Stand Density Management for Optimal Volume Production

The relationship between volume production and stand density, often termed the 'growth-density relationship', has been studied since the beginnings of forestry and yet no conclusive evidence about a general pattern has been established. Throughout the literature claims and counterclaims concerning the growth-density relationship can be found. Different conclusions have been attributed to the diverse range of definitions of volume and stand density among problems with study design and other pitfalls. Using data from two thinning studies representing non-intensively and intensively managed plantations, one spacing trial, and one thinning experiment a comprehensive analysis was performed to examine the growth-density relationship in loblolly pine. Volume production was defined as either gross or net periodic annual increment of total, pulpwood, or sawtimber volume. These definitions of volume production were then related to seven measures of stand density including the number of stems per hectare, basal area per hectare, two measures relative spacing and three measures of stand density index. A generalized exponential and power type function was used to test the hypothesis that volume production follows either an increasing or unimodal pattern with stand density. These patterns were tested using all combinations of the six definitions of volume production and the seven measures of stand density. Significance of the parameters indicated that different patterns existed depending on the type of management (intensive vs. non-intensive), if thinning is performed, and depending on the definitions of growth and density. The growth-density pattern was generally the same between gross and net production although different patterns emerged when comparing total, pulpwood, and sawtimber volumes. The definitions of stand density which used diameter as a measure of average tree size were more highly correlated with volume production and produced similar patterns while the number of stems per hectare was the least correlated. Further analysis was performed to evaluate Langsaeter's hypothesis which states that volume production is constant and optimal across a wide range of stocking. A mixed-model approach was used to test the equality in mean volume production across a range of planting densities and thinning intensities. To account for the effects of age, the equality in mean volume production was tested separately across a range of ages from 8 to 25 years within the spacing trial data and across a range of one to six years since thinning within the thinning experiment. A multiple comparison test indicate that pattern of volume production and stocking is highly related to the two stages of self-thinning. In young stands, within the distance-independent mortality stage, volume production increases with increasing planting density and therefor increasing stocking. During the distance-dependent mortality stage the assumption of constant and optimal volume production across a wide range of stocking is generally correct. However when mortality began to reduce canopy closure to the point that the residual stand could not recover gaps in the canopy a decline in volume production occurred resulting in a decreasing relationship with increasing stocking. Finally, a system of equations were constructed to describe volume production at the individual tree and stand levels. From this model it was determined that stand level volume production follow an increasing pattern with stand density.