Early growth responses of loblolly pine varieties and families to silvicultural intensity

Abstract

Varietal forestry may increase the productivity and quality of loblolly pine (Pinus taeda L.) stands. However, little is known about the genotype by environment interaction when varieties are established under different silvicultural intensities and at different sites. A block-plot study with a split–split plot design was established in 2009 on the Virginia Piedmont (VA) and North Carolina Coastal Plain (NC) of the southeastern U.S. The treatments hierarchically arranged were two levels of silviculture (operational and intensive), six genetic entries (1 open-pollinated family (OP), 1 control-mass-pollinated family (CMP) and 4 clonal varieties) and three planting densities (617, 1235 and 1852trees per hectare). We measured tree height, diameter at breast height (dbh), crown width, and height to the base of the live crown (HTLC) annually for four years. In general, the varieties grew faster than the OP family across all treatments and sites, but the differences were greater at VA than NC (20% and 33% greater height and dbh, respectively). The CMP family generally performed among the varieties and the OP family. There were no differences in growth among the varieties. However, crown characteristics varied between the varieties due to site, silvicultural treatment, and planting density effects. Varieties responded differently in crown width and HTLC, and this was influenced by treatments and site. These data suggest that the crown development of varieties is plastic, allowing them to respond to environmental conditions and management practices. At NC, intensive silviculture increased crown-width, height and dbh by 33%, 14%, and 23%, respectively. At VA, intensive silviculture increased crown-width, height and dbh by 41%, 10%, and 23%, respectively. Planting density had a greater effect on crown attributes than on height and dbh. The highest planting density significantly decreased crown width (11.5%) and increased HTLC (20%) relative to the lowest planting density. Our results demonstrate the complex interactions between genetic and environmental effects, which may have a significant impact on the operational gains expected for varietal forestry.