Effects of planting density and cultural intensity on stand and crown attributes of mid-rotation loblolly pine plantations

Loblolly pine (Pinus taeda) is an important timber species in the southeastern US and abroad. A better understanding of loblolly pine crown relationships with growth has implications for improving productivity estimates using process-based models or remote sensing techniques. Four study installations in the Upper Coastal Plain and Piedmont of Georgia were used to examine the effects of planting density and cultural intensity on loblolly pine stand growth and crown attributes. Treatments included six planting densities, ranging from 740 to 4440 trees ha−1, in a split-plot design with two different levels of fertilization and competition control. The “low intensity (LI)” cultural treatment included relatively high nutrient inputs and early competition control. The “high intensity (HI)” cultural treatment included even greater nutrient inputs and complete sustained competition control. Treatment effects on stand and crown attributes were examined at age 13. Fertilization and competition control did not have a major influence on stand and crown attributes. Stands planted at lower densities resulted in significantly greater DBH and height but less standing volume per hectare, basal area per hectare, and current annual increment (CAI) volume growth compared to stands planted at higher densities. Stand-level foliar biomass, peak projected leaf area index (LAI), foliar nitrogen (N) content, specific leaf area (SLA), and intercepted photosynthetically active radiation (IPAR) were significantly greater for stands planted at higher densities, while live crown length and crown ratio were significantly greater for stands planted at the lower densities. IPAR efficiency (CAI per IPAR) was significantly affected by planting density, with values of 0.32-0.42 m3 %IPAR−1 for 740 and 4440 trees ha−1, respectively. At this stage of stand development, light limitations due to high stocking have a greater influence on growth than soil resource limitations for the loblolly pine plantations analyzed in this study. Higher density stands resulted in increased SLA and IPAR efficiency, supporting the idea that higher density stands utilize light more efficiently than lower density stands.