All forest ecosystems are subject to endogenous and exogenous disturbance events which influence species composition and stand structure and drive patterns of stand development. Disturbances may be categorized by a variety of characteristics, but they are most often classified along a continuum according to their spatial extent and magnitude. This continuum spans from broad-scale, stand-replacing events where most of the overstory is removed to fine-scale events resulting from the removal of a single canopy individual or a small cluster of trees. Although all forests may be exposed to stochastic, exogenous disturbances, the disturbance regimes of most oak stands in the Eastern Deciduous Forest are characterized by fine-scale, endogenous events. At the stand scale, these localized and asynchronous disturbances may create a patch-work mosaic of microsites comprised of different tree species, ages, diameters, heights, crown spreads, and growth rates. Through the modification of fine-scale biophysical conditions, these localized canopy disturbances promote heterogeneity and biodiversity in oak forest ecosystems. The gap formation rate in oak stands is typically 1-2% year-1. However, some studies have indicated that gap-scale disturbances were historically more common and a reduction in gap events may in part explain the regeneration failure of oak across the eastern USA. Management based on natural disturbance processes should consider elements of the gap-scale disturbance regime such as frequency, size, shape, and closure mechanisms and the historical range of variation associated with these characteristics. Silvicultural prescriptions are available for gap-based management designed to promote oak regeneration and mimic natural disturbance processes.
Dept of Geography; University of Alabama, Tuscaloosa, Alabama
33 • Steven Oak1, Martin A. Spetich2
Oak Decline in Central Hardwood Forests: Frequency, Spatial Extent, and Changes to Forest Composition and Structure
Pathogens and insects regulate forest composition and structure by causing growth loss and mortality in host trees. One such change agent distributed widely in the central hardwood region is oak decline. Episodes have been reported since the early 1900’s throughout the region. Oak decline is a stress-mediated disease resulting from the interactions of physiologically mature trees; abiotic and biotic stressors that alter carbohydrate physiology; and opportunistic root disease fungi and insects. Symptoms include reduced radial growth and slow, progressive crown dieback from the top downward and outside inward in upper canopy trees occurring over years to decades, often ending in death. Species in section Leucobalanus are less susceptible to oak decline mortality than species in section Erythrobalanus. Patterns of oak decline on the landscape vary widely. Mortality patches can range from a few trees in stands with relatively diverse species composition and age structure, to several thousand hectares in landscapes with a more uniform composition of physiologically mature Erythrobalanus species subjected to drought in combination with repeated defoliation by the non-indigenous gypsy moth. Past disturbances shaping current species composition and age structure are inextricably linked to oak decline vulnerability. Noteworthy examples are the functional extirpation of the American chestnut by the non-indigenous chestnut blight pathogen and changing disturbance patterns including fire suppression and reduced harvesting, all occurring during the early 20th century. Data from regional long-term monitoring surveys were used to develop models predicting the probability and severity of oak decline events as part of the Forest Vegetation Simulator (FVS).
1 USDA Forest Service, Southern Region, Forest Health Protection, Asheville, NC; 2 USDA Forest Service, Southern Research Station, Arkansas Forestry Sciences Laboratory, Hot Springs, Arkansas
34 • John Nowak, William Carothers, Kier Klepzig
Southern Pine Beetles: Frequency, Spatial Extent, and Changes to Forest Structure Across the Southeastern Upland Hardwood Forest Region
The southern pine beetle is a major disturbance factor in pine forests throughout the range of southern yellow pines, mostly impacting Coastal Plain and Piedmont forests of the Southeast. However, this insect also has significant influence on forests throughout the southeastern upland hardwood forest region because it has been known to attack and kill Eastern white pine, table mountain pine, pitch pine, shortleaf pine and red spruce. This disturbance does impact Southern Appalachian forests by converting stands to other species types, changing the age and stand structure. Southern pine beetle can create disturbance in stands in large clusters of pine trees or by hastening the succession from pine-hardwood forests to the climax forest. Southern pine beetle populations are cyclical and have traditionally impacted upland hardwood forests about every 20-25 years. The last outbreak in this ecoregion occurred from 1997-2002, impacting more than 1 million acres and causing an estimated economic loss of $1 billion across 5 states. In this chapter we will examine the spatial extent, frequency and severity of disturbance that this insect has caused to forests in this region over the past 50 years and how this disturbance has altered the resultant forests. Additionally, although this insect has been known to be a factor in upland hardwood forests since it was first intensively studied in the early 1900’s, we will discuss the possibility that this disturbance has become more frequent and more intense due to changes in climate in this ecoregion.
USDA Forest Service, Southern Region, Forest Health Protection, Asheville, NC
