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Long-term loblolly pine aboveground growth, canopy dynamics, and economic implications from throughfall exclusion, fertilization, and thinning in southeastern Oklahoma, USA

Shephard, Noah T.
Loblolly pine (Pinus tadea L.) is the most commercially important timber species in the southern USA. Climate change induced drought, due to longer periods without rainfall, will alter forest growth in the region. Loblolly pine occurs on 21 million ha in the southeast and represents 87% of the regions timber production. The species productivity is likely to face new tests as climate change makes growing conditions more adverse. How climate change might affect common silvicultural practices, like fertilization and thinning, that typically increase stand productivity, is not known. This study, located in the more xeric southeastern Oklahoma, aimed to understand if a plantation regime shift could occur under drier conditions from a growth and efficiency standpoint. A 30% throughfall reduction (drought) treatment from age 5 to 12, fertilization at age 5 and 10, and thinning at age 10 were examined. From stand age 5 to 12, drought treatment decreased standing volume by 7% and fertilization increased standing volume by 8%, offsetting one another, and thus fertilization compensated for potential drought conditions. Additionally, drought-induced plots had +10% basal area growth after meteorological drought conditions subsided. Under current management strategies and potential compensatory growth, loblolly pine plantations appear to be sustainable under a drier climate. Further, efficiency analysis was leveraged to examine all treatment's ability to turn volume growth and stand density into timber products, i.e., pulpwood, chip-n-saw, and sawtimber, at 21, 26, and 31 rotation ages. At all rotation ages, fertilized-thinned stands were perfectly efficient, yet overall fertilization had no effect, and showed negative synergistic interactions with drought (-24% efficient). Thinning had the greatest ability to maintain effective production; non-thinned stands demonstrated a 32% decrease in efficiency. Drought treatment decreased efficiency by 11% after 26 years. Efficiency scores support thinning as a regime staple and fertilization to be ineffective in the long-term. Together, growth analysis supports fertilization to biologically compensate for drought, but efficiency analysis suggests fertilization unable to compensate on a resource-use basis.