Effects of climate variability and management on shortleaf pine radial growth across a forest-savanna continuum in a 34-year experiment

We investigated the radial growth response of shortleaf pine (Pines echinata) to climatic variation and management using tree cores collected in southeastern Oklahoma at the drier, western limit of its range. Beginning in 1984, experimental units were created by various combinations of pine harvest, hardwood thinning, and fire return intervals (1, 2, 3, 4 years and none) that produced ecosystems ranging from mature, closed canopy forest to open savanna. Monthly and seasonal weather for previous- and current-year as well as growing seasons since fire were used to determine the relationship between radial growth and climate variability (1987-2018) for different management regimes. Across all treatments, growing season precipitation (similar to 5% decrease per 100 mm decrease in precipitation), average summer temperature maximum (similar to 7% decrease with 1 degrees C increase), and previous year's average October minimum temperature (similar to 6% increase per 1 degrees C increase) were the variables most frequently correlated with variation in ring width. Annual wood and latewood growth increments were correlated (R-2 = 0.60) and generally responded similarly to climate variability, with latewood more sensitive to late growing season conditions. Management with frequent fire that resulted in savanna ecosystems reduced growth sensitivity to annual variation in precipitation relative to trees in a closed-canopy forest condition. Suppressed trees were also less responsive to climate variability than intermediate or co-dominant trees. Both annual wood and latewood growth were reduced by 21-33% the first year after prescribed fire for treatments with a 2- and 3-year fire return interval. Multiple regression combing temperature and precipitation variables as well as time since fire accounted for 55% of the variability in annual ring growth. Our findings indicate that a drier climate with hotter summers will likely reduce the growth of shortleaf pine growing at the western margin of its range while warmer temperatures in October, by extending the growing season, may help ameliorate the effects of warmer summers. Management to reduce stand density, either through thinning or by prescribed fire, may dampen some of the variation of growth in response to climate variability.

Automated summary

The study found that shortleaf pine trees growing at the western margin of its range are sensitive to variations in precipitation, summer temperatures, and previous year's October minimum temperature. Management strategies involving frequent fires can reduce the sensitivity of tree growth to annual variations in precipitation.