Crown physiological responses of loblolly pine clones and families to silvicultural intensity: Assessing the effect of crown ideotype

Clonal forestry must be linked with intensive silvicultural practices to increase forest productivity of loblolly pine (Pinus taeda L.) in the southern United States. Although the positive growth responses due to intensive silviculture have been reported extensively, much less is known about the physiological processes that drive these responses. This study assessed the responses of growth, leaf area, leaf-level gas exchange and foliage morphology of 4 year-old loblolly pine clones and families to changes in silvicultural intensity on the Virginia Piedmont (VA) and North Carolina Coastal Plain (NC). Four clones (differing in crown ideotype), 1 control-mass-pollinated (CMP) and 1 open-pollinated (OP) family were evaluated in two levels of silviculture (operational and intensive). The operational silvicultural treatment included only banded weed control, whereas the operational silvicultural treatment included broadcast weed control, fertilization and tip moth control. The effect of genotype and silvicultural intensity were site-specific, and expressed mostly at VA. The intensive silvicultural treatment increased stem volume by 68% and 36% relative to the operational silvicultural treatment at VA and NC, respectively. At VA, the differences in the leaf area responses to the silvicultural treatment among genotypes differed between 60 and 146%, which suggested great differences in growth efficiency among the genotypes. These responses were not linked to changes in leaf physiology and morphology. The seasonal variation of gas exchange parameters was similar between sites, but significant differences in leaf physiology and morphology were found among the genotypes. However, this variation was neither attributed to the genetic source (clonal versus non-clonal) nor the crown ideotype (broad- versus narrow-crown) as hypothesized. Understanding the differences in the crown physiological processes among loblolly pine genotypes may be required to optimize the gains expected from clonal forestry.