Stressors of Pine Forests
Littleleaf disease (LLD) affects shortleaf and loblolly pines in the Piedmont plateau of the southern United States. It is caused by a soil fungus that attacks the feeder roots of older trees growing in eroded soils with poor internal drainage. Feeder roots are killed at a faster rate than the tree can generate new ones, and tree growth slows.
After several years, infected trees show sparse and yellow crowns of dwarfed and tufted needles (Figure 18). Abnormally large cone crops of undersized cones are often produced. The cones contain smaller seeds with decreased viability. These distress cone crops are indicative of the tree's loss of recuperative vigor. Eventually, affected trees die, either from the disease alone or from increased susceptibility to pine bark beetles, particularly the southern pine beetle.
The historical range of LLD encompasses 177 counties comprising some 50 million acres from Mississippi to Kentucky and Virginia (Figure 19). This range represents the largest area where older, susceptible pines and littleleaf-disease soils once coincided. The area was determined from the historic distribution of LLD described in published literature and from consultation with State and Federal cooperators.
Today, high and moderate risk soils predominate in 125 of the counties in the historical range of LLD, with a total of 33.6 million acres. The soil hazard for LLD was determined from State soil maps.
At present, 87 of these 125 counties with high- and moderate-risk soils have low LLD potential because they have a low volume of loblolly plus shortleaf, defined as less than 100 cubic feet per acre. Data on volume were from work units of Forest Inventory and Analysis of the Southeastern and Southern Forest Experiment Stations. These counties were identified by overlaying data for host type within the disease range on counties with predominately high- and moderate-risk soils. The LLD potential could increase in these counties if pine volume increases in these areas.
In only ten of the 125 counties, totaling 3.6 million acres, does high volume of loblolly plus shortleaf pine coincide with littleleaf soils. For this analysis, high volume was defined as more than 200 cubic feet per acre and is used as a surrogate for older, littleleaf-prone trees. The ten counties with the highest LLD potential, shown in Figure 20, are located in Alabama, Georgia, and South Carolina.
This hazard rating is based on Forest Inventory and Analysis data and on soil information. Its accuracy is directly related to the completeness and accuracy of these information sources. Further refinement might be made by enhancing pine-volume maps with information on size and age classes that gives a better indication of prime susceptibility. Soil survey data collected consistently with identical intensities in all States would also improve overall quality. With the passage of time, we can expect to witness these improvements.
Littleleaf disease probably will continue to decrease on private land because the trend for industrial owners is toward shorter rotations, thereby avoiding older stands prone to the disease. Control of tree density and species composition through artificial regeneration will also contribute to this downward trend. Further, increasing urbanization in the Piedmont region probably will divert land from forest uses and reduce the acreage of high-hazard soils that support susceptible pine types.
In contrast to private land, the trend on public land is toward longer rotations and increasing noncommodity uses, such as recreation and wildlife habitat. On these lands, LLD will probably continue to be a significant management concern as stands age. If susceptible shortleaf and loblolly pines are regenerated on public lands where the objectives are continued long rotation for wood products and noncommodity values, LLD will surface as a management concern as these stands approach vulnerable ages. If no intentional pine regeneration is undertaken, then low-quality hardwoods that are immune to LLD will gradually dominate the landscape as pines are replaced by the combined effects of nonintervention, LLD, and fire control.