acorn crops for reproduction and the size of acorn crops from one to several wildlife food. On the other hand, low years, and in all parts of the world where oak forests occur. Both timber and wildlife and phorate were effective in reducing managers are intensely interested in the acorn losses by the acorn weevil (Cu~culio seed production of oaks, the timber manager spp.) when the insecticides were injected because of the reproductive needs of the into the boles of the trees. forest, and the wildlife manager for the The percent of sound acorns capable of store of food used by important game birds germination is closely related to size of and animals. crop (Beck and Olson 1968; Korstian 1927; It has been commonly observed by all of Minckler and Janes 1965). In poor years, these investigators that acorn crops are ex- nearly all acorns are consumed. The num- tremely variable from tree to tree, from ber of new seedlings added to the advanced species to species, from location to location, regeneration each year is also directly re- and from year to year (Beck and Olson lated to size of the acorn crop. 1968; Christisen 1951; Christisen and Kor- In addition to consumption by organisms, schgen 1955; Downs and McQuilken 1944; acorns are destroyed by other means, such Gysel 1956; Minckler and Janes 1965; as freezing, drying out, burning in wild- Tryon and Carvell 1962a, 1962b). The fac- fires, and flooding. These destructive forces tors causing this variability are not known, account for relatively large losses in poor though many attempts have been made to crop years and relatively small losses in discover them (Gysel 1958; Williamson good crop years. 1966). On a land-area basis. acorn crops vary GERMINATION OF ACORNS from nearly zero to 250,000 or mori acolGs Sound, undamaged acorns have a germi- per acre. Many times the white oak group native capacity between 75 and 95 percent will produce heavily in a particular year, (USDA Forest Service 1948). Acorns of and the red oak group will have prac- the white oak group germinate in the fall tically no yield, and vice versa (Barrett immediately after falling to the ground. 1931; Gysel1956; Beck and Olson 1968). In Acorns of the red oak group overwinter mountainous terrain, the wide variability in on the ground and germinate the following microclimate due to elevation and aspect spring. For germination to occur, the moist- results in comparable variability in acorn ure content of acorns must not drop below production. There is need to develop an 30 to 50 percent for white oaks, and 20 to index of production that can be used to 30 percent for red oaks (Korstian 1927). predict mast for animal food from both the Litter or soil cover affords protection white and red oak groups (Gysel 1918). against drying and is necessary for success- Since we do not have practical means for ful germination (Barrett 1931; Korstian controlling the size of acorn crops, wildlife 1927). Germination is hypogeal, and under managers advocate the culture of mixed favorable conditions is generally complete oak stands over a range of sites to minimize in 3 to 5 weeks (USDA Forest Serwice the likelihood of complete crop failure 1948). (Christisen 1955; Collins 1961). In a West Virginia study, Carvell and Tryon (1961) found that, from equal num- DESTRUCTION OF ACORNS bers of red and white oak acorns, the Acorns are consumed by birds, animals, number of white oak seedlings produced insects, and microorganisms (Korstian was five times as great as the number of 1927). Two groups of insects, nut weevils red oaks. This situation appears to be the and gall insects, cause the largest losses of usual one, but the reason for the difference acorns (Barrett 1931; Beck and Olson 1968; is not known. White oak acorns generally Downs and McQuilken 1944). Brezner fall earlier than red oak acorns and are (1960) found that in Missouri the weevils smaller than red oak acorns. Since the of the genus Curculio were the principal smaller white oak acorns germinate im- agents of acorn depredation. Dorsey (1967) mediately and are covered by deeper leaf showed that the systemic insecticides Bidrin litter than are the red oaks, white oaks are probably afforded greater protection from ing a study of cutting intensity. He ob- damaging agents. served a dvnamicJ fluctuation in the advance The establishment of oak seedlings in regeneration as seed germinated, seedlings West Virginia was found to be more lived a few years, died, and were replaced closely related to favorable site factors than by new on&. to size of acorn crops (Tryon and Carve11 There is a high mortality of newly estab- 1958,1962a, 19626). However, this relation- lished oak seedlings on all sites. Death is ship varies widely for the different species highest in the first 6 years of the seedling's of oaks and regions of the country. life. Few seedlings survive the first year without dying back and sprouting at least EARLY GROWTH OF SEEDLINGS once. Some seedlings (mostly white oak) AND SEEDLING SPROUTS survive under dense canopies for periods up The principal factors that influence the to 50 or more years by a repeated process development of oak seedlings are light in- of sprouting and dying (Mem and Boyce tensity and soil moisture (Bourdeau 1954; 1956; Merz and Boyce 1958; Minckler and Kozlowski 1949). The five upland oak Janes 1965). This survival is made possible species under discussion in this symposium by the large number of buds that occur on are intermediate in shade tolerance and new seedlings and subsequently develop reach maximum rates of photosynthesis at near the ground line of the long-surviving lower light intensities than intolerant species individuals. Near the base of new shoots a such as the southern pines (Kramer and large number of buds is formed in the axils Decker 1944). As a result, these oaks can of rudimentary leaves. These leaves look compete favorably in a broad range of light like tinv hairs. The buds in the leaf axils. climates if soil moisture is adequate. The which &ay remain in suppression for many exclusion from poor sites of the oak species years, and the three buds in the axils of normally found on good sites (northern each cotyledon (Kumes and Boyce 1964), red, scarlet, and white oaks) is because of form a reservoir of buds for repeated insufficient drought resistance of the better sprouting of shoots. oaks. The exclusion from good sites of When the top of a young seedling dies poor-site oaks such as blackjack oak is ex- back, one or more of the suppressed buds plained by the low shade tolerance and grow. These buds are often below the soil slow rate of growth of blackjack oak. surface, and are protected from damage.

Merz and Boyce (1958) found that 88 Such characteristics make it IDossible for percent of the oak reproduction became oak seedlings to survive after the tops are established prior to both light and heavy destroyed by animals, insects, disease, log- harvest cuts. The amount and distribution ging, and fire. In this manner, seedling of oak reproduction found in the harvest sprouts of oaks accumulate beneath the cuts is not necessarily related to the cutting canopies of existing stands (Liming and intensities. A majority of oaks in the new Johnston 1944). stadds grow from seedling sprouts that Various forms of partial cuts prior to have accumulated over several decades final cuts have been recommended to re- prior to the harvest cut. The major oak generate the oaks from current seed crops, species in these areas was white oak. Some but there are no certain ways to manage of the red oaks, mostly northern red and stands of oak for the abundant ~roduction scarlet oak, do not persist as advanced re- of acorns. Methods of manag&ent have 2 production for more than a few years. been developed to favor the accumulation McGee (1967) found abundant oak re- of advanced oak regeneration. These generation under eight mixed oak stands in methods are discussed in other symposium the Southern Appalachians prior to install- papers. Literature Cited

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