Production dynamics of intensively managed loblolly pine stands in the southern United States: a synthesis of seven long-term experiments

Abstract

Results from seven long-term experiments in the southern US were summarized to understand production dynamics of intensively managed loblolly pine plantations. Replicated studies that spanned a wide range of soil and climatic conditions were established (North Carolina—NC; Georgia—GA (three sites); Florida—FL; Louisiana—LA; Oklahoma—OK). All experiments received some combination of silvicultural treatments, including fertilizer application, understory competition control, irrigation, and thinning. Accumulation of stemwood biomass at age 15 years varied among sites and ranged from 70 Mg ha −1 (NC) to 180 Mg ha −1 (GA). All sites that received fertilizer and weed control applications were responsive to treatments, and growth responses ranged from about 2-fold (GA) to 3.5-fold (FL) at age 15 years. Variation in fertilizer responses among sites reflected differences in soil types, inherent site quality, and stages of stand development. Site carrying capacity, expressed as maximum basal area attained in closed canopy stands, was surprisingly consistent among sites and climatic conditions, reaching 45–48 m 2 ha −1 under the most intensive silvicultural treatments. Across studies, competition-related mortality was initiated as stand basal areas approached 30–35 m 2 ha −1, and annual stemwood biomass increment peaked as basal areas reached about 20–35 m 2 ha −1. These data suggest that a regional maximum in stand density occurs for loblolly pine when regular fertilizer additions are made or inherent soil nutrient supply is high. Periodic density management will be required to minimize reductions in individual tree and net stand growth. Wood specific gravity at a standard ring age of 10–11 years varied both among sites and by treatment within sites, and ranged from approximately 0.45 (OK site) to 0.56 (GA sites). Maximum leaf area index (LAI) among sites varied from about 2.5 to 6.5. The relationship between stemwood biomass increment and LAI across sites was strong at LAI levels <3.0, but was considerably more variable at higher LAIs. Variation in stemwood increment at higher LAIs was substantially reduced, however, when extreme data points were removed; a logistic function explained 75% of the variation in stemwood increment across all sites. Differences in productivity among sites were not strongly related to site water balance. Taken together, these results support other work which suggests that soil nutrient availability is the dominant driver of loblolly pine productivity across its natural range. Future research should seek to better understand the mechanisms of nutrient demand, uptake and utilization efficiency in developing stands, as well as the factors controlling intraspecific tree competition.