Graduation Date
Spring 2026
Document Type
Thesis
Program
Master of Science degree with a major in Natural Resources, option Forestry, Watershed, & Wildland Sciences
Committee Chair Name
Andrew Stubblefield
Committee Chair Affiliation
Cal Poly Humboldt Faculty or Staff
Second Committee Member Name
Lucy Kerhoulas
Second Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Third Committee Member Name
Alana Chin
Third Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Keywords
Ecohydrology, Tree water sources, Stable isotopes, Soil moisture dynamics, Mediterranean-climate forests, Hydrologic connectivity, Forest resilience, Seasonal water use
Subject Categories
Natural Resources
Abstract
Increasing drought intensity and declining summer water availability in Mediterranean- climate forests raise concerns about forest resilience and streamflow persistence. This study evaluated how tree water sources vary with hillslope position, season, species, and watershed context across three coastal northern California forests. Stable isotopes of hydrogen (δD) and oxygen (δ18O), soil moisture time series, linear mixed-effects models, and site-specific Bayesian mixing models (MixSIAR) were used to estimate proportional contributions of shallow soil, deep soil, stream-associated, and fog water to tree xylem water. Across sites, xylem water isotopic signatures overlapped primarily with soil water, indicating dominant reliance on subsurface sources, while contributions from stream-associated water were present but variable across species and sites. Soil moisture data showed that shallow soils dried rapidly during summer while deeper soils remained comparatively stable. Consistent with these patterns, linear mixed- effects models and mixing models indicated a seasonal shift from shallow soil water use in spring to increased reliance on deeper soil water in summer and fall. Hillslope differences were generally small and inconsistent, suggesting partial rather than strongly partitioned hydrologic connectivity. Species differed in water use strategies: Douglas-fir and California bay were more strongly associated with deep soil water, coast redwood used a combination of shallow soil water and fog water, and tanoak showed greater stream-associated contributions in some lower slope positions. Watershed-level differences were also evident, with the coarse-textured Standley watershed showing greater stream-associated contributions, while Jacoby and Schatz watersheds were more strongly dominated by soil water. These findings highlight the dominant role of seasonal soil water dynamics in structuring tree water use, while also indicating that vegetation can access hydrologically connected water under specific conditions. Differences in water use strategies among co-occurring species suggest the potential for complementary use of available water sources across soil depths and seasons, which may influence how water is partitioned within mixed-species forests. This ecohydrologic understanding provides a framework for evaluating how forest composition may contribute to drought resilience and vegetation–water interactions under increasingly dry conditions.
Citation Style
APA
Recommended Citation
Briggs, Kaitlyn, "Following the water: Tree water sources in coastal northern California forests" (2026). Cal Poly Humboldt theses and projects. 2550.
https://digitalcommons.humboldt.edu/etd/2550