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. / Master of Science

During the third growing season after thinning, eight-year-old loblolly pine gas exchange parameters were measured. Full sunlight and stand light conditions for upper (sun) and lower (shade) crown foliage were examined. Xylem water potential, litterfall dynamics and tree growth were monitored. Nutrient availability, chlorophyll content and chlorophyll a/b ratio were assessed. At the end of the third post-thinning growing season (eleventh season since planting), thinned trees had larger stem and crown diameters, and live crown ratios. Thinned trees grew 11 cm more in height, and approximately 35% more in diameter, basal area and volume. Only crown diameter differences were statistically significant at p=0.05. Upper crown foliage was found to have somewhat higher net photosynthesis, stomatal conductance and transpiration when measured under saturating light conditions. Lower crown needles displayed features usually associated with shade foliage: lower photosynthetic capacity, lower light compensation point and dark respiration rates. Observed physiological differences were not related to plant water status, chlorophyll content or chlorophyll a/b ratio. The higher light saturated gas exchange rates for upper crown foliage were paralleled by lower stomatal limitations to photosynthesis, possibly the effect of better potassium supply facilitating more efficient stomatal function. Light conditions in thinned stands did not differ significantly between the upper and lower crowns, even though light levels in the lower crowns of unthinned stands were nearly a third of that in the upper crowns. This had an effect on in situ photosynthetic rates, which were similar in the upper and lower crowns of thinned trees. In situ photosynthetic rates in thinned stands were also similar to those measured under saturating light. In the unthinned stands, because of light differences, gas exchange rates were significantly reduced in the lower crowns, with only upper crown foliage fully utilizing its potential for photosynthesis. The physiological activity in the upper crowns of unthinned trees tended to be even higher than in the upper crowns of thinned trees. Litterfall timing did not differ between the thinned and unthinned stands. Litterfall per hectare was higher in unthinned stands, but when expressed on a per tree or unit basal area basis it averaged higher in thinned stands. A larger photosynthetic apparatus together with full utilization of photosynthetic potential in both crown positions are likely the major reasons for better growth of thinned trees. / Master of Science

Although there remains some controversy as to the benefits of individual tree growth and yield prediction models over stand-level models, individual tree models still have wide acceptance. A generalized thinning response variable which can account for the intensity of thinning, as well as the age of the stand at the time of thinning and the time elapsed since thinning, was applied to two existing models for loblolly pine (Pinus taeda) in cutover site-prepared plantations. A site index equation for predicting mean total height of dominant and co-dominant trees and a diameter increment model were developed to incorporate the thinning response variable. New fits of height increment and mortality functions to the available data were also completed. Separate mortality functions were fit to data from unthinned and thinned stands. The base models for this analysis were from the individual tree growth simulation model PTAEDA2. Evaluations for predictive ability of these models were done in a reduced version of the growth simulator which was modified to accept external data. The mean total height model had improved predictive ability over the original PTAEDA2 model for this variable. The diameter increment model produced no significant improvement in simulation comparisons. Fitting the two mortality functions to the multiple observation data resulted in the reduced predictive ability of the simulator compared to the original mortality model from PTAEDA2 which was fit to data from unthinned stands only. / Master of Science