Effects of drought severity on physiological and morphological responses to future climate in jack pine (Pinus banksiana)

Abstract

Climate change is expected to expose boreal trees to higher atmospheric CO₂, warmer temperatures, and more frequent drought. This study examined how drought severity and future climate affected jack pine (Pinus banksiana) seedlings. Seedlings were grown for eight weeks under current climate conditions (600 μmol mol⁻¹ CO₂ and current temperature) or future climate conditions (1000 μmol mol⁻¹ CO₂ and current temperature +4 °C) and 3 soil moisture conditions: control, moderate drought, and severe drought. Foliar gas exchange was measured under 600 μmol mol⁻¹ CO₂ for the current climate treatment and both 600 and 1000 μmol mol⁻¹ CO₂ for the future climate treatment in the early, middle, and late stages of the drought treatment. Photosynthetic CO₂-response curves were measured to estimate Vcmax, Jmax, and Rd. Seedling height, root collar diameter, and biomass were measured after the completion of all gas exchange measurements. It was found that Vcmax and Jmax were significantly affected interactively by drought and climate treatments, while Rd was not significantly affected by either treatment. The future climate conditions resulted in downregulations in Vcmax and Jmax in the control and severe drought treatment. Under the moderate drought treatment, however, Vcmax was higher under the future climate while Jmax did not show a significant response to climate treatment. The net photosynthetic rate of the seedlings in the future climate treatment measured at 600 μmol mol⁻¹ CO₂ was significantly lower than the corresponding measurement in the current climate treatment in the moderate and severe drought, another indication of photosynthetic downregulation. When measured at the corresponding treatment CO₂, the stomatal conductance and transpiration were significantly lower, but intrinsic water-use efficiency was significantly higher, in the seedlings in the future climate treatment, especially under moderate and severe drought. Drought and climate treatments had no significant overall effects on biomass or morphological traits. Under future climate conditions, seedlings tended to allocate more biomass to leaves and showed a trend toward greater height. Moderate drought tended to shift biomass allocation from aboveground to belowground tissues, whereas severe drought showed a stronger trend toward reduced seedling size and biomass accumulation. Overall, these results indicate that future climate conditions may improve water-use efficiency but cause photosynthetic downregulation and shifts in biomass allocation and accumulation in jack pine.