Influence of spacing regimes on the development of loblolly pine (Pinus taeda L.) in Southern Brazil

The economic potential of loblolly pine has long been recognized due to the ability to achieve growth levels in commercial plantations that far exceed their expected natural growth. Interest in this species is on the rise as the potential for increased production is becoming more feasible in Brazil and particularly in the US. As production increases and cultural treatments are adopted, understanding long-term outcomes based on similar practices are essential for managers. Long-term spacing studies using a variety of initial densities for different tree species show that initial density has little effect on total wood yield for cycles longer than 20 years. On the other hand, initial density has important implications when considering the intended product and its respective value as well as the overall costs of cultural treatments. This paper reports the 24-year growth of Pinus taeda - loblolly pine - in Southern Brazil in response to five cultural regimes. Five initial spacing regimes (2.5 × 1.2, 2.5 × 2.0, 2.5 × 2.8, 2.5 × 3.6 and 2.5 × 4.4 m) combined with cultural procedures generally used in commercial stands were studied. Dendrometric variables analyzed include diameter at breast height (dbh), average and dominant height, site index (SI), basal area, volume per tree and per hectare, and assortment volume; variables were tested using analysis of variance and Tukey test. The results indicate a final lower dbh average in denser spacing regimes but no significant difference in relation to volume per hectare and stand basal area at the end of the 24 year cycle. Our results demonstrate that it is possible to obtain the same volume per hectare, on average 385.7 m3 ha−1, at the age of harvesting by combining different initial spacings with thinning intensities. The mean annual increment (MAI) was analyzed by reconstructing growth (volume) using the software Pisapro. The simulations showed that MAI has a positive correlation with initial spacing in which the densest spacing obtained a MAI approximately 45% higher than the widest treatment. The results provide managers with long-term data that can be used in forest management planning, e.g. by allowing companies to adjust their operations depending on the costs of planting, maintenance and other cultural treatments. The results also point out that depending on the aim of the production, initial spacing and thinning can be adjusted to meet target product specifications (e.g. larger dbh, lack of knots).