The Effect of Mid-Rotation Fertilization on the Wood Properties of Loblolly Pine (Pinus Taeda)

Abstract

Mid-rotation fertilization is a common practice in the management of loblolly pine (Pinus taeda L.) plantations, typically providing large improvements in growth. However, concerns exist about the quality of wood produced following fertilization. The objective of this study was to develop an understanding of wood property changes following fertilization. Wood samples from a study involving four levels of fertilization applied to a thinned mid-rotation loblolly pine plantation located on the lower coastal plain of North Carolina were sampled. The study was laid out in a randomized complete block design involving four blocks and four levels of nitrogen fertilizer: Control-000, 112, 224 and 336 kg/ha, along with 28 kg/ha of phosphorus with all treatments. Thirty-two trees were felled and disks were cut at five heights from each tree. Wood properties including modulus of elasticity, air-dry density and tracheid anatomical properties were measured for each of the three post-fertilization annual growth rings using near infrared (NIR) spectra obtained from the radial face of strips cut from the disks. An analysis of variance was conducted on three-year basal area weighted average stiffness, air-dry density, and tracheid anatomical properties. A decrease in stiffness, air-dry density, tracheid wall thickness, and an increase in tracheid radial diameter were observed for the heaviest fertilizer treatment (336 kg/ha) compared to the control and 112 kg/ha of nitrogen. Microfibril angle (MFA), cell tangential diameter, and tracheid perimeter showed little change. Wood properties of trees receiving fertilizer rates of 112 and 224 kg/ha were not significantly affected.

Models for Predicting Specific Gravity and Ring Width for Loblolly Pine from Intensively Managed Plantations, and Implications for Wood Utilization

“…The radial and longitudinal (i.e., at different tree heights) variations of SG in this study are consistent with previous studies on loblolly pine from plantations [69,70,77,78]. The within-tree variation in SG was captured in a model that predicted SG values against cambial age (Figure 1), while incorporating disk height in the model gave a better fit and showed that predicted values decreased with height in the tree.…”

“…Including the effects of silviculture may have improved the model fit at the expense of model complexity. Antony et al [78] modeled latewood SG and showed that latewood SG decreased with increasing fertilization intensity. Included in their paper is a plot of mean latewood SG with the x-axis in years instead of cambial age to illustrate the decreasing trend.…”

“…The influence of silviculture on these properties can often be very different than for SG [46]. For example, Clark et al [16] found that outerwood designated using SG was produced at approximately half the age than when designated using MFA.…”

“…For loblolly pine, several studies have shown that SG decreases with increased growth rate in response to silvicultural treatments [46][47][48]. One challenge with building models to account for silvicultural differences is having enough data to enable construction of plausible models.…”

“…These models should include the effects of cambial age and/or height within a tree, as well as the influences of silviculture and/or genetics. Regarding wood property variation in response to silviculture for loblolly pine, Antony et al [46] and Love-Myers et al [47,48] reported that as mid-rotation fertilization intensity increased, both ring SG and latewood SG decreased, although the magnitudes of the changes varied by study. Clark et al [49] found that vegetation control did not result in decreased ring SG properties; however, early rotation fertilization led to a 6% decrease in outerwood SG and a 62% increase in the amount of corewood (juvenile wood), versus the control treatment.…”