Available at: https://digitalcommons.calpoly.edu/theses/3054
Date of Award
6-2025
Degree Name
MS in Environmental Sciences and Management
Department/Program
Natural Resources Management
College
College of Agriculture, Food, and Environmental Sciences
Advisor
Richard Cobb
Advisor Department
Natural Resources Management
Advisor College
College of Agriculture, Food, and Environmental Sciences
Abstract
The rapidly emerging disease sudden oak death, caused by the non-native pathogen Phytophthora ramorum, is transforming coastal California forests by driving extensive mortality of tanoak (Notholithocarpus densiflorus) and increasing fire severity. As a novel disturbance, sudden oak death and its interaction with wildfire may catalyze unexpected shifts in successional trajectories and reconfigure forest understory communities. Furthermore, how tanoak influences the forest understory and how its decline might reshape understory dynamics remain poorly understood. We hypothesize that disease alone and the compounding disturbances of disease and wildfire alter successional dynamics, and that the potential functional extinction of N. densiflorus will promote increased understory density. Leveraging 17 years of plot-level vegetation surveys that span three landscape-scale wildfires in the highly diverse and disease-impacted Big Sur region, we modeled the effects of disease and fire severity on understory diversity and structural composition over time. We found that shrubs gain a successional advantage when high-severity wildfire occurs in stands already affected by disease, indicating a synergistic interaction between disturbances. We also confirmed and extended previous findings that tanoak canopies suppress understory growth, suggesting that a decline in tanoak may favor a denser understory, further shifting fuels structure and likely wildfire dynamics. To mitigate these cascading effects, we recommend integrating disease dynamics into outplanting decision-support tools, expanding the application of landscape-scale prescribed fire, and continuing targeted pathogen management.