Negligible effects of severe organic matter removal and soil compaction on loblolly pine growth over 10 years

The long-term soil productivity (LTSP) study was initiated to examine the effect of soil porosity and organic matter (OM) levels on net primary productivity (NPP). The study design calls for three levels of OM removal (bole, whole tree and whole tree plus forest floor) and three levels of compaction (none, moderate and severe) being imposed on harvested sites prior to planting. Additionally, the effect of understory control on NPP was examined as a split-plot treatment. The study has been installed on 62 sites covering a range of climates, soil types and tree species across the United States and Canada. The inclusion of 46 closely related affiliated sites has created the world's largest coordinated research network devoted to investigating the relationship between land management and sustainable forest productivity. In the forefront of the LTSP network are the installations in North Carolina (NC) and Louisiana (LA) which focus on loblolly pine (Pinus taeda L.) growth. The rapid growth at these sites has resulted in the trees reaching crown closure much earlier than any other LTSP installation. Consequently, these sites may provide the earliest glimpse as to the treatment effects on NPP. Tenth year data from these sites show that large OM removals and severe soil compaction (main treatments) had no significant (α ≤ 0.1) effect on stand volume but control of the understory (split-plot treatment) resulted in greater stand volume compared to plots with no understory control. Plots ameliorated by a one-time fertilizer addition (NC and LA) and bedding (NC only) resulted in small (+8 and −8% for NC and LA, respectively) to large (+69 and +15% for NC and LA, respectively) changes in mean stand volume compared to the experimental plots. Although there were no main treatment effects on stand volume, extractable soil phosphorus in NC and LA were reduced with increasing OM removal. The reduction of extractable soil phosphorus with increasing OM removal on these already phosphorus deficient sites may affect tree growth in the future but have not yet translated to stand volume differences over 10 years.