Long-term growth effects of simulated-drought, mid-rotation fertilization, and thinning on a loblolly pine plantation in southeastern Oklahoma, USA

Loblolly pine (Pinus taeda L.) is the most productive commercial softwood species in the southeastern USA. More frequent drought due to climate change will likely alter loblolly pine productivity, especially in areas where the climate is already highly variable such as the Upper Gulf region of the south-central USA. In a long-term study, we examined the effects of drought treatment (30% throughfall exclusion), fertilization, and thinning on a loblolly pine plantation growing in southeastern Oklahoma, USA to determine how nutrient availability and stand density interact with drier conditions to affect productivity. Our treatments were applied at mid-rotation: throughfall reduction ages 5 to 13, fertilizer ages 5 and 10, and thinning age 10. Treatment effects on diameter and height increment depended on year, with drought treatment tending to decrease diameter and height growth in drier years and fertilization having a stronger positive effects on diameter growth in the years closely following fertilization events. The net effect on standing volume was that fertilization (+7%) and simulated-drought (-8%) countered each other by age 13. Positive fertilization effects were supported by increased foliar nitrogen (N) and phosphorus (P) concentrations, along with increased leaf area index (LAI, +14% measured age 12). Three and four years after thinning, diameter increment increased by 38%, and positive fertilization effects on gross current annual increment were greater in thinned compared to non-thinned stands. Basal area increment in droughttreated stands relative to non-drought-treated stands decreased more during drier years and exhibited recovery during wetter years. The fairly small growth decline in response to 30% throughfall reduction, positive fertilizer effects, and possible post-drought recovery indicate continued plantation viability in the future even at the drier, western fringe of loblolly pine's commercial range.