Graduation Date
Summer 2026
Document Type
Thesis
Program
Master of Science degree with a major in Natural Resources, option Forestry, Watershed, & Wildland Sciences
Committee Chair Name
Jeffrey Kane
Committee Chair Affiliation
Cal Poly Humboldt Faculty or Staff
Second Committee Member Name
Alan Tepley
Second Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Third Committee Member Name
Camille Stevens-Rumann
Third Committee Member Affiliation
Community Member or Outside Professional
Fourth Committee Member Name
Kyle Merriam
Fourth Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Keywords
Baker cypress, Hesperocyparis bakeri, Fire refugia, Serotiny, Immaturity risk, Post-fire regeneration, Post-fire reforestation, Assisted gene flow, Cupressaceae
Subject Categories
Natural Resources
Abstract
Increasing wildfire frequency threatens serotinous, fire-obligate species that depend on sufficient fire-free intervals to develop seed banks for reproduction. These species require fire intense enough to trigger seed release and create favorable substrate and light levels for regeneration, yet consecutive fires that reburn populations before they reach maturity can cause local population contraction or extirpation. Understanding the environmental drivers of natural regeneration and mature tree survival, as well as when and how active reforestation should supplement natural recovery, is critical for managing fire-adapted species under altered fire regimes.
This thesis uses Baker cypress (Hesperocyparis bakeri [Jeps.] Bartel), a rare serotinous conifer endemic to northern California and southern Oregon, to investigate post-fire recovery through survival in fire refugia (areas that experience lower fire frequency or severity), natural regeneration, and active planting. Chapter 1, we surveyed post-fire regeneration and fire refugia across five Baker cypress populations that experienced recent fires. Using Generalized Additive Mixed Models (GAMMs), we quantified how topography, fire severity, and pre-fire stand structure influenced regeneration density, refugia presence (whether any trees survived), and refugia quality (proportion of cypress that survived fire). Regeneration density was greatest in areas with high pre-fire basal area and high fire severity, but it was 77% lower in reburns compared to single burns. In contrast, refugia presence and quality were highest at low fire severity, with quality further influenced by topographic position, wetness, heat load, and slope, peaking on hot, dry, exposed ridges. These findings reveal spatial decoupling of regeneration and refugia sites, demonstrating that resilient landscapes require heterogeneous fire effects, where some areas burn to trigger regeneration while others escape burning until adequate seed banks are developed.
In Chapter 2, we evaluated post-fire planting of Baker cypress at the Mud Lake Unit of the Mud Lake Research Natural Area (RNA), where consecutive high-severity fires in 2007 and 2021 nearly extirpated the local population. We planted 558 seedlings representing six seed source populations in spring 2024 and monitored survival, height, and root collar diameter over two growing seasons to evaluate how performance varied in relation to site and microsite factors, and seed source population. We additionally explored climate transfer distance, defined as the climatic difference between the planting site and seed source, and soil moisture as potential mechanisms to explain observed patterns in performance. Overall survival was high (76.1%), despite both growing seasons receiving approximately half the normal summer precipitation and being 1.4-1.7°C warmer than normal, though possibly aided by above-normal snowpack in the preceding winters. Seedling size at planting was the most consistent driver of performance across all three response variables, with a minimum initial root collar diameter of 2.5 mm corresponding to 80% predicted survival. Distance from shrub canopies was a positive predictor of radial growth, suggesting competitive suppression through light reduction or belowground competition for soil moisture near shrubs. Block-level patterns consistent with aspect-driven microclimate differences emerged, with substantially greater survival and growth on the north-facing planting block. Survival did not differ significantly among seed source populations, suggesting that non-local seed with moderate transfer distances may be viable when local seed supply is insufficient. These findings provide a baseline for guiding Baker cypress reforestation while offering insights relevant to restoration of other serotinous conifers facing altered fire regimes and climate change.
This work demonstrates when natural recovery is likely and when intervention becomes necessary, establishing a framework for conservation of fire-adapted species under increasing fire activity. Together, these findings highlight that resilient management of serotinous conifers requires both protecting sites where natural regeneration and refugia occur and strategically supplementing recovery through active planting where consecutive fires have depleted seed sources. As fire regimes continue to shift, species- and site-specific information of the kind generated here will be increasingly important for guiding restoration decisions.
Citation Style
Chicago Manual of Style
Recommended Citation
Moskowitz, Olivia Lorraine, "Baker cypress (Hesperocyparis bakeri) in a changing fire landscape: Characterizing fire refugia and guiding post-fire reforestation" (2026). Cal Poly Humboldt theses and projects. 2608.
https://digitalcommons.humboldt.edu/etd/2608