Abstract
Increasing drought frequency and intensity pose significant threats to forest regeneration in the western United States. This dissertation investigates drought adaptation mechanisms in two conifer species of ecological and economic importance, western larch (Larix occidentalis) and interior Douglas-fir (Pseudotsuga menziesii var. glauca), with a focus on seedling-stage responses that inform nursery production and reforestation strategies. Through a combination of controlled greenhouse and field experiments, this work explores how water stress intensity, water stress duration, and seed source origin influence seedling physiology, morphology, and survival.Chapter One examined the water stress responses of western larch seedlings from six distinct seed provenances. The study found that moderate water deficit significantly changed root architectural traits and promoted a drought avoidance strategy centered on shallow lateral branching. Provenances from dry environments displayed greater root plasticity and higher photosynthetic rates under stress, although some mesic-origin seedlings also showed unexpected adaptive traits. These results emphasize the importance of selecting for functional root traits alongside climatic origin in nursery stock development.
Chapter Two focused on the response of interior Douglas-fir provenances to five different water deficit treatments combining intensity and duration. Results revealed that high-elevation provenances exhibited superior biochemical and morphological adjustments under extreme water stress. While low-elevation seedlings showed constitutive traits favoring drought resistance, high-elevation seedlings demonstrated greater osmotic plasticity through increased soluble sugar accumulation. We also found that water stress duration resulted in greater physiological plasticity in the higher-elevation provenance. These findings suggest that metabolic flexibility may confer resistance under extreme stress and support the strategic use of diverse seed sources for drought-prone planting sites.
Chapter Three complements the study in Chapter Two by investigating the role of drought preconditioning treatments in improving Douglas-fir seedling performance in the field. Short-duration moderate drought significantly enhanced root growth and morphological traits while prolonged extreme drought increased overwinter survival at the cost of early growth. These findings support the application of the Target Plant Concept in nursery conditioning to produce stocktypes adapted to dry and variable field environments.
Together, the three studies underscore the critical role of provenance-specific plasticity, root system architecture, and physiological plasticity in shaping seedling responses to drought. This dissertation provides actionable insights for climate-smart nursery practices and reforestation planning, advocating for integrative approaches that combine seed source selection, trait-based screening, and adaptive nursery protocols to enhance forest restoration success under changing climate.