Graduation Date
Spring 2022
Document Type
Thesis
Program
Master of Science degree with a major in Natural Resources, option Forestry, Watershed, & Wildland Sciences
Committee Chair Name
Lucy Kerhoulas
Committee Chair Affiliation
HSU Faculty or Staff
Second Committee Member Name
Rosemary Sherriff
Second Committee Member Affiliation
HSU Faculty or Staff
Third Committee Member Name
Erik Jules
Third Committee Member Affiliation
HSU Faculty or Staff
Fourth Committee Member Name
Christopher Lee
Fourth Committee Member Affiliation
Community Member or Outside Professional
Keywords
California, Drought, Forest, Mixed conifer forest, Mortality, Regeneration, Recovery, Resilience, Northern California, Climate change, Recruitment
Subject Categories
Forestry
Abstract
The 2012-2016 California drought was the most severe in the state’s recorded history, contributing to the death of millions of trees. While the effects of this drought on forests are relatively well studied in the central and southern Sierra Nevada, less is known about its effects on the heavily timbered and diverse forests of northern California. Through sampling 54 0.25 ha plots in northern California, this study compared tree mortality and regeneration patterns before, during, and after California’s most recent record-setting drought. This study evaluated 1) the influence of habitat and competitive covariates on mortality and regeneration trends using ridge regression analysis; and 2) tree death and seedling/sapling establishment dates using dendrochronology and Superposed Epoch Analysis to explore the influence of climate on forest demographics. Montane drought-induced tree mortality occurred primarily in trees smaller than 40 cm diameter at breast height (DBH), with no coastal drought-related mortality in trees with DBH greater than 80 cm. The highest rates of overstory mortality across all sites were observed in Abies grandis (51%), Pinus lambertiana (43%), and Pinus monticola (37%). Picea breweriana (6%) and Picea sitchensis (9%) had the lowest average mortality rates. In montane environments, years with high rates of mortality were positively associated with climatic water deficit (CWD; drier than expected conditions) in the 1-2 years preceding and during tree death dates. Pre-drought montane mortality was greater at wet sites than dry sites, and recent montane mortality (~2013-2020) was positively related with canopy openness. In coastal environments, recent tree mortality was positively associated with maximum temperature and topographic position. Regeneration was dominated by advanced regeneration (median age of 32 years) of shade-tolerant species. In montane environments, regeneration dates were significantly associated with lower-than-average CWD the year proceeding. In coastal environments, regeneration was greater at dry sites than wet sites, and was positively associated with stand density and maximum temperature. These data demonstrate that these forests are not actively perpetuating as diversely into the future, especially in montane environments where more mortality is found in white pine species (Pinus lambertiana and P. monticola) and where the regeneration is weighted towards advanced regeneration of shade-tolerant fir species. This work indicates a need to implement targeted management aimed at generating disturbances to foster balanced and responsive regeneration. This management should preferentially retain medium to larger trees, as these size classes seem to be the least vulnerable to mortality. Such management would be promising for supporting the resilience and diversity of northern California landscapes.
Citation Style
Forest Ecology and Management
Recommended Citation
Lemmo, Sophia LB, "Drought, tree mortality, and regeneration in Northen California" (2022). Cal Poly Humboldt theses and projects. 541.
https://digitalcommons.humboldt.edu/etd/541