First Advisor

Jennifer Morse

Term of Graduation

Summer 2021

Date of Publication

8-25-2021

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Environmental Science and Management

Department

Environmental Science and Management

Language

English

DOI

10.15760/etd.7638

Physical Description

1 online resource (viii, 74 pages)

Abstract

Forest Park is a 5,100-acre urban forest located in Portland, Oregon, that has been impacted by various anthropogenic stressors including logging, fragmentation, invasive species, air pollution and recreation use due to its proximity to the urban environment. This legacy of land use coupled with natural disturbances has resulted in changes to forest structure, composition, and function--threatening the long-term sustainability of the park. Past research in Forest Park has identified a lack of regenerating shade-tolerant conifers, particularly western hemlock and western red cedar species, in the section of the park closest to the city. Typically, western hemlock and western red cedar typically establish later in Douglas-fir-western hemlock forest types in the Pacific Northwest and the successful regeneration of these species is a critical development process that leads toward multilayered canopy and structurally complex old-growth stand conditions. Achieving this old-growth condition in the park is one of the goals of current management activities include which include, invasive species removal and replanting shade-tolerant conifers in degraded sections of the park. Since conifer recruitment dynamics are less understood in novel urban forests, these management actions would benefit from science-based guidance on current ecological conditions in the park.

In order to better understand conifer recruitment dynamics in this urban forest, a targeted sampling approach was utilized to find and monitor existing shade-tolerant conifer juveniles to characterize and quantify the multi-scale environmental habitat conditions at those sites as possible drivers of juvenile presence. Microsite factors associated with the presence of each juvenile species were modeled using a boosted regression tree approach. Vigor was qualitatively assessed for each juvenile sampled and vigor rankings were analyzed using topographic and stand level factors. Juveniles were typically observed in areas with less understory fern cover (6 - 22%) than the surrounding area (26 - 73%). Coarse woody debris was heavily associated with the presence of western hemlock juveniles with 84% of juveniles found established in downed nurse logs or stumps. Decreased litter cover was also significantly associated with western hemlock juveniles within the park units (56 - 65%). At the stand level, decreased canopy cover density was associated with higher juvenile vigor for both species (~95%) and decreased overstory density (191 trees/ha) was associated with higher vigor for western red cedar juveniles. Juveniles of both species were observed more frequently on north facing slopes and in park management units farther from urban environments. Based on these results, microsite, stand level, and site topographical factors need to be considered when implementing restoration techniques to promote natural regeneration of shade-tolerant conifers and identifying locations to plant new seedlings.

Rights

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Persistent Identifier

https://archives.pdx.edu/ds/psu/36317

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