Morphological and physiological response of conifer seedlings to drought conditioning

Increased frequency, severity, and duration of droughts and increased wildfire severity are impacting many conifer forests globally. Reforestation in these changing disturbance regimes requires tree seedlings capable of establishing in hotter and drier climates. We evaluated the morphological and physiological effects of drought conditioning on second-year ponderosa pine (Pinus ponderosa), western white pine (Pinus monticola), and western larch (Larix occidentalis) seedlings. Treatments included a well-watered control (75% of container capacity) and a water-limited (40% of container capacity) drought treatment. Ponderosa pine exhibited no significant treatment effects to above or below-ground biomass while treatment caused a significant reduction of height and root-collar diameter for both western white pine and western larch coupled with a significant reduction in root mass for western larch. The drought treatment resulted in significant reductions of gas exchange rates across all species, although mid-day water potentials and δ¹³C indicate lack of cumulative stress. Drought conditioned ponderosa pine seedlings resulted in decreased total root nonstructural carbohydrates (NSCs) and droughted western white pine displayed increased total needle NSCs driven by increased needle soluble sugars. Drought conditioning responses in second-year seedlings in large pots proved to be species dependent allowing for further investigation of treatment intensity and duration for drought-adapted stocktypes.