Environmental stresses such as drought are predicted to increase in many parts of the world. This study investigates the physiology and morphology of two major timber species, Douglas-fir (Pseudotsuga menziesii) and loblolly pine (Pinus taeda), from wet and dry provenances (coastal, inland, and disjunct, respectively) in Oregon and the southeast of the United States associated with low, moderate, and high putative drought tolerances. The study was designed to investigate a) the physiological and morphological responses of potted seedlings of the various provenances to water stress and b) the extent and mechanisms by which an early drought affects performance of the different provenances in a second drought during the same season. Loblolly pine germinants and one-year old Douglas-fir seedlings were pot-grown in a greenhouse for one summer under one of four irrigation regimes: no drought, an early drought, a late drought, or two droughts. Physiological measurements such as A/Ci curves, electron transport rates, fluorescence, and water potentials were used to assess seedling performance under water stress. Biomass and heights were measured at various points throughout the experiment. Infrared imagery was used to aid in visualization of water stress. After harvest, xylem specific conductivity and xylem anatomy were assessed.
Results/Conclusions:
Overall, drought tolerances were consistent with expectations related to the seedlings’ provenances, as shown by water potentials, growth, and physiological measurements. Preliminary results from this study suggest that previous drought exposure moderately enhanced seedling performance during subsequent drought in loblolly pine, as evidenced by electron transport rates and fluorescence measurements. Physiological plasticity appeared to be greater in provenances associated with low drought tolerance compared to provenances associated with high drought tolerances. Drought-tolerant seedlings may already be operating near the upper end of potential drought performance, or alternatively may have experienced less selection pressure for acclimation to water stress over the course of one growing season. Exposure to moderate growing-season drought stress may assist seedlings with low drought tolerances during future drought incidents. These results have implications for forest regeneration in a future where drought may become increasingly prevalent.