Mixed Mesophytic

Distribution and Description of the Mixed-Mesophytic Forest Type

Mixed mesophytic forests occur in the Appalachian Plateau, Ridge and Valley, and Blue Ridge physiographic provinces.  In the mountains, this forest type occurs on lower north-and east-facing slopes and mesic coves up to about 5,000 feet [1524 m] in elevation (McLeod 1988).  In less mountainous terrain, mixed mesophytic forest may cover the entire landscape where conditions are suitable (Greenberg and others 1997).

The most common characteristic tree species are sugar maple, beech, hemlock, silverbell, yellow-poplar, red maple, white ash, white oak, northern red oak, yellow birch, yellow buckeye, and basswood.  Yellow buckeye and basswood are indicator species for mixed mesophytic forests.  Many of the characteristic mixed mesophytic forest tree species are associated with high pH, and with high cation exchange capacity, high percent base saturation, and high contents of certain nutrients (Muller 1982, McLeod 1988). Depth of organic soil averages 0.4-3.0 inches (1.1-7.5 cm) in nonrocky areas (Greenberg and others 1997).

Structure, Composition, and Dynamics of Old Growth

Many of the structural features of typical old-growth apply to the mixed-mesophytic forest type, including a complex vertical structure, a "shifting mosaic" of age and size-class patches due to continual canopy gap formation, a high percentage of decadent trees, presence of snags and coarse woody debris, and thick organic soil layers with abundant macropores.  Species frequently found in a state of decadence are dogwood, beech, red maple, sugar maple, yellow birch, and basswood. Canopy height of eastern old-growth forests ranges from 98 to 131 feet (30 to 40 m).  Only 0.3 - 2 percent of incident light penetrates to the herb layer due to overlapping crowns.  Deciduous subcanopy and shrub layers are sparse (5 - 30 percent cover); but ericaceous shrubs can be dense.  A multilevel herbaceous layer may be present under a deciduous canopy but sparse to nonexistent beneath dense evergreen shrubs or in excessive shade.  Whittaker (1966) estimated aboveground biomass of 223 to 272 tons per acre [500 to 610 metric tons per hectare] in mature climax mesic forests (Greenberg and others 1997).

See: Standardized Table of Old-Growth Attributes for Western and Mixed Mesophytic Forests.

Diversity

Mixed mesophytic forest types are among the most biologically diverse ecosystems of the United States and perhaps of temperate regions worldwide (Hinkle and others 1993).  High species diversity, richness, eveness, and low dominance of canopy tree species appear to characterize old-growth, mixed mesophytic forests (Martin 1992).  Shrub, tree seedling, and herbaceous species richness also tends to be high.  McLeod (1988) reported an average of 51.1 vascular plant species per 0.25 acre (0.1 ha), with herbs contributing 72.2 percent of the total flora (Greenberg and others 1997).

No plant species is currently known to be restricted to old-growth, mixed mesophytic forests (Meter and others 1996), but Frasers sedge (Cymophyllus fraseri) and spotted mandarin (Disporum maculatum) are possible candidates (Martin 1992).  It is likely that if old-growth-dependent plants exist, mycorrhizal fungus species limit their distribution (Martin 1992).  No vertebrate species is known to be restricted to old-growth forests, but several require the structural features and microclimate of mature mesophytic forests (Haney and Schaadt 1996, Meter and others 1996, Pelton 1996, Greenberg and others 1997).

Disturbance

Disturbances that influence forest dynamics of mixed mesophytic old-growth forests include damage from fire, tornadoes, hurricanes, microbursts, ice storms, insects, fungal infections, floods, and landslides.  Forest patches dominated by shade-intolerant species, such as yellow-poplar, suggest that large-scale disturbance influences tree regeneration and species composition of these forests.  Infrequent intense fires are likely the primary large-scale disturbance type, although smaller fires may have been more frequent on xeric ridges before fire suppression was begun by State and Federal agencies in the 1930s.  Lorimer (1980) suggested that a heavy wind created the old-growth poplar cove at Joyce Kilmer Memorial Forest near Robbinsville, NC, over 300 years ago (Greenberg and others 1997).

Canopy Gap Formation

The creation of small canopy gaps by the death of single trees, or groups of trees accounts for the nearly constant tree turnover and species composition of old-growth, mixed mesophytic forests.  Actual gap size may range from 10 to 16,000 square feet (1-1490 m²) but commonly does not exceed 4,304 square feet (400 m²).  Estimates of background tree mortality for all species range from 5 to 10 percent per decade in old-growth, mixed mesophytic forests.  Estimates of canopy turnover rates vary from <0.4 percent to 1 percent annually (Greenberg and others 1997).

Age Structure and Diameter Distributions

Old-growth stands differ from younger, uneven-aged forests in having a greater range of tree sizes, a greater maximum tree age, and more large-diameter trees.  Few studies cite age exceeding 250 years for trees in old-growth, mixed mesophytic forests, although occasional trees much older than 250 years have been reported.  Irregular age distributions are common in old-growth stands and reflect severe natural disturbance or irregularities in seed production (Lorimer 1980). Estimates of background tree mortality for all species range from 5 to 10 percent per decade in old-growth, mixed mesophytic forests.  An additional 6 to 8 percent mortality caused by disturbance within a given decade is sufficient to create peaks in diameter distributions as more seedlings and saplings survive and grow into the canopy stratum (Lorimer 1980, Greenberg and others 1997).

Old-Growth Dynamics

Species composition and structural attributes of mixed mesophytic old-growth forests continue to change after the development stage is reached.  Gap-model projections by Clebsch and Busing (1989) predicted a shift in species composition from yellow-poplar to sugar maple over 250 years.  Death of American chestnut due to chestnut blight has dramatically altered canopy species composition of old-growth stands.  Busing (1989) reported a 52-year increase in basal area from 173.7 to 195.5 ft²/ac (39.9 to 44.9 m²/ ha) and from 118.0 to 167.7 ft²/ac (27.1 to 38.5 m²/ ha) in two old-growth stands in the Great Smoky Mountains National Park.  Increases were mainly in sugar maple and hemlock or silverbell, primarily in response to American chestnut mortality.