Date of Award

9-2017

Degree Name

MS in Forestry Sciences

Advisor

Sarah Bisbing

Abstract

Forest densification in response to a century of fire suppression in Sierra Nevada mixed conifer forests has decreased conifer regeneration and survival. Increases in overstory biomass and decreases in canopy heterogeneity, along with decreases in shrub cover in the understory, has created unfavorable establishment site conditions for conifer species. Establishment site conditions are key in promoting germination and establishment of conifers seeds and in determining the survival of these seedlings into the overstory. These changes in establishment site suitability resulting from the removal of disturbance from these forests has decreased conifer regeneration and survival into other age classes. In Sierra Nevada mixed-conifer forests, the relationship between establishment site conditions and conifer regeneration is multifaceted, and changes to microsite conditions as a result of fire suppression further confounds our understanding of conifer regeneration requirements and survival probability. Additionally, the relationship between Sierra Nevada shrubs and conifer seedlings is complex and not clearly understood, as these studies have been over relatively short time frames, and no research has examined the relationship between shrub cover and conifer seedlings throughout time in the Sierra Nevada. Using a historic dataset beginning 85 years ago and a re-measurement of the same plots, I examined changes to conifer regeneration dynamics from historic to current forests in response to forest densification following a century of fire suppression. I also analyzed the importance of establishment conditions on seedling survival into the canopy and how these factors have changed throughout time. The relationships between seedlings and their establishment site conditions will influence regeneration and survival, which will ultimately determine the structure and composition of future forests.

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