Gains from one-time mid-rotation thinning on loblolly pine plantations in the Western Gulf region

A region-wide mid-rotation thinning study of multiple sites was installed in intensively managed loblolly pine (Pinus taeda L.) plantations in the Western Gulf region (WG). At each site, three thinning intensities, thinned to 370 (T370), 555 (T555), and 740 trees per hectare (T740) using a method of geometric thinning paired with removing undesirable trees, and an unthinned control were implemented. Trees were measured annually for the first five years and thereafter up to year nine for some sites. Data were analyzed using quadratic models. Compared to the control, thinning (1) improved tree DBH growth; nine years post-thinning, trees at T370, T555, and T740 plots were on average 20.93%, 12.91%, and 7.11% greater than that of control; (2) reduced per hectare basal area ( \(\:{\text{B}\text{A}}_{\text{h}\text{a}}\) ) without changing its growth rate significantly regardless of thinning intensity. The temporal trajectories of \(\:{\text{B}\text{A}}_{\text{h}\text{a}}\:\) of the thinned plots were in parallel, yet at a lower level, to that of the control. T370, T555, and T740 each had about 21, 14, and 10 m² less \(\:\:{\text{B}\text{A}}_{\text{h}\text{a}}\) than the control at each age; (3) decreased volume per hectare \(\:\left({\text{V}\text{O}\text{L}}_{\text{h}\text{a}}\right)\:\) and its growth rate, in particular T370. Thus, the \(\:{\text{V}\text{O}\text{L}}_{\text{h}\text{a}}\) of the thinned plots diverged from that of the control with elapsing time post-thinning; (4) shifted the Weibull function-based DBH distributions more to the right end, skewed more to the left, and displayed less variation. The study was established following the current thinning practice for loblolly pine plantations in WG, and, therefore, the results should benefit decision-making on silvicultural regimes for the regional foresters.