A post-harvest evaluation of mechanized thinning in natural loblolly pine in the coastal plain of Arkansas

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Virginia Tech


Commercial thinning in the South is a highly efficient mechanized operation which operates year round. Southern winters are typically wet; therefore, the potential for soil rutting and compaction exists when heavy machines are used. This study was undertaken to determine if mechanized thinning in wet weather impacted the soils and affected tree growth.

Soil and tree growth data were collected from two natural loblolly pine stands located in the coastal plain of Arkansas that were thinned 4-5 years previously. The Demonstration Area was 1.9 acres in size and 26 years old, with a site index of 50 feet (base age 25). The second study area, the Deer Camp Area, was 4.0 acres in size and 31 years old, with a site index of 60 feet (base age 25). The soil physical conditions in both study areas were not significantly impacted by the mechanized thinning operation. In general, the soils had bulk densities below 1.3 Mg/m³, approximately 15% macropore space, 30% micropore space, and ruts were generally less than 6 inches deep.

Trees growing greater than 12 feet from the skidding corridors were compared with trees growing 0-12 feet from the skidding corridors. In both study areas, radial growth of the trees next to the corridors exceeded that of those between corridors after thinning. Other results varied by site. On the Demonstration Area the trees in the 0-12 foot zone had larger DBHs and crown widths than the trees between corridors; but the trees growing more than 12 feet from the corridor were taller than the trees growing within 12 feet of the corridor. In the Deer Camp Area, the trees within 12 feet of the corridor had larger DBHs, total heights, and heights to the live crown than the trees growing more than 12 feet from the corridor. The main reason tree growth next to the corridors exceeded that of trees between corridors ( > 12 feet from corridor) was because of heavier thinning in the areas closer to the corridor.

The only post-thinning growth reductions were found in trees growing near the deepest ruts (i.e. >6 inches deep). In both study areas, these trees had extremely poor radial growth responses after the mechanized thinning, increasing only 1.6% in the Demonstration Area and decreasing 4.7% in the Deer Camp Area. The trees located on ruts less than 6 inches deep had the highest radial growth responses to the thinning operation. The trees on 3-6 inch ruts responded 20.2% in the Demonstration Area and 28.6% in the Deer Camp Area; on 0-3 inch deep ruts, the trees responded 15.0% and 23.3%, respectively.

Bole damage was also found to reduce the growth of residual loblolly pine. Damaged trees growing within 6 feet of the rut had the lowest increase in radial growth after thinning. On both study areas, the damaged trees located more than 6 feet from the rut did not seem to be adversely affected by the bole damage. Overall, mature loblolly pine seems tolerant of small amounts of soil disturbance and basal damage, but, if both occur, then tree growth is severely reduced.