Effects of planting density on canopy dynamics and stem growth for intensively managed loblolly pine stands

Leaf area index (LAI), intercepted photosynthetically active radiation (IPAR), and foliar nitrogen content are related to stand growth. However, they are all a function of canopy size, and their relative importance is hard to determine. We used intensively managed, 4-year-old loblolly pine (Pinus taeda L.) stands planted at densities ranging from 740 to 4440 trees ha−1 that possessed very different canopy structures to separate the relationships between IPAR, LAI, and foliar N content. Our objectives were to determine whether stem growth was best correlated with LAI, IPAR, or foliar N content and to determine how the relationship between LAI and IPAR varied among stands of different planting densities. While stem growth rates increased significantly with planting density, from 12.0 to 35.2 m3 ha−1 y−1, increases were not proportional, indicating the presence of competition-induced limitations to growth. Annual IPAR (863-2345 MJ m−2 y−1), LAI (2.5-4.9), and foliar N content (67-122 kg ha−1) all increased as stand density increased from 740 to 4440 trees ha−1. Stem growth rates were better correlated with annual IPAR (r2 = 0.90), than LAI (r2 = 0.67) or foliar N content (r2 = 0.55). The efficiency of LAI to capture radiation (IPAR LAI−1) increased with stand density even though the higher density stands possessed greater LAI, which increased the likelihood of within canopy shading. The increase in IPAR LAI−1 corresponded to significantly decreased variation in light interception within plots as planting density increased. These results indicate that stem growth is best correlated with annual IPAR and underscore the importance of stand structure since the more even distribution of foliage within canopies of higher density stands increased the efficiency of light interception.