COS 262-6 - CANCELLED - Root system function of eastern and western loblolly pine genotypes in response to simulated drought

Abstract: Tree vigor is sustained by balance between water loss during C-fixation and water uptake by the root system. Eastern sources of loblolly pine are often planted in the western range of this species because they have superior stemwood growth. As climate change accelerates evapotranspiration, the risk of growth-limiting water deficit increases. We need knowledge of drought acclimation in eastern and western sources of loblolly pine to guide future seed source choices in the western Gulf region.

Belowground responses of loblolly pine to throughfall exclusion and genotype were assessed in a central Louisiana plantation. Six 2-tree experimental units (EU) of three genotypes from the eastern (E7, EC9) or western (WLA) loblolly pine range were established in 2018 at age 13 years. The 18 EU were partitioned into three blocks and two levels of throughfall exclusion (no TFE, TFE) were applied in 2018-2019 and maintained until August 2022. In 2020, a hydraulic probe was used to extract two 50 cm soil cores by EU. Mean EU argillic horizon (B_t) depth, and plant-available water-holding capacity (PAWHC) above and below the B_t were determined. EU fine root biomass (FRB) to 50 cm was measured 2 years after treatment initiation. After 4 years of TFE treatment, one tree per EU was felled and stem and crown biomass were quantified. The taproot and all attached woody lateral roots were excavated from the 4 m³ soil block, 1 m deep, around each stem. Ten small woody roots, approximately 1 cm in diameter, were excised by EU and native (K_Snat) and maximum (K_Smax) hydraulic conductivities were measured by a pressure chamber and cavitation sleeve. Using two of these roots, mean EU xylem tensions associated with a 50% loss of K_Smax (P₅₀) were determined by the flushing method.

Preliminary results indicate TFE treatment did not affect FRB, K_Snat, K_Smax, or P<sub>50 and genotype did not affect K_Snat or K_Smax. FRB and P<sub>50 differed significantly among genotypes. FRB of the eastern genotypes, E7 and EC9, were 110 and 127 percent greater than that of the western genotype, WLA. While allocating less to FRB, WLA (-1.7 MPa) maintained a significantly more negative P<sub>50 compared to E7 (-1.2 MPa) and EC9 (-1.2 MPa). Root system architecture and biomass analyses are underway and together with root hydraulic conductivity and soil property information, we will assess the effect of genotype origin on root system function in the western Gulf region.