Evaluating Stand Density Measures for Regulating Mid-Rotation Loblolly Pine Plantation Density in the Western Gulf, USA

Efficiently quantifying stand density is crucial in sustainably managing mid-rotation loblolly pine (Pinus taeda L.) plantations. While various stand density measures, including basal area (BA), stand density index (SDI), relative spacing (RS), and live crown length ratio (CR), are used, ambiguity persists among these measures: are they each biologically sound and are they on par with each other in terms of density management? These topics were investigated by examining the relationships between measures and stand age, between measures and tree growth, and between measures using data from numerous long-term permanent plots established in loblolly pine plantations in east Texas. A strong trend of increasing density with age was found for all the measures. The trend followed an asymptotic trajectory when density was expressed as BA, SDI, or RS, adhering to biological expectations, but the trend showed a gradual decrease for CR. Strong and biologically sound relationships between DBH periodic annual increment (PAID) and BA or SDI were observed, suggesting that both measures match true DBH growth. However, PAID linearly decreased with decreasing RS and with decreasing CR in a smooth curve, biasing from the biological expectation. Strong relationships existed between the measures, suggesting that these seemingly disparate measures are not independent of each other. Site index affected all investigated relationships in a manner of having higher densities at a given age or a greater PAID at a given density for higher site index sites regardless of measures. The effects of initial planting density on the relationships were mostly negligible, having no practical significance, with few exceptions (the relationships of SDI–age, RS–age, and CR–RS). Among the measures evaluated, our results advocate for the use of BA to regulate mid-rotation loblolly pine plantation density such as determining the approximate biological timing for thinning in the Western Gulf region due to its biological soundness, ease of measurement, and feasibility of incorporating effects of site quality and planting density.