Growth in loblolly pine plantations as a function of stand density and canopy properties

Abstract

Interrelationships between forest-canopy properties, stand growth, and Reineke's stand density index (SDI) were investigated for unthinned plots of a loblolly pine, growth-and-yield study. Gross, periodic-annual increment ( I v) and mean-tree, gross, periodic-annual increment ( I mv) were calculated for the intervals between 17, 22, 27, 32, and 37 years of age. Data to calculate canopy variables were available only after age 22. Regression analysis indicates that a second-degree polynomial of SDI is statistically related to both growth variables during the first two measurement intervals but not the last two. The shape of the significant equations generally agreed with conventional growth-growing stock relationships, and I v, adjusted for SDI, decreased significantly with age. Leaf area index ( L) and foliage density ( F) were linearly related to SDI for each measurement period. While the equations relating F and SDI were not significantly different between measurement periods, the intercepts of the fitted equations for L and SDI generally decreased with plantation age. Mean-live-crown ratio ( C r) was significantly related to SDI for all measurement periods, with the exception of age 32, and canopy depth ( C d) was statistically related to SDI only at age 22. Significant multiple-linear regression models were found between the growth variables and canopy properties with one exception. With that one exception, I v was significantly related to L during each measurement interval and to F and C r during the first two intervals. Mean, gross, periodic-annual increment was statistically related only to those canopy variables that described canopy structure. With the exception of F, the overall average value of the canopy variables decreased with age in these loblolly pine plantations, probably leading to the systematic reduction in I v with age. Although growth-growing stock relations were not significant in these plantations after age 27, the relationships between canopy variables and canopy variables emphasize the importance of early density management to maintain vigorous crowns and growth rates as plantations age.