A Multi-Scale Approach to Forest Restoration: Endophytes, Environmental Gradients, and Seedling Performance in the Pacific Northwest

Abstract

Pacific Northwest (PNW) forests face growing pressures from legacy management practices, wildfire, and drought intensified by climate change. This dissertation reframes forest restoration as an investment in critical infrastructure, arguing that the long-term public benefits of resilient forests warrant sustained, incentive-backed support for the science and practice of their stewardship. To provide an evidence-based blueprint for such investment, this work uses a multi-scale approach, employing factorial experiments to evaluate seedling performance from the micro-scale (the phytobiome) to the meso-scale (forest structural conditions following silvicultural intervention), aiming to improve artificial regeneration and strengthen forest resilience. Chapter 1 synthesizes literature on resilience science, plant holobionts, and gap-based silviculture, identifying three knowledge gaps for species endemic to the PNW: (1) the performance of endophyte-inoculated seedlings under key abiotic stressors like drought and nutrient limitation, (2) species-specific physiological and morphological thresholds along light and moisture gradients, and (3) strategies for integrating these factors into adaptive management. Chapter 2 addresses the micro-scale by testing multi-strain bacterial and fungal consortia as bioinoculants for Pseudotsuga menziesii and Thuja plicata. Bioinoculation raised T. plicata survival by up to 80% under the driest regime, enhanced root development, and stabilized key physiological indicators. The findings show that these empirically selected consortia can improve seedling performance on water-limited sites. Chapter 3 presents the meso-scale, reporting on a 1,080-seedling experiment that simulated the light and moisture gradients found in canopy gaps. Light availability was the dominant driver of seedling performance. Notably, moderate canopy retention (~40% shade) maximized the Dickson Quality Index, a key measure of seedling vigor, in six of twelve species, including shade-tolerant taxa like Tsuga heterophylla and pioneers like Larix occidentalis. These results challenge traditional shade-tolerance rankings for artificially regenerated seedlings and support the use of partial-retention silviculture. Collectively, these results demonstrate the potential of integrating micro- and meso-scale interventions: (1) bioinoculation of seedlings enhances their drought fitness, while (2) partial overstory retention creates a more favorable establishment environment than open conditions. These findings expand the toolkit for artificial regeneration, opening new possibilities for utilizing a wider range of silvicultural prescriptions and species mixtures. This research provides a practical basis for the "critical infrastructure" framework by showing that while ecologically superior, these restoration strategies are often less economically efficient than traditional methods. Unless the risk reward balance changes and industrial forest managers begin to adopt more costly investments in rebuilding forested landscapes, we are unlikely to see large scale integration of these approaches. Therefore, justifying investments in these practices through subsidies or other financial incentives (new economies based on valuing forests for carbon or their biodiversity and therefore deriving new more expansive revenue streams) is essential for securing long-term societal benefits, such as reducing the regional reforestation backlog, maintaining biodiversity, and ensuring the continued flow of ecosystem services that support rural economies.