Effects climate variability, stand density, and prescribed fire on relative growth response of shortleaf pine (Pinus echinata) and post oak (Quercus stellata) in southeastern Oklahoma

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Arjun Adhikari, Omkar Joshi, Chris B. Zou and Rodney Will, Natural Resource Ecology and Management, Oklahoma State University, Stillwater, OK, Ronald E. Masters, College of Natural Resources, University of Wisconsin-Stevens Point, Stevens Point, WI, Henry D. Adams, School of the Environment, Washington State University, Pullman, WA

Background/Question/Methods
Management to alter stand structure may affect resilience of trees to drought and potential climate change. We investigated the interaction between climate variability, stand structure, i.e., forest vs savanna, and prescribed fire on the relative radial growth response of shortleaf pine (Pinus echinata) (~24 cm average dbh) and post oak (Quercus stellata) (~ 36 cm average dbh) using tree cores collected in southeastern Oklahoma near the drier, western limit of their ranges. Beginning in 1984, experimental units were created by combinations of pine harvest, hardwood thinning, and fire return intervals (1, 2, 3, 4 years and none) that produced ecosystems ranging from closed-canopy forest to savanna. Weather for previous- and current-year, and years since fire were used to determine the relationship between radial growth and climate variability (1987 – 2018) for different management regimes.

Results/Conclusions
Relative radial growth of shortleaf pine increased with growing season precipitation, decreased with average summer temperature maximum, and increased with previous year’s average October minimum temperature. Radial growth of shortleaf pine decreased by ~25% the first year after prescribed fire for 2- and 3-year fire return intervals. Suppressed shortleaf pine were less responsive to climate variability than intermediate or co-dominant trees. Shortleaf pine growing in savannas appeared less sensitive to annual variation in precipitation. Relative radial growth of post oak increased with precipitation May-July, decreased with higher summer maximum temperatures, and increased in response to warmer September minimum temperatures the previous year. Burning regime and stand density did not influence the response of post oak radial growth to climate variability. Overall, a drier climate with hotter summers will likely reduce the growth of shortleaf pine and post oak in southeastern Oklahoma, but warmer temperatures in autumn may extend the growing season. Management for lower stand density may increase resilience to climatic variability for shortleaf pine, but not post oak.