The occurrence of hardwood types often relates to the direction of the slope (aspect) as well as to the elevation of the site. In the Piedmont, for instance, the post oaks or on sites characterized by impervious and plastic clay within a few inches of the surface of the land. Seldom are northern red, black, scarlet, and white oaks found in such locales. The more moisture-demanding white and red oaks occur on cooler, wetter, usually elevationally higher north-facing slopes.6
Shade intolerance excludes many xeric species from sunless northern slopes, while insufficient drought resistance prevents mesic species from dominating dryer south-facing sites. Although neither soil fertility nor pH seem influential in species location, microclimate plays a role. The climate near the ground affects overwintering seed, availability of light for photosynthesis, the occurrence of competing plants, and—by affecting soil moisture—resistance of the soil to root penetration.
Fire induces changes in the spatial pattern of broadleaf trees. This is especially apparent for turkey oak in longleaf pine stands in the Florida sandhills. There, fires reduce clumping by species, creating a more random pattern that thus increases segregation of the oak and pines. Repeated burning diminishes the number of clumps, while the patchiness of the oaks may be directly attributed to fire intensity.7
Birds and rodents distribute seeds that establish oak and hickory forests. Blue jays notably disperse black oak acorns over wide areas. While seed germination may not be improved by digestion in mammals, many seeds passing through alimentary canals do remain germinable. Birds, however, hasten the breaking of dormancy as the thickness of seed coats is reduced by abrasion within the gizzard. Acidic fluids and bacteria aid the process; seed coats then are more permeable and better able to absorb water and oxygen when later deposited on the soil. Birds distribute seeds with fleshy fruit, like those of cherry and persimmon trees, often getting drunk and falling from tree limbs as they consume fermenting fruit. Some birds are selective; robins improve the germinative capacity of black cherry, grouse carry poison ivy seeds, and pheasants and bobwhite quail enhance germination of the black locust, dogwood, and choke cherry seeds on which they feed. (Seed treatments in nurseries often mimic these natural processes. Turning seeds in sandpaper-lined portable cement mixers scarifies the seed coats and soaking the seeds in dilute solutions of hydrochloric acid softens their coats.)
Acorn production varies little from mountain to mountain, though rainfall between the summits might differ by 20 inches. Year-to-year trends in acorn production follow a common pattern for all species; white oak mast is abundant in the same years that black oak fruit are plentiful. This is especially interesting because species of the white oak Quercus subgenus leucobalanus mature mast in one year while those of the red oak subgenus erythrobalanus require two years.
When seeds fall in the autumn, 20 to 60% are sound. Birds and squirrels destroy one-fourth of the acorns on a tree, while insects take another 30%. A typical oak tree might disperse 1300 sound seeds, only 100 of which will germinate.8 Acorns and hickory nuts, being relatively large seeds, germinate to produce large seed leaves (cotyledons) that are able to withstand much damage and still provide adequate initial nutrient and carbohydrate supplies for seedlings. Once on the ground, nut weevils, moth larvae, and gall-forming cynipids feed during the insects' larval stage on oak and hickory mast.
Deer consume entire acorn crops, although sometimes vainly trying to fend off piney-woods' rooters for this food. Wildlife seem to prefer acorns of the white oak subgenus over those of the red oak group, perhaps, as wildlife specialists speculate, because the latter are cathartic.
Oak seeds germinate readily in a litter layer about one inch thick. With deeper layers of undecomposed organic matter, hickory species win out. Under droughty conditions, thick mats of fibrous material conserve moisture for use by tender seedlings. When conditions are right, as many as 900,000 seedlings per acre arise from oak acorns.9 After germination, long-tailed wood mice sever seedling roots in their search for food, while soil-burrowing rodents cut roots as they tunnel the ground.
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