Structural characteristics of an old-growth longleaf pine stand on Horn Mountain, Alabama

Background/Question/Methods Components of forest structure critical to understanding stand dynamics include age and stand structure (tree size and location).  Age structure provides information on past disturbances and the process of replacement of individuals.  Stand structure relates to current conditions, useful in projecting future growth.  Components of forest structure provide the basis for models of the dynamic nature of forest stands. Longleaf pine (Pinus palustris Mill.) forests once dominated the uplands of the Coastal Plain where stand structure and dynamics are relatively well understood.  In contrast, dynamics of longleaf forests north of the Coastal Plain (montane longleaf) are poorly understood.  Stand and age structure were evaluated in a fire-excluded old-growth longleaf pine montane forest on Horn Mountain in central Alabama, USA.  This site is thought to be the largest remaining old-growth longleaf stand in the montane region.  We measured age and stand structure to evaluate variability in the stand and shed light on past disturbance and replacement patterns of old-growth mountain longleaf pine stands. All living longleaf pines > 2.54 cm DBH were stem-mapped and measured.  Hardwoods and non-longleaf pines > 10 cm DBH were sub-sampled.  All longleaf pine > 10 cm DBH and sub-sample of non-longleaf pines and hardwoods are being cored.

Results/Conclusions The Horn Mountain stand covers approximately 10 ha.  Diameter distribution for longleaf pine is relatively uniform.  The majority of trees are 25-50 cm in size with few trees less than 10 cm.  Lack of trees in smaller size classes is an indication of fire exclusion.  The Horn Mountain stand contains several age classes ranging from 25 to 200+ years old.  Lack of trees less than 25 years also suggests that fire has been excluded for many decades.  This is in contrast to descriptions of Coastal Plain old-growth stands that have been documented to support a relatively high proportion of younger ages.  Age class distributions of natural Coastal Plain stands often resemble a reverse J-shape curve, an indication of relatively consistent recruitment.  Preliminary data from Horn Mountain suggests that recruitment in montane areas may have been more episodic.  Unlike fire-maintained Coastal Plain sites where the canopy is dominated by longleaf pine, old-growth montane forests may have support hardwood and non-longleaf pine species even when fire was not excluded.  At Horn Mountain, several oak species over 100 years old are present but in addition fire intolerant hardwood and non-longleaf species also are now an unnatural component of the stand.